Hazelwood-FJ-1966-Phd-Thesis.Pdf

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THE PHYSICO-CHEMICAL PROPERTIES 'OW OF LOW MELTIN SALTS.

A thesis submitted to the

University of London

for the degree of Doctor of Philosophy

FRANK JULIAN HAZLEWOOD, B.A. (Cantab).

January, 1966.

Department of Chemical Engineering and Chemical Technology, Imperial College of Science & Technology, London, S.W.7. 2.

Acknowledgements.

The author wishes to express his thanks to

Professor A.R. Ubbelohde, F.R.S., for his advice, and for many ideas which have stimulated this work.

Many thanks are also due to Dr. E. Rhodes, who has not only been a source of encouragement, but has also provided much valuable practical advice.

A maintenance grant from the Ministry of Aviation is also gratefully acknowledged. 3.

Abstract.

A survey of the salts of organic acids was made in order to assess their stability in the molten state, with a view to investigating their properties as ionic melts.

Acetates of the group IA metals proved the most suitable.

A range of physical properties was measured for the sodium and potassium salts, and for some mixtures. In the absence of detailed crystal structures for the solids interpretation of the results in terms of a structure of the melt was somewhat hindered. However a simple lattice model was advanced for the melt which was compatible with the observed data.

In part IV a report is given of an investigation of a transition in crystalline potassium nitrite at 40-50°C.

This part of the work has been published as a paper, con- jointly with Dr. E. Rhodes and Professor A.R. Ubbelohde, F.R.S. and a copy is bound in at the end of the thesis. 4.

Table of contents Page No.

Acknowledgements 2 Abstract 3 List of symbols 6

PART I • INTRODUCTION. 7 CHAPTER 1 Theories of the liquid state. 7 -

CHAPTER 2 Theories relating to transport processes in 26 ionic melts.

CHAPTER 3 The thermodynamics of the solid state I relating to the structure of the solid.

CHAPTER 4 Survey of previous work on the physical, structural - and thermodynamic properties of salts containing organic anions.

PART II EXPERIMENTAL AND RESULTS. - rrt,

CHAPTER 5 Preliminary investigation of the stability 12 of organic salts.

CHAPTER 6 General experimental techniques. RG CHAPTER 7 The Measurements of the freezing points of 04. the melts. .

CHAPTER 8 The Measurement of the electrical conductance 02 CHAPTER 9 The measurement of VlscosIty.

CH =ER 10 111-)4, measurement of the molar volume of **A i3o the solids.

CHAPTER 11 The measurement of the molar volume of the melts.

CHAPTER 12 The me&s14.11L-picnt of the volume change on 15'1 fueon.

CRAFTER 13 The measurement of ultra-violet spectra. 1? 5. Contents - contd. Page No.

PART III : DISCUSSION. I7s- 216

CHAPTER 14 The general properties of acetates as melts. t7 5

CHAPTER 15 The transport properties of molten acetates.

CHAPTER 16 The structural and thermodynamic properties 211 of acetates.

PART IV : THE THERMAL TRANSITION IN POTASSIUM NITRITE. 227 -2ki

Chapter 17 17.1 Introduction. 227

17.2 Experimental. 2-21

17,3 Discussion. 237 List of symbols used

This list is not comprehensive but covers those symbols which constantly recur throughout the work. Where symbols are used infrequently, they are explained on their introduction.

A Helmholz free energy G Gibbs free energy ) per mole V Volume k S Entropy Subscripts 1, 2 refer to components 1,2; refers to partial molar quantities 4 G1 Partial molar free energy of component 1. In Natural logarithm (base e) log Common logarithm (base 10) Z Molar partition function. f Molecular partition function. Subscripts tr, r, v refer to translation, rotation and vibration, respectively. No Avogadro's number. k Boltzmann's constant. Faraday's constant. e Charge on 1 electron. z Valency. Viscosity. Specific conductance. A Equivalent conductance. Fluidity. D. Diffusion coefficient of species i. T Temperature. Tf Temperature of fusion. (Of etc. Heats of fusion. kcm-1 Thousands of wave numbers. 7 CHAPTER 1 THEORIES OF THE LIQUID STATE The exact evaluation of the physical properties of liquids, in terms of their molecular parameters is hindered by the large variation in the properties of the liquid, between its freezing point, and its critical point. At the critical point there is a continuous transition between the liquid and vapour phase, with a continuity of properties. At present, no liquid is known which exhibits a solid/liquid critical point. However, the Uscontinuous step in properties between the solid and liquid, is normally much less than that between the liquid and vapour (when far from the critical region). Thus a theory of the liquid state must account for a nearly regular structure at low temperatures passing into a nearly random structure at temperatures near to the normal boiling point. In many cases more detailed theory has been worked out for the so called "dense gases", than for liquids at the higher densities characteristic of molten salts. 1.1. The relationship between the macroscopic thermo— Aynarnic_properties and the microscopic properties of liquids. The Helmholz free energy A, of a material can be related to the microscopic properties by the equation 49 A =-kT in Z (T,V) Z 10 the molar partition function of the assembly and is defined by Z(T,V) = exp ( Er kT Er is the energy associated with the rth state of the system. From the first equation the other thermodynamic parameters can be derived by the formation of the appropriate partial differentials. Notably an equation of state can be defined by P - 1 'DV /T atnz 4.1

Thus, the evaluation of the molar partition function is of vital importance in the determination of the equilibrium properties of the material. 1.2. The molar partition function of liquid systems An array of particles distributed over the available energy levels in accordance with a Boltzmann type of distribution law is considered. Quantum— mechanical effects are neglected. Eaah particle is free to move throughout the whole of the liquid volume, provided that it does not overlap with any other particle. In a liquid, as distinct from a gas, this exclusion effect will be important, due to the higher density of the system. For this reason also, the interparticle potential energy cannot be neglected when dealing with liquids. Thus the total energy of the assembly will take the form 57 E = 1 n(px2 py2 Pz2) U(r) + 2m Px,Py, Pz are the momenta of the particles along three orthogonal axes, U(r) is the interparticle potential energy, and I the total potential energy due to internal degrees of freedom. Normally either monatomic particles are considered in which case. I = 0, or alternatively the molecules are supposedly so rigid that I is unaffected by the external field in which the molecule moves. In this latter case the contribution of I to the partition function can be treated independently. If, in addition it is supposed that the trans— lational degree of freedom of the particles can be described by classical statistics the contributions of the kinetic and potential energy terms to Z can be separated, leading to N 3/2 = 1 ( Tr mkT ) exp( __U(r) trans FT! ur h3 kT - drN The term in square brackets is the molecular translational

partition function with the volume divided out — ftr V ID

Thus the complete partition function of the liquid can be represented by N Z = ftr fv fr f..... 1 - exp(-1:EcH dri...drN 171.! { V The first term contains all the molecular partition functions, and is only dependent on the temperature of the system. The second term contains the additional terms due to the peculiar nature of the liquid system and is normally known as the configurational integral of the system (3 (3 is the most difficult of the two terms to deal with, due partly to our lack of knowledge of the exact form of U(r) and partly to the mathematical complexities of such an integration. Various approximate methods have been used to overcome this difficulty, amongst which are:- 1)Expansion of the integral, in terms of interactions between small numbers of molecules. 2)Use of a physical model for the liquid so defining the relationship between the particles. 1.3. Distribution function theories of liquids Such an approach has been used to predict p—V—T data for fluids in the region of the critical point (1,2,3). An attempt is made to synthesize the configul— ational integral by combining the n—particle distribution St functions. These functions define the probability that a certain number of particles exist in a certain spatial relationship. Assumptions have to be made to relate the pair distribution function to the triplet one, and higher. For example, Kirkwood has supposed that the probability of three particles being in a particular configuration, is the product , the probability of eacb of the three pair configurations occurring independent1 (Super-position, approximation). This approach,,with its attendant mathematical complexities, has been used for Tluids consisting of inert monatomic particles, but not for ioni, melts, where the electrostatic effects will lead to short rany order. 1.4. The use of phygical models to evaluate the configuration integral. Many of the models used particularly for ionic melts take as a basis the structure of the solid state, This is reasonable in view of the data on X-ray diffraction of liquids.4 There are indications that short range order is preserved, though long range order disappears. These observations, together with the increase in molar volume on fusion, lead to a simple lattice model. /2 6 1.4.1. Simple Lattice model. Frenkel 1935.5 Bresler 1939 Basically this retains the solid lattice, but introduces a large number of Schottky defects (sufficient in number to account for the change in volume on fusion) The equilibrium properties have been evaluated by Miller 7) a and Stupegia! Th t this model is over simplified is shown by the predicted values of the entropy of fusion being only 50% of the observed values. 8 1.4.2. Hole theom. Altar. : Ftrth.9 This is closely related to the former theory except that the holes are not Schott:iiy defects but arise as a result of micro—fluctuations in density. The size of the holes was related to the surface energy, However it is necessary to consider carefully the meaning of the macroscopic surface energy, when dealing with holes of molecular dimensions. This model yields good results for the properties of the halide melts. 10 Another hole theory is due to Bernal and arises from a consideration of the packing of spheres. It is possible to arrange spheres in a 12 co—ordinated manner but in such a way that they form a dodecahedron. This leads to a higher degree of short range interactions, but an assembly of such solids cannot be completely space filling. Thus the macroscopic volume of the system is increased. This type of model introduces the idea of long range disorder in a liquid, while a high 3. degree cf coherence is still retained. As yet however no quantitative.predictions of the behaviour of molten salt systems have been made using this model. 1.4.3. Cell models Much early development on this type of model was done by Lennard—Jones and Devonshire,11 first for hard sphere particles and later for particles with a more realistic potential function. The liquid is imagined to consist of an array of N microscopic cells, each containing one molecule. The configurational integral for the system is replaced by a product of N integrals, since it is assumed that long range forces are absent. Development of the theory is concerned with the best approximations for the shape of the cell, and for the intermolecular force field. The shape of the cell is closely allied to the concept of "free volume" which is that volume throughout which the centre of the molecule can move freely. It is necessarily less than the cell volume. In the original theory of Lennard—Jones and Devonshire the total free volume is equally divided between the cells, and is closely identified with the configurational integral. A problem which arises with this simple cell theory is concerned with the concept of restriction of molecules to specific sites. In a perfect crystal each molecule is this restricted and the molar partition function is expressed by Z = fN When the same material is vapourized the molecules are free to move over all the sites, subject only to their not overlapping. This leads to a reduction of the molar partition function

Z = fN N!

Though this N! does not affect the statistical distribution of the particles over the energy levels, it does effect the absolute magnitude of the predicted entropy and hence the other thermodynamic parameters. This effect — the communal entropy — arises by allowing all the molecules access to the whole free volume of the system. In the simplest cell theory the communal entropy is introduced wholly at the melting point, and clearly will not lead to satisfactory results. Attempts have been made to overcome this problem, by the inclusion of 12 more cells than molecules and by the idea of cell 13 clusters. However none of these approaches leads to liquid—like p.V.T. data rather than to data more suitable for dense gases, and for this reason the simple models have not been applied to ionic melts. 5

1.4.4.Cohen and Turnbull — Free volume model14 This model uses the same idea for free volume as above, with one important difference. The free volume is not associated equally with each cell, but is thermally distributed over the whole of the total volume (in this way it begins to resemble the hole model). This model has been used to interpret transport properties for ionic melts.15 1.4.5. The significant structures model Eyring,Ree,Hirai16 Essential this combines elements of the vacancy models (derived from the solid state) with elements of a compressed gas model. Melting involves the creation of defects in the solid which partially breaks up giving particles of a modified Einstein solid, plus regions which are treated like a gas. This model has been used to predict thermodynamic data for ionic melts, with some success.17,18. Not all of these models have been used to predict data for molten salts, and naturally the amount of comparison between theory and experiment is small. However, in general terms the mathematical complexities of the distribution function theories are such that they have not been applied to salt melts. Despite this, it would still be perhaps the most intellectually sat— isfying'approach. Of the physical models, either the significant structures theory, or the Cohen and Turnbull free volume', "lead to results which are quantitatively in agreement with experimental data. Even so most of the comparisons have been made for the alkali metal halide melts rather than for the type of melt with polyatomic anions which have been the subject of this research. 1.5. A corresponding states approach to molten salts. By dimensional analysis of the configurational 19 integral for molecular liquids Pitzer was able to show that they all ought to fit the same set of p.V.T. data, provideC, that reduced variables related to the critical point, were used. This analysis rests upon the interparticle potential energy being a two parameter equation of the form W = A R Ro) where W is this potential energy R the interparticle separation and A and Ro the adjustable parameters.

Reiss, Mayer and Katz have applied a similar analysis to ionic liquids. Initially it was necessary to justify the assumption of a two parameter potential function for all types of interactions (anion/anion; cation/cation, and anion, cation). This was done by noting that

energetically it is very unfavourable for anions to have anions as nearest neighboursl . and similarly for cations. Therefore, the short range effects due to ions of like charge could be neglected. Then the potential energy could be represented by W = 2: where tiij is the pair interaction energy of the form uij (t (ij)) = Do for t (ij) 1 -(ze)2 fort (ij) ›/ 1 aK (ij)

-i-L-e)2 for ti.e,0 j?

The two adjustable parameters are thus, the function t (ij) and )a distance parameter. ze is the charge on the ion and k the local dielectric constant. This leads eventually to a set of reduced variables in terms of 1 . While A is not a readily available parameter it can be found if it is assumed that the reduced melting point is a universal constant, 2 i.e. A Tm = Z TM = const. 3 x 105 for alkali halides

For the alkali halides this seems to be the case. However other data indicate that this parameter is only constant within a group of salts, but varies from group to group. Using this approach a set of reduced variables &re derived as shown T/Trn (z 6 P I i T 4 K3 m ‘51- KT T 3 V z2. m In the work by Reiss et al. comparisons are made of data in this form for the alkali halides. Self consistent results were found. However, when attempts were made to extend the comparison to other melts (molten group IIA oxides) results were less consistent. This work indicates that while the melting point is not an ideal basis for the definition of corresponding states for molten melts, it has some theoretical justification.

1.6 Theories of melting A less thorough, but none the less instructive, approach to these melts, is to consider them as randomized solids, and to discuss the increase in entropy on melting. Formally, this can be related to the number of ways in which the ions in the liquid or the solid, can be distributed over the available energy levels.

S f R tnt W k7— s In the melt, there will be a greater number of ways of arranging the ions, due to the less ordered configuration of the material. In addition to this positional randorl- 1.9 ization it is possible that non—symmetrical ions may randomize their rotational axes. In melts with particular types of molecules, an increase in entropy due to skeletal flexing (for example long chain organic moleculos)5 or due to the formation of complexes, not necessarily long—lived, and well defined chemical entities (KSCN, KHSO and AgNO3 are cited as examples) is possible. 4 21 Finally, a vibrational term could be included in the entropy of fusion. For simple monatomic salt melts, such as the alkali halides, only the positional entropy is available to determine the entropy of fusion. When polyatomic ions are introduced, other possible modes of entropy uptake may become available, as above. This is often cited as a. reason for the lower melting points of such salts. 1.8 Theories of solutions Solutions are normally discussed in terms of departures from ideality, an ideal solution being one fo.: which the heat of mixing and volume change on mixing are zero. The free energy does not follow a linear inter— polation between the values for the pure components, and can be expressed by

G" = G —(1—x )G1 G 2 where Gm is the free energy of forming a binary solution of components with partial free energies G1 G2°' at a 20: mole fraction $ of component 2. For the ideal solution the partial molar free energies are related to the free energy of the pure component by

G ° G1 1 + RT1n etc. Thus

Cm = RT(1-4)1n(1-x) + RT* in x . and, by differentiation, 5,1v,= R( f-i)ln (ii in (ir-) Deviations from these laws are expressed by the use of activity coefficients I •

G 1 G1 + RT ln y When x is not unity, then the enthalpy, and volume change on mixing are no longer zero, and the free energy and entropy of mixing no longer follow the ideal law. Deviations from these laws are discussed in terms of excess functions. Molten salt mixtures of the type investigated in this work (two cations plus a common anion) have been discussed in terms of the random mixture approximation (cf 22). To evaluate the configurational integral and hence the free energy of the mixture, involves knowing the form of U(r), the interparticle potential energy. This will contain terms due to the interactions between + a the cations A nd and of the cations C of the type 2, cl

AA, BB, CC. To simplify the computation of U(r)7 a cell model is assumed, with an interpenetrating lattice:, of anion and cation cells. The two types of cation are assumed to be randomly distributed over the network cation cells. This model leads to a free energy of mixing given by

Am = (2EAB — EAA. EBB') RT ((1—x)ln(1-70 x.lntx))/ per mole

where 21 is the ionic fraction of B+. The terms EAA, Ewa

and EAB represent the total cationic potential energies for each type of interaction. This type of model allows a prediction of the fit*. energy and entropy of mixing provided that the energy of mixing (effectively the first item) is known. An estfmak of this quantity was made for the alkali halides by /1014..a04, and for a 1-dimensional hard sphere salt mixture by Blander(24) These results showed that provided that fke, structural changes on mixing were small, then the ene.-:gyeY mixing was negative and resulted mainly from the change cationic repulsion energy as a result of a change in cationic spacing.

This type of lattice theory makes no allowance ferr a change in molar volume on mixing, since the cells ax.e... assumed to be of fixed dimensions. It is possible tc allow for small changes in cell size, without disruptive 22 scheme of the argument but if there is a large excess volume of mixing, then it is necessary to consider the potential function in more detail, and better definition of the model is required. The use of this model is also invalidated if the distribution of the cations over the available sites is non—random and in general a mixture having a non zero energy of mixing will also deviate to some extent from a random cation distribution. Guggenheim(25) has treated such mixtures by an approach which utilizes the equilibrium constant of the quasi—chemical reaction A—Apair +B—Bpair 2(A—B ) pair Expressions for the excess energy, free energy and entropy of mixing are developed in terms of the exchange energy of this reaction. Other factors which may influence the size of tL mixing parameter include the change in polarization of te. anion when one cation is substituted •for another. It has been suggested that for the alkali halides this effe_c1.7 is of the same order of magnitude as the observed heat of mixin06). Finally the Van—der—Waals—London dispersioo forces may influence the form of the interionic potential and so affect the heat of mixing. REFERENCES (1) Mayer. Journal of Chemical Physics (1945) 13 276,

( 2 ) Kirkwood. • U " (1946) 14 180. (3) Green. Molecular theory of Fluids. N.Holland Publishing Co.1952. (4) Levy et al. Ann. N.Y.Acad.Sci. (1960) 12 762. (5) Frenkel. Acta Fhysicochem URSS. (1935) 3 633 & 913. (6) Brasier. Acta Physicochem.URSS. (1939) 10 491 (7) Willer & Stupegia. Journal of Chemical Physics. (1957) 26 1522. (8) Altar. Journal of Chemical Physics.(1937) .2. 577 (9) Ftrth Proc.Cambridge Phil.Soc. (1941) 32 252,276, 281. (10) Bernal. (Scientific American. (1960) August p.124 (Nature 2222 68. (11) Lennard—Jones & Devonshire. Froc.Roy.Soc.(1939) A163,53 et seq. (12) Oernuschi & Eyring J.Chem.Phys. (1939) / 547 Ono Mem. Faculty of Eng. K ushu Univ. (1947 10 190 Peek & Hill. J.Chem.Phys. . (1950 173 1252 Rowlinson & Curtiss J.Chem.Phys. (1951) 12 1519 cf Barker—Lattice Theories of the Liquid State — Pergamon. (13) De Boer Physica (1954) 20 655 (14) Cohen & Turnbull. Journal of Chem.Phys. (1957) 22 1049 (1959) 21 1164 (15) Angell. Journal of Physical Chemistry(1964) 68 1917 (16) Eyring, Ree & Hirai. Froc.Nat.Ac-Id.Sci. 44 683 (17) Carleion,Eyring & Ree. Proc.Nat.Acad.Sci. 46 333 References (Cont'd)

(18) Blomgren Ann.N.Y.Acad.Sci. 22 781 (19)Fitzer. Journal of Chem.Physios(1939) 583 (20) Reiss, Mayer & 12 820 Katz (21)Plester,Rogers Proc.Roy.Soc. A235, 469 (1956) & Ubbelohde (22) Fp(rland Chapt.2. Fused Salts. Ed.Sundheim McGraw Hill. (1964) (23) Fe(rland Jerkontorets Ann (1954) 148 455 (24) Blander J.Chem.Phys. (1961) 34 697 (25) Guggenheim Mixtures O.U.F. New York 1952. (26) Lumsden. Disc.Far.Soc.1961. The Structure and properties of some melts. 2.

CHAPTER 2

THEORIES RELATING TO TRANSPORT PROCESSES IN IONIC MELTS

A study of the behaviour of liquids under various types of applied stress should lead to an understanding of the mechanisms by which the liquid adjusts itself to the stress. Ultimately a knowledge of the transport behaviour, combined with the equilibrium thermodynamic properties will lead to an understanding of the molecular motions and of the forces which control them. 2.1 Types of transport phenomena A full macroscopic description of a fluid in motion is embodied in the hydrodynamic equation of state. Normally however it is usual to consider singly the various types of perturbation applied to a fluid, and so characterize the various transport processes. A difference in chemical potential between two points leads to diffusion. In this case there is an unambiguous transfer of molecules down the potential gradient. Temperature gradients lead to the flow of heat energy. Since this can be related to the kinetic energy of the molecules, the redistribution 2. of this energy brought about by collisions is sufficient to transfer the energy. It is not necessary to visualize long range movements of the molecules. An externally applied shearing stress leads to the deformation of the material, and in a liquid as distinct from a solid this deformation is a continuous process — the flow of the material. For the great majority of liquids, the velocity dv gradient dx set up is proportional to the shear stress, ,f, the constant of proportionality being the viscosity coefficient 1. dv f dx On a molecular scale viscosity is visualized as the tranport of momentum, and again collisional mechanisms are sufficient to ensure this. In an electric field the ions in a molten salt tend to acquire a drify velocity towards the oppositely charged electrode. These velocities cannot be measured for a single type of ion, but must necessarily be related to some property such as the conductance of the liquid. V =

u+ and u are the ionic mobilities (the drift velocity in a field of unit strength); T Faradays constant, and VE the volume containing 1 gm equivalent of ions. The two forms of conductance noted are )4? the normally defined specific conductance, and It the equivalent conductance which relates always to the same number of ions between the electrodes. 2.2 Relationships between transport parameters and temperature Chapter 1 has shown that there is a considerable degree of short range order in a liquid, particularly when near to its melting point. Thus the potential field will show a regular variation about a selected molecule over 3-4 molecular diameters. It has been suggested(1) that even for ionic liquids,forces of the Lennard-Jones type describe the situation near to the central ion since in this region the gradient of the coulombic field is much less than that of the field due to -1 r.6 other forces (proportional to r and respectively). Thus while the coulombic field provides the overall cohesive energy it appears as a nearly constant effect when considering the hopping of individual ions into vacant positions. If this is the case, then the treatment of transport processes by the theory of absolute reaction rates(2) can be expected to apply to molten 2.

salts. Eyring and co—workers have considered the perturbation in the periodic potential field around a particle in the liquid, caused by the application of an external stress. A molecule adjacent to -a vacant site is now in the situation where by making an activated hop it can lower its potential energy. By setting up the partition functions for the normal and activated states of the molecule the rate constant for the process can be found. Ultimately expressions are derived for the viscosity and equivalent conductance (3).

12N St VM exp exp RT

23 z ,x,d 2 (If exp St exp (—A H ) RT

AH &AS are the enthalpy and entropy of activation

V ivi is the molar volume of the liquid. is Faradays constant z is the ionic charge d is the half ditance between successive potential minima in the direction of motion X . is the local potential gradient in an external field of 1 volt cm and can be -set equal to

D+2(4) where D is the local Cielectric constant, 3 within the accuracy of this derivation. If the molar volume and entropy of activation are teaperature independent, then these expressions reduce to the traditional exponential form.

p = Ap exp11J21 RT / where p is any transport parameter. For molten salts, accurate experimental data do not substantiate this exponential relation— ship over an extended temperature range. Figure 2.1 shows the slopes of the semi—logarithmic plots

da-A = A H for the conductances d (1/T) A of the alkali halides(5) and indicates that the slope varies for all the alkali halides, and that for some there is a considerable change. Some workers have tried to explain this ley introducing temperature dependence into Ap and ki Hp. Thus 2 R d(tn p) Hp 4 d(1 Hp) * RT d( 1n AP) d(;) d(1) d(i)

The Arrhenius expression requires that both of the latter terms are negligible. However for the alkali halides and probably for many other melts this is not so. An alternative approach is to consider the effect of volume on the transport process. While this is dealt with more fully below, it must be remembered that the theory considered above used Figure 2.1 30 6.0

Cs13.- 5.0 CsI Csa

RbCI. 1400

A ppctrent actmatson Na1137- energy Lin Al Nal aciiTi 3.° Alo.CL

1$0

50 J00 150 • ZOO Fi,ure 2.2

Self diffusion coefficient, calculated from Stokes- Einstein eva.t yi 104

Experimental self clifFusLon coefficient .x 106 activation at a constant volume in its derivation, while molten salt experimentS are normally done at constant pressure. Thus it may be that as the results of high pressure studies become available, the activation energies at constant volume will show better constancy than those in figure 2.1. For 'ion polar liquids it hoe been found Chet activation energies at constant volume are very much less than those at constant pressure suggesting that a volume dependent activation does apply. In its present form this type of theory is useful since it can be related to the postulated energy barriers to ionic movement. Results partiou;axly atowaloue onefil at constant pressure should however- be treated with a degree of caution. 2.3 Relationships between transport properties and volume The above theory has considered that the factor controlling the movement of molecules or ions into an adjacent hole in the liquid, is the potential barrier due to the partially regular arrangement of particles. An alternative approach to the problem, relates the transport parameters to the amount of free space in the liquid. Such a correlation, between viscosity and 3.2 molar volume was first noted by Batohinski(6) who noted that the fluidity () of 80 organio liquids was related to their molar volumesVm by 1

Such a correlation has been shown to be valid for ionic melts such as Cd Cl Cd Br2, Pb Cl 2 2 Pb Br2 Ag Cl Ag Br(7). It was reasoned that the case with which a liquid flowed should be related to the amount of free space in the liquid. This would be less than the molar volume of the liquid, by an amount b cc

i.e. 1 ' V —b

This volume Is was envisaged as similar to the 'co— volume' b in the Van der Waals equation of state. The value of B is reasonably constant from liquid to liquid, though it shows some variation which can be correlated with molecular shape. A. table of values for molten salts is given in chapter 15. The thermal expansion of liquids is due to two main effects. Firstly the vibrational part due to an increase of the vibrational frequency of molecules about their mean positions, and secondly a contribution due to the increase of defects (holes, .3 3 loss of long range order) in the structure. While the second of these unambiguously increases the space in which the particles can move, the effect of the first is not so clear. The Batchinski expression correlates the fluidity with the whole of the thermal expansion, whereas it appears that only a part of this expansion should be used. 2.4 Correlation between transport properties and free volume In chapter 1, the model due to Cohen and Turnbull was mentioned briefly as applicable to the equilibrium properties of the liquid state. This model has been used with some success by the above authors to predict transport phenomena in liquids, particularly for those liquids which can be super— cooled, and which eventually pass into an amorphous glass(8). The idea underlying this theory, is that movement of molecules requires little activation compared 'to kT, but that before a molecule can move from its site, a cooperative movement in several other degrees of freedom, is necessary. These ideas were present in theories by Barrer(9) and Beuche(1o) but were not developed fully. This development postulates that above a certain temperature 3 it

the free volume is not associated in discrete portions with each molecule as in various cell. theories, but can be distributed throughout the liquid in larger or smaller portions, in such a way that there is no change in the energy of the system. Transport then occurs when a molecule and a suitably large portion of the free volume become adjacent. Within the accuracy of this theory there is no necessity to allow for the movement of the molecule from its cell, into the vacant space and then back again as has to be done when considering diffusion in solids(11) Each particle moves in a cage formed by its nearest neighbours. Hard sphere iand Lonnard— Jones particles have both been shown to give rise to an approximate square well potential function by the authors, as have ionic particles(12) Free volume is defined in a particular way. Cohen and Turnbull define an excess volume as the total thermal expansion of the hypothetical liquid from 0°K to the working temperature. This is split into two parts correlated with the form of the potential energy function .within the cell. The main part, corresponding to the flat section of the 3.4 potential energy curve is their free volume 7-f, and the other part, corresponding to the steeply rising potential energy is designated Ak A v = of -Fittvc of is the part of the cell in which the molecule can move freely with its gas kinetic energy 1 velocity u . 3kT 2 ( m The temperature variation of of is supposedly of the form of =m um ,( T — To ) cr, is the mean volume per molecule, derived from molar volume data o f is then the total thermal expansi*n of the liquid from a temperature To. Conversely TQ is the temperature at which the free volume is zero. Cohen and Turnbull equated this value of To with the glass transition temperature of the system. A consideration of the movement of the par— ticle in its cage, togetherwith the statistically most probable distribution of free volume leads to an expression for the diffusion coefficient. D =gau exp (--y V*_ of g and y are numerical factors to allow for the probability that the molecule is moving towards a

3 6

site where diffusion could occur, at the right time, and overlap of various portions of the free volime, respectively. y is the critical sized portion of free volume necessary before transport occurs and a its diameter. These can be related to the size of the diffusing species.

u and of have been defined previously as the gas kinetic energy velocity of the molecule and

( T To)] respectively.

The viscosity and electrical conductance of the material are derived from the diffusion coefficients by the application of the Stokes- Einstein, and Nernst-Einstein relationships(13) (see also section 2.5). 1 — 2 = exp Af - o ) 1 — 2 and A A T exp - 6ft A TQ

for the temperature dependence of viscosity and electrical conductance. These equations reduce to an Arrhenius type dependence but with a temperature dependent pre-exponential factor only if To . 0°K or T To Equations of this type fit the electrical . 37 conductance of glass forming nitrate mixtures better than an Arrhenius expression(13) and were previously found to fit the fluidities of a variety of organic liquids(14). This treatment has attempted to avoid the criticism of the Batchinski expression by splitting the thermal expansion into two parts. However this has necessitated the introduction of an arbitrary parameter To, which can only be equated to a known property, the glass point, for certain systems. In its present form this theory can be as a semi-empirical relationship for fluidities and conductances. 2.5 The validity of the Stokes-Einstein and Nernst-Einstein relationships for molten salts. The original derivation of the Stokes equation considered the classical hydrodynamics of a moving sphere in a viscous continuum. Later derivations based on a more realistic molecular (15) model lead to an equation of the same form i.e.DB.1 kT A air) i where Di is the self-diffusion coefficient of species i and ai is the radius of the moving particle The value of A depends on the method of derivation. A rate theory approach predicts A 1, whereas Stokes equation gives A = G-r- Despite the differences over this constant a simple proportionality relation does seem to apply for those melts whose diffusion coefficients are known. An example is shown in figure 2.2, taken from reference (15) :)The situation as regards the Nernst-EinstAn relation Di RT A, where Di, A i are the diffusion coefficient and equivalent conductance of the ith species, is Faradays constant and z is the charge on the species, is more oomolex. Many attempts have been made to justify it using a molecular, rather than a continuum model, both for dilute solutions, and for ionic melts(15 Amongst the Factors which may invalidate the direct proportionality of D and A are the following. The movement of anions and cations are opposed under the influence of an electrical field, but are in the same sense in a chemical potential gradient. Thus interactions between anions and cations may affect A rather more than D. Amongst the conceivable ionic movements contributing to diffusion (and incidentally to viscosity) are those where a neutral ion pair moves 3 9 as a single unit. Such a movement would not contibute to the electrical conductance of the melt. These effects can lead to large discrepancies between the actual diffftsion coefficients, and those predicted by the Nernst equation, an effect which normally increases with temperature (table 2.1).

Salt T°C I (D +D-)x 104 D .1- D- prtdicted from/i measured deviation

Na Cl 838 ' 1.39 1.63 17 977 1.82 2.47 36 Rb Cl 737 0.75 0.88 17 890 1.17 1.46 24 Cs Cl 670 0.60 0.73 21 790 0.93 1.15 23 Na 1 670 1.05 1.13 8 794 1.44 1.47 1.4

Data from (15). 0 REFERENCES

(1)S.A. Rice Transactions of the Faraday Society 1962 58 499. (2)Eyring J. Chem. Phys. (1936) 4 283. Eyring,. Kincaid Chem. Rev. (1941) 28 301 & Stearn Glasstone Laidler "The Theory of Rate Processes" & Eyring McGraw-Hill 1941 ( 3 ) Bockris, Kitchener Transactions of the Faraday Ignatowioz & Society (1952) 48 76. Tomlinson L (4) Yaffe & van J. Phys. Chem. (1955) 59 118. Artsdalen Huckel Z. Electrochem. 1928 34 546. (5) Yaffe & van J. Phys. Chem. 1956 60 1125 Artsdalen (6)Batchinski Z Physik. Chem. (1913) 84 643 (7)Harrap & Heymann Chem. Rev. 48 45. (8) Cohen & Turnbull J. Chem. Phys. (1958 29 1049. 11 II (1959 21 1164. (1961 li 120 (9)Barrer Trans. Faraday Soc.(1949 1.1.3.. 322. II (1943 39 48. (10)Beuche J. Chem. Phys. (1953) 21 1850 (1956) 24 41,_8 (1959) 75 748 (11)Bardeen & Imperfections innearlyperfect Herring crystals. Wiley. New York (12)Stillinger, J. Chem. Phys. (1960) 32 1837. Kirkwood & Woltowicz (13)Angell J. Phys. Chem. (1964) 68 1917 1 (14)Doolittle J. App- Phys. (1951) 22 1471 (15)of Bloom Chapter 1. Fused Salts.Ed.Sundhcim McGraw-Hill (1964). CHAPTER 3 THE THERMODYNAMICS OF THE SOLID STATE RELATING TO THE STRUCTURE OF THE SOLID

3.1 The nature of the solid state Essentially a crystalline solid is formed by a regular repeating array of its constituent molecules atoms, or ions. The symmetry of the crystal is a consequence of the regularity of its atomic architecture, repeated many times. The forces which lead to the cohesion of such a solid are of several types. Attractive forces of the Van-der-Waals type caused by the mutual polarization of neighbouring particles are present in all solids, as are the repulsive forces, which are effective only at short distances. The presence of permanent dipole moments will also lead to cohesive forces. In addition, for the ionic solids, electrostatic forces will also contribute to these cohesive forces. Those forms having the lowest free energy avoid packing ions of like charges as nearest neighbours. It can be shown that in such a case, the attractive forces between neighbouring ions of opposite charge outweigh the repulsive forces due to ions at greater distances and such a lattice is stable and coherent. The cohesive lattice energy, E, of such a crystal

if 2. can be shown to be

E = — N 2 o ea where e is the electronic charge, No Avogadro's number a, the lattice spacing at 0°K. Ni is the Madelung constant, and can be found in terms of the geometry of the crystal lattice. p is a term which arises from the form of the interparticle repulsions, and can be related to the compressibility of the solid. Lattice energies are high for ionic crystals (table 3.1), much greater than the heats of fusion. There is also little difference between the lattice energies of high and low melting point salts; thus other effects must be sought to explain differences in melting point. Until the crystal structure of the acetate L is known, their lattice energies cannot be calculated. Table 3.1

Salt M.p oC Heat of fusion Lattice Energy k cal mole 1 k cal mole- 1

Na Cl 801 6.69 (3) 183.1 (2) K Cl 770 6.34 (3) 165.4 (2) Na Br 747 6.24 (3) 174.6 (2) K Br 734 6.10 (3) 159.3 (2) Na NO 306 3 3.49 (3) 177.6 (1) KNO (3) 3 338 2.80 162.5 (1) KNO2 429 - 166.6 (1)

3.2 The statistical thermodynamics of crystals As stated in chapter 1, in principle, the key to the thermodynamic properties of a system lies with the partition function Z, given by Z (TiV) exp ( i4E) In a perfect crystal the different energy states, Er, depend on the vibrational frequency of the ions. For a crystal containing 1 mole of monatomic ions. , *ID There are(3 - qvibrational degrees of freedom. The determination of the normal frequencies of such a large number of modes, and the consequent summation to find Z would be an impossible task, and various approximations have been 'used, the best known of which are due to Einstein, and to Debye. The Einstein model assumed that all the — 6) frequencieswere the same, and that further— No more the oscillations were independent, and harmonic. Heat contents derived from this model are satisfac— tory at high temperatures (T III-ie.) but fall off more rapidly than experimental values at. lower temperatures. Such a model is an obvious over simplifica— tion of the true state of the solid. Debye used a more satisfactory model in which a range of frequencies was allowed. This spread from zero to an upper limit set by the total number of possible degrees of freedom. The distribution of oscillators amongst the available frequencies was assumed to be of the type prevailing in black body radiation — a Boltzmann type of distribution. The expressions derived for the energy and heat content of solids depend on the maximum frequency of oscillation V Max e.g. h -9max kT x 4 = 3 No k (kT e x dx h y max (0x...1 )2

where x = by kT

Y max can be said to define a characteristic or Debye temperature E) D such that

e, h max

The Debye equation gives good agreement with results for a large number of solids, down to temperatures in the region of 0°K. 3.3 The thermal expansion If the oscillators used in the above argument are purely harmonic, then since the mean position of each particle would always be on its lattice point, no expansion would result. Anharmonicity in the oscillations leads to a movement of the mean position of the particle with increased frequency and so to an expansion. Since the frequency is dependent on the temperature, a temperature dependent molar volume will result from the use of enharmonic oscillators. The Dobye expression can be treated in this way by allowing a volume dependence of E)D, and forming an equation of state. The volume dependence of can be expressed in terms of the compressibility and coefficient of expansion of the solid. Predictions are in general agreement with observed data, but this extension of the theory is not so successful, or useful as the prediction of heat content. •Accordingly the effects of internal degrees of freedom of the ions, and of defects in the lattice, will be discussed in terms of the heat content, and 1+6 generalizations made to cover the other properties.

3.3 Other sources of heat content in solids If the ions or molecules can occupy two alternative positions in the lattice, with different energies, then this leads to the modification of the heat content curve. The simplest of such cases was considered by Schottky, where the energy of the state was considered independent of the number of molecules or ions already in that state. A simple case like this is not usual; more often the energy of the states depends on how many of the neighbouring molecules are already in that state. The Schottky case however shows that a peak is imposed on the normal specific heat of the material in the neighbourhood of the characteristic temperature of the transition. i.e. 6) = E where E is the energy difference of the states. Calculations for the more usual case are more difficult but it can be shown that a similar peak in the heat content curve is to be expected, though its shape is markedly different from that in the simple Schottky effect. When the particles in the crystal are polyatomic, then it is possible that energy absorbtion 4-7 occurs as a result of rotation of these molecules. Various orientations of the molecule, with respect to the lattice will have different energies, and such a rotation or libration will lead to similar effects to those discussed above. All such transitions in the solid lead to a peak in the specific heat/temperature curve. Since the upper energy level usually allows a more random distribution of molecules, an increase in entropy in the transition region is expected. Molar volumes show no change for a simple Schottky effect transition; however when the energy levels are known to be dependent on the inter—particle interactions then a change in volume, or of thermal expansion is characteristic of the transition. 3.4 Lattice flaws In addition to the above treatment, which has visualized a perfect lattice, heat can be absorbed by the lattice in the production of defects. These normally require an activation energy for their production but above the characteristic temperature for their formation, they are in thermal equilibrium with the lattice. This leads to an additional term in the overall partition function, with consequent changes in the thermodynamic functions. Defects can be of many types, amongst which are: Schottky defects; effectively vacant lattice sites Frenkel defects; caused by the transfer of an ion to an interstitial position Dislocations; Foreign atoms or ions : may be either on lattice sites or interstitial positions In addition to affecting the thermodynamic parameters of the solid, defects can have a profound effect on other properties. In particular their Presence can affect the normal transitions in the solid. Firstly a modification of the energy difference between the states may occur in the presence of defects. This will affect the temperature at which the transition occurs. Secondly, they may affect the movement of molecules, which may occur at a transition, and thus the range of temperature over which the transition is observed. This latter effect is particularly noticeable for the so called 'higher order transitions'. 3.5 The classical thermodynamics of transitions Equilibrium properties of materials are determined by the shape of their free energy surfaces. A transition in the material is visualized as the result of the intersection of the energy surfaces of the two forms. These surfaces are functions of both pressure and temperature but can be represented as in figure 3.1 by sections at constant temperature or pressure. In this diagram form 1 is stable at high temperatures, and 2 at low temperatures, since the fi stable form must have the lowest free energy. The changeover point B at temperature TB is the only one at which both phases can coexist in thermal equilibrium. figure 3.1

Other thermodynamic properties at the transition point B can be found from the appropriate derivatives of G. In certain cases however the free energy surfaces may intersect in such a way that one or more of the derivatives of G is zero. There may be a zero G- ‘1 Gs1 volume change iT or entropy change — / p in such a transition, known generally as one of a higher order. Even in these cases however there is still only one temperature where Gi = G2 and both phases can coexist in thermal equilibrium. Experimentally many transitions of this kind show a region of coexistence 5.0 of both phases, and a hysteresis if the temperature is cycled about the mean transition temperature. In such a region it has been proposed that hybrid crystals consisting of domains of both phases exist, and that the free energy must include terms due to the surface energy (,9) and strain energy (t ) of this hybrid material(4).

i.e. G1 = G(p, T 12'112) etc. These additional terms can lead to a 'thickening' of the free energy surfaces in the region of B and cause a loss of the precise definition of the intersection point. Moreover, since the terms and t will normally be different for phase 1 forming in a matrix of phase 2, and for phase 2 forming in phase 1, then a hysteresis in the thermodynamic functions can occur, about the mean transition temperature, REFERENCES

(1) Morris J. Inorg. Nuclear Chem. (1958) 6 295. (2) Partington Advanced Treatise on Physical Chemistry Vol. 3 Longmans Green (1954). (3) Blander Chapter 3. Molten Salt Chemistry Ed. Blander Interscience (1964). (4) cf Ubbelohde Reactivity of Solids. 4th Int. Symposium. Amsterdam (1960) p 249. 52_ CHAPTER 4

SURVEY OF PREVIOUS WORK ON THE PHYSICAL, STRUCTURAL AND THERMODYNAMIC PROPERTIES OF SALTS CONTAINING ORGANIC ANIONS.

4.1. The properties of this type of salt which have been previously reported can be grouped under the following main headings:-

a) Investigations of Phase equilibria.

b) Structural Investigations.

c) Other physical and thermodynamic propertieta.

4 investigations of the solid-liquid phase equilibria.

The form of the liquidus has been recorded for many systems containing organic anions. Included are systems with more than two components. The majority of workers in this field are Russian, and indeed the earliest recorded work is (1) Russian, by Baskov . This reports the phase diagram, and also the electrical conductivity for the binary system sodium/potassium/acetate. No further data appears in the (2, 3) literature until the work of N.M. Sokolov

This is claimed to be the first systematic investigation of the binary phase diagrams of mixtures of sodium (2) carboxylates , and of sodium carboxylates and sodium nitrate, (3) or sodium thiocyanate . Detailed tables of the systems reported in this and other work, together with the melting, *-"3 points of the pure materials are given at the end of this chapter (Tables 4.1. and 4.2.).

A brief report on the nature of these materials as melts is also given and reiterated in much of the following work. No particular reason is given for interest in these melts in these papers. Many of the salts formed stable melts and could be repeatedly melted and frozen. This applied particularly to carboxylates with short chain lengths. In mixtures with nitrates, they did not explode until the temperature was high enough to cause decomposition of the nitrate.

A whole range of types of phase diagrams were observed, ranging from the formation of a simple e:ttectic (e.g. sodium formate-sodium acetate) to the formation of two liquid phases (e.g. sodium caproate-sodium nitrate).

The method used in this, and most of the other

Russian work, translates as "Visual Polythermal Analysis".

The temperature when crystals are first observed in a slowly cooling well stirred melt is taken as the liquidus temperature.

Other binary systems reported are:- sodium hydroxide/ (4) sodium formate ; 24 miscellaneous salts and organic acids(5); (6) potassium carboxylates and potassium nitrate . Respectively, these are concerned with:- the chemical technology of the manufacture of sodium formate; analytical methods; a systeMatic investigation of the phase diagrams of potassium carboxylateS and potassium nitrite.

Most of the remaining work in this field has been motivated by double decomposition in the absence of solvents, or by interest in these materials as reaction media.

A large range of quaternary reciprocal salt systems with cations ranging from Lithium to Caesium, inorganic anions including nitrates and halides, and organic anions containing one to five carbon atoms have been studied (Table4.1), and references 7 --- 24. A.G. Bergmann(27) reviews some of the effects which may be expected in such systems.

An attempt is made to correlate the equilibrium constant for the reciprocal system

AX + BY --N. AY + BX with the melting points of the components, and the form of liquidus surface found.

Other workers report a few ternary systems (28-30 & 63-64). 4.3. Previous work on the structure of the solids 4.3.1. Infra-red spectra

The widest range of salts studied are those of acetic acid. Spectra have been recorded in the usual manner in Nujol mulls. pressed potassium bromide discs, and also in aqueous solution.

A series of papers by Lecomte and other workers(45-67) record the infra-red spectra of solid crystalline carboxylates of many metals. They found that the characteristic band at approximately 1700 cm-1 due to the stretching of the C = 0 bond in other carbonyl compounds, incldding acetic acid, is missing and is replaced by bonds assigned to the symmetric (1400 cm-1) and anti-symmetric stretching (1550 cm-1) of C-0 bonds in a symmetrical -0.,0Or group. This work has been extended and repeated by other workers 02_34 and 68). These two new frequencies are found to be very dependent on the ionic character of the cation. On this basis, carboxylate group is supposed to be a symmetrical resonance hybrid 0 x/O- O.4 —C 0 \'0 with an 0— C —0 bond angle of 110-130°.

Spinner(69) however suggests that this is not in fact so. He has considered the effects of various electrophilic groups attached to the carboxyl group, on the bands at -1 1400 cm and 1550 cm-1. While the first of these is noticeably affected by these groups, the second is not, suggesting a non-resonating ion R- C . Some evidence for a covalent metal-acetate bond is found in the behaviour of a bond at -1(31). 935 cm Spectroscopic data are summarised in Table 4.3.1.

Other salts for which infra-red spectra are recorded are formates of alkali and alkaline earth metals (35, 36), benzoates (37, 38) and dichloro and trichloro acetates (39).

4.3.2. X-ray crystal structure determinations.

Two papers, one by R.E. Jones and D.H. Templeton the other by Orville Thomas(41) are concerned with the di- (40) mensions of the carboxyl group. gives the dimensions in (41) acetic acid and considers a variety of other situations. (42) W.H. Zacharissen presents a complete analysis of the structure of sodium formate in the solid state at room (43) temperature while S Haussuchl gives unit cell dimensions for calcium and cadmium formates. Unit cell dimensions and space groups are again given for anhydrous and hydrated (44) (45) lithium acetate , hydrated sodium acetate and uranium (IV) acetate

A ferro electric transition at -150°C. in ammonium (47) monochloroacetate is reported by R. Pepinsky et al. together with the unit cell dimensions for the two forms.

Some work has also been done on lithium trichloroacetate (48) The crystal data for the acetate ion are collected in

Table 4.4. Its sparsity hinders interpretation of much of the later work.

4.5.3. Other miscellaneous structural studies.

The unit cell of lithium benzoate has been measured by electron diffraction(49) .

Proton magnetic resonance has been used to study (50) (51) sodium stearate and potassium acetate

Ultra-violet absorption spectra are reported for aqueous solutions of acetates and for acetic acid(52' 55)4

444. Other physical and thermodynamic properties.

In contrast to the Russian work on phase diagrams, no other systematic work has been done on other properties of these materials. Heat capacity entropy and free energy have been measured for sodium formate over a range of 5°K

--4 350°K (54).

For sodium acetate the specific heat of the melt is recorded(55). No temperature is given; the present work suggests that it would be in the range 330°C. to 350°C.

Occasional Russian papers contain references to sources of thermodynamic data for the salts. Thermodynamic data are collected in Table 4.5. 52 Transport properties have been measured for some salts. 56, 1) For *sodium acetate the electrical conductivity( and the viscosity and density(57) have been measured. (1) However` refers to the original work by Baskov in 1915, the only record of which is the entry in Chemical Abstracts, and the rest refer to the 'molten' trihydrate. Dielectric loss and electrical conductance have been measured for a series of soaps:- zinc d:ecanoate, laurate and oleate; magnesium oleate; copper oleate, and lead stearate(59)

Surface tension measurements have been done for potassium acetate (1+2.0 dyne cm-1 at 315°C.) and potassium propio- -1 (70) nate (25.6 dyne cm at 315o C.)(59) . Also reported is the effect of small additions of organic salts (up to 6 mole

56 of sodium acetate) on the surface tension of a lithium sodium, potassium nitrate eutectic at 166°C. In this latter case, the effects found are very similar to those in the corresponding aqueous solutions. Solute molecules were absorbed at the surface, and similar surface pressure/area relationships were found.

Finally, there is some work on the velocity of sound in dilute aqueous solutions of acetates, and in the molten 60) hydrates(71' Table 4.1. Systems studied by Russian workers. 4.1.1. Binary systems and eutectic temperatures

(1) NaY/CH3CO2

Na+/HCO2-, CH3CO2- 242°C. 10.5% Na+/HCO2-, CH3 (CH2)2 CO 1)252°C. 2.5% 2)318°C. 89% Na+/HCO; (CH3)2 CH CO; 1)252°C. 1.3% 2)250°C. 96.5% 4)252°C. Na+/HCO2 (CH 3) 2CHCH 2 0.75% 2)245°C. 94.5%

No solubility Na+/HCO2 C5S 11 CO 2 tt ft Na+/HCO2 C6 H 5 CO 2 it I Na+/HCO2 C17 H35 CO

Na+ CH (CH ) CO- 1)266° /CH 3CO 2 3 2 2 2 c. 33.5% 2)250°C. 695 Na+/CH3CO2 (CH3)2CHCO2 208°C. 58%

Na+/CH3CO2 (CH ) CHCH2 Co2t 156°C. 73% /CH CO- C H 1)268° 34.5% Wa+ 3 2 5 11 CO2- C. 2)260°C. 49.5% 315°C. Na+/CH3C02-., C6 5 5 02- 2.6% Na+/CH 3CO 2' C17 H 35CO 2- immiscible

b 0

Na+/CH3(CH2)2CO2 (CH3)2CH.0O2 continuous series of solutions. /CH (CH ) CO - 257°C. Na+ 3 2 2 2' (CH 3) 2CH CH2 CO 2 90.5% 22.5% Na+/CH 3(CH 2 ) 2 CO 2' C5 H 11 CO2 1)317°C. 2)517°C. 27.5% - 248°C. 15% Na+/CH 3(CH 2 ) 2 CO 2 C17 H 34 co 2 Na+/CH (CH ) CO- C-H CO- °C. 0.15% 3 2 2 6 5 2 330 Na+/(CH5)2CH CO'2' (CH 3 )2 CHCH2 CO 2- continuous solutions 160°c. Na*/(CH3 )2 CHCO2' C5 H11 CO 2 23.5% 162 Na+/(CH3)2CHCO2- C17 H 35 CO 2 °C. 25.5% Na C H CO- 228° +/(CH3)2CHCO2- 65 2 C. 3.5% Na+ - /(CH3 )2 CHCH2, C5 H 11 co 2 continuous solutlors Na ° l-/(CH )2 CH CH2 GO-2 C17 H35CO- 14o c. 17.3% Na+/(CH3)2 CH Cl.!2 CO; 96H5C0.2. 261°C. 3% Na+/C H CO- C H co 239°C. 17.5% 5 11 2' 17 35 2 Na4 1 "/C5 H11 CO 2' C6 H 5 CO 2 371°C. Na/C H CO C H CO- 17 35 2 6 5 2 301°C. 1.3% All of the above are in reference (2). Compositions are given in moles percent referred to the last ion tabulated. 4.1.2. Organic Salt .Eu.tectic temperature E'tectic temp. & & mole percent NaNO3. percent NaSCN. ° HCO2Na 180c. 49% 187 c. 36% CH3CO2Na 224°C. 58% 244°C. 54.5%

C2H5CO2Na 255°C. 56.5% 258°C. 54% CH5(CH2)2CO2Na 267°C. 50% 262°C. 48.5% 4.1.2. Organic Salt Eutectic temperature mole % KNO2 HCO2K 107°C. 33.5 ° 45 CH3co2K 210 C. C2H5CO2K 283°C. 55 C3 H7 CO2 K 1)315°C. 33.5 2)306°C. 45 C4H9CO2K 1)321°c. 37 2)323°C. 47 CO K C. c5H11 2 1)356° 58 2)390°C. 78.5 C H CO K 47.5 6 13 2 1)391°C. 2)389°C. 74 c7H15.co2K 1)320°c. 26 2)344°C 60.5

C8 H172CO K 332°C. 7.5 Reference (6). 4,1.3. Ternary systems

CH3CO2Na: CH3co2K : CdBr2 (28) 2 S (29) CH3CO Na: NaCNS : Na2 203 CH3CO2K : K CNS K2203 (29) CH co Li : CH3CO2Na: CH3CO2K (30) 3 2 CH3CO2Li : CH3co2Rb: CH3CO2Cs (63) CH3CO2Na : CH3CO2K : (c1i3co2)2Cd (64) 4.1.4. Reciprocal systems + + Na K Cl- CH CO- (7) 3 + + Na K Br- CH CO- (8) 3 2 + + Na K I- CH CO- (9) 3 + + Na K CNS HCO- (10) + + Na K CNS n-C CO- (11) 3 H7 .+ + Na NO- HCO- (12) 3 a . + + Na NO- CH CO (13) L 3 3 2 Li+Na+ NO3 CH CH CO- (14) 3 2 .+ + Na NO- CH (CH )C0- (15) L 3 3 2

NO3 HCO- (16) Na+ le 2 Na+ IC+ NO CH CO- 3 3 2 (17) _ + + NA K NO- CH CO- 3 + + Na K NO- H CO- (18) 3 C22 + + Li K NO- CH CO- (19) 3 3 2

Na R + NO- CH CO- (20) 3 3 K+ Cs+ NO- CH CO- (21) 3 3 + + Na K CH CO- C2 H CO- (22) 3 5 2 + Na K CH CO- CH (CH CO- (23) 3 3 2 )2 + + Na K CH CO- (CH CH CO (24) 3 2 3 )2 2 + + Na K CH CO- CH (CH CO- (25) 3 3 2 )4 2 + + Na K CH CO (CH CH.(CH ) CO- (26) 3 3 )2 22 + + Na K CH CO (CH CH CH .00- (62) 3 2 3 )2 2 3 Table L.'2 Mau" points Of soli* contains or9anitc anions. ( It).

LithLuni. 'R 446 ich C acs tam . 4. • F.0 rmate. 273_ 258 167

2c2" 2.9170 324." L s i. . zo 21 Acetate 33i ; 302- 310 23 6 t80 328 ' 30V . 2,4.0" 185" 327 3011

Propionate 3294 2983 3 65 *

Butyrate 329's 3301 4.04.°

6 iso y 2603 365 butyrate 16224 36o 24

3 Valerate 357 Lt JO*

is o 1623 3966 ualerate 2bo 33 396"

Cafroote 365 3 44.4-•5 6

Capril ate

Pclar,yongete 421 Table 4.3. 4.3.1. Summary of the infra-red spectral data for acetates.

Frequency Assignment

2980 cm-1 CH anti-symmetric stretch. -1 2930 cm CH symmetric stretch. -1 ,, 1550 cm CO2 anti-symmetric stretch.

4-CH

0 '0

e..! 1400 cm O0 symmetric stretch

Many bands at lower frequencies due to bond bending.

4.3.2. Ultra Violet Spectra r Band edge at 228 mg 114 (43.86 kcm-1) with a molar absorption of t2„ 1.0 for a solution -2 containing 2.2 x 10 moles 1-1

Ref. (52).

Zto

5 Table 4.4. Crystal structure data for acetates CH3CO2Na. 3H20

Space group C2h - C2/M Unit cell a 12.4 R b 10.5 R p = 112.1° 10.3 R density = 1.45 gm.co-1. molecules per unit cell. Ref.(45). Table 4.5. Thermodynamic parameters of organic salts Heats of formation (k.cal./mole) Salt 0PC./1 atm. 18°011 atm. 400°0./1 atm. HCO2 Na - 160(10) HCO2 K _ 157.7(10) -171.16 (73) -175.10 CH3CO2Na - 169.8 (73) (73) -384.71(73) CH3 CO2 Na3H20 -383.50 (73) (73) CH3CO2K -173.2 -174.48 LI (20) CH3 CO2 -184.6 Heats of reaction LINO + CH CO K CH3CO2Li4KNO3 ilH = -1.26 k.cal (19) 3 3 2 + 12.65 k.cal (20) LiNO3 + CH3CO2Na CH CO Li NaN0 4,H = -9.6 k.cal (20) C-- 3 2 3 6L

Miscellaneous H CO Na 2 -CH3002Na Specific heat Cp 19.76 cal.mole 41.7 cal/mole. Entropy 24.80 eu. At 298.15°K (55) Enthalpy 3766.9 cal.mole-1 Gibbs function 141.4 cal.mole 67 REFERENCES

(1)Baskov J. Russ Phys.Chem.Soc. 1915, 47, 1533. (Not available in U.K.).

(2)N.M.Sokolov Zhurnal Obschei Khimii 1954, 24, 1581-93

it it tt (3) 1954, 24, 1150.

(4)G.D. Sirotkin J. Applied Chem. USSR 1950, 23, 285. (5)M.Kh. Gluzman & Zhur. Fiz.Khim. 1960, .21, 2742-7. V.P. Rubtsova (6)N.M. Sokolov & Zhur.Neorg.Khim. 1961, 6, 2558. M.A. Munch

(7)I.I. Il'yasov & Zhur. Obschei Khim. 22, 355-8 A.G. Bergmann

(8) Ibid. 11 tt 31, 368. (9)G.G. Diogenov & Nauk Doklady Vysshei Shkoly Khim. A.M. Erlykov. Khim. Tekhnol 1958, No.3, 413-6. (10)N.M. Sokolov & Zhur Obschei Khim. 28, 1391-7. E.I. Pochtakova (11) Ibid. It It 28, 1693-1700. (12) N.M. Tsindrik Zhur. Obschei Khim. 28, 830-8. (13) G.G. Diogenov Zhur. Reorg. Khim. 1, 799-805. (14)N.M. Tsindrik & Zhur Obschei Khim. 28, 1404-10. N.M. Sokolov.

It It (15) Ibid. 28, 1728-33.

it (16) I. Dmitrevskaya it If 28, 299-304.

(17)A.G. Bergman & Isvest-Sektora Fiz.Khem.Anal.Inst. K.A. Evdokimova Obschei i Neorg Khim Acad Nauk S.S.S.R. L, 296-314. 68

(References - contd.).

(18)0.I. Dmitrevskaya & Zhur. Ohschei Khim. 28, 2920-6. N.M. Sokolov.

(19)G.G. Diogenov, Zhur.Neorg Khim 2, 1596-1600. N.N. Nurminskii & V.G. Himel'stein

(20)V.G. Himel'stein & Zhur. Neorg Khim. 3, 1644- ' G.G. Diogenov.

(21)N.N. Nurminskii & II II " 5, 2084 G.G. Diogenov. (22) N.M. Sokolov & E.I. Zhur. Obschei Khim. 28, 1397-1404. Pochtakova.

(23) Ibid II II 11 30, 1401-5. (24) Ibid 11 II II 30, 1405-10. (25) E.I. Pochtakova u u u 29, 3183-9. (26) Ibid It 11 11 33, 342-7. (27)A.G. Bergmann & Soviet Research in Fused Salts. G.A. Bukhalova Chemistry Collection No.1. Consultants Bureau. p.245. (28)I.I. Ilyasov & Zhur. Obschei Khim. 1075-8. A.G. Bergmann. (29)M.S. Golubeva, Zhur. Neorg Khim. 4, 2606-10. N.N. Aleshkina & A.G. Bergmann. (30)G.G. Diogenov " " 1, 2551-5. (31)F. Vratny, C.N.R.Rao Anal. Chem. 33, 1455. & M. Dil3ins. (32)A.I. Grigorev. Zhur. Neorg Khim. 8, 302. (33)L.L. Shevchenko Ukrain Khim Zhur 29, 1247-56 Uspekii Khimii 32,(4) 457-69.

(34)R. Goto & T.Takenaka J. Chem. Soc., Japan, 84, 392. 69 (References - contd.). (35)K.B. Harvey, B.A. Morrow •Can.J.Chem. 1963, 41, 1181-7. & H.F. Shurrell. (36)C.J. Shutte. & Spectrochimica Acta 20, 187-95. K. Buijs. (37)J.H.S. Green, Spectrochimica Acta, 17, 486-502. W. Kynaston & A.S. Lindsey. (38)D. Hadzi & A.Novak. Nuovo Cimento, 11, Supplement 3, 715-22.

(39)Cl.Duval, J. Lecomte Bull.Soc.Chim. 9, 263-74. & Mme.F. Douville.

(40)R.E. Jones & Acta Cryst. 11, 484. D.H. Templeton. (41)Orville Thomas. J.C.P. 18, 761. (42)W.H. Zachariasen J.A.C.S. 62, 1011 (43)S. Haussuchl. Fortschritte der Mineralogie 221 345. (44)Carol P. Saunderson Acta.Cryst. 14, 321. & R.B. Fergusson.

(45)V.M. Padmanabhan Current Science (India), 21, 97. (46)I. Jelenic,: Grdenic Acta Cryst. 17, 758. & A.Bezjac. (47)R. Pepinsky, Y. Okaya Acta Cryst. 10, 600. & T. Mitsui. (48)D. Tuomi. University Microfilms Pub.No.24517, 192.

(49)M.M. Stasoya Kristallografiya 4, 243. (50)M.R. Barr & B.A.Dunell. Can. J. Chem. 42, 1098. (51)Mashashi Yagi & Science Reports of Tahoku University, Yuichiro Ishikawa. 1st series, 43, 148-51. 70 (References - contd.).

(52)E Buck, Samang Analytical Chemistry, 26, 1240-2. Singhadeja & L.B. Rogers.

(53)Organic Electronic Spectra Data Vol.I, Ed.Kamlet.Pub. Interscience.

(54)E.F. Westrum Jnr., J.P.C. 64, 1553-4. Shu-Sing Chang & N.E. Levitum.

(55)M. Bizouard & F. Panty. Compt.Rendu2. 252, 514,-5. (56)Augustus Levi Atti r inst Veneto 74, 1167-78.

(57)G.L. Kobus Trudy Odessk Gidrometeorol Inst. 1956 No.8, 29-35. (58)P.G.T. Fogg & R.C.Pink J.Chem.:Sc?.. 1959,1735-39.

(59)V.K. Semenchenko & L.P. Shikobalova Khim. 21, 613-22 (1947). (60)P.R.K.L. Padmini & Nature, 191, 694. B. Ramachandra Rao.

(61)International Handbook of Physics and Chemistry, 34th Edn. (6- )

(63)G.G. Diogenov & Zhur. Neorg. Khim. 9, 482-7. I.F. Sarapulova.

(64)I. Il'yasov Zhur.Obschei Khim. 32, 347-9- (65)J. Lecomte & Comptes Rendues. 208, 1401- R. Freymann.

(66)C. Duval, F. Donvill‘ Comptes Renducs 212, 953- & J. Lecomte. (67)C. Duval, J.Lecomte& Annales de Physique 11%erie 17, 5. F. Douville.

(68)Nakamura J. Chem. Soc., Japan (Pure Chem.) 79, 1411. 7 ' (References - contd.).

(69)E. Spinner J. Chem. Soc. (1964) 4217-26. (70)K.F. Guenther J.Phys.Chem. 67, 2851-3. (71)M. Suryanarayana. J. Sci. Ind. Res. 21B, No.2, 57-61. (72)Wells Structural Inorganic Chemistry Table of Covalent Radii. (73)Handbook of Physics and Chemistry (Rubber Publishing Co-) 37th Ed. pp.1682 & 1707. 7Z cHAE12_5 PRELIMINARY INVESTIGATION OF THE THERMAL STABILITY OF ORGANIC MELTS. 5.1. Bearing- in mind the results of previous workers (Chapter 4) and the type of measurement which was intended a list of potentially interesting salts was compiled. A series of preliminary experiments were done on these salts to determine the thermal stability of each salt in the molten state. 5.2. Salts examined Nominal Melting Point

Potassium trichioroacetate >400°C. Sodium trichioroacetate >400°C. Potassium trifluoroacetate 135-7°C. Potassium chlorodifluoro acetate decomposes 170°C. Sodium trifluoro acetate 200-205°C.

Potassium mono fluoroacetate 197°C.

Potassium Benzoate •v 440°0. Potassium Benzenesulphonate >400°C. Potassium orthotoluate 169°C. Potassium orthochlorobenzoate 235-40°C. Sodium paratoluene sulphonate decomposes 350°C. Potassium Methyl Sulphate 208-9°C.

73 Salts examined Nominal Melting Point.

Sodium Methyl Sulphate 194°C.

Sodium Acetate 328°C.

Potassium Acetate 204°C.

Sodium Formate 258°C

Potassium Formate 0".,150°C.

5.3. Preparation of samples for testing.

The acetates, formates, methyl sulphates, potassium benzoate, benzene sulphonate and sodium paratoluene sulphonate were supplied by chemical companies. With the exception of sodium acetate, which was A.R. grade, they were all general purpose reagent grades. Many of the salts were supplied in a moist condition, and some, notably potassium acetate, were deliquescent.

All of the salts were dried before use under a vacuum of 0.01 mm. of mercuryibr a minimum period of 12 hours. Drying was carried out at room temperature in order to minimise the risk of hydrolysis. The remaining salts in the list were prepared by the addition of the appropriate alkate metal hydroxide to the acid. Approximately 5N aqueous solutions were used. In order to minimise the chances of hydrolysis of the salt, only 95g of the theoretical quantity of alkali was added to the acid, thus ensuring that the solution was always acid. 74_ From this solution, salt was extracted by evapor-

ration under a presssure of 1-5 mm. Hg. The wet mass of

crystals so obtained, was drained and dried in the same

manner as the commercially obtained salts.

5.4. The effect of melting the materials.

A Gallenkamp melting point apparatus was used to

observe the melting points and thermal stability of most

of the salts. This consisted of electrically heated

aluminium block, pierced vertically to hold 2 mm. diameter capillary tubes of the material, and a mercury in glass

thermometer. A horizontal aperture allowed observation

of the samples. This did not give such accurate results

as more sophisticated methods, but was self-consistent and

appeared to be in error by not more than 2-3°C. at 300°C.

All melting points quoted in this section refer to

this method unless otherwise stated.

In many cases the thermal instability of the salt

was immediately noticeable during the observation of its melting point. Where this was not the case, a sample was

sealed under vacuum into an ampoule and maintained at a

temperature of 10-20°C. above its melting point in an

electrically heated steel block. Observations were made

at intervals as decomposition occurred. 5.5. Results. 5.5.1. Potassium trichloroacetate.

The melting point was higher than 400°C. Charring

of the solid was observed on heating at 300°C. One sample

burst its container, indicating gas evolution.

5.5.2. Sodium trichloroacetate.

Some softening of the solid was noted at 250°C. but the material did not melt or char up to 400°C. 5.5.3. Potassium trifluoroacetate.

This melted over the range 135-7°C. Maintained at 14515°C. the melt started to char after 4 hours, and gas evolution was noted after 12 hours.

5.5.4. Sodium trifluoroacetate.

This salt melts with decomposition at 200-205°C. as

reported by other workers(l).

5.5.5. Potassium chlorodifluoracetate.

This salt began to decompose and char atev170°C. No

signs of melting were observed.

5.5.6. Potassium monofluoroacetate.

Melting, with simultaneous decomposition at 197°C.

was observed for this salt.

5.5.7. Potassium Benzoate.

A melting point, with simulateneous decomposition

was observed at 440oC. 7' 5.5.8. Potassium benzene sulphonate.

This salt decomposed atoy400°C. without melting, though some sintering of the material occurred at 360-370°C. 5.5.9. Potassium o-toluate. Some sintering was observed at 120°C. and the material melted, without decomposition at 169°C. Maintained at

180 ± 5°C. the material charred within a period of 12 hours. 5.5.10. Potassium o-chlorobenzoate.

This salt molts at 235-240°C. to an amber liquid. Within 30 minutes, the melt begins to darken, and gas bubbles are formed.

5.5.11. Sodium p-toluenesulphonate. Decomposition without melting occurred at 350°C.

5.5.12. Potassium methyl sulphate.

This softened at 197°C. and melted, with simultaneous gas evolution, but no charring at 208-90C.A melting point, with decomposition of 233°C. is recorded in the literature(2). 5.5.13. Sodium methyl sulphate. Considerable expansion of the solid was noticed at 120-140°C. and the material finally melted with simultaneous decomposition at 194°C. 5.5.14. Sodium acetate. This material, sup-)lied as A.R. grade melted to a clear liquid at 324-5°C. In a sealed tube, the melt showed 77 little charring until 24 hours had elapsed.

5.5.15. Potassium acetate.

A melting point of 297 C. was noted. The melt remained c7.emr at 330°C. for approximately 24 hours, when confined in a sealed tube. It could be briefly heated to

35000.9 before gas evolution became apparent, without caus-

ing decomposition.

5.5.16. Sodium formate.

A stable melt was formed at 258°C. Gas evolution

was not observed until 280°C. and charring at 310°C.

5.5,17. Potassium formate.

This material, like potassium acetate, is very

hytroscopic. A melting point of 150°C. was recorded which

is lower than the literature value of 167.5°C. It is poss,

ible tere.was some contamination by a hydrate. The

melt w7,s clear and remained so until gas evolution began

at 250-280°0.

5.6. Cbemicaa reactions which may be occurring in 5.5. Since these reactions are observed at 200-300°C.,

it is to be expected that the overall reactions will be

complicated by competing side reactions.

Decomposition of the.trifluoro acetates has been

investigated(1) and the following main products identified, -18 depending on the conditions. In the absence of sodium hydroxide, trifluoracetyl fluoride is formed in about 90% yield. This product is replaced by tetrafluoroethylene as the major product in the presence of sodium hydroxide.

Pyrolysis of potassium methyl and ethyl sulphates has been studied(2) by thermogravimetric analysis, and gas evolution. Decomposition began at temperatures just above the melting point to give the dimethyl ether. At higher temperatures sulphur trioxide was also formed by decomposition of the intermediate potassium salt. i.e. 2 K (CH )60 K S 0 0 3 4 2 2 7 + (CH3)2 K S.0 --+ K2SO4 + SO 2 2 7 3 Normal carboxylates can give a variety of products on pyrolysis. The most likely ones from the acetates are methane, and higher hydrocarbons, or acetone and other ketones. Discussion of this decomposition is covered more fully in Chapter 14.

5.7. Selection of materials and further experiments on thermal stability.

The results given in Section 5.5. indicate that the only salts in the list which have reasonably low melting

points (<400°C.) and form liquids with any degree of thermal 79 stability are the formates and acetates and, as a borderline

case, potassium trifluoroacetate. It was felt that the two acetates were the most suitable salts for further study.

The formates were in fact the most stable salts, but since this work was motivated by a desire to discover the properties of melts with ions of different shapes from the nitrates, halides and sulphates previous studied, it was apparent that the salt with the largest anion, would be the most suitable.

The use of potassium trifluoroacetate (presumably a larger and more irregular anion than the simple acetate) was dis- couraged partially on the grounds of cost, partially on the

grounds of inconvenience (the salt was much less stable than the acetates) and partly on the grounds of toxic hazard

(fluorinated olefines are amongst the gaseous decomposition

products).

Further stability tests to ascertain the effects of impurities and additives on the thermal stability of the acetates were undertaken. Recrystallization of both salts was attempted. 75% of Industrial Alcohol + 25% distilled was was a suitable solvent for the sodium salt. The potassium salt was not so readily crystallized but a

moderate yield of plate-like crystals could be obtained from a mixture of 66.6% industrial alcohol and 33.3% acetone. o When the recrystallized material had been intensively dried, the melting points were of the order of 1°C. higher than the original dried samples. However, the melting point of crystals obtained by evaporating the mother liquor to dryness was not substantially lower than that of the original materials, which indicates that very little impurity was being rejected into the mother liquor.

In view of the limited accuracy of this method of melting point determination, the observed increase in melting point with recrystallization is of little value.

Thermal stability tests were carried out using both recrystallized and "as supplied" sodium acetate, using various additives. Later, the significant experiments were repeated with the potassium salt.

Glass tubes approximately 12" long and 1/4" internal diameter open at the top, containing the salt were placed in a molten salt thermostat.bath (Chapter 6.5) maintained at 335 t 0.1°C. By using the commercially supplied material as a control, it was possible to correlate the effect of various additives.

As expected from general knowledge of the chemistry of carboxylic acids and their salts, a small proportion of sodium hydroxide markedly reduced the time needed before gas evolution occurred, presumably by causing decarboxylation by:-

CH3COONa + NaOH --* CH4 + Na2CO3

Logically the addition of sodium carbonate slightly increased the stability with respect to the untreated material.

The addition of a few drops of acetic acid, or water noticeably increased the stability of the melt. Typically, the untreated material started to darken in colour after 1/2 1 2 hours and material tested with water remained clear for 3 hours. Small quantities of B 0 and NaH PO 2 3 2 4 were added in order to see if very slight acidity, or alka- linity could be used to stabilise the material, but both led to a more rapid decomposition of the salt than the control.

A trace of cupric acetate caused rapid decomposition.

During the course of these experiments, it gradually became apparent that in all cases, the recrystallized material was rather less stable than the material as supplied.

Furthermore, when any aqueous solution was added, either to the melt, or to the solid, an increase in stability was noted with respect to dry material. The water vapourised rapidly, and then condensed on the colder part of the tube where it remained for-2-3 hours before it finally disappeared. 82.. Under these circumstances, all of the air would be forced from the tube and would not begin to diffuse back in until most of the water had distilled away.

A sample melted under vacuum decomposed immediately by gas evolution, which contained so long as the pump was connected to the tube. As soon as the pump was disconnected, it died away, and recommenced when the tube was again evacuated.

Arrangements were made, whereby a supply of "white spot" nitrogen (Chapter 6.1) could be passed into the tube by a capillary and so out to the atmosphere. In a typical case, while the normal dried salt showed a marked discolouration after 3 hours, when nitrogen was passed through the tube no discolouration was apparent for about 6 hours.

The inference, that the decomposition is caused by the presence of oxygen, was proved satisfactorily by passing oxygen through the melts. Within minutes of starting the flow of oxygen the melt rapidly turned to a dark brown colour (Table 5.1.at end of Chapter).

Further experiments showed that paracetic acid

CH C 3 s'‘O — OH caused discolouration almost instantly. It would seem possible therefore, that the decomposition of inorganic acetates in g. 3 the molten state and, in the absence of alkali, proceeds by free radical chain mechanism which is catalsed by the presence of oxygen (See also Chapter 14).

Experience has now shown that acetate melts can be handled satisfactorily if oxygen is excluded rigorously.

The best procedure is to evacuate the container of solid and refill with white spot nitrogen several times, before finally melting the material under white spot nitrogen.

It is preferable that this gas is passed through the melt immediately after melting (this also facilitates final removal of water) and whenever else possible during a run.

Eventually the melt will begin to discolour and evolve gas bubbles. The time that the melt can be preserved is a function of both temperature and time. About 370°C. is the highest temperature that has been reached before gas evolution became serious, but at lower temperatures melts can be kept for 12-24 hours and occasionally longer.

To give an order of magnitude comparison of the effect of oxygen and nitrogen on the melts, a typical run for sodium acetate at 335°C. is shown (Table 5.1.). S 4 Table 5.1.

Time in Sodium Acetate Sodium Acetate Sodium Acetate hours. Nitrogen passing Oxygen passing through melt. through melt.

0.0 All materials molten, giving clear melts. nio All melts clear.

Turned on nitrogen supply.

All melts clear. Turned on Oxygen supply. 20 o 6o Clear Clear Brown.

Very slight Clear Dark Brown. discolouration.

1.0 Slight brown. Clear Black.

22 Brown. Clear Black.- 2 discontinued.

3 Brown Clear

22 Dark Brown Clear liquid though the surface & walls of tube are discoloured. 10 22g-0- Replaced nitrogen supply with oxygen supply. Almost instantaneous blackening of melt. REFERENCES

(1)"Aliphatic fluorine compounds" A.M. Lovelace, P.A. Ranck & W. Posteluck. ACS (Reinhold) 1958. (2)J.W. Wendlandt & E. Sturm. J. Inorg. and Nuclear Chem. 25, 535 (1963). CHAPTER 6 GENERAL EXPERIMENTAL TECHNIQUES.

6.1. Specification of materials used Sodium Acetate

Hopkins & Williams 'Analar' Anhydrous Maximum limits of impurities:-

Chloride 0.002%

Sulphate 0.01% Water 2.0% After drying as described, freezing points as determined in Chapter 7, taken at various times and on successive batches, varied between 329.4°C. and 329.0°C. Potassium Acetate British Drug Houses 'Laboratory Reagent'.

Not less than 99% potassium acetate.

Chloride 1°. 0.05% Heavy Metals 0.001% Iron 0.001% Sulphate 0.05% The dried material had a freezing point which varied over the range 304.9°C. to 304.0°C. taken at various times and on various batches. Nitrogen

British Oxygen Company 'White spot'. 87

Nitrogen 4 99.9% Oxygen 4: 10 p.p.m. Carbondioxide 4, 5 p.p.m.

Water - average - 0.01 gm. per cubic metre.

6.2. Drying of salts.

Since both pure salts were hygroscopic, potassium acetate in particular, it was necessary to confine periods of handling the dried salts, in the laboratory atmosphere, to a minimum.

Immediately before use, the salt to be used in a particular measurement was dried by evacuating to a pressure of 0.001 --4-0v01 mm.Hg. for a period of at least 24 hours, and generally for longer. No heat was used to aid the drying, as it had been shown (Chapter 5) that the molten salts decomposed under a moderate vacuum and it was felt that decomposition might be caused by heating the salt in a high vacuum.

6.3. General considerations related to the design of apparatus and handling technique.

Preliminary experiments have shown that the exclusion of oxygen from the melt is vital to ensure a reproducible stable system. A secondary consideration is that since the salts are hygroscopic, the solids should not be exposed to 88

laboratory air for long periods.

These considerations dictate the use of closed systems for measurements wherever possible, using 'white

spot' nitrogen as an inert atmosphere. Furthermore, it is important that all oxygen should be removed from the jailer-

-sUcts of the solid-and replacednbynitrogen'befere melting is com-

menced. In all cases a similar procedure was followed. The dried solid was quickly transferred to the measuring

apparatus in a glove box filled with nitrogen, if any

prolonged manipulation was required. Following this, the

whole apparatus, including any relevant nitrogen supply

lines was evacuated. Depending on the particular apparatus

in use, a pressure of 0.01 to 0.1 mm. Hg. could be reached.

By means of a system of two way taps, the whole apparatus

was filled with dry, white spot nitrogen. This procedure

was repeated four or five times after which melting of

the salt could be undertaken.

6.4. Preparation of the mixtures.

In order that the determinations of molar volume,

viscosity and conductivity, could be made on material of

the same composition, 500 gm. of the 53.7 CH3C0K mix-

ture were prepared in the apparatus shown in Fig:6.1. Ft u. re 6-1 Vacuum connection

Mixtures apparatus.

Thermocouple pocket . Stirri, device.

F2. :Ire 112 The melt ••• bath. R0 The dried salts were weighed into the flask, weighings being done by difference, while the flask was kept in the dry atmosphere of the glove box. The lower melting point, potassium salt was placed in the foot of the flask. After flushing out the oxygen as described above, the flask was immersed in a molten salt bath at

31500., while a vigorous flow of nitrogen was maintained to stir the acetate melt. As more of the sodium salt dis- solved, the temperature of the bath was lowered towards

24000. until all the salt was molten. After stirring for

30 minutes to allow for complete mixing, the salt was solidified, transferred to the glove box and broken out of the apparatus.'

For the determination of the molar volumes of the mixed melts a slight modification of the above procedure was used for the smaller samples required. A small furnace was built for use in the glove box. This was fitted with facilities for stirring the melt and for passing nitrogen.

This was more convenient for the preparation of small samples

(approximately 70 gm.). After the sample had been thoroughly mixed, it was frozen, crushed and treated as described.

6.5. The Thermostat baths.

For most of the measurements a thermostat bath, as illustrated in 6.2. was used to provide a constant temperature 40 environment during the time of a measurement.

A 5 litre pyrex beaker formed the container for the ternary eutectic (KNO3 : NaNO3 : NaNO2 : 40 :- 7 :53 by weight) which was used as the heating fluid. This liquid is clear and does not corrode stainless steel, glass or ceramics in the range 160-450°C. The bath is insulated by a 2" layer of vermiculate (A). Heat is supplied by a

1 500 watt inconel sheathed coiled heater B whose input is controlled by a variac. A stainless steel propellor C driven at a constant 200 r.p.m. and angled at 10° from vertical, provides efficient circulation of liquid. Tem- perature control is by a bridge type, saturable r3actor temperature controller. The sensing element is a glass encased, 50 ohm, platinum resistance thermometer D, and the control heater a 500 watt ceramic sheathed heater (E).

A viewing slit F is provided in the front of the insulation and the thermostat is illuminated by a light bulb G.

For measurements from 50°C. to 200°C. a similar thermostat bath filled with silicone fluid (Midland Silicones

M.S.550) was used. This particular oil remains comparatively colourless, and does not start to volatilise until temperatures of above 220°C. are reached.. In order to q z

maintain temperature control at low temperatures (50-120°C.)

a water cooling coil had to be used.

For temperatures in the range, ambient 4 50°C. a

Townson & Mercer water thermostat bath was used.

Temperature fluctuations in the molten salt bath

amounted to no more than ± 0.025°C. after the bath had

been allowed to equilibrate. The silicone oil bath could

be controlled to this accuracy at the higher temperatures

but showed greater fluctuations at lower temperatures

( ± 0.05°C.). The specification of the water bath

claimed a temperature control to ± 0.03°C.

6.7.. The measurement of temperature.

Johnson-Matthey 'Pallador' thermocouples (Pd/Au)

(Pt/Ir) were used in the range 50°C. —4 450°C. They

were calibrated against an N.P.L. standardised platinum

resistance thermometer reading to 0.01°C. Taking into

account various errors in calibrating, and reading a

thermocouple, the error in temperature measurement from

this source was estimated at ± 0.05°C.

Since the thermocouple was maintained at temperatures

of the order of 350°C. for several weeks at a time, it was necessary to re-calibrate it at approximately 9 month intervals, to allow for the change in e.m.f. due to diffusion at the hot junction. In a typical case, the e.m.f. at 300°C. changed from 14.380 mV to 14.220 mV, over the course of ai years.

Temperatures in the range 0-40°C. were measured by means of mercury in glass thermometers graduated 0-40°C. by 1/10°C. -4- CHAPTER 7

MEASUREMENT OF THE FREEZING POINT OF THE MELTS.

7.1. The principle of the method.

The usual procedure of allowing a well stirred sample to cool slowly while recording its temperature,

was followed. At any transition point in the system, at which latent heat is evolved, a change in slope of the cooling curve is observed. By a careful choice of cooling rate the sample can be made to remain at its trans- sition point for some time and so give a horizontal

portion on the temperature/time graph.

7.2. Apparatus.

A double jacketed pyrex container was used

(Fig.7.1.). Through the conical joints in the cup

were passed a thermocouple and two long tubes for supply- ing nitrogen. One of these was adjusted so that it was

1-2 cm. above the melt surface and the second so that it passed right to the foot of the tube. This latter was sealed at its end, and then pierced by several small pin-holes so that when nitrogen was passed through it, the bubbles agitated the whole of the liquid volume.

The use of nitrogen to stir the melt will result in the freezing point recorded being that of a nitrogen saturated q5

Ft9ure I . Cryoscapy apparatus. 91, melt, but has the advantage that no sliding or rotating seals are required for the stirrer, and the whole apparatus can thus be made gas tight.

P.T.F.E. sleeves were used to seal the B.7 and B.10 joints in the cup, rather than grease, which might otherwise have run down and contaminated the melt. For this same reason, the B.20 joint between the cup and inner tube was inverted.

7.3. Method

A 50 gm. charge was placed in the inner tube, and the B.7 and B.10 joints sealed with stoppers. Final drying and flushing with dry nitrogen were carried out as detailed in Chapter 6.2. The inner tube was placed in the melt bath, the temperature of which was 5-10°C. above the expected melting point of the salt. As soon as was possible, first the short nitrogen tube, then the long one, and finally the thermocouple were substituted for the appropriate stoppers. During this changeover procedure, a flow of nitrogen in excess of 200 ml/minute was maintained in through the side arm. Both the nitrogen supply tubes were purged with nitrogen for 30 minutes before fitting. Finally the gas supply was disconnected from the side arm, which then reverted to use as an exhaust. q7

When the salt was fully molten, the inner tube was placed into the outer tube, also immersed in the melt bath and the controller re-set so that the bath would equilibrate

,.10°C. below the melting point of the acetate melt.

The fall in temperature of the acetate melt was followed as a function of time. The bath temperature was also occasionally recorded by means of a separate thermocouple.

7.4. Results.

Typical cooling curves are shown in Fig.7.0

2; 3; 4; . Initial cooling rates were of the order of

0.5°C/minute, followed by an arrest at the liquid/solid transition, lasting for 20-30 minutes. In some cases, the melt supercooled by up to 1°C. However, the amount of supercooling, compared with the length of the arrest portion of the curve, makes it unnecessary to apply correc- tions or take precautions to prevent this.

The reproducibility of the results, estimated from all determinations, spread over a period of time was ± 0.2°C.

Examination of the phase diagram data for acetates revealed that there were two alternative reports regard- ing the eutectic composition and temperature of the binary mixture. Figure 7 a

Sections of wain, curves .

Soaiu.m acetate.

329.2- - -329

328

Potassium acetate.

506

_705

304-7- -

_704. T Temperature °C

J03

Time nunutes 3?

270 79.4.7 m7+ Cii)COI K + 20-53 ni 14, CH5co2Na.

265

Timeminutes. —t is 30 irg 101 These are 53.7*CH3CO2K and 2350C.(1) and 50 m% CH3CO2K and 240°C.(2). Mixtures of these two compositions were prepared, and cooling curves plotted for each one (Fig.7.2)..

It is obvious from these curves that the 53.7 M% mixture is the true eutectic, though the temperature recorded is lower 231.50°C. than that in reference (1). The 50m% mixture exhibits a typical cooling curve for an off-eutectic composition, with a break in slope at 235.30°C. for the liquidus and an arrest at 231.8°C, for the eutectic temperature.

The freezing points recorded by this method are :

CH3CO2Na 329.2°C. +- 0.2°C. CH CO K 3 2 304.7°C. ± 0.2°C. 53.7r& CH3 CO 2 K mixture 231.5°C. - 0.2°C.

The ranges of previously recorded freezing points are:

CH CO K 292 - 310°C. 3 2 CH CO Na 324 - 331°C. 3 2

References

(1)N.M. Sokolov. Zhurnal Obschei Khimii, 28, 398. (2)A.G. Bergmann. Zhurnal Obschei Khimii, 30, 355. I 0 2. CHAPTER 8

THE MEASUREMENT OF THE ELECTRICAL CONDUCTIVITY OF THE MELTS.

8.1. The Method

It is normal to use alternating current methods to measure the conductance of electrolytes in order to avoid the effects of polarization at the electrodes, or electro- lysis of the material. For molten salts, which have a fairly high specific conductance, a capillary type of cell is normally used, so that the current path is long in comparison to its cross-sectional area. This will give a resistance which is sufficiently high to be measured with some precision.

8.2. The Cell Design

For convenience in manipulation a dipping cell assembly, illustrated in figure 8.1. was used. The electrodes were of platinum. The upper one was a disc, approximately 5 mm. in diameter, and slightly convex downwards, so that the tendency for gas bubbles to be trapped on it was minimised. This was separated by a 5 cm. length of

2 mm. bore capillary from the lower electrode, in the form of a loop of 0.8 mm. diameter wire 12 mm. across. Thus, the surface areas of the two electrodes were approximately equal. Since measurements were to be made at high frequencies

(up to 100 k.cs.) it was necessary to use screened co-axial to3 tshielaed lead to ri419e " ...lector. I 1, II Asbestos Lead to bridse :_packuy.- trans-former. 1

Fiure 8.1

Conductance cell:

ElectriCal. connections. Glassware.

Elearoaes.:- IG pifa.rW section. 1 04 leads wherever possible. With this in view the cell was

made in such a way that copper foil completely surrounded

the central conductor, and the capillary current path,

providing an extension of the shielding over this part.

The lower electrode was always connected to the transformer lead of the bridge, and was thus relatively insensitive to

pick-up of stray fields, and stray capacitance to earth,

due to the lower impedance of the transformer arms and

their tight coupling(1).

This dipping cell fitted into the same melt container,

as the freezing point apparatus. It was found during preliminary experiments, that unless clearances of 0.5 cm.

beneath the capillary, and either side of the ring electrode

were maintained, then the resistance measurement was very sensitive to small movements of the cell, relative to the

outer tube. For greater clearances than this, the effect

was negligible. For the cell used, there was a 1 cm.

clearance under the capillary and 0.7 cm. radially.

The usual provision was made for evacuation of the

cell and also for passage of nitrogen through the electrode assembly and melt.

8.3. The electrical circuits associated with the cell.

A schematic diagram of the General Radio, type to s- 1605-AH bridge, and the associated circuits is given

(Fig.8.2.). The internal, stabilised R-C

is coupled to the bridge at the primary of the transforMer.

Pre-set frequencies of 0.1, 1.0, 10.0 and 100.0 k.csi are

available all with an accuracy of 3%.

The secondary of the transformer consists of two

windings, forming ratio arms of the bridge. They are con-

structed so as to have equal impedances to within 10,000 5'6' The common point of these windings is connected to a

terminal on the front panel of the instrument and forms the

common earthing point of the whole circuit. It was connected

to a mains water pipe.

The cell formed the unknown arm of the bridge; a

resistance box (Muirhead D.825N) and precision air con-

denser (General Radio 722.FS.5) in parallel formed the

standard arm. Whencapacitances greater than that of the

condenser were required, a Muirhead Decade capacitor (B.21.F)

would also be inserted in parallel.

The "common" terminal, which formed the junction of

these two arms was connected internally to a phase sensitive

detector, which measures the 'in phase' and 'quadrature'

components of the voltage which exists between the common

terminal and earth. These are displayed as Impedance •

• 7:17 107

differences, AZ, and 'phase angle differences'69., between the unknown and standard arms. Normally the overall range of these deviations were AZ, and - 0.03 radius tie respectively. If either of these values was exceeded, a relay cut the detector out of circuit.

In use, the bridge was brought as near as possible to balance by variation of the external standards, and a small correction calculated from the meter readings as

follows:-

0 x

In this figure, Os, Ou represent the vector impedances of

the standard and unknown arms of the bridge.

The impedance difference meter reads:-

A Z lzu / -lz I lzui lz )Zu) 1 Zs') I Zs

if AZ is small. The phase angle meter reads =9-9 u s

The circuit in the standard arm of the bridge can

be represented by two imedances in parallel

The total impedance of the circuit is:-

2.)

1 = 1 Z- 7 4- -T- 1

or 1 2 / E 1 2

E is a pure resistance R and Z a pure capacitance 1 s 2 1 using the vector notation, where co is the frequency j Wcs -1 of the signal in radiup sec and j = ,/-1

10q

Thus 1 rio . = Rs j 6.7 Cs Z' R + 1 s jwcs R s (1 - jc3RsCs) 1+AR2C2

Resolving this into real and imaginary parts gives 2 2 2 -1 = Rs (1 + R s C ) 2 z s 1 8 = tan (-OR C ) s s

A similar argument applies if the cell is assumed to consist of a resistance (that of the salt) and capacitance, in parallel.

Thus:- .7-1/ -1 2 2 2 2 2 2 /2 R (1+ R C R (1+ R C ba u u u ) s s s z = 2 2 R (1+ R C W2)-1/2 s s s Re-arranging and squaring 2 2 AZ )2 R R (1 + u 2 2 2 2 2 1+R C 142 1+ R C u u s s

Now 9 =8-eu s

u = es +A e

tan G + tant s tan u 1 - tan 8 tan 6e s

a C + tanA e -s s . . C - u u+g,42R tan AG s Cs

, 2 2 2 2 2 2 2 a (1+R CW tan AG) (1 +l,-) R C ) . 1 + 2R C‘otan AG + R C tan4 A s s u u s s s 2 2 2 2 + (.,) R C - 2R CC&ItanAG + tan AG s s s s • 2 2 2 2 f , + 2R,2„. C2) . (1 + tan AG) (1 + L.3 RR C ) • 4 ‘i u s s , (1+ R C t4tan S s AG)

Thus 2 ,2 2 2 1+R ( R Cc,a tan AG) R (1 +AZ) u s s s (i+SR2 C2) (1+tan269)(1+4,h2sC2s) S S

R (141,Z) sec 6G R s u (1+ R C Wtan s s

In actual use, it was always possible to null the AZ

meter. AG never exceeded 0.1 radina, even at 100 k.cs. Tkje

worst possible case therefore gives a correction factor of

0.1% and in general it was not necessary to correct for the

small residual phase angle differences observed.

8.4. Calibration of the cell.

The cell constant was determined by measurement of

the resistance of solutions of potassium chloride, 1.0 Demal

and 0.1 Demal solutions were used at 25°C., following Jones (2) and Bradshaw . It was found that even at 25°C., water from I I 1 the solution begain to condense in the top part of the

cell after approximately 6 hours with a consequent change in the resistance of the solution. For this reason several

separate solutions were each used for about 1 hour each in the cell, when taking the series of readings for the cell

constant.

The cell constant can be found from the equation

if =

where Aris the known specific conductance of K.Cl solutions, -1 R the measured resistance, and g.cm the cell constant.

A value of 193.47 0,44 was used for this series of mea-

surements.

8.5. The measurement of the conductance of molten acetates.

Sufficient solid to give a depth of melt of,v7 cm.

was placed in the outer part of the cell, and purged with

nitrogen in the usual way. For this a blank cap replaced

the electrode assembly. When the salt had been melted the

blank cop was changed for the electrode assembly. During the changeover, ingress of oxygen was minimised by the maintenance of a flow rate of nitrogen of 500 cc./min. through the side arm A, aad out to atmosphere. The electrode assembly was flushed with nitrogen through the tap at its head before and during its positioning. .12 Measurements of the resistance of the melt, were taken at each of the four frequencies, over a range of

temperature while the bath was heating or cooling, and while waiting for the temperature to stabilize, nitrogen was passed through the electrode assembly ( 50 cc/min.).

Two to three minutes were allowed for the electrode assembly to come to thermal equilibrium with the melt before readings were commenced. At least two separate samples were used to cover the whole temperature range, and the readings were arranged in such a way that deviations due to progressive decomposition of the sample towards the end of the runs would be detected. No such deviations were observed..

8.6. Results.

In figure 8.3. the effect of frequency on the resistance for typidal cases is shown. It can be seen that very little variation with frequency due to polarization was found. To calculate the specific conductance, the value of R extrapolated to -- = 0 is normally used.

Very little error is introduced by using the value of R 4 at 10 c/s as can be seen.

Values of loglok and logioA were plotted against

103/T°K and are shown in figures 8.4. and 8.5. k is the CH3CO2K .339.2 `C r~ur~ 8·~· s·pec.ifle cot14l1laa.ncU

specific conductance, andA the equivalent conductance .--k •VM where Vm is the molar volume (chapter 11)] . No curvature of these plots is evident over the short temperature range accessible to the pure salts, but when this range is increased as in the eutectic mixture, definite curvature is apparent. Polynomials were fitted to these lines by a computer, and are given below :- Potassium acetate

logioX = [1.304 - 1.170 (± 0.016) 1°3T 1+-- 0.005 3 logioA = 3.304 1.257 --10T- ] Sodium acetate - log = [1.165 - 1.148( ± 0.021)11T t 0.002 logioA= {2 n3 . .974 - 1.148 (1.-T-)] Eutectic mixture logiok = [1.262 + 1.854 (--yr-03 ) - 0.916 (5-)13 i 0.001 (03 ) logioA = (0.742 + 1.717 - 17- - 0.894 (1071-,-3) 2- ± 0.004

From this data, activation energies and pre-exponential factors have been calculated. Potassium acetate 3 -1 = 20. 14 exP 5.353 x 10 1 .1f.es-1 CM RT 73° x 1031 -1 2 -1 A= 20.11 exp[ 5. RT _Q. cm mole V( 7 Sodium acetate 5.252 x103 -1 -1 k = 14.62 exp cm 1. RT 2 -1 A = 942.3 exp [5.235 x 10 Ja71cm mole RT Eutectic mixture

Effective activation energy derived from the slope

of the curve.

Just above Well above melting point melting point 103 , 103 = 1.6 T = "7

7.441 K.cal. 4.926 K.cal.

I\ 7.665 K.cal. 5.219 K.cal.

8.7. Assessment of accuracy.

Standard analysis of the regression lines of log IC 3 o on 10 /T K leads to values of the standard deviations for

logX, and for the slope of the line, as shown in section 6.

Using the relation k. g/R, it is also possible to estimate

the error in Y caused by errors in g and R independently.

The calculated standarddeviation for g is 0.23%, and the

estimated accuracy of resistance readings is 0.02%. The

latter figure is rather worse than the absolute accuracy of

measurement claimed by the manufacturers of the bridge, but

takes into account random errors due to temperature inhomogeneity rEg and fluctuations in the sample. The error to be expected in4i(from these sources is 0.3%, which is of the same order of magnitude as the standard deviation of the points about the regression line.

Thus conductance values can be quoted to approximately ± 0.3%, and similarly the activation energy, derived from the slopes of the regression lines to approximately

± 1.7% The only sources of systematic error within my

control would be inaccuracy in the cell constant due to incorrect preparation of the standard potassium chloride solution. Since four independently prepared solutions,

., were used during this determination, all of which gave the same results, very little systematic error can be attributed to this source.

References

(l) General Radio Manual for 160;-AH bridge.

(2) Jones and Bradshaw. Journal of the American Chemical society, 55, 1780. CHAPTER 9

THE MEASUREMENT OF VISCOSITY.

9.1. The principle of the method.

Various methods have been used for the measurement of viscosity in molten salts. The damping of the oscillations of a disc suspended from a torsion wire, in the melt, (1) • has been used by Dantuma . A similar inertial method, but using the damping of oscillations of a crucible con- (2) taining the melt, is reported by Janz and Saegusa . A falling sphere method has been used by, for example, Rogers (3), (4) and Ubbelohde and Ogawa The method used in this work, the capillary method, has previously been used 1y a (5) number of workers, for example Bloom Harrap & Heymann , (6, 7, 8) Ubbelohde and co-workers . Discussions of the possible sources of error and of correction terms, for various methods of measuring viscosity appear in the mono- (9) (10) graphs of Barr and Bingham .

The principle of the capillary viscometer is embodied in the well known equation of Poiseuille : V ma4 t 8171

V/t is the volume rate of flow through a capillary of radius a cm. + lengh 1 cm. of a liquid whose viscosity coefficient Y2 0

2 is n poise. The pressure drop across the tube is p dynes/cm .

Thus, Tca4 ' NT • pt

A viscometer of this type can be used as a comparative instrument, even if its dimensions are not known, provided that the volume of liquid flowing is the same in each case.

This was the method used in this work.

9.2. Apparatus.

The viscometer was of the Ubbelohde type (fig.9.1.)

modified for working in the complete absence of oxygen.

The calibrated volume of melt which is timed in its passage through the two capillaries, occupies the volume between the constrictions A and B. Sufficient material has to be put into the viscometer to fill it completely from the constriction A, to constriction C at the lowest temperature used.

An overflow device D allows a small adjustment of level to be made.

In the normal instrument, the bulbs E and F are closely similar in shape. In this way, the effect of the added hydrostatic pressure of the melt as it is forced from A to B, in the first half of the flow-time, is exactly Fiure 9.1 Cold. trap. N Ftlte Viscosit, apparatus. 4— Needle. valve. Mc.i.nosneYer.

To liner supply . To Const.calt vacuum heca device.

absorbent . A avert X 22 cancelled by the effect to be subtracted in the second half of the flow. Due to the necessity of accommodating all of the solid to be melted in the viscometer, below the level of the melt bath surface, the bulb F was widened, and thus the cancellation of hydrostatic pressure terms will not be exact. However a calculation shows that this error will only be 0.1% of the total driving pressure.

Furthermore, since all measurements were taken as a mean of runs from B to A and A to B, the magnitude of this error is reduced to negligible proportions.

The constant driving pressures were obtained by selecting one, or more, of the overflow tubes in the water operated constant head device (fig.9.1.). The pressure of nitrogen supplied by the cylinder was adjusted so that it was just sufficient to maintain a steady stream of bubbles through the water. From this device, the pressure supply for the viscometer, was led through a filter, a cold trap, a 22 litre "buffer" volume and mercury manometer, to a two-way tap system to allow either side of the viscometer to be connected to the pressure supply, while the other was connected to atmosphere through a small Dreschel bottle containingev0.5 cm. head of an oxygen absorbing solution.

(Sodium hydrosulphite 8 gm.; sodium hydroxide 7.5 gm.;

Sodium anthracianone sulphonate 0.4 gm.; in 50 ml.water). Z 9.3. Calibration.

The constant c in the equation EL- was found by calibration with 40% w/w sucrose solution at 25°C.(11).

The sucrose solutions were prepared gravimetrically, correcting all weights to vacuum. Freshly boiled distilled water was used and the solution was not heated to aid solution of sucrose,as previous experience has shown that inversion of the sucrose can occur under these conditions.

Several flow times in each direction were recorded, using all the available driving pressures and the viscometer constant calculated as a mean of all results was found to be 113.94 t 0.22 cm.Hg.sec. millipoise-1. Within this order of accuracy it is not necessary to correct the calibration constant for the thermal expansion of the viscometer, since this correction is not greater than 0.001%.

9.4. Measurements of viscosity of melts:

Before use the whole of the connecting system was flushed with nitrogen and thereafter kept sealed. Those connections which carry the driving pressure were carefully checked to ensure that they were leak-free. Finally the whole apparatus was tested to the highest pressure used.

26 cm.Hg.). No significant diminution pressure ( 4:0.005 cm.Hg.) was observed in the sealed system over the course, of 30 minutes. 12 4 Sufficient solid salt to fill the necessary parts of the viscometer was finally dried, and purged of oxygen in the wide limb F before melting. After melting,the temperature was maintained just above the freezing point of the salt, while the material was forced into the capillaries and intervening bulbs until it reached the overflow D. It is necessary that the viscometer is filled completely from the tip of the jet, D, to the constriction C, at this low temperature, to ensure optimum cancellation of hydrostatic effects.

At higher temperatures, the liquid will expand and occupy a larger volume, thus altering the 'V' in the calibration constant. This could be compensated by re-levelling between the marks C and D, at each temperature. However, if this is done, then the repetition of results after going to a higher temperature cannot be undertaken. Since the thermal expansion effect is small (*0.1%) no adjustment of this type was made during this work in order that the potentially more serious effect of decomposition could be checked for by cycling the temperature.

The viscosity of the melt was measured as a mean of readings of the flow times from B to A and A to B under at least two different driving pressures. Since each measurement necessarily took quite a long time, several separate samples of salt were used to cover the accessible temperature range, as, at the higher temperatures, the salt began to decompose after only 3 to

4 hours. A different "temperature pattern" was used for each run to check for possible errors caused by prolonged exposure of the melt to high temperatures. No significant effect was found within the experimental periods used.

9.5. Results. 103 Plots of log10 (millipoise) against t1-57 are given in figure 9.2. No accurate measurement of the viscosity of the pure sodium salt, over an extended temperature range, was possible because of the formation of tiny gas bubbles in the melt, and their consequent interference with flow.

No significant curvature can be detected over the

70°C. temperature range for the potassium salt. When the range of temperature covered was increased with the eutectic mixture,a slight curvature of the best line through the results was noticeable.

Polynomials were fitted to the results and are given below. · f

r? ? (enin.pOlse) I

,-,

'-7 '0 s

12. 7 Potassium acetate

log0 (millipoise) -0.982 + 1.597(t o.o4)--103 1.z 0.004

Eutectic mixture "in3 ) ( (m.p.) = [2.005 - 1.903(if + 1.056 103 )1. -+ 0.004

Sodium acetate

At 335.6°C. - just above the melting point of the salt, = 72.83 millipoise = 1.862: g = 1.644

From these results, activation energies and pre- exponential factors can be calculated and are shown below.

Potassium Acetate. 11= 1.042 x lo-4 7. 6 x 0 exp( 30RT )poise

Eutectic mixture Activation energies calculated from the slope of the curve are:- 103 at -- = 1.9 (just above melting point), activation energy = 9.684 K.cal. 223 = 1.6 (well above melting point) activation energy =6.751 K.cal.

Assessment of errors. The equation l = EL c allows an estimate of the amount of random error in due to the combination of errors in p. t, 1 2 9 and c. The result, 0.3% is of the same order as the

observed standard deviation of the points about the regres-

sion lines, and it is therefore felt that an accuracy of

0.3 - 0.4% for random scatter can be quoted.

There remain possible systematic errors, some of

which have already been discussed. The effects of the

re-shaping of the viscometer limbs amounts to no more than

0.1% and that due to the difference between the working

temperature and the calibration temperature is negligible.

The most serious error arises from the thermal expansion of the melt resulting in an increase in the volume of

material flowing, at higher temperatures. This can be as much as 1.5% at high temperatures, but is proportionately less nearer to the melting point.

Finally, the correction for the kinetic energy of (12) the liquid issuing from the capillary has been shown to be of the order of 0.1% for this type of viscometer.

To check that flow was in fact laminar so that the

Poiseuille equation could be used. Reynolds number was calculated for the worst possible case to be A/150. The critical value above which laminar flow begins to become unstable in "0200. 120(

With”the accuracy of measurement the viscosity of

the melt, at any temperature, was independent of shear rate

within the limited range of shear rates available; that

is Newton's law of viscosity is obeyed.

References.

(1)Dantuma Z. Anorg. Chem(1928) 122, 1 (2)Janz & Sagusa E.iec.tre.chem So. (OW) (ID 461 A (3)RogerS & Ubbelohde. Trans..Faraday Soc.(1950),46,1051. (4)Ogawa Japan Inst.Metals B(1950),141 49. (5)Bloom, Harrap & Beymann. Proc. Roy.Soc.(1948) A/94, 237.

(6)Davis, Rogers & Ubbelohde. Proc.Roy.Soc.(1953) 220A, 14. (7)Plester,Rogers & Ubbelohde. Proc.Roy.Soc.(1956) 235k, 469. (8)Frame, Rhodes & Ubbelohde. Trans.Faraday Soc.(1959) 55, 2039. (9)Barr Monograph:Viscometry (Oxford) 1930. (10)Bingham Fluidity and Plasticity (McGraw-Hill) 1922.

(.111) Bingham & Jackson. Bull.Bur.Std. 14.

(12) Frame Ph.D. Thesis, University of - London, 1961.

CHAPTER 10 t3

THE MEASUREMENT OF THE THERMAL EXPANSION OF THE SOLIDS.

This method was used both for potassium acetate, and for potassium nitrite as reported in part IV of this thesis. Since single crystal material was available in neither case, the average cubical expansion was measured by a conventional solid/liquid dilatometric method using a silicone oil as a confining fluid.

10.1. Construction of dilatometers.

Dilatometers of the form shown in figure 10.1. were used for the solid, and a similar instrument, without the side arm to determine the thermal expansion of the silicone oil.

The stem of the dilatometer was formed of 2 mm. precision bore "Veridid'capillary, the diameter of which was checked at seveill placed along its length by measuring and weighing a mercury thread contained in the tube. The bore of the tube conformed to the manufacturers limits of 2 ±0.01 mm. diameter.

10.2. Measurement of the thermal expansion of the silicone fluid.

Before either oil was used a check was carried out to test the inertness of the oil with respect to the solid. 131

Figure 10.1

Construction. of aitatorneters.

Precision bore cc.p~l~c~r~:• Bore, : 2-0 Imo. -For• CH, CO3 K 05 min -for KNO2

FiatiCica incAr .

Bull, volume 2'0 cc for CH3CO3 k 7•5 cc -For K NO2

The M.S.200 oil (supplied by Midland Silicones Ltd.) was kept in contact with the potassium nitrite at 60°C. for

10 dayS with no change in density. This test was also successfully carried out with the M.S.550 oil (also sup-

plied by Midland Silicones) at 250°C. As an additional chebk in this latter case, it was found that potassium acetate could be melted in contact with the oil and kept molten for 2-3 minutes before decomposition becarie evident.

The dilatometer was initially calibrated by weighing, filled at 25°C., with freshly distilled water. Oil or water, as appropriate, was introduced into the bulb of the dilatometer by means of a capillary funnel to avoid forming a film, or drops on the upper part of the capillary.

The thermal expansion of the oil was observed using a precision cathetometer reading to 0.0010 to measure the height of the meniscus above the fiducial mark. Readings were taken as the temperature was raised, and again as it was lowered to check for loss of oil at the higher temperatures.

The equation -4 -7 2 V oil . (0.9275 + 7.0172 x 10 T 4.1312 x 10 T ) ± 0.001 cc/gm. was computed to fit the data for M.S. 550 shown in figure 10.2. Results for M.S.200 are given in part IV.

10.3. Measurement of the molar volume of potassium acetate crystals.

Two separate samples were used. The first (runs I

and II & III) was the normal crystalline material dried in

the usual way. When degassing the mixture of oil and

crystals this tended to be carried up the capillary with

gas bubbles, leading to a possible loss of solid from

the weighed amount introduced. A second sample was formed

into pellets and was free from this disadvantage.

A weighed quantity of solid was introduced into

the bulb through the side arm, followed by sufficient

oil to cover the crystals. The side arm was sealed off

in an oxy-coal gas flame in such a way as to minimize

heating of the sample. The apparatus was degassed by

connecting to a vacuum line (0.01 mm. Hg. pressure) and

repeatedly evacuating until no more gas bubbles could::bo

forced to the surface by sharply tapping the bulb with a

spatula. After topping up to the mark with silicone oil

and weighing, the space above the oil was filled with

nitrogen.

All manipulation in the above sequence were done in

.a glove box filled with dry nitrogen, or with the dilatometer stoppered. 3 5-

In order to determine the calibrated volume of the

bulb it was necessary to separately measure the density of the solid at 25°C. by means of a density bottle method again using the silicone oil as confining fluid. This

method is chosen since it is not certain at the outset of the experiment whether the dilatometer will survive unbroken and clean so that its volume could be determined conventionally.

The thermal expansion of the material was observed by measurement of the height of the meniscus after allowing 30 minutes for the material to come to thermal equilibrium with the appropriate thermostat. Additional readings were taken as the temperature was lower'ed to check for hysteresis, caused by thermal transitions, or loss of material at high temperatures, and within the accuracy of the experiment none was observed. No significant differences were noticeable between the crystalline, and the pelletted sample.

10.4. Results.

Fig.10.3. shows a plot of the molar volume V of M the solid, calculated from the equation M.V • M 2 98.1 - VD (1 + (T-25)) + nr h aglass T 25 -4 - M(oil) (0.9275 + 7.0172 x 10 T -7 2 + 4.1312 x 10 T ) 3 where M = weight of solid used.

VV,D calibrated volume of dilatometer at 25°C.

a = coefficient of cubical expansion of pyrex. glass

T°C = temperature of observation.

radius of the capillary.

h = height of meniscus at T T °C. MO1l = weight of silicone oil used.

It can be seen that within the experimental scatter

of points there is no difference in behaviour between the

powdered and pelletted samples, not is there any hysteresis between the heating and cooling samples.

The highest temperature reached was 286.35°C. On trying to increase the temperature above this, darkening of the solid began to appear. (This is in no way inconsistent with the earlier observation that the oil and soild could be taken to higher temperatures. Decomposition of the material is influenced by the total time of exposure of the sample at high temperatures).

Linear equations were calculated to fit various sections of the curve and are given below:

25-75°C.

Vt, = (61.34 + 0.0281 T) ''0.15 cc/mole. I

V = (60.47 + 0.0446T) ± 0.47 cc/mole 1311- A feC V = (64.47 + 0.0196T) * 0.14 cc/mole 1 2t6°C. V = (58.39 + 0.0452T) 1: 0.13 cc/Mole.2i6 187 60 There is a sudden increase in volume at approximately 80°C. of 0.5 cc/mole.

Also shown in figure 10.3 are the molar volumes of the solid end liquid at the melting point as measured by the Landon method (Chapter 12) and the molar volume of the liquid as measured in chapter 11.

The changes in behaviour of the salt are perhaps shown more clearly in figure 10.4., in which the deviation of the results from the initial linear behaviour are plotted. 10.6. Erru s.

As can be seen from the graph 10.3., there is a considerable amount of random variation of the results.

This is inherent in an experiment of this type for a variety of reasons. Firstly, the actual expansion of the sample is

small (^/0.1 cc.) since the volume of the sample is necessarily small. Small errors of observation will therefore amount to quite a large percentage of the expansion

and will be reflected in the results as a large amount of random scatter. Unfortunately the design, of the dilatometer Figure

Molar yam-he

200 250 300 350 400 Nure 0 •

Thermaa expansion of CH3 CO2 K riottea as Cl• aifferenCe fLaieti011.

T °C. --ti So g00 150 200. 250 300 350 lop I I I 1 1 'I '4111 yap

and ancillary equipment is such that the size of the

sample cannot be increased significantly. The limiting

factors are the length of the capilkry stem and the size

of the constant temperature zone in the thermostat.

A second source of error is inherent in the prin-

ciple of solid/liquid dilatometry. The expansion of the

sample is measured with reference to thk, known expansions

of the container, and the indicating fluid, i.e.

V =V r solid container liquid In this particular case, the expression can be reduced to 2 solid =BhA - CT - DT where A, B, C, D are constants and h and T the height

of the meniscus and the temperature respectively. By

comparison with the other terms DT2 is small. However Bh

and CT are of the same order of magnitude but of opposite

signs. The thermal expansion of the solid thus depends

on the small difference between two nearly equal numbers

(Typically, these two terms are 3.7 and 2.1 at room

temperature and 34.6 and 26.3 at 200°C.). Again, small

errors in either of these two terms will be reflected as

large percentage errors in the difference function, and

therefule in the calculated expansion of the solid. 114

Another criticism of this technique is that there is no way of knowing whether the oil totally fills all the interstices in the solid. if it does not, then too large a value for the molar volume, and the expansion parameter will be recorded. The repeated evacuation employed during filling will help to eliminate this error with respect to interstices which come to the surface of the solid, but which cannot fill any voids which are totally within the solid. The powdered and pelletted samples have the same molar volumes within the limits of random variation of the experiment. This must indicate that there are very few wholly internal voids as it is unlikely that the two differently prepared samples would have the same percentage of unfilled spaces.

The calculated standard deviations from the straight lines on figure 3 amount to 0.6% to 1% of the total molar volume and it is considered that this reflects the accuracy of the experiment as a whole. CLAPTER 11 4 .

MEASURE= OF TEL MOLAR VOLUME OF T}3 MELTS.

11.1. Principle of the method.

The method used was that of balancing the hydro-

static pressure due to the liquid against that due to a

sr.itable refere_ce liquid at room temperature. If,the

dimensions of the densitometer are accurately known, the

change in hydrostatic pressure can be related to the

change in volume of the 1.quid as the temperature is varied.

11.2. Construction of apparatus.

The densitometer was built to the design of (1) (2) Husband , as modified by McAuley and 'shown in

•figure 11.1. It consists of a bulb surmounted by two

pieces of precision bore capillary. The shorter one is

marked with a calibration index, to which the meniscus

is levelled each time. The liquid is allowed to expand

up the longer one.

Above the first fiducial mark is a small bulb,

and other short, marked capillary section. This allows

a larger temperature range to be covered than with the

original instrument. The volume of the small bulb is

adjusted to be equal to that of the long capillary so that

when expansion from mark 'A' has used up the whole of the

long capillary, the change to mark "B" will allow the process To c)bnder.

Cold. trap.,

Fip.tre 11.1

Densitomet, apparatus. Adjustable constant head. device. t Lf. to be repeated, and again for mark "C". The pressure required to bring the meniscus to the appropriate mark is developed by the constant pressure device also indicated in figure 11.1.1. Nitrogen is supplied through an overflow device to maintain the constant head, then through a cold trap, and ballast volume to the manometer, filled with di-n-butylphthalate, and to the densitometer. The two-way tap system allows pressure to be applied to either limb of the densitometer as appropriate.

Provision is also made for the evacuation of the

pressure lines and densitometer so that flushing with

nitrogen can be undertaken.

11.3. Derivation of formula.

The pressure difference set up in a system as shown

is given by p = h(2,0 = x

7s. h c where x and h are the differences in levels as indicated, and po and p are the appropriate densities of the two liquids.

If the left hand U tube consists of a volume V.0 surmounted by a precision bore capillary of cross-sectional area A cm2, then the volume of liquid in this tube is given

V= + Ax Therefore, V - Vo x = A

V -0 A Now since

= fir where DI is the weight of liquid. contained in the volume V of the densitometer in use, MV tihaM o o

MVo • V - H-Ahp0

If the effective molecular weight of the salt or salt mixture is Wes, then its molar volume is given by

W.V

WV0 V1.1 = ivi-Jdap 0

Thus, once the densitometer has been calibrated, only the external pressure needed to bring the liquid level to the appropriate mark in the wide limb, needs to be measured.

This pressure is considered positive if applied to the wide limb, and negative if applied to the narrow limb. 11.5. Calibration. The 2 mm. bore Veridia capillary was measured by weighing a thread of mercury of known length contained in the capillary. At room temperature the diameter was 2.002m., within the manufacturers tolerance of 0.01 mm. A graph

was constructed, giving the cross-sectional area of the capillary at higher temperatures. (figure 11.2).

Vo was determined for each fiducial mark by filling

to that ma2k with freshly distilled water, at 25°C. The

results were corrected to the working temperature range using a value of 1.08 x 10-5 for the coefficient of cubical

expansion of 'pyrex' glass(3) and are shown in figure 11.3.

The values for the density of di-n-butylphthalate

were those measured by Cleaver(4) and re-measured by (2 McAuley ) , and are shown in figure 11.4.

11.6. Measurement of the molar volume of acetate melts.

Approximately the correct quantity of dry salt or

salt mixture was initially weighed into the wide limb of

the denaitometer. This was followed by the usual procedure •3•15S

2 . Arec.. P1411.

3.155 •

- 3.154

.3.153

Tenveratt‘re 100 260 300 350 1,00 . f)·f)1

t . (

Vo1.um~ o-r st";Jes.: A • .5'33 us. G: UJ-96". C. = Ib ·S£, I.,.

Te.mfe,.~t ...re Dc. ~

'J50 300 350 17 rijure 114 4

Patisit, cubiAty i phth cat& e

Temperdturo: ®C

9511,CC —p

1•041 1.042 1•01$3 1.044, ;50 of evacuation and flushing the densitometer, and associated gas lines with nitrogen.

The densitometer was transferred to the thermostat bath and when all the material had melted suitable manipulation of the two way taps displaced all the nitrogen from the lower bulb of the densitometer.

Measurements of the density of the liquid were taken by adjusting the overflow of the constant head device until a steady stream of bubbles emerged, and the meniscus in the densitometer was levelled at the appropriate mark. The pressure difference in the system was measured on the manometer by means of a cathetometer reading to 0.001 cm.

When a sufficiently large number of readings have been taken, or when bubbles began to form in the melt, the run was discontinued. All of the salt was forced into the wide limb of the densitometer, and the bulk of it carefully emptied into a tared porcelain basin, cooled in a desiccator and weighed. The densitometer was cleaned externally and weighed after cooling in a desiccator, and from the combined weights, the weight of salt used in the experiment determined.

Occasionally, during the preliminary flushing, particles of salt were blown out of the densitometer. s..,

For this reason the weight of salt after the experiment, rather than the weight introduced into the densitometer9 was the one giving the highest accuracy. 11.7. Results.

Figure 11.5. shows the molar volumes of a range of mixtures of sodium at potassium acetate. Linear equations have been calculated to fit these lines and are given below. The effective molecular weights of the mixtures and the mole percentages were calculated on the basis of one gram ion of acetate for each mixture. Potassium acetate

vM = [70.583 + 0.0349(t 0.001)(T-300)] - 0.077 cc 304 37900. 79.47 m% potassium acetate : 20.3 m% sodium acetate VM = [69.351 + 0.0309 (± 0.0009)(T-300)i ±0.073 cc 270 —)350 °C. 53.70 ril; potassium acetate : 46t30 m510 sodium acetate

VM 467.537 + 0.0293 (± 0.0008)(T-300)] ± 0.013 CC 232 _4. 350°C. 35.23 m% potassium acetate : 64.77 ra% sodium acetate VII 4166.386 + 0.0294 (± 0.0008)(T-300)] ± 0.056 cc 270 ---> 340°0. 22.76 m% potassium acetate : 77.24 sodium acetate' VM -165.413 + 0.0305 (± 0.001)(T-300_1 1: 0.084 cc 240 350°C. I 2.

12.63 m% potassium acetate : 87.37 m% sodium acetate

VM = L64.586 + 0.0312 (± 0.0006)(T-30qt 0.024 CC

310 350°C. Sodium acetate

Vn = L63.472 + 0.0323 (- 0.002)(T-300)jt 0.059 cc

330 --) 355°C. 11.8. Errors. An estimate of the effect on Vm of small variations in the quantities on the right hand side of the equation can be made by logarithmic differentiation of the expression

- V(31Ar V° " " M-Apoh - M(1TZ giving A SITy. (!IT_Q. ) - 8(1 - fil°h ) TIT - -73 (1 _ A poh ) (1/1

An average value of A Poh is 0.02 so the second term in M this expression can be approximately replaced by g (A p oh)

Logarithmic differentiation of allow an estimate of n h 8 A—1-52_ to be made from the supposed accuracy of measurement of A. h and M. Errors in A (-1. 0.7ra) and h (t 1%) were much greater than those expected in either r, or M.

Ft :Are . n• 6 Excel s volume g of acetate melt s.

. Sto.ticicira. cleviat tor..

t Encegg volume, cc. thole" • 250°C

// // /

Mole fraction of pota5sium scat. -P 0.7 0.8 0.9 04 0.1 0.3 0.4 0.5 -3.5

-341,

- 3.3

Expew 5ieri t er

-3'1

- -0

Mole 'rr cti,or 043C04 1<, 01 0.2 0:3 0-4 0-5 0.7 0 :9 ' 16-6

Combination of these values with an estimated accuracy of t 0.01 ml for Vo, leads to a figure of ±0.25 for the maximum error in ym. In fact the standard deviations derived from the statistical treatment of the points are of the order of 0.1% so it is felt that the assessment of errors is fair.

Statistical treatment of the regression lines also yields a value for the standard deviation of the slopes of approximately ±yg.

Excess functions calculated from these regression lines will, by the rules for combination of errors, be Subject to errors of ± 0.1 cc. Such excess functions are shown in figures 11.6. and 11.7.

References.

(1)D.J.B. Husband. J. Scientific Instr. 1958, , 300. (2)McAuley, W. M.Sc. Thesis (London) 1965. (3)Peters & Crayoe National Bureau of Standards, 1920. (4)Cleaver, B. Ph.D. thesis. University of London, 1963. 15 -7 CHAPTER 12

MEASUREMENT OF THE VOLUME CHANGE ON FUSION.

The extrapolated results for the molar volume of

solid potassium acetate indicate that the material would

have a greater molar volume than the corresponding liquid. It was desirable to check this unusual phenomenon by a

totally independent method.

12.1. The principle of the method. (1) The method was previously developed by Landon (2) for work on cryolite and by Plester for work on the

thiocyanates. A pyknometer of the form shown in figure

12.1. can be used to measure either the volume of the melt,

or the solid, at the melting point by suitable application. To determine the volume of the melt the instrument is placed in a temperature gradient, such that the temperature is constant at a value just above (ideally infini-

tesimally above) the freezing point of the salt, over the

length of the bulb, but falls just below that temperature over the length of the capillary stem. Melt is drawn into

the bulb and part of the stem thus forming a solid plug

in the capillary. If now the whole pyknometer is detached

from the bulk of the liquid, and moved vertically into the

colder zone, a sample of the melt which just fills the bulb

at the freezing point is trapped. Subsequently this melt .1 58.

Flex i Poo i.t ion irt r device.

Figure 124

Psknometer

e" r ."" Sur•fute level of thorost

Position in tube

Fa in teinpercctinz. ao 20 30 159. solidifies and can be weighed. If the bulb and capillary stem are calibrated, then the density of the salt at a temperature infinitesimally above its freezing point is determined.

The density of the solid at a temperature frac- tionally below the freezing point is found by a slight variation on this procedure. Suppose that the temperature in the upper part of the tube is also constant, at a value infinitesimally below the freezing point, then a solid plug is formed in the stem as before. The pyknometer is withdrawn vertically through the temperature gradient, this time however keeping the lower tip of the bulb in communication with the bulk of the melt. As each element

:of volume of the bulk reaches the level at which freezing

occurs, solid is formed. However any change in density

of the melt results in liquid being expelled or sucked

into the bulb from the body of the melt. The net result

of this process is that the final weight of the pyknometer

results from its being filled with solid at the freezing

point. Hence the density of the solid at this temperature,

and the change in molar volume on fusion, can be found.

12.2. Construction of the apparatus.

The pyknometer is shown in figure 12.1. The bulb

has a volume of approximately 2.5 cc. and is surmounted 6, 0 by a 5 cm. length of 0.05 cm. diameter precision bore capillary, and a long stem. The jet at the base of the bulb was ground flat and has a diameter of 0.05 cm. The whole was suspended by a lo9p of thread from a small windlass inside an outer sheath containing the melt (figure 12.1.).. This windlass served to adjust the pyknometer so that its jet touched, or was clear of the melt as appropriate.

Also connected to the top of the pyknometer was a short length of flexible rubber connected with the outside by way of the stopcock This allowed nitrogen to be blown through the melt, and also permitted salt to be drawn into the bulb and into the capillary stem by suction from a small teat.

The outer sheath was provided with the usual facilities for evacuation and flushing with nitrogen. It ran through a guide and was suspended in the molten salt thermostat from a winch driven by a synchronous motor through a six speed gearbox. This gave rates-of withdrawal of 0.05, 0.1, 0.15, 0.25, 0.5, 0.75 cm. per minute.

12.3. Calibration.

The diameter of capillary stem was measured by weighing a measured length of mercury thread containded in the

tube. This was within the manufacturers tolerances of 16 1 0.05 ±0.01 cm. at all points along its length.

The volume of the bulb up to the fiducial mark

was determined by weighing the amount of mercury contained o in it at 25 C. A mean of 3 readings gave 2.522 - 0.001 cc. The temperature profile inside the sheath was measured using a thermocouple probe, and is shown in figure 12.1. Initially an attempt was made to provide a more nearly ideal "step function" profile, by winding a heater

around the guide. This proved rather erratic in behaviour, and was disContinued. Errors due to this temperature effect

are discussed in section 12.7.

12.4. Measurement of the molar volume of the liquid at its freezing point. Approximately 15 gm. of dry salt were placed in the outer sheath, the pyknometer inserted, and the whole apparatus evacuated to 0.01 mm. pressure through one side arm. Repeated filling with dry nitrogen and evacuation followed.

The apparatus was transferred to the melt bath and dry

nitrogen passed through the pyknometer to stir the material

as it melted, and to aid further drying. When the material was fully molten, the temperature was set to control as o close above the freezing point as possible (305° °C., 0.5 C.

above the freezing point of the potassium salt, and 330° °C.,

0.7 °C. above the freezing point of the sodium salt). By manipulation of the windlass and the teat, the capillary stem was brought into the region where the temperature fell below the freezing point, and a plug of melt was formed in the capillary. The tip of the pyknometer was carefully detached from the melt surface by readjustment of the windlass. Since the jet diameter was small, very little tendency for liquid to run from the bulb was noted. The whole apparatus was lifted vertically so that the pyknometer was totally above the freezing level, and held there until the sample had frozen. Careful observation established that no liquid was expelled from the jet during this period so that no errors can be attributed to this possibility. Removal of the pyknometer for weighing was carried out as quickly as possible, using a strong outward flow of nitrogen (m, 500 ml/Min.) to prevent ingress of air.

From the weight of the cleaned pyknometer the weight of salt which occupied the volume just above the freezing point can be found. A small correction for the difference in volume between the fiducial mark and the actual position of the solid in the stem was calculated after measuring the difference in position using a travelling microscope. This whole process was repeated several times so

that an estimate of the variance of the result could be made. 16 3 12.5. Measurement of the molar volume of the solid at the freezing point.

The same procedure for filling and melting the material was followed. With the bath controlling at a temperature just above the freezing point of the salt under investigation, the pyknometer was filled, and a solid plug-formed in the capillary as before. The windlass was adjusted so that the tip of the pyknometer was just immersed in the melt (1 mm.), and clamped in position. Slow withdrawal of the apparatus through the temperature gradient by means of the electric winch followed. Various rates were used for different samples in order to check whether or not there was any correlation between the withdrawal rate and the weight of material frozen in the bulb. No significant effect was found. During the withdrawal period, careful observation established that no gas bubbles appeared at the melt/solid interface until over 2 hours had elapsed. For this reason, the two slowest winch speeds were not used during definitive runs. When a complete solid ingot had been formed, the pyknometer was carefully detached from the crust of solid on the bulk melt, removed, and weighed and measured as before. A4p..in, a series of measurements were taken so that the variance could be estimated. 12.6. Results. Potassium acetate liquid Individual results - Molar volumes. 70.889 70.882 70.770 70.800 70.875 70.823 70.889 70.847 70.867 70.813 70.881 70.893 These give V = 70.85 ± 0.04 cc. The bath temperature was 305°C. Potassium acetate - solid

Individual melts - Molar *volumes. 71.106 winch speed 71.389 winch speed 70.808 45 72.197 15 72.785 winch speed 71.024 70.6221 30 71.484 winch speed 72.233 9 These give V . 71.6 t 0.7 cc. Change in volume on fusion = -0.75 ± 0.7 cc/mole. Sodium Acetate - liquid Molar volumes 63.985 64.227 64.105 64.199 63.977 63.946 64.111 64.235 It, Thus

V = 64.10 i 0.12 cc.

The bath temperature was 330°C. Sodiwn acetate - solid 61.719- winch speed 61.525 winch speed 61.0594,1 45 61.781 30 62.275 winch speed 61.22 .,

61.450 9 62.670 winch speed 15 Thus V = 61.7 0.5 cc. Change in volume on fusion = + 2.4 ± 0.5 cc/mole. 12.7. Assessment of errors. Random errors in the volume of the liquid are mainly due to the loss of a small amount of material from the pyknometer while detaching it from the melt. This is reflected in the variance of the results of,v0.1%.

The first systematic error iv due to the temperature of the melt necessarily being somewhat above the freezing point. This difference in temperature was never greater than 10C., and would lead to an error of 0.03 cc. per mole.

This is less than the observed standard deviation.

A second systematic error arises from the expansion of the pyknometer between the calibration temperature and t1' working temperature. This would lead to the calculated zolar volume being too small by approximately 0.1%. Since the object of this experiment is to determine the change in volume on fusion, and since the technique, when used for the solid yields more random results, neglect of this correction can be justified.

Errors arising in the measurement of the volume of the solid at the melting point are more serious, revealed by the larger standard deviation of the points (Aft is).

This could be attributed to two main sources.

The solubility of gases is generally much lower in the solid than in the corresponding liquid. This it might be expected that as solid was formed, the concentration of gas in the melt would rise, and eventually bubbles would appear in the liquid phase. This did in fact occur in the original work of Landon, and visible bubbles appeared. Except in a few cases (which have been neglected in the calcUlations) gas bubbles did not appear in this

work, and errors due to this source must be small.

Ideally the temperature of the solid ingot formed

should be infinitesimally lower than the freezing point,

along the whole length of the material. In fact it is not,

and falls as shown in figure 12.1. The effect of this will

be that the solid et the top of the ingot will be under

a strain due to its contraction. This strain could,be

relieved either by cracking of the solid,or by movement of IS the ingot as a whole, in an upwards direction. The former will have little effect on the results as cracks will not spread through the relatively unstrained material new to the interface. The latter effect will cause more liquid to be drawn into the bulb, increasing the weight, and decreas- ing the apparent molar volume. The random variation,of the results is most probably due to this effect. The errors present in these two measurements lead to a high estimate of the volume of the melt, and a low one for that of the solid. Thus systematic errors would lead to an over-estimation of positive volume changes on fusion, and an under-estimation of negative ones.

References. (1)Landon & Ubbelohde Trans.Far.Soc.(1956) 52, 647. (2)Plester Ph.D. Thesis (Belfast) 1955• CHAPTER 13

THE NEASUREMEla OF ULTRA-VIOLET SPECTRA.

Ultra-violet spectra were measured for samples of pptassium nitrite, and potassium acetate. The detailed results for the former are presented in section IV. 13.1. Method.

The modifications made to a Unlearn S.P.700 recording, double-beam, ultra-violet spectrophotometer have been described by Cleaver, Rhodes and Ubbelohde(1). Basically a small thermostatted furnace has been built into each cell compartment in which films of salt contained between a pair of 'Spectrosil' quartz plates, can be heated. Tem- perature uniformity was f 0.5 0C. over the beam area, at

300°C. and varied proportionately at lower temperatures. Over a period of time, the temperature could be maintained constant to within * 0.1 °C.

Before use, the whole of the optical path of the

.instrument was flushed with dry nitrogen, and this gas was passed through the instrument continuously while it was in use. This precaution was particularly necessary with the acetate, where the only absorption occurs near to the short wave-length limit of the instrument where absorption by oxygen begins to obscure any other features. i ct

When a check on the transmission at 54,000 cm-1 revealed no further change, indicating that the oxygen concentration had been reduced to a minimum, the baseline of the instrument was adjusted to be flat from 15,000 cm-1 to 52,000 cm-1 to ± 1,4.

The wave number scale of the instrument was also checked periodically by measurement of the single beam spectrum of the hydrogen lamp at 15,000 - 15,400 cm-1. -1 The observed line was always within the specified ± 20 cm of the true value of 15,237 cm-1 (3).

Samples were prepared by heating a pair of Spectrosil plates on a small electric hot plate, inside a glove boxy to a temperature above the melting point of the salt. A few particles of the salt placed on one of the plates was spread to form a thin uniform film when molten by pressure from the second plate, superimposed on the first. Several samples of varying thickness were prepared so that one of the correct thickness (determined by subsequent events) was available. Samples which looked non-uniform in thickness, or containing bubbles could generally be modified by remelt- ing and freezing several times.

The "sandwich" so formed was held in a stainless steel holder and transferred to the sample compartment of the spectrometer. In the reference compartment was 1 7 0 placed a similar holder containing two more quartz plates, separted by thin pieces of platinum foil.

A sample of the correct thickness was chosen.

"Correct" in this context implies that any feature in the spectrum should preferably lie within the percentage transmission limits of 10% to 90%.

Spectra were measured according to the particular experiment undertaken. In this chapter results are given for potassium acetate. Detailed presentation of the spectra of potassium nitrite has been reserved for section ilr

13.2. Results - Potassium Acetate. -1 -1 The complete spectrum from 15,000 cm to 52,000 cm was measured for liquid and solid samples and is shown in figure 13.1. The only feature of note in this range is the band edge at approximately 45,000 cm-1. It is compared with other reports of ultra-violet spectra.

Spectra were plotted for a sample initially in the liquid state, and then in the solid state down to room temperature. The transmission levels on the high end low

Vequency sides of the absorption edge tended to change as the experiment proceeded, and in order to relate all readings to the same scale, an arbitrary transmission scale was used. This scale used as a 0% level of transmission and the chart reading at 51,000 cm-1 and, for a 1000 level, that 45 420 30 ' 25 20 Frequent, !tern` -47•6

47-0

442'S Fipure 13.2

U. V 5Ficzt1"u:rn: Ge9uaric.):: 4411"

5O O/43 transanisSlon. CH5C°2K 46.0

V). Isen )

45%5

t nt. j .

300 Sp J00 ISO 200 .250 I 7 3 at 359 000 cm-1. On this basis, the 510 transmissions level was calculated and from this the frequency at which

5414 transmission occurred read from the chart. A cathetometer was used to interpolate between the wave-number markings at 1000 cm-1 intervals on the edge of the chart.

Figure 13.2. shows the relationship between this frequency, at which 50% transmission occurs, and the temperature of the sample. As the solid melts, the band edge moves towards lower frequencies. This behaviour, by contrast with the nitrates, is discussed in Chapter 17.

13.3. Errors.

During measurement of charts for interpolation purposes, it became apparent that the wave number scale, marked by special pens on the edge of the chart, was not even. Correlation of the whole set of results established a standard deviation for frequency measurements of i 16 cm-1.

Also present is an error due to change in the calibration of the instrument in relation to the frequency of the hydrogen time of t 20 cm-1 and a possible absolute error due to mis-shaping of the prism drive cam. This latter amounted to -42 cm-1 at 31,993 cm-1 (2) and could not be checked at the higher frequencies due to lack of sufficiently precise line sources whose spectra could be measured using this instrument.

Within these limits of accuracy, a smooth line may be drawn through the points as shown.

References. (1)Cleaver, Rhodes and Ubbelohde Proc.Roy.Soc. A276, 437, 1963.

(2)B. Cleaver Ph.D. Thesis. University of London 1963. (3)Handbook of Chemistry & Physics, Rubber Publishing Co.

(4)R.P. Buck, Samang Singhadeja & L.B. Rogers. Analytical Chemistry, 26, 1240. 176 CHAPTER 14 THE GENERAL PROPERTIES OF ACETATES AS: MELTS

14.1 Thermal stabilily- The experience of this work on the molten acetates, is in accord with the results of the Russian workers who found that salts with fewer than 4 carbon atoms in the chain "could be repeatedly melted to form quiescent melts". (1) One important point has been found necessary to qualify this statement. This work shows that the exclusion of oxygen from the melts is of the utmost importance. That this fact is not reported by the Russian workers seems strange. However, none of the Russian papers gives lengthy details of the equipment used, probably due to the inherently simple nature of the apparatus necessary for phase studies. It is possible that either sealed or narrow sample tubes were used, which are shown in Chapter 5 to inhibit the effect of oxygen on the melt. The actual mechanism for the thermal degradation of molten acetates has not yet been fully established. While the action of heat on a mixture of a carboxylate and sodium hydroxide as a method of synthesizing hydro— carbons is well known, the products of the thermal degrad— ation of the pure salts have not been thoroughly investig— ated. Yakerson (2) has suggested that there are two '76 possible products from the thermal decomposition of pure sodium acetate; methane or acetone. Stoichiometric equations (which do not necessarily represent all of the possible reactions) can be written for these processes. 2 CH CO Na > CH CO CH + Na 3 2 3 3 2 and 2CH CO Na + H 0 + CO + Na CO 3 2 2 4 2 2 3 Yakerson noted that methane formation did not appear to occur except in the presence of water, or of course sodium hydroxide. Kinetic studies of the reaction, in the absence of water seemed to indicate that two molecules were involved in the activated complex. In the present work gas evolution was initially noticed at 350-360°C. This initiation temperature drops gradually as the materials are kept in the molten state, suggesting a certain amount of auto—catalytic action. In addition to gas evolution, discolouration of the melt, and eventually the formation of a black solid deposit after three to twelve hours was noted. These times depended on the conditions 'of temperature, and exposure to air. This diversity of products suggest that there may be competing reactions with comparable activation energies 177 In view of the ionic nature of the medium, and the observed effect of water, some of these may have wholly ionic mechanisms. However, the action of oxygen, and of peracetic acid, sugsest a mixed ion—radical chain mechanism, similar to the Haber—Weiss decomposition of hydrogen peroxide by ferrous ions (3). A reaction of this type will explain many of the features noted for this decomposition. Initiation leo 024 CH 3 COil- 4 CH 3c- '••• 04'

Prafacsext L 0 irr •

rb C H3 —C -- 0

x44- 3 C r 0

• • 11. 0 C.''`

CH3‘).C.dee

N. \ C.en er • • • 0=c -•• CH coo en : 0, s

The colouration of the melt, and the solid products can easily be explained by 0 —C. —CH- O 3 DJ? C143—C— CH3

rit))etc C7g

Leading to what are traditionally known as "high molecular weight gums and tars". If such a mechanism occurs, then there would appear to be scope for the use of antioxidants, which would quench the chain reaction by combining preferentially with the radicals. However, most of the usual anti— oxidants would not be thermally stable at 350°C, and moreover, being covalent compounds may not dissolve in the ionic melt. 14.2. The freezing points of the melts For the pure salts, the freezing points fall within the range already noted by other workers (Table 14.1) The liquidus temperatures of the mixtures also lie within a few degrees of those noted by other workers (figure 14.1.). Table 14.1. Comparison of freezing points

Salt This work Other work (See Chapter 4)

Potassium 292°C; 301°C; 302°C: Acetate 304.7°C 306°C; 310°C . Sodium Acetate 329.2°C 324°C; 331°C; 326°C; 328°C;

It is probable that part of this variation is due to techniques of measurement, and part is due to the differing purity of the samples. Assuming for an order of magnitude calculation, that the depression of the -350

r19 u re IIt

Phase aia.9ram C1i3C0:11 1(t rn i %tures .

o sah.te, Podit..60„, .9,a eer,mosln • -300 This worts.

1" °C

2 30

Caiiii3oLlai or; , .4» To CH.3[02.1i . to Ito 30 1,0 SO 60 70 Co (10 IGO I freezing point is proportional to the concentration of impurity over the first few percent of the phase diagram, these ranges of freezing points could correspond to 0-4 mole % impurity for the sodium salt, and 0—il mole % impurity for the potassium salt.

In principle, latent heats of fusion can be determined from the initial portions of phase diagrams. However, the simple theory assumes that the activity coefficient of the solute is unity, that the latent heat is not a function of temperature, and that pure solids, and not solid solutions are deposited. In molten salts it is doubtful if the first of these assumptions can apply. The second is generally untrue, but its effect can be minimized by considering only the initial part of the phase diagram. The affect of the third assumption can be rigorously calculated, provided that the solidus line in the phase diagram is known. Blander (4) has discussed these effects for molten salts and concludes that accurate values of the latent heat of fusion for molten salts can only be measured calorimetrically, However, with the reservations noted above it is possible to calculate an approximate value for the heat of fusion of the acetates from the freezing point depression data using the relation — S g t 2,303 lot y A CA = H( 1 RfA To T where )1 A is the activity coefficient of the acetate in solution, HA is the latent heat of fusion, and To and T the liquidus temperatures of the pure acetate, and the solution, respectively. 1 Thus a plot of to% 0 A against ToK should be linear if Y A is unity. If this is not so, then since A 1 as CA 1, the initial slope of such a plot should give an estimate of the value of — A IlfA 2.303R Data are available for the depression of the 6 freezing point of acetates by nitrates, 1,5 nitrates, 7,8, thiocyanates(1) 7 chlorides(8)1 and acetates containing different cations, and are plotted in diagram (14.2). Second order polynomicals have been fitted to this data, suitably weighted so that the curve passes through the freezing point of the pure salt, and the latent heats of fusion calculated from the initial slopes of these curves. That a range of values for the latent heats of fusion is found, is apparent from the graphs, and is shown in table (14.2). The variation observed partially reflects the lack of precision of this method, and partially the tendency to form solid solutions, which lead to apparently low values of A Hf. For this latter reason the true value of the latent heat is probably F"orei,," ion. Cf\fS N0.t,. (,t - -t . ~ l'J~ or K NO - ..J l- Na or f< greater than the arithmetic mean of the results in table 14.2. In table 14.3 this mean value is compared to latent heats of fusion of other salts. Entropies of fusion are also shown. Table (14.2). Latent heats of fusion from cr osco ic data

Additive I Salt A and Hf Kcal Potassium Acetate Sodium Acetate

Ea+ 4.46 5.44 6.16 + K 4.63 01— 6.11 NO 3 2.87 7.54 - NO2 4.04 ONS— 8.42

Mean Value 4.6 + 1.3 6.7 + 1.7 Table (14.3)., Latent heats and entropies of fusion

T °K 1 A S eu. Salt f Hf kcal mole- Sodium acetate 601 6.7 11.0 Potassium acetate 576 4.6 8.0 Lithium chloride(4) 883 4.76 5.39 Sodium chloride(4) 1073 6.69 6.24 Potassium chloride(4) 1043 6.34 6.08

Lithium bromide(4) 823 4.22 5.13 Sodium bromide(4) 1020 6.24 6.12 Potassium bromide(4) 1007 6.10 6.06

Lithium nitrate(4) 525 6.12 11.65 Sodium nitrate(4) 579 3.49 6.03 Potassium nitrate(4) 611 2.80 4.58

Sodium thiocyanate(9) 583 Potassium(t4io— cyanate'9) 450 3.39 7.56 1 Sodium bisulphate(10) 45 Potassium bisulphate(10)483 20

The calculated entropies of fusion of the acetates are high, compared to those for simple halide melts. If the entropy of fusion can be represented by 6 s - A s A -+ A s f positional Sorientat ion config

(chapter 1), then these high values of the entropy of fusion, will correspond to randomization of the melt in modes additional to the positional one which is the only one possible with the halides. This finding is in general agreement with data on the melting of other salts with non—spherical ions. In view of the very approximate nature of the

estimate of tSf for the acetates, and also the lack of knowledge of the crystal structure of the solids at any temperature, it would be premature to place any more emphasis on this aspect of the work.

14.3. The freezing points of the homologous series of carboxylates Table 4.2 gives the reported freezing points of all carboxylate melts so far considered. In figure 14.3 these are plotted to show the relationship of melting point to chain length. To refer all melts to a common origin, a difference function is used as ordinate defined by:— Freezing point of the melt — Freezing point of the format, with the same cation. The number of carbon atoms in the chain (i.e. n in

Cn H2n+1 COO M) is plotted as abscissa. Insofar as the data allow, it appears that the melting points rise, to reach a limiting value, as the :;00

t :'aOO

Tr-T,tb" ."

'l1 in Ci1 "1.".1 COl n

IJ 7· '&'~ ~J. 3 ". 5 b 9 ~QJ -qt ~ t::1 ~~ ~OJ .""OJ ~ o~ .....,tl' ,..p ~ It.0~ ,~l)' I.~ lor:: ':l ,~ ~ . 1,.0 'N~ . ii:. 1I-}1 ""f1J ~ l~ t:c!.J .(0 (j)~ ~t:J r.:J.fJ.. ~QJ ",(J ~ ~J' Q"QJ , I V ~ ~ \'" ~

~. ig7 chain length is increased. This value is dependent on the cation. The results for the potassium salts seem to indicate that for longer carbon chains than 6-7 units, the melting point may begin to decrease, as further —CH2 — units are added to the compound. Again assuming that the total entropy of fusion can be related to various randomization processes in the melts, it is possible that this trend in the melting points represents the introduction of a configurational entropy term of increasing importance. Since the hydrocarbon chain can be expected to keep itself remote from the ionic part of the ion, the addition of one or two CH2 units to a formate ion will not allow a large change in the 'configurational entropy'. As the number of —CH2— units is increased however, each one should be able to allow more variations in the conformation of the carbon chain without bringing it near to the ionized part. Thus the entropy increase on melting will eventually increase faster than the enthalpy change, as CH2 units are added, resulting in a decrease in melting point. A second reason for this melting point behaviour could be connected with the ionic nature of the melt. As the number of carbon atoms is increased, the ions in the melt become effectively diluted by the hydrocarbon lEg chains. The forces between the molecules will cease to be mainly ionic and will be replaced by other forces similar to those in non—polar liquids. This might also cause the observed melting point behaviour. REFERENCES

(1) N.M.Sokolov Zhur.Obschei.Khim. (1954) 24, 1150 (2) Yakerson. Kinetika i Kataliz (1961) 2.1. 172 (3) Weiss & Haber Proc.Roy.Soc. 1934. A147 332. (4) Blander Chap.3 "Molten Sqlt Chemistry" Ed.M.Blander. Inter science (5) G.G.Diogenov & N.N.Nurminskii (6) N.M.Sokolov & M.A.Munch. Zhur.Neorg.Khim. (1961) 6 2558 (7) N.M.Sokolov & E.I.Pochtakova. Zhur.Obschei Khim. (1958) 28, 1398. (8) T.T.Il'ynsov & A.G.Bergmann. Zhur.Obschei Khim. (1960) 30, 355. (9) Plester, Rogers & Ubbelohde. Froc.Roy.Soc. 235A 469 (1956). (10) Rogers. Ph.D. Thesis. Belfast 1951. CHAPTER 15 140 THE TRANSPORT PROPERTIES OF MOLTEN ACETATES

15.1 Comparison of data with that for other systems With the limited amount of data available the interpretation of the transport properties of these melts can only be attempted on a qualitative, and comparative basis. This approach is also indicated by the state of the theory of transport in salt melts. No detailed mechanisms of transport which might apply generally have yet been advanced. The problem of comparing the data for various groups of liquids is confused by the problem of defining corresponding states for reference. The use of equal temperatures cannot be justified since the viscosity/temperature, or conductance/temperature curves of various materials can cross. Thus the comparison would depend on the temperature chosen. Also extrapolation of data over long ranges of temperature would be necessary to include some salts in this scheme of comparison : an extrapolation which the data is not sufficiently accurate to justify. The most acceptable basis for 'corresponding states' particularly for molecular liquids is to relate all temperatures to the critical temperature. Even for these liquids this comparison is not always possible as critical temperatures are unknOwn. For sq molten salts critical temperatures are expected to be very high and are unknown for any of the salts considered here. Reiss etal (chapter 1.5) have considered another basis for corresponding temperatures in ionic melts, which they justify by dimensional analysis of the configurational integral. This is to refer all measurements to the melting point of the salts concerned. Intuitively, this is at least satisfactory, since all molten salts have molecular packing densities comparable with that of the corresponding solids. Yaffe & van Arstdalen(1) have also used such a reduced temperature i.e. 'r = T/Tf. For many salts this leads to adequate intercomparison of data, but it is naturally not so satisfactory as a corresponding states principle based on the critical point. In table 15.1 a comparison is made at values of r of 1.00 and 1.05. Certain of the salts (e.g. KONS(0)1 are known to show marked anomalies in the transport parameter close to the freezing point and comparisons based on T = 1.00 might be misleading. Higher values of T than about 1.10 are not possible for the acetates, due to decomposition. Angell(3) has suggested that To (see chapter 2) rather than Tf is a suitable ba6is for the definition Table 15.1 Comparison of Transport data at corresponding temperatures r = 1.00 1C` = 1.05 -1 - - 1 - Rohm 2 milli- x ohm an A ohm cml Salt poise mole poise mole Sodium acetate 72.6 0.181 11.67 0,224 14.39 Potassium acetate 60.67 0.189 13.49 45.4 0.242 17.14 Eutectic mixture 239.9 0.066 4.29 148.5 0.094 6.35 Sodium carbonate 2.77 153.8 3.09 168.3 Potassium carbonate 2.03 150.0 2.20 165.6 Sodium sulphate 2.18 149.5 Potassium sulphate 1.79 161.0 1.90 178.2

Sodium nitrate 31.0 0.956 26.3 1.067 Potassium nitrate 29.2 0.674 25.2 0.718

Sodium nitrite 31.3 1.223 46.4 26.7 1.411 54.2 Potassium nitrite 19.8 1.109 17.5 1.239 Sodium bisulphate 0.069 0.091 Potassium bisulphate 586 0.044 320.6 0.067 Contd. Table 15.1 (Coned)

T = 1.00 14" = 1.05 - -1 , -1 -1 -1 ,2 milli- m ohm cm t\ ohm-lcm 1 milli- g ohm em ohm em-1 Salt poise mole' poise mole

Sodium thiocyanate 35.6 0.574 - 26.1 0.688 - Potassium thiocyanate 132.2 0.142 ._ 93.8 0.193 _ Potassium dichromate 138.6 0.206 26.6 100.5 0.274 36.0 Sodium chloride 16.2 3,09 130 13.2 3.15 140 Potassium chloride 15.5 1 2.89 106 12.9 3.25 115 Sodium bromide 15.8 2.90 126 12.30 3.06 130 Potassium bromide 15.98 1,60 90 13.24 1.75 100 of corresponding states. While this may be so for the glass forming mixtures studied by him, it has little relevance here since To cannot be located with any accuracy for acetate melts. Furthermore it introduces an even more hypothetical basis for comparison. Another method of correlating data which is honoured by long standing use, and is backed by its close relationship to the theory of absolute reaction rates is the comparison of the slopes and intercepts of the Arrhenius plots. This can be criticized since accurate data show that the semi- (1) logarithmic plots are not linear in many cases These departures from linearity cannot be explained without questioning the foundations of the theory (chapter 2). Over short temperature ranges the semi-logarithmic plot can be described in terms of two constants, viz p = Ap exp ( where p is a RT transport parameter and such a comparison is made in table 15.2. Curvature of the -Arrhenius plot is exhibited by acetates when the accessible temperatUre range is increased, by forming a eutectic mixture, as is shown in figure 15.1. For comparison the viscosity of a glass forming Ca(NO3)2/kNO3 mixture, and a "normal" Table 15.2 Comparison of Arrhenius parameters for ionic melts

Material Melting A Point K poie kEa.1 ,A31-1cm-1 Kti 0.11-lcm'2 ITEAl 44 1 4e.7S mole 1 Y CH 00 Na 602 14.62 5.25 942.3 5.24 3 2 1 - CH3CO2K 578 1.04x10-4 7.31 20.14 5.35 2011 5.73 1.36 -1.21 Eutectic( 505 1.001x10_5-4 6.75x 11.92 4.93,x 1063 5.22x 1.37 -3.4e,cr, mixture ( 1.512x10 9.68XX 110.7 7.44-x 9539 7.67xx 1.30 +2.74-- Na2CO3 1131 13.757 3.87 K2C03 1172 - - 11.014 3.89 - - - - Na2SO4 1157 - - 9.92 3.47 I 950.6 4.24 - - K2SO4 1349 - - 8.30 4.10 1197 5.29 - - 3.95 10.i 2.76 - - 1.43 -5.95 Na NO3 579 9.87x10-4 K NO 611 14.12x104 3.66 11.2 3.49 - - 1.05 -5.75 3 1 - Na NO2 557 8.56x104 4.00 19.83 1 3.0691 I - 1.30 -4.18 K NO2 692 16.94x10-41 3.37 f 8.39 1 2.755 t - - 1.22 1 -5.45 Na HSO 451 - 30.59 5.44 r-_ - 4 -6 KHSO4 483 2.3x10 12.00 33.95 6.2 - 6.5 1.94 - -4 1.9x10 , ''' 5.80 A 1.37 -1.65 RaCNS 560 4.8x10-' xx 4.70 xx 22.59 4.25 - 4.51 1.15 -4 K CNS 1.78x10 x 5.72x' 6.02 0.95 -4.28 446 1.53x10-5xx 8.00y4 106.2 5.84 1.37 +2.91 K2Cr207 671 2.011x10-4t 8.68 t 73,0 7,8 - - 1.11 -3.70 (Contd) Table 15.2 (Con-bill

Material Melti A.:7 E41 A x _i _i I -Ey -EA l_a AS Point poise K chl ohm 'cm ' K cal 1cm+ K 'da1 --E; e. j.? mole-1 . Na Cl 1074 1.95x10-4 9.4 7.3 1.54 - 2.70 6.1 -1.28 K Cl 1043 3.55x104 7.8 6.5 2.30 3.26 3.4 -3.05 Na Br 1020 18.13x10-4 10.63 7.4 1.84 - 2.58 .5.8 -4.38 K Br 1007 2.95x104 7.96 5.8 2.55 - 3.42 3.1 - xx Near to melting point x Well above melting point. 17

References to tables 15.1 & 15.2

The acetates present work The carbonates — from data in chapter 3; fused salts. Ed. Sundheim The sulphates — from data in chapter 8. 'Nolten Salt Jhemistry. Ed. M. Blander, Interscience. The nitrates — From Frame Rhodes & Ubbelohde nitrites dichromates Trans. Faraday Soc. 55 2039 (1959 Data marked 4 have been re— calculated from the original results of J.P. Frame. Ph.D. Thesis. Un. of London 1959. The acid sulphates From Rogers & Ubbelohde. Trans. Far. Soc. (1950) 46 1051. The thiocyanates From Plester Rogers and Ubbelohde Proc Roy Soc. A.235 469 (1956) and Plester Ph.D. Thesis (Belfast) (1955) The halides Bloom & Heymann Proc. Roy. Soc. A188 392 (1947). and Yaffe and van Artsdalen J. Phys. Chem. 60 1125 (1956). nitrate mixture(4) are also shown. Despite the considerable curvature shown by the acetate no tendency towards glass formation was noted for any mixture of sodium and potassium acetates. When making a comparison of the electrical conductance of melts it is more satisfactory to use the equivalent conductance as a basis. This removes any spurious effects due to the variation of the number of ions per cc due to thermal expansion. Effectively A is the conductance associated with 1 mole of ions. Inspection of the tables shows that the viscosities. and conductances are similar in magnitude to those for other melts. It can be supposed then, that acetate melts are ionic in nature. A more rigorous differentiation between electronic and ionic conductivity would be to compare the conductance:: of the solid and the liquid. Ionic conductors show a large increase in conductance on fusion, not shown by the electronic conductors. Since the solid state conductance has not been measured the evidence for the ionic nature of the melts must rest on the comparison with known ionic melts. Materials showing electronic conduction, with the exception of metals, which are a rather special case, normally have much lower conductances than observed here. 0'0

Inter comparison of the data in tables 15.1 and 15.2 shows that transport processes in acetate melts are hindered. Viscosities are high, and conductances low; the closest parallels can be drawn with the thiocyanates, and bisulphates. Activation energies for both transport processes studied are high, again comparable to the thio— cyanates and acid sulphates. For viscous flow, the activation energy for acetate melts is also comparable to that for the halide melts. However the ratio El/Ex shows that the acetates fall into the general class of low temperature melts, and are fundamentally different in structure from melts of the halides. It has been suggested(5) that conductance is controlled by the small cation, while viscous flow is controlled by the large anion. If this is true for the halides, then to account for the low values of E /E for the low melting salts, )' additional mechanisms for the relaxation of shear must be proposed. In melts containing only monatomic ions, hopping of ions into Iholest is the only relaxation process which can be envisaged. When polyatomic, ions are present, another mechanism involving the 2 Of rotation of one or even several, of these ions can relax shear stresses(8) and this may cause the observed differences in the ratio Ell/Ex. If such a mutual rotational mechanism does account for the viscosity of the low melting point salts, then intuitively it would seem natural that ions having a high degree of rotational symmetry would undergo relaxation of shear more easily, than those ions which are less regularly shaped. Since the acetates, thiocyanates, and bisulphates, have ions of low symmetry (based on the structural formulae) then these melts might be expected to have higher activation energies for viscous flow than the nitrates. The unsymmetrical nature of the acetate ion is discussed later (chapter 16) where it is fottnd to be consistent with the molar volumes of the melts. k5,2,Entropies of viscous flow The absolute rate expression for viscosity and conductance can be cast in such a form as to relate the pre-exponential factor to an entropy of activation (chapter 2). The conductance equation contains terms such as the local dielectric constant in the neighbourhood of the moving ion. Uncertainties of this nature limit the validity of discussion of the entropies of activation in the conductance process. The .correspondin5 202. term in the viscosity equation does not give rise to such uncertainties and can be discussed. Formal Entropies of activation calculated from A is Avogadro's number A Noh exp —RS- 1 where No im h is Plancks constant are given in table 15.2 For most of the salts listed the entropy is negative, implying that the activated state has a more particular geometric form than the normal liquid. Positive entropies of activation are only found in two cases; potassium thiocyanate, near to its melting point, and the eutectic acetate mixture, near to the eutectic temperature. In both of these cases, the energy of activation at these temperatures is greater than at higher temperatures. Thus while the geometric requirements for activation appear to have been relaxed, the lower free volumes under these conditions force an increase in activation energy. Those ions showing the highest rotational symmetry seem to require a more geometrically specific activated state (large negative t s), than ions with less symmetry. Again, this is consistent with a rotational mechanism of shear relaxation. 15.3 Relationships between transport and volume In chapter 2 three possible relationships

2-03

between the volume, or free volume of the melt, and the appropriate transport parameters were discussed. Two of these, due to Batchinski, and Doolittle are purely empirical, while the third is related to a theory of liquid structure, though the parameter To is at present only able to be determined empirically. ts- J-4The Batchinski Equation 1 9 B The fluidity. -47; has been plotted against molar , volume in figure 15.2. The lines obtained are parallel, but not straight, though the curvature is only just detectable for the potassium salt. The slope of the lines are closely similar for both materials (B = 0.166). This factor is supposedly related to the size and shape of the flowing particle(6). Thus the indications are that the flow process io controlled by the same ion, the acetate, in both cases. Values of B are compared with those for other materials in table 15.3 l?:",tcai, \\1ixtl.'\Ae•.

o o

of) 09 7(J "I' 71 2. 0 S' Table 15.3

Salt B (per mole)

Potassium acetate 0.166 Sodium nitrate 0.080 Potassium nitrate 0.097. Potassium thiocyanate 0.137 Sodium thiocyanate 0.069 Sodium chloride 0.051 Potassium chloride 0.078

Values of B seem to be greater for melts containing non spherical anions. The second parameter b. can be estimated by (6) extrapolation of the line: to zero fluidity. McLeod in his explanations of the Batchinski equation has supposed that fluidity was proportional to the amount of "free space" in the liquid, VIM—b. Thus b ought to be equal to the total space actually occupied by ions (rather analogous to the Van der Waals co—volume). For potassium acetate 1) is found to be 68.1 cc compared to the volume of the solid at the melting point of 71.6 cc (chapter 12). The actual volume of ions in the solid (i.e. the solid volume less the volume of the spaces in the crystal lattice) would of course be less than 71.6 cc. 'LOb fA.

I.~

~ t.'t:""l-~ 20L

However, though the Batchinski equation is approximately followed by molten acetates, since it does not provide a great deal of useful structural information, deviations from it will not be discussed further. 5.5 The Doolittle Equation

1 A exp (- Vo • VM -Vo / This can be plotted in a semi-logarithmic form and ought to give a straight line. Such plots are shown in figure 15.3 indicating that these melts do not fit the Doolittle equation particularly well. However, estimation of the value of Vo involves extrapolation of the thermal expansion data for the melts to 0°K, and over such a range small errors in the expansion coefficients can load to large errors in the value of Vo. The equation was derived on purely empirioal grounds by the author, and its only significance lies in the fact that many non-polar liquids for which bettor estimates of Vo are available, do fit it. This has led to the application of Cohen and Turnbull's free volume theory to these liquids. J‘liThe Cohen and Turnbull Theory 1 In A T2 = Al k T - To 411 k ir In 0. T2 . T - To 207 The above equations show forms in which the fit of the data to this theory can be tested. Plots are given in figures 15.4 - 5 for varioua, values of To, and it can be seen that a value of 200°T gives a linear plot for these melts. This value of To has been selected on a purely arbitrary basis, a limitation of this theory in its present form (chapter 2). According to the authors To is the temperature at which the free volume in the system is reduced to zero. It should correspond with the glass transition temperature of the super-cooled liquid. However there is at present no unequivocal way of relating this temperature to the molecular structure of the system. Despite the fact that no super-cooling was observed in the acetate melts it is of interest to extrapolate the computed curve for the viscosity of the eutectic mixture, to a value of 1013 poise - the supposed glass transition point. This would occur at 2400K, in reasonable agreement considering the nature of the extrapolation, with the value found from the Cohen and Turnbull plots. Thus the transport parameters of acetate melts can be reproduced by an expression of this type. At the present state of' development of this theory, no further information is supplied, concerning the nature of the acetate melts. t9.9u.re 15 .

6.1161 Tu-.;71)Ltil.

2 -6

•I -2.7 FL ure, 15.6

C h Ch17 a Tur,4716.4it riot

F-tuicittce.5 of etstezt miz;ture...

.,•3

2.1

2-0 2.0 3.0 5.0 2 1 1' REFERENCES

(1) Yaffe & Van Arstdalen Journal of Physical Chemistry (1956) 60 1125 (2)Plester, Rogers Proc Roy. Soc, (1956) A235 Ubbelohde 469 (3)Angell J. Phys. Chem.(1964) 68 1917 (4)Smith, Rhodes & Proc Roy. Soc. 1965 A285 Ubbelohde 243 (5)Bloom & Heymann Proc. Roy. Soc. (1947) A188 392. iQ s. rare ha Leott re4,4 41'9 spe_. (I4z3) (6) G .

(7)cf Plester Ph.D. Thesis. Belfast. 1955. (8)Barrer. Trans Faraday Soc. (1943) 39 59. 2 II

CHAPTER 16

THE STRUCTURAL AND THERMODYNAMIC

PROPERTIES OF THE ACETATES

16.1. Comparison of the data with that for other materials Melts of potassium and sodium acetate, and those mixtures studied in this work show normal thermal expansion characteristics in the temperature range accessible (231°C 360°C). There appears to be no tendency for the molar volume of each melt to deviate from a first—order linear relationship with temperature (Figure 11.5). For mmparison with other materials therefore, it is sufficient to quote the molar volume at the melting point (this is discussed as a basis for' corresponding states in chapters-(14) and (1) and the expansion parameter lb' in the eqUation V = a 4- bT. Such a comparison is made in table (16.1);included in this table are the values of the discontinuous change in volume at the melting point, and the entropy of fusion, where available. Immediately noticeable is the contraction in volume on melting for the potassium salt, which has been substantiated by two separate methods of measurement (chapters 10 and 11, and chapter 12). This unusual property of potassium acetate is shared by few materials,

2. 1 2. Table 16.1. Molar Volumes of Ionic Melts

m.p.oc 2 AS Material Molatt. Molar Vol. 10 b 100elf f at M.p. Vs cc/mole

Sodium acetate 329 82.04 64.407 3.23 +3.9 11e6 Potassium acetate 305 98.1 70.723 3.49 -1.05 8.0 Sodium carbonate 858 106.00 53.72 1.28 Potassium carbonate 899 138.20 72.84 1.73 SOO •••••• Sodium sulphate 884 142.06 68.62 1.69 - Potassium sulphate 1076 174.26 92.44 2.72 Sodium nitrate 306 85.01 44.30 1.70 +10.7 Potassium nitrate 338 101.01 54.04 2.27 3.3 4.5 Rubidium nitrate 310 147.49 59.36 2.52 -0.2 Sodium nitr.te 284 69.01 38.08 1.70 Potassium nitr,te 429 85.1 Sodium bisulphate 178 120 Potassium bisulphate 210 136.1 65.7 4.37 44.0 20 Sodium thiocyanate 287 81.00 67.0 +7.2 Potassium thiocyanate 173 97.10 60.20 410.0 7.56 Potassium di- chromat e 398 294.21 128.7 4!07 2_13 Table 16.1 (Oont'd)

Material M.p.°C Mol.Vit. Molar 102b 100A Vf% ASf Vol. at M.p.cc/ Vs mole

Sodium chloride 801 58.45 37.56 1.44 25 6.24 Potassium chloride 770 74.55 48.75 2.01 17.3 6.08 Sodium bromide 747 02.91 43.93 1.64 22.4 6.12 Potassium bromide 734 19.01 55.90 2.36 16.6 6.0E Sodium iodide 660 149.92 54.62 2.06 18.6 6.04 Potassium iodide 681 166.02 67.71 2.93 15.9 6.0',' 21 4. References for Table 16.1

The thiocyanates Plester. Ph.D.thesis. Belfast 1955. Plester Rogers & Ubbelohde. Proc.Roy.Soc. (1956) A235 469 The bisulphates Rogers. Ph.D.thesis. Belfast 1951. Potassium dichromate Frame, Rhodes, & Ubbelohde. Trans.Faraday Soc.(1959) 55 2039 V for nitrates and Sauerwald & Schinke.. f halides Z.Anorg.Chem. (1956) 287, 313. V and b Klemm. Chap.8, Molten Salt m Chemistry. Ed.Blander. Inter— ecience 1964. S Blander. Chap.3. Molten Salt . Chemistry. Ed.Blander. Inter— science 1964. for RbND cf Sauerwalde & Schinke. Z. Anorg. 3 Chem. (1960). 32A, 25 2.) water (2), rubidium nitrate (1), gallium (2) and a few intermetallic compounds are the only well documented examples. Though sodium acetate expands on fusion, the volume change is small by comparison with many other melts. In general salts containing non—spherical V ions have lower values of A f than those salts V containing only spherical ions. Also apparent in table 16.1 are the high values of the molar volumes and expansion parameters of the acetates, compared to other salts of similar molecular weight. A sufficiently large number of mixtures of molten acetates have been observed to allow isotherms of molar volume against composition to be drawn. Points evaluated from the quoted lines have been plotted in this way in figure 16.1. Included in this figure are mixture iso— therms for a variety of molten salt systems, and also non—ionic mixtures. A definite positive deviation from linearity is shown by the acetates at all accessible temperatures. This deviation is not however symmetrical about the 50% mixture, the maximum being at about 40 mole % of the potassium salt. The phase diagrams fig.(14.1) shows no peculiarity at this composition, though at higher percentages of potassium there are some irregularities 11 6

O.0 Pc.".tra 164 .

Ci2 4 Ku • CbOt (3)

ca; g .., 700°C (3 ) 0 .14, Trr C C71 4. Kel. 75o °C(3)

0.2 No KNO 551,t(3) 0.0 3 0.0 03 ir NO5 4.00t(3) °C(3) 0.0 tr (NO K Or SO 0-2 Ktk103 f 13c1„NO3) 14.10°C(3) 0-0

0-2 6enzene 2 0° c (3)

•CC1.4 11 0 peztain e (3) .

neopentane 4 baTazerie 0°C (3) 0. 0.4 C 1-1...,, Col C07.4.c 0.2 zocfc, c co2 4, 4. Ccoz ac 0.0 • 350C

0.0 0-0 8 r Gr. 750°C (4) - .0 2.0 - 3.0

1;.0 V00

a, 0 accrac,... corg-ppma 211 which might be interpreted as possible compound formation. The mixture isotherms also appear to show an inflection in the region of the eutectic composition. This effect, if it persisted to higher temperatures would eventually lead to an isotherm showing both positive and negative deviations from ideality of the type reported for KBr Tl Br (4). For the acetates this effect is of the same order of magnitude as the standard deviation of the results and it may be that a direct method of measurement would not substantiate the anomaly. It would thus be imprudent to base any conclusions on this fact. The molar volume of solid potassium acetate was also measured (figures 10.3 and 10.4). Though the experimental error is larger than for the other measure— ments several sections can be seen in this curve. At 80°C. there appears to be an increase in volume of 0.7 cc/mole, spread over a range of 8°C. Within the experimental accuracy, no hysteresis was observable. At higher temperatures there are changes in the slope of the curve, at 170°C and 230°C. No observable discontinuity in the molar volume occurs at either of these points. There are literature reports of transitions at 58°C and 155°C (5), detected from the cooling curve of the solid. No comment on the re— producibility of these points or of the detailed 2.1,9 method used is given in (5) and it is possible that the differences arise partly from the purity of the samples, and partly from the difference in technique. The significance of the volume changes is discussed later. In figures 16.2, ' the movement of the U.V. band edge for potassium acetate is compared with the movement of the edge of the strong band for the nitrates (6). There is a movement of this edge to lower wave— lengths in the region 285 — 305°C and apparently no sudden change on melting. For the nitrates a shift of the band edge to shorter wavelength occurs on melting, except in the case of rubidium nitrate, where no change can be detected. The weak band of the nitrates shows a movement to higher energy on melting for all nitrates (7). No volume premelting which correlates with the movement of the band edge from 285° — 305oC was observable since in this region it was not possible to measure the molar volume of the solid. When attempting to infer structural information from the data restraint should be exercised on two counts. Firstly, while a relationship between the U.V. spectrum and structural parameters (the anion—cation ts Co.

Lira 16 •1 F%-d91.12sac, lot- - 0010 travwvai55ton 4Q-. n Ar ai 04,0 Cfl CO tc C.113 COI it x fkl &m0 + A.103 SI'fron3 (51) C.mo o nta403 4.7- 0-

Cet.:./ ✓ !

.9 .• ;;;.11 11:5.0-

9.0

50 10Q 150 200 2p0 300 350 • 2 10 distance) has been established for the halides and the nitrates, it is not so certain for these materials. It is likely that this band arises from a type of charge transfer as postulated for the halides, and for the strong band of the nitrates, (8) in which an electron is transferred from the anion to an extended orbital over— lapping the nearest neighbour cation shell. Such a band would be influenced by its nearest neighbour environment. Secondly it is not certain that the movement of the band edge is fully correlated with the movement of the corresponding band maximum, though results for the nitrates suggest that this is normally true for salts (6).

16.2 A tentative structure for the acetate ion Lack of detailed X—ray measurements for acetates at any temperature hinders the interpretation of the thermal expansion curve. Infra red spectra (cf chapter 4) for the crystals, Nujol mulls, and for aqueous solutions, indicate that the most likely form for the carboxylate is a symmetrical one. The corresponding formate ions have 0 an 0-0-0 angle of 124° and a 0-0 distance of 1.27A . If these dimensions are retained in the acetate ion, and the other bond lengths in the ion are the normal ones found in organic chemistry, then a variety of models can be

22!

drawn for the acetate ion (figures 16.3 A,B,C). The first of these neglects the size of the hydrogen atoms, the second and third use repulsion envelopes for the atoms derived as indicated. These models indicate that while the linear dimensions of the ion are quite small, the minimum circumscribing sphere is somewhat larger. An even larger sphere is required for free rotation of the ion in all directions, about its centre of mass, since this point is markedly offset towards the carboxyl groupt As a first approximation to the structure of the melt two interpenetrating, cubic close packed lattices of spherical cations, and rotating anions can be considered. The molar volumes derived from these models, are compared with the observed molar volumes of the acetates, at their respective melting points in table 16.2. Table 16.2.

Model Sodium Potassium Observed Salt salt molar vol. at m. p. A 1) (Rotation in Sodium min.vol). 31.0 44.5 salt 2) (Rotation about C.of G.) 39.0 64,4- 54.4 i.. B 1) , 38.0 53.4 1,2otassiuril salt 2) 58.5 72.2 - 70.7 C 1) . 67.8 79.5 2) 89.4 96.0 212 Fib tire 1 b.3

flocle14 of the 4, ci.cetate ion. Scale. : : I H

This simple groad ne.51e4t.s tile size of ttie drooen A

o Centre of mininumn cia-cle • Centre- of pravii›.. coz 'roil, as in •corrricae ion . CH3 1.0,12 as Ln edger 12.5a.roc.o.rhons. (rd. 2 )

0 Project on of r: 2-97 model 14.51.n5 Catain atomic models. r.:2-71 A •

(ref. 9 ) C__ 223 The agreement between these values for model B (which is probably the most realistic model for the ion) is sufficiently good to show that this crude model is worthy of consideration. 16.3. Correlation of this model with other data. Such a model would require that thermal trans— formations occurred in the solid phase to allow for the increased space required for the rotation of the acetate ion. Several such transformations occur for the potassium salt, and others are reported for the sodium salt (2 ). One of these may account for the onset of free rotation of the methyl group about the C—C bond, though there is some evidence to suggest that this already occurs at room temperature (11,12). Thus the observed solid behaviour may be due to the crystal lattice adjusting itself to allow for lib— ration, and rotation of the ion about mutually perpendic— ular axes (possibly the ones indicated in 16.3.B.) The red shift of the ultra violet absorbtion band is characteristic of a positional disorder mechanism of melting as in the halides. Using the above model, the randomization of the cation positions would lead to a highly disordered state of the material, which is compatible with many theories of melts. The approx— imate entropies of fusion (Chapter 14) do, however, Zip indicate that the melting mechanism involves more than just positional disorder. With such a model, no large volume change would be necessary to allow for the onset of translational movement of the ions, as there is already a large amount of free volume, associated with the rotating acetate ion. Co—operative movement of ions of the type proposed by

Barrer could lead to transport processes. Activation energies for these processes might be higher than for those with simpler mechanisms in view of the larger number of ions involved. 16.4 The mixed melts The mixing isotherms indicate that the replace— ment of potassium ions by smaller sodium ions does not lead to a uniform, proportional shrinkage in the melt, as occurs with perfect solutions and mixtures of the type A NO /B(N0 ) 3 3 2' At high temperatures however, the excess function decreases, and may indicate that this state would eventually be attained. The structure of the melts of the sodium and potassium acetates may thus differ in detail by more than just the size and consequent packing of the particles present. For example the smaller sodium ion, may be able to come closer to the anion, and lead to a different polarization of the carboxylate group. Alternatively it may be that the 225 inter—particle interactions in the mixed melt lead to preferential packing of certain proportions of the two cations around the larger anion leading to the momentary existence of larger particles in the melt, which would distort the uniform change of structure between the two extremes. 16.5. Conclusions A simple model for the acetate ion has been proposed, which qualitatively will satisfy most of the available data. More sophisticated models of the melt, and the melting mechanism will require a knowledge of the crystal structure of the salts, and of the detailed inter— atomic distances and bond angles, which would result from X—ray work on the crystals, 22(,. REFERENCES

(1) Sauerwald & Schinke. Z Anorg.u.Allg.Chem. (1960) 204 25 (2)Handbook of Physics & Chemistry — Chemical Rubber Co. 46th Ed. (3) cf Cleaver,Rhodes & Proc.Roy.Soc. 1961. A262 435 Ubbelohde (4) Buckle, Tsaaussoglou & Trans.Par.Soc. (1964) 60 684 Ubbelohde (5) N.M.Sokolov & El Zhur.Obschei Khim. 32 1401 Pochtakova (6) B.Cleaver. Ph.D. Thesis. London 1963. (7) Cleaver,Rhodes & Disc.Faraday Soc. 1962. No.32 Ubbelohde p.22. (8) Smith. Chap.6. Molten Salt Chemistry. Ed,Blander Interscience (1964) (9) Catalin Molecular models cf. Stuart. Z.Phys.Chem.(1934) B.27 350 (10) N.M.Sokolov & El Zhur.Obschei Khim. 28 1398 Pochtakova. (11) Grigor'ev. Zhur.Neorg.Khim. (1963) 8 802 (12) Nakamura. J.Chem.Soc.Japan. Pure Chem. (1958) 12 1411 211 CHAP1NR 17

A DILATOMETRIC AND SPECTROSCOPIC EXAMINATION

OF A THERMAL TRANSITION IN POTASSIUM NITRITE

17.1. Introduction The nitrates of group IA of the periodic table are remarkable for the number of polymorphic transform— ations that are shown by the solids. Less well known is the behaviour of the nitrites of the corresponding cations. Sodium nitrite undergoes a first—order transition at (1) about 16000 . There is no discontinuous volume change at this point, only a change in slope of the thermal expansion curve. The electrical properties show a corresponding change from a ferroelectric to paraelectric phase at this temperature. Potassium nitrite has a reported phase change at WC. detected by thermal analysis of a saturated solution of the salt (2). This transition was also previously observed by means of shifts of the ultra—violet spectrum ( f Cleaver,Rhodes and Ubbelohde (3)). In this section of the work, the behaviour of the ultra—violet spectrum was correlated with the thermal expansion of the solid, and the effect of purification on the material• was investigated. 17.2. Experimental The purest commercially available material was 22..g was used, supplied by Hopkins and Williams (G.P.R.grade). The specification gives a 97.5% minimum purity, and the main impurity is expected to be potassium nitrate for which there is no satisfactory method of analysis. A solution in distilled water was neutral to B.D.H. universal indicator paper. The crystals were markedly deliquescent and could not'be dried by heating, as this led to the compaction of the material into a form almost impossible to grind by normal methods. Drying was accomplished by evacuation to 0.001 mm Hg for a Minimum period of 16 hours, and sub— sequent handling took place in a glove box flushed by dry white spot nitrogen. 17.2.1. Zone refining In order to study the effect of impurities, a sample was zone refined under a vacuum of 0.001 mm Hg, 15 passes of the molten zone through the material being applied. This zone refined material had a higher melting point (425.5 +,0.5°C) than the normal material (420.6 ± 0.5°C) (as measured by direct observation of the slowly heated samples in the molten salt bath). Addition of 6% potassium nitrate to a zone refined sample lowered. the melting point to 412°C. Assuming that the depression of the freezing point is approximately proportional to the percentage of impurity, this 5° rise corresponds to 2,29 the removal of Ai 2% of impurity from the material as supplied_ by the zone refining process. Tests with the usual analytical reagents showed that a trace of carbonate present in the original simple was removed by zone refining. A consideration of the process of zone refining shows that if the distribution coefficient of impurity between the solid and melt is unknown then the optimum position for sampling the ingot to minimize impurity levels, is in the central section (4). Such samples were used in this work. 17.2.2. Measurement of the thermal expansion of the solid. The techniaue employed is described fully in chapter 10. Dilatometers had a bulb volume of ,v 6cc, and the bore of the capillary stem was checked and found to be within the manufacturers tolerances of 0.05 cm:±p.,o,14,,,.. The Midland Silicones M.S.200/10 fluid used as indicator liquid had a measured specific volume of 1.0419 + 0.001079 T cc gm-1 (T in 00) (figure 10.2). The dried crystals of the impure sample (Sample I) and pellets of the zone refined sample (Sample II) cast by melting the material in the glove box, and allowing it to drop on to a porcelain dish, were used to fill the bulb. — 4.14.• S 0

Fiy4re 17.1

,157 re-? i ;lad 1.0102

- •75

ei Tam perat ure. r iSit, . Tampereiture.-canal".

TamireGrcitura °C. L!2 Rs3 101. try 136 Q

M- C'I

ti.,9" n~ 17·.t

Zone. refilled. KN01.

Irtr·50 f f101Cl.. volume " 11101~ -I II

a Temperature. 1*'5Ll' •

. & Temper"tLlre. .f"lh~.

4& 50 SI 51 Molar volumes determined by this technique are shown in figures 17.1 and 17;2 where it can be seen that there is a definite hysteresis in the behaviour of the heating and cooling samples. 17.2.3. Ultra violet absorbtion spectra The technique described in chapter 13 was used to determine the position of the absorbtion bands for both samples. Typical spectra for samples above and below the transition temperature are shown in figure 17.3 and a plot of the position of the second absorbtion maximum against temperature in figure 17.4. While this shows a discontinuity in the region 40-5000., the very small change in E 2(max) coupled with instrumental errors do not make this a sensitive parameter by which to follow the transition. Figure 17.3 shows that at 30-40 kcm there occurs a relatively large change in the percentage absorbtion of the sample. Thus the absorbtion of the sample at a fixed frequency forms a more sensitive test of the optical properties of the material in this region. Plots of this type at a fix d frequency of 35 km _1 are shown in 17.5 and 17.6 for samples I and II, Two complete traverses of the transition are included in both cases showing that there are no ageing effects in the sample. During the course of this investigation it became apparent that the rate of transformation was low. Fi9ure 17.3

LW spectrum of 10102 o 4.1.25 °C. A 53.00 °C

-100

-60

-40

25 40 35 4— frequency kanis 30 Fi,ure 17.

-174 lb

•1 E 2 61/W4 CM

-274 Temperature rising . I Temperatmre 27.7

-.27.6

- 27.5

Temperature °C. -4 100 ISO 30 40 SO 60 70 80 90 rJ

ri5vre 17'5-

linrelinea KNO2

100 a* °a A-4-1-

-so

Trull smistgon (0rbitrar3 SCcae) -60 is Temperature risly .

TeMparettl.tr0 4C.tnin, - I o

TelliperatUre 36 —Q-44 4S 46 1.7 Fi5Jure 17'6.

:Zone refinea KN01 •. a Te.mpera.tu.re rlSLrt".

4 TemJ1erCl.t~l"e f"lhn,.

100 ' a N II

80 t Tran5\nisstol1 ot 35" ken;' &0 ("rbit,~"l'"' swle.)

1'1 236

The rate of temperature change had to be kept below 2°C per hour in the region of the transition if consistent results were to be obtained. 17.2.4. Electrical properties When drying the outside of the dilatometer after its immersion in the oil bath, with pieces of 'Kleenext tissue, a marked agitation of the crystals was observed. It was inferred this might indicate that the crystals were electrically anomalous. To test this, 9 sample was frozen between two parallel platinum plates and sealed under vacuum. This sample was subjected to high alternating voltages (1-2 kV) on a "ferro—electric loop tester" which measures any anomalous behaviour of the dielectric constant. Results on this sample, though rather confused due to electrical breakdown of the sample, and its comparatively low insulation resistance indicated that the material was probably ferroelectric in the room temperature form though not in the high temperature form. This section of the work was carried out in conjunction with Professor J.C.Anderson of the Science of Materials section of .the Electrical Engineering Depart— ment, and Dr. G.S.Parry of the Department of Chemical

Engineering and Chemical Technology, at Imperial College. Further work has been carried out on this subject establishing the presence of an anomaly in the conductance 2%7 • and dielectric constant of this material in the range 40- 50°C., and giving further evidence for the ferroelectric nature of the low temperature form, by Mr.J.Woore in the Materials Science laboratories of the Department of Electrical Engineering at Imperial College. That this transition is from a ferroelectric to paraelectric phase has been confirmed. .17.3. Discussion These results confirm the existence of a thermal transition in potassium nitrite in the region 40-50°C. The mean temperatures of the transition as recorded dilatometrically are 41.8°C for the impure sample (I) and 47.4°C for the pure sample (II). Both dilatometric and spectroscopic data show that the transition in sample II occurs at higher temperatures than in sample I, and furthermore, in the former case, the hysteresis observed extends over a longer range (8.5°C) than in the latter case (5°C). X—ray and optical crystallographic studies5 on a recrystallized (but not zone refined) single crystal also indicate the presence of a transformation at •-• 4200, with hysteresis extending over as much as ± 3.5°C. The material used above corresponds to the non—refined sample I in this work. The X—ray results confirmed the structure of the 6 room temperature form as being monoclinic. This form 238 passes into the high, temperature form, with a complete persistence of axes, though it is evident that consider— able strain is generated in the process. A complete structure for the high temperature form was not derived, but indications were that it was either tetragonal, or more probably cubic.

The optical studies showed that in the transform— ation region, zones of both the high temperature and room temperature forms existed within the crystal. Conductance and capacitance measurements? (figures 17.7 and 17.8) shoW an anomaly with hysteresis, in the range 42-44°C. Both Woore and Parry report that the hysteresis range is narrowed by successive cycles of the loop. Such behaviour was not observed in this work. The whole of this behaviour is consistent with the type of continuous thermodynamic transition discussed by Ubbelohde (cf chapter 3), caused by a i thickening'of the free energy surface in the neighbourhood of a transition point. This allows for the co—existence of the high and low temperature forms over a range of temperature instead of at only one temperature, as pre— dicted by classical thermodynamics. Furthermore, since the surface energy and strain energy characteristics of the material when passing to and when passing from the high temperature form are different, then the properties of the material will not normally follow the same path 26 .

t ... 'Tr'~1'hJ'e"&4't4~r~ yiSip,* 'c.J)C~'i.tc.I1U o Taml'e.r~ture. f~l1i", . pI=". 11

.: 20

o -re111l'.c..."t",~e- "{, ~ 30 1," 50 1,,0 .70 80 90

CD n;L",ac.",e. f'Hh~. 0·''''

l' Tempera.t ....Y*~r 1.S&'I1, • o Te.,..perc..1:&.Ao....a t"Utn,.

30 240 when passing through the transition in different directions. The crystallographic data show that only a small change of structure, consistent with the small volume change, occurs at the transition. However, while the high temperature form possesses a centre of symmetry, the monoclinic low temperature form is of the type which can show ferroelectricity. Other results suggest that at this transition, the change from a ferroelectric room temperature form, to a para—electric high temperature form does occur. This would be in accord with corresponding behaviour in sodium nitrite at 160°. The effect of impurities on this transition (lowering its temperature, and sharpening the hysteresis loop) suggests that the impurities are present in solid solution. This would lead to increased disorder in the material. In such cases, the impurity can promote the movement of dislocations, and thus facilitate the transition. 24-1 REFERENCES

(1) Abdullaeva, Annageiv & Kristallografiga (1961) 6 733 Ismailzade, (2) Ray. J.Inorg.& Nuclear 0hem.(1960) 15 290 (3) Cleaver,Rhodes Proc.Roy.Soc. A276 453 (1963 Ubbelohde (4) Pfann. Zone Melting. Wiley. New York. 1958. (5) Parry, Schuyff & Prov.Roy.Soc. A285 360 (1965) Ubbelohde (6) Zeigler. Z.Krist. (1936) 94 491 (7) J.Woore. Unpublished observations. Offprinted from the Transactions 0/ The Faraday Society, No. 491, Vol. 59, Part 11, November, 1963

THERMAL TRANSFORMATION OF POTASSIUM NITRITE Thermal Transformation of Potassium Nitrite

By F. J. HAZLEWOOD, E. RHODES AND A. R. UBBELOHDE Dept. of Chemical Engineering and Chemical Technology, Imperial College, London, S.W.?

A thermal transformation in solid KN02 at approximately 40:C has been studied by dilatornctric methods and by ultra-violet spectroscopy. Both methods show marked hysteresis, extending over a range of about 5 deg. for the less pure sample, and 8 deg. for a sample purified by zone refining.

Nitrates of group I of the periodic system are remarkable for the number of polymorphic transformations they show; a variety of slightly different crystal packings of the ions, with somewhat different thermal properties, give rise to solids with the same free energy below the melting point. Less is known about polymorphic transformations in nitrites of the same cations. Sodium nitrite is reported to undergo a first-order ferroelectriceepara-elcctric phase transformation at about 160°; lattice volumes are said to increase continuously with rising temperature with only a change of slope at l60°C.l The phase transition has also been studied by polarized infra-red radiation and results indicate that it is due to rotation and relocking of the nitrite ions. 2 Potassium nitrite shows a transformation at about 40°C first detected by thermal analy sis in a saturated aqueous solution of the salt) The ultra-violet absorption spectrum of the solid salt.! pointed to marked hysteresis and to a continuous transformation. Probably the same transformation accounts for the fact that at room temperatures KN02 shows a double peak, unlike NaN02, in the \'2 infra-red frequency, which disappears at higher temperaturcs.> KN02 also exhibits a low-temperature first order phase transformation at -13°C.6 X-ray studies on single crystals 7 suggest that the monoclinic room temperature form undergoes a continuous transformation to a high-temperature structure, probably of higher symmetry. The present paper describes detailed ultra-violet absorption studies on this salt, coupled with thermodynamic studies of transformation. In view of the marked hysteresis, dilatometric studies are more convenient than calorimetric observations for general thermodynamic characterization of this transformation. Previous findings, about the effect of stubbornly retained water and of other possible impurities on the course of a " continuous" transformation in other systems,8-10 made it desirable to investigate effects of purification on the transformation in KN02• As described below, zone refining broadens the hysteresis loop and makes the transformation more sluggish, as well as shifting it to higher temperatures.

EXPERIMENTAL The purest potassium nitrite commercially available was of 97'5 % minimum purity (Hopkins and Williams, G.P.R.). It was in coarsely crystalline form. Tests of an aqueous solution, which was neutral (pH 7,0), gave a faint opalescence with a solution of barium nitrate which disappeared on the addition of HCl, indicating traces of carbonate. The crystals were dried by pumping for 16 h under high vacuum. Heating to ac celerate drying led to compaction of the material into a form almost impossible to grind into a fine powder by the usual methods. Dried crystals were manipulated in a dry-box kept purged with oxygen-free nitrogen (02~ 10 p.p.m.) dried by passage through a trap packed with glass wool and cooled in liquid oxygen. When necessary, counter-currents 2612 F. J. HAZLEWOOD, E. RHODES AND A. R. UBBELOHDE 2613 of this gas were also used to prevent access of moisture to the apparatus. Crystals were stored over phosphorous pentoxide in a desiccator. The melting point of these dried, but otherwise unpurified crystals (I) was 420·6±0·2°C. To obtain purer material the crystals were zone refined in a horizontal tube 1·2ern diam., 80 em long. The crystals were melted in this tube in a current of nitrogen, and high vacuum was then applied. Nine fairly rapid passes were first carried out, with a molten band about 4 em broad, moving at about 5 ern/h. These were followed by six slower passes with a narrower molten band, about 2·5 em broad, moving at about 2 ern/h. The product after cooling was cut into five equal segments: segments 3 and 4 from the start were used for the measurements described. Since it was not practicable to pulverize the segments, these were melted, and the melt was cast into small pellets by dropping on to a porcelain dish. These operations raised the melting point of the solid (II) to 425·5± 0'2°C; dilute aqueous solutions gave no opalescence with aqueous barium nitrate. If the rise in melting point is attributed to impurities soluble in the melt but not in the crystals, it corresponds with the removal of about 1·5 %of impurity on the basis of a heat of fusion of 2·8 kcal/mole. However, the accompanying shift upwards in the transition temperature, as well as the chemistry of nitrites, suggests that the main impurity in the commercial sample was nitrate, part of which remains in solid solution in the solid nitrite, so that cal culations of the impurity content from cryoscopic data are not valid. Direct addition of 6 % KN03 to the zone-refined KN02 lowered the melting point to about 412°C. Ditatometers of conventional form were constructed of Pyrex, using a bulb of about 6 ml capacity sealed to precision bore capillary, nominally of 0·05 em diam. The bulb was two-thirds filled with solid, taking precautions to prevent entry of moisture or com bustion products in sealing off. Silicone oil as supplied (Hopkin and Williams, M.S., 200/10 cs) was used as confining fluid. Before completely filling the dilatometer, any entrapped gas bubbles were removed by pumping under vacuum for 2 h. The dilato meter was protected by a silica gel tube during use. With the dilatometer immersed in an oil bath thermostatted to ±0·02 deg., readings of the meniscus and of a fiducial mark were taken at intervals of 0·5 deg. by means of a cathetometer. To calibrate the volume of the bulb from the various weighings, the density of the potassium nitrite was determined separately in a specific gravity bottle, using the same confining fluid, whose density and thermal expansion coefficients were likewise measured in separate experiments. The specific volume of the silicone oil used was found to be 1·0702cm3 gm"! at 26'20°C and followed the equation, V = 0'001079(T-26'20)+ 1·0702. At any temperature up to 55°C, the thermal expansion of the Pyrex bulb was taken to be 0·0001 cm3 deg.-l or 0·0012 %and could be neglected.

RESULTS

THERMAL EXPANSION OF POTASSIUM NITRITE (SAMPLE I AFTER DRYING) By calculating the lines ofclosest fit for the results obtained above 44°C, the thermal expansion of the high-temperature form was computed and followed the equation,

Vmolar = 44·7S+0·0077(T-44·70), (44-48°C) Above the transition temperature the volume expansion coefficient y is thus 1·74 x 10-4 deg.r! Insufficient results were obtained below the transition to establish the thermal expansion coefficient of the low temperature form.

(SAMPLE II) For zone-refined KNOt, the corresponding line of closest fit for the high tempera ture form gave Vmol ar = 44·SS+0·00863(T-Sl·00), (SI-SS0C) The thermal expansion coefficient y is thus 1·94 x 10-4 deg.-l 87 2614 THERMAL TRANSFORMAnON OF KN02 Dilatometric plots of two cycles of the transformation in the hysteresis region are recorded in fig. 1 for sample I and fig. 2 for sample II. For both samples the volume

44'8°1

44·78

44·76 .-.. E .2, 44·74 d) E ::3 "0 > 44·72 "'"('j "0 E 44·70

44·68

44·66~~,;)9~--:-l::---:-l:---4~2~--4~3--~--~~-~--4-'.7

ternp.v'C temp. rising, • ; temp. falling, 0 FIG. l.-Dilatometric observations showing hysteresis for unrefined KN02.

44·58r---_r_----,.---r---r--~-.,._-~--~-_r_-__,--,.__..,

44'50

44·54 .-.. E 44'52 ~ d) E ::3 44·50 '0 > .... ('j 44'48 '0 E 44'40

44'44

44'42 42 temp.TC temp. rising .; temp. falling 0 FIG. 2.-Dilatometric observations showing extension of hysteresis and shift of transition range, due to zone refining.

difference between the Iow- and high-temperature forms of KN02 is found to be about 0·12 em- or 0·27 %. F. 1. HAZLEWOOD, E. RHODES AND A. R. UBBELOHDE 2615

ULTRAVIOLET ABSORPTION SPECTRA These were observed for samples I and II using procedures previously described.s Fig. 3 records a pair oftypical complete absorption spectra, well below and well above the transformation, for sample II. There is a steep change in the position of the maximum of the second absorption band (E2 max) at 47·50°C as is shown in fig. 4.

\, \ \ \ \ \ \ \ \ \ "...... _-- - - 40 35 30 frequency (kcrrr-t) -- 41'25°; --- 53'Ooe FIG. 3.-Ultra-violet absorption curves for KNOz above and below the transition temperature.

27·8

-r 27·7' eu ~ '-' >C -~ 27·6 '-'e t!1 27·5

27'4 20 temp.oe I, temp. rising; • temp. falling .l FIG. 4.-Shift of band maximum on travelsing the transformation temperature. Length of vertical lines is a measure of the inaccuracy in determining £z max.

The fall in E2 max on traversing this transition is about 140 em-I. Uncertainties in determining the position of E2 max from the absorption curves are shown by the length of the vertical lines in fig. 4. The overall uncertainty in the band peak position is ±20 cm-I (4). It was not possible to detect hysteresis by this method, due to the unavoidable inaccuracy in the measurements of E2max. As can be seen from the records for the complete absorption spectra, a more sensitive way of following the 2616 THERMAL TRANSFORMATION OF KNOz progress of the transformation is to measure the % absorption or transmission at different temperatures at a fixed frequency, where the difference between the two crystal forms is large. For the present purpose, 35,000 cm-1 was chosen. By taking readings at intervals at constant temperature, it was found that steady states were attained in the neighbourhood ofthe transition, provided the rate ofchange oftempera ture did not exceed 2 deg.jh.

temp.OC temp. rising. ; temp. falling At. FIG. 5.-Changes in the ultra-violet transmission at a fixed frequency (35 kcrrrt, corresponding with the dilatometric results for unrefined KN02 (fig. 1).

Successive temperature cycles for sample I are recorded in fig. 5. The transmission is plotted on an arbitrary scale making the low-temperature form 0 and the high temperature form 100. The corresponding plot for sample II is illustrated in fig. 6.

..-.. () "2 100 ~J-&'-I-~_·_··-~~~'-'l u ~~ V> /'" :>. g1-0 80 :e -.e 60 t: .~ 40 .~ .r c 20 · ..f g ._.-~~~~! L~ 0 I I I I 43 44 45 4~ 47 48 49 50 51 52 53 54 5S temp.TC temp. rising e; temp. falling A. FIG. 6.-Changes in the ultra-violet transmission at a fixed frequency (35 kcrrrt) corresponding with the dilatometric results for zone-refined KN02 (fig. 2).

In order to verify that these hysteresis loops corresponded with steady states over long periods of time, sample I was kept at a fixed temperature (41'8°C) on the upper limb of the loop, and showed no change of %transmission in 24 h. F. J. HAZLEWOOD, E. RHODES AND A. R. UBBELOHDE 2617

DISCUSSION The transformation in KN02 shows a number of features also found in other thermal transformations in solids. If the mean value of the transformation tempera ture Tc is taken from the middle of the transformation loop, this may be estimated as approximately Tc= 41·8°C for sample I, and Tc = 47·4°C for sample II. Effects of purification are not much less on the transformation temperature than on the freezing point. A sharpening of the transformation on passing to the less pure sample is evi5cnt from a comparison of fig. I and 2. On the viewthat the transformation occurs by/~ay of single crystals that are hybrids of two structures 11 this sharpening could be attributed to increased disorder and greater ease of internal nucleation in single crystals ofthe less pure sample. This may arise from the presence of foreign ions or water molecules in solid solution. In general, ultra-violet absorption spectra give information about the course of the transformation between the two forms of KN02, which corresponds with the dilato metric results. Fig. 5 and 6 make the great detail that can be conveniently achieved by spectrophotometry very evident. They also permit much more precise estimates, than are possible by dilatometry, of the onset and completion temperatures for the transformation. Careful comparison of fig. 2 with fig. 6 indicates a " tail" for the purified sample at the high-temperature end of the hysteresis loop, at about 53°C, which extends as much as 2° beyond the limit made probable by dilatometry, where the results unavoid ably show greater scatter. By contrast, for the less pure sample I, dilatometry (fig. I) and spectroscopy (fig. 5) agree, and indicate a high-temperature cut-ofTof the hysteresis loop at about 43·5°C. More complete interpretation of these changes at the trans formation awaits the outcome of X-ray studies of this transformation on single crystals, which are in progress.

Thanks are due to the Royal Society for the loan of the spectrophotometer and to the Ministry of Aviation for other support.

1 Abdullaeva, Annagiev, IsmaiIzade, Kristallografiya, 1961, 6, 733. 2 Gesi, Sato and Takagi, J. Physic. Soc. Japan, 1961, 16, 2172. 3 Ray, J. Inorq. Nucl. Chem., 1960, 15,290. 4 Cleaver, Rhodes and Ubbelohde, Proc. Roy. Soc. A, 1963, in publication. S Greenberg and Hallgreen, J. Chem. Physics, 1960, 33, 900. 6 Ray and Ogg, J. Physic. Chem., 1956,60, 1599. 7 Parry, Schuyff and Ubbelohde, unpublished observations. 8 Kennedy, Ubbelohde and Woodward, Proc. Roy. Soc. A, 1953, 219, 305. 9 Brown and McLaren, Proc. Roy. Soc. A, 1962,266,329. 10 Rhodes and Ubbelohde, Trans. Faraday Soc., 1959, 55, 1705. 11 Ubbelohde, Proc. Roy. Netherlands Acad. Sci. Lett. B, 1962, 65, 5.

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