Electroanalytical Chemistry of Some Organometallic Compounds of Tin, Lead and Germanium

ELECTROANALYTICAL CHEMISTRY OF SOME ORGANOMETALLIC COMPOUNDS OF TIN, LEAD AND GERMANIUM

Nani Bhushan Fouzder M.Sc. (Rajshahi)

A Thesis Submitted for the Degree of Doctor of Philosophy of the University of London.

Chemistry Department, Imperial College of Science and Technology, London S.W.7. September, 1975. 11

ABSTRACT.

The present thesis concerns the investigation into the electrochemical behaviour of some industrially important organometallic compounds of tin, lead and germanium and development of suitable electrochemical methods for the analysis of these compounds at formulation and at trace level.

The basic principles of the electrochemical techniques used inthis investigation have been given in the first part of the 'Introduction', while the various factors which control the electrode process have been discussed in the second part of the 'Introduction' in chapter 1.

The electrochemical behaviour and analytical determination of some important organotin fungicides and pesticides such as tri-n-butyltin oxide, triphenyltin acetate, etc., some antihelminthic compounds such as dibutyltin dilaureate and dibutyltin dimaleate and some widely used PVC-stabilizers such as di-n-Octyltin dithioglycollic acid ester (Irgastab 17 MOK), Irgastab 17M and Irgastab 15 MOR have been described in the following three chapters. For each type of compound a detailed mechanism of the electrochemical process has been proposed and established.

The electrochemical behaviour of organolead compounds and of the organogermanium compounds have been described in the next three chapters. In each case, the mechanism of reduction of these compounds has been established and methods 9fc their determination at ordinary and at trace level have been developed.

Finally, in the eighth chapter a brief introduction into the highspeed liquid chromatographic technique has been given and analysis of organotin compounds by this method using a wall-jet electrode detector has been described. iv

ACKNOWLEDGEMENTS.

The research work described in this thesis is entirely original except where due reference is made. I would like to thank Professor T. S. West for his interest and encouragement and for placing the facili- ties available in the Analytical Chemistry Department of the Imperial College at my disposal. I am particularly indebted to my immediate supervisor. Dr. B. Fleet for his help, advice and guidance throughout the entire research programme. I would also like to thank all my colleagues particularly Drs. S.Dasgupta and G.P.Bound and Messrs. J.Bland, T.Berger and J.Andrews for many stimulating discussions, Dr. C.J.Little of Roche Products, Welwyn Garden city and Dr. P. Bristow of I.C.I. (Pharmaceuticals Division) Macclesfield for their help in connection with HPLC experiments. My thanks are also due to E.D.T. Research, London, for the loan of a PAR Polarograph during HPLC experiments. My grateful thanks are due to the Ministry of the Overseas Development, Govt. of the U.K., for the award of a study fellowship, to the British Council for their cooperation and to the University of Rajshahi, Bangla- desh, for study leave. I am also grateful to my wife, Gita, for her constant encouragement throughout this investigation and particularly for her active and very useful help during preparation of this thesis.

N. B. Fouzder. To

APU with love and affection. CONTENTS

Page.

Title page. Abstract. ii Acknowledgements. iv CHAPTER 1. INTRODUCTION 1 1.1 D.c. polarography 3 Different types of limiting currents. 8 Interpretation of the polarogram and elucidation of electrode process 10 1.2 Diffusion controlled electrode process at solid electrodes: Determination of diffusion coefficient 12 1.3 Determination of n—values and identification of products. 13 Controlled Potential Electrolysis. 13 Controlled potential coulometry. 15 Microcoulometry. 18 1.4 Reversibility of the polarographic wave and identification of the intermediates 20 Oscillographic polarography and cyclic voltammetry 22 A.c. polarography 26 1.5 Polarography as a trace analytical technique. 27 Pulse polarography 27 Differential pulse polarography 29 1.6 Factors influencing the electrode process. 32 The electrical double layer and the role of adsorption. 32 The solvent effect. 39 vi

The pH-effect. 42 The influence of the electrode material 45 Effect of temperature. 46 Effect of background electrolyte. 46

CHAPTER 2. ETECTROCHEMICAL BEHAVIOUR OF Bis (TRI- n-BUTYLTIN) OXIDE AND TRIPHENYLTIN COMPOUNDS AND THEIR ANALYTICAL DETERMINATIONS BOTH AT FORMULATION AND AT TRACE LEVELS. 2.1 Introduction. 49 2.2 Experimental technique. 50 2.3 Results and discussions 53 General polarographic behaviour. 53 Mechanism of reduction. 61 2.4 Analytical determinations. 66

CHAPTER 3. ELECTROCHEMICAL BEHAVIOUR OF DIBUTYLTIN DILAUREATE AND DIBUTYLTIN DIMALEATE AND THEIR ANALYTICAL DETERMINATIONS. 3.1 Introduction. 68 3.2 Experimental technique. 71 3.3 Results and discussions. 73 General polarographic behaviour. 73 Mechanism of reduction. 80 3.4 Analytical determinations. 82

CHAPTER 4. ETRCTROCHEMICAL BEHAVIOUR OF ORGANOTIN SULPHUR COMPOUNDS AND THEIR ANALYTICAL DETERMINATIONS. 4.1 Introduction. 90 4.2 Experimental technique. 91 4 3 Results and discussions 91 ,Differential pulse polarography. 94 Mechanism of reduction process. 96 4.4 Analytical determinations. 100 vii

CHAPTER 5. ELECTROCHEMICAL BEHAVIOUR OF TRIPHENYLLEAD ACETATE AND ITS ANALYTICAL DETERMINATION. 5.1 Introduction. 102 5.2 Experimental technique. 105 5.3 Results and discussions. 105 General polarographic behaviour. 105 Voltammetry at glassy carbon electrode. 117 Mechanism of electrode process. 119 5.4 Analytical determinations. 123

CHAPTER 6. ELECTROCHEMICAL BEHAVIOUR OF DIBUTYL- LEAD DIACETATE AND ITS ANALYTICAL DETERMINATION AT ORDINARY AND AT SUBMICROMOLAR LEVEL. 6.1 Introduction. 126 6.2 Experimental technique. 128 6.3 Results and discussions. 128 General polarographic behaviour. 128 Voltammetry and cyclic voltammetry at a glassy carbon electrode. 136 Mechanism of reduction at the DME. 139 6.4 Analytical determinations. 140

CHAPTER 7. ELECTROCHEMICAL BEHAVIOUR OF TRIPHENYL - GERMANIUM BROMIDE AND ITS ANALYTICAL DETERMINATION .

7.1 Introduction. 145 7.2 Experimental technique. 148 7.3 Results and discussions. 149 General polarographic behaviour. Mechanism of reduction. 158 7.4 Analytical determinations. 160 viii

CHAPTER 8. THE ANALYSIS OF ORGANOTIN COMPOUNDS BY HIGH PRESSURE LIQUID CHROMATOGRAPHY USING A VOLTAMEETRIC DETECTOR SYSTEM. 8.1 Introduction. 163 8.2 Theoretical principles. 164 8.3 Polarographic and other voltammetric detectors. 168 Wall-jet electrode principles. 170 8.4 Application of the WJED to the analysis of organotin compounds by HPIC method. 174 Experimental technique. 174 Results and discussions. 176 Conclusion. 183

REFERENCES. 187 LIST OF PUBLICATIONS. 207 1

CHAPTER 1. INTRODUCTION

The formation of organic derivatives of metals having carbon-metal 6 -bond is a general phenomenon among the metals. Consequently a very large number of organometallic compounds are known. The organometallic chemistry is supposed to have begunl with the famous researches of Robert Bunsen on cacodyl, (CH ) As in 1841. Bunsen found the chemical beha- 3 4 2 viour of this compound to be very interesting. Since then many significant contributions were made for the newly discovered organometallic compounds of mercury, silicon, Tin, Lead and many other metals. Early industrial interest in these compounds was aroused from a series of brilliant discoveries in the application of these compounds in chemotherapy2 and in organic synthesis3. Midgley's discovery4 in 1922 of the highly effectiveness of organolead compounds as "antiknock" agent and subsequently its wide application as gasoline additive in internal combustion engines brought organometallic chemistry very much into public view. The Ziegler-Natta discovery that organometallic compounds can be used very effectively as catalyst for low pressure polNhmeri- zation of olefins5-7 has further enhanced the industrial importance of these compounds and has led 2 to the growth of many new chemical industries during recent years. Finally the preparation of ferrocene and other metallocenes8'9 and elucidation of their structures10-15 have opened up a new area of organometallic chemistry which has broadened the horizon of our knowledge about the nature of chemical bon- ding as well as industrial applications16 of these compounds. The ever-increasing use of organometallic compounds in industry, medicine and agriculture has also increased the dangerous risk of environmental pollution and consequent ecological concern17. This as well as the quality control factor in industrial concerns have demanded highly sensitive specific methods of analysis of these compounds particularly at trace level. Several analytical techniques have so far been tried more or less successfully for this purpose. The present thesis is concerned with the development of suitable electrochemical methods of analysis of some industrially important organometallic compounds of Tin, Lead and Germanium at both macro and trace level.

For the development of electroanalytical procedures it is desirable to elucidate as completely as possible the mechanism of the electrode process. So a detailed investigation into the mechanism of electrochemical reduction of these compounds was undertaken. A variety of 3 electrochemical techniques have been_ extensively employed for this purpose. So a brief account of each will also be given. Finally, a general introduction will be given to the principles of high performance liquid chromatographic (HPLC) technique coupled with Wall-jet electrode (WJE) detector system, which was developed in this laboratory18 and its application to the analysis of organotin compounds will be described.

1.1. D.c. Polarography:

Polarographic technique was first developed by Heyrovsky'19 in 1922. It involves measurement of current as a function of applied voltage and Inter- pretation of the resulting current-potential curves. A dropping mercury electrode (DME) is most widely used for polarographic electrolysis. The unique feature of the DME is that fresh electrode surface is generated with the formation of new drops of mercury. Polarographic measurements are carried out in unstirred solutions and under conditions in which convection and migration of the depolarizer is neg- ligibly small. Since polarographic measuremrnts with organometallic solutions are mostly carried out in non-aquous or in mixed solvents of high resistance a three-electrode polarograph having "potentiostatic" control of the working electrode 4

is necessary because this system gives a true current-potential curve. The rising portion of the current-potential curve is known as polarographic wave, the slope and position of which provide both qualitative and quantitative information about the electrode process. For a simple reversible process

k 0 + ne f kb in which 0 is reduced sufficiently rapidly , compared to diffusion, to R the limiting current is controlled by diffusion of the depolarizer (0) to the electrode and the limiting current is then called diffusion current. An expression for the diffusion current at the DME was derived by Ilkovic20 and is given by

m31 t1/6 id = 607 neD2 where the current is expressed in P'A, m is the rate of flow of mercury in mgm/sec, t is the drop-time in seconds, Do is the diffusion coefficient of the 2 depolarizer (0) in cm sec-1 and c is the bulk concentration of the depolarizer in mmoles/lit. The proportionality between current and concentration is the basis for the analytical applications of the diffusion current.

The shape of the current-potential curve for the reversible reduction of one soluble species to another is given by the equation21 .

2 RT id-i DR E = E + nF Do /

where E° is the standard potential, i is the current for any potential E, and id is the diffusion current. For the case in which both oxidized and reduced forms are present there are both anodic and cathodic limiting current and the general equation is given by

2 RT ((id)a-i / DR nF In i-(id)'c Do

The potential at which the current equals half the limiting current is known as "half-wave potential" and is given by

RT ln (D / D ) 2 = Eo nF o R

At a given ionic strength in which the diffusion- coefficient of the oxidized and the reduced forms are practically equal,

El- = E° or, E4 = - AG-7 nF

where L1 G° is the standard s free energy change.

This equation is the basis for the relation of polarography with thermodynamics. Half-wave potential is seen to be independent'of concentration of oxidized and reduced species. However, it depends upon the concentration of other components of the reaction_ which for organic/organometallic system are usually protons. Thus for the reversible reaction

0 +. me 4- ne

E = - RT R T nF in ( Do DR)7. m laF In alit o if Do == DR El7 -- 0.059 111- pH

where E is the half-wave potential at pH = 0. This equation shows the necessity of well-buffering the solution so that the electrode process does not appreciably change the pH at the interface. Most organic compounds do not, however, behave in a polarographically reversible manner, Many reactions may involve a reversible electron-transfer step often followed by irreversible chemical reactions. In the irreversible process, the polarographic current is still determined by the rate of diffusion of the depolarizer to the electrode surface but the current- potential relation includes an overpotential term, the value of which increases with increasing current throughout the wave to the diffusion-limited condition. The equation of the irreversible polarographic wave22-25 is given by

0.81[(k (Id)o /50) + kT(Id)allicR ]rt"-- I irrev = 1 + 0.81 [(k/Do) + (k t/ /TR) J ft

More vigorous treatments of slow irreversible processes have been given by several workers26-29. The half-wave potential for an irreversible process is given by30

203RT.3 1 El = E° + In 0.886 kh (t/Do)2 where 0( is the transfer coefficient and kh is the heterogeneous rate constant for the electrode process at the standard potential, E°. The relation between (E2)irrev and the free energy of activation, AG*, is given31 by

A =E° cxn in 0.886 (t/D0)1

This equation shows that many important information about the structure and energy distribution of the transition state during electrode reduction may be obtained from half-wave potential values. 8

1.2. Different types of limiting currents.

Although diffusion-controlled waves are most frequently encountered in polarographic analysis other process may occur which may control the limiting current. Limiting currents may be catalytic or kinetic in nature or may involve adsorption phenomena. Kinetic currents arise when the product of a slow chemical reaction is electroactive while the reactant itself is inactive. The limiting current is then governed by the rate at which the electroactive species is formed. Since chemical processes are usually pH and temperature dependent this type of polarographic current is influenced by change in the pH and temperature of the medium. Kinetic current is charac- terised by being independent of the mercury reservoir height. There are two important types of catalytic currents. One involves an increase in the limiting current due to the regeneration of the original depolarizer from the products of reduction whilst the other involves a shift on half-wave potential towards more positive values during reduction. The well-known catalytic hydrogen waves belong to the second type of catalytic currents. Both effects are accomplished by a catalyst which is either electroinactive or undergoes reduction at more negative potentials. Cata- lytic currents in most cases are nonlinear functions 9

of concentration and show variations with pH and buffer concentration. The adsorption waves arise from adsorption of the electroactive species or its products. The adsorption wave is linearly dependent on concentration upto a limiting value, at which the electrode surface is covered by a layer of absorbed material. The adsorption current is given by the equation_32

2 1 is 0.85 nFZ mat-3,

2 where Z is the number of moles absorbed per cm of the electrode surface. For sufficiently rapid adsorption, is is proportional to the rate of growth of the mercury surface i.e., to the mercury reservoir height (h). Adsorption waves can also be distinguished from diffusion wave by the characteristic shape of their current(decay)-time curve for a single drop in the potential region corresponding to the plateau of the polarographic wave.

Polarographic Maxima:

Polarographic waves are often accompanied by maxima and are belived to be due to tangential strea- ming motion of solution past the surface of the mercury drop. Polarographic maxima can be easily elimi- nated by adding some suitable organic surfactants such as gelatin or Triton X-100. 10

Interpretation of the polarogram and elucidation of electrode process.

D.c. polarography is very useful for a rapid evaluation of electrode process over a wide range of . potentials. The mechanism of the electrode process may be elucidated by studying the effect of mercury reservoir height, concentration of the depolarizer, temperature and pH of the medium, composition of the background solution, etc. on the limiting current as well as on the half-wave potential. The instantaneous current-time curves and the electrocapillary curves also provide valuable information about the type of the limiting current and adsorption process. The shape and height of the polarographic wave and the position and behaviour of the half-wave potential are also indicative of the electrode process. As already stated, for a reversible diffusion- controlled process the polarographic wave is described by Heyrovsky -Ilkovic equation so that plot of

E vs. log ( , .a ) is linear with slope of 2.303RT )c-1 nF • For such cases the half-wave potential is independent of the concentration of the electroactive species and the pH of the solution. When the slope of the wave corresponds to a diffusion-controlled reversible process but the half-wave potential is a function of concentration dimerization of the products after a 11

rapid electron transfer process is to be anticipated. The half-wave potential values are shifted with change of pH of the medium if there is any electrolytic dissociation of the oxidized or the reduced forms in a reversible system. The magnitude of the El -- shift 2 depends on the number of electrons transferred, the number of protons dissociated, the pH of the solvent and the standard potentials of the depolarizer. For many organic irreversible systems, it has been observed that the half-wave potential, the limiting current and the shape of the wave are all influenced by pH. These changes may be due to the effect on acid-base equilibrium preceeding or following the electrode process or due to the effect on a chemical process converting an electroinactive species into an electroactive one. In a reversible process the number of electrons transferred can be calculated by using Ilkovic equation. But as the exact values of the diffusion constants under the polarographic conditions are not known this procedure can not be recommended. However, n-values are sometimes determined by comparing under identical condition the wave height of the compound with those having similar molecular shape and weight having almost identical diffusion coefficients and for which n-values are known. The general practice for the determination of n-value is however the use of controlled potential coulometry, Microcoulometry 12

as well as the identification of products of electrolysis carried out at constantly controlled potential. The principles of these techniques will be discussed in a later section.

1.2. Diffusion controlled electrode process at solid electrodes: Determination of diffusion coefficients.

A rotating disk or a ring-disk electrode is generally used for examining electrode process at solid electrodes under reproducibly controlled mass- transport conditions. The rotating disk electrode provides a hydrodynamically controllable, forced convection arrangement which brings the depolarizer from the bulk of the solution to the electrode surface. The unconsumed reactant is sucked into the central region of the disk electrode and the products and the unconsumed reactant are spun out across the surface of the rotating electrode. For a diffusion-controlled process, the limiting current at a rotating disk electrode is given33 by Levich equation:

-1/6 id = 0.62 nFCoD33t w where, V is the kinematic viscosity of the solution in cm? /sec, w is the angular frequency in radians/sec, D is the diffusion constant, and C o is the bulk concentration in gramequivalent/ cm3 . Riddiford and Gregory34 introduced some minor 13

corrections to the above equation so that it becomes

. V -16 w2 0.62 nFCoD3 id - 1 + 0.34(D/W )0.36

With the help of this equation it is possible to determine diffusion coefficients of many organic depolarizers for use in polarographic calculations. Frumkin and Nekrosov35 introduced an important development in the rotating disk technique. The modi-- fied technique consists of a disk electrode surroun- ded by a narrow annular region of insulation. The electron-transfer process is carried out at the disk; any electroactive diffusible intermediates, or products formed are transferred across the annular regions in the flowing solution and is reoxidized or reduced at the ring which is provided with a sepe7 rate potential controlling circuit. Also, further reduction (or oxidation) can be achieved by using appropriate potentials at the ring. This methcid provides important means of indicating the exist- ence of any intermediates formed during the electrode process.

1.3. Determination of n-values and identification of products.

Controlled potential electrolysis:

In all electrochemical reaction mechanism studies analysis and characterisation of the 14

electrolysis products and/or byproducts is very essential. Controlled potential electrolysis is extensively used for this purpose. In controlled potential electrolysis the potential of the working electrode is held at a fixed potential with the help of a 'potentiostat' on the plateau of a polarographic wave and the electrolysis is carried out to completion. In order to achieve complete electrolysis in a reason- able period of time an working electrode of a large surface area such as mercury pool electrode or a platinum gauze electrode is generally used and the solution is well—stirred. In order to separate the electrolysis pr'oduct formed at the cathode or anode from those formed at the counter a divided or two— compartment cell is used. A third compartment or probe must be provided to accomodate the reference electrode. The products of controlled potential electrolysis may be identified in situ or after isolation where the compounds are stable. Various techniques such as Electron—spin resonance, Ultraviolet and infra red spectrophotometry and cyclic voltammetry are generally used for identification of unstable (radical) species. If the products are stable and can be isolated in a reasonably pure state nuclear magnetic resonance and mass spectrometry can also be used36,37 15

Controlled Potential Coulometry:

When the total quantity of electricity flowing during the controlled potential electrolysis is determined, the technique is called controlled potential coulometry. Many of the basic principles of controlled potential coulometry was laid down by Sand38 and his contemporaries as early as 1907. But the method recei- ved relatively little attention because of experimental difficulties involved until in 1942 when A. Hickling39 reported the design of his "potentiostat" -- the automatic potential control device. Since then this technique has found increasingly important applications in electrochemistry. In controlled potential coulometry a current integrator is used in series with the electrolysis cell. A constant speed motor should be used with the stirrer for maintaining a smooth decrease in current during the electrolysis. The value of the initial current is given by

nFDAC° b

where io is the initial current (mA), D is the diffusion coefficient of the electroactive species (cm2/sec),, A is the area of the electrode, C° is the initial concentration (m moles/lit) and S, the thickness of the diffusion layer. During electrolysis the current falls exponen- tially according to the eq4tion 16

i = io exp ( -pt) where i is the current after time t and p = DA/Vg , V being the volume of the solution. It can be seen from this equation that the plot of log i vs. t should yield a straight line of slope - e/2.303 and an intercept of log o • The total quantity of electricity required for complete electrolysis is given by

Q = f i dt = nFVC° where n is the number of electrons involved in the electrode process, C° is the molar concentration_ of of the electroactive species at the start of electrolysis and T is the volume in liter. In controlled potential coulometry it is often observed that the electrolysis current may decay to a negligible value or to an appreciable one. In the latter case if it is identical with the one obtained at the end of the pre-electrolysis a correction term should be included42 in the above equation and n-value can be obtained from

n = ( Q - ift ) / C°V where Q is the quantity of electricity accumulated during the electrolysis period (t sec) and is the steady final current. 1 7 '

If the steady final current is appreciably larger (or smaller) than the current at the end of the pre-electrolysis, catalytic process must be involved and correctiam40-48 must be made for the extent of catalysis. Like many electrochemical methods controlled potential coulometry appears deceptively simple in principle, but in fact it requires clear insight, deep understanding of possible experimental complications and skill in the interpretation off -the data. For example if the electrode process involves a slow intermediate chemical step such as in

tn1e slow +n e A > B .) c 2 D chemical

Controlled potential coulometry and polarography will give different n-values. If the intermediate chemical step is too slow compared to the drop-time of the DME The reduction at the DME will proceed upto B and will involve only n1 electrons, whereas controlled potential coulometry will give the value (n1 + n2). Similar- ly for the mechanism

A + ni e B

1 2B A 4 C

The polarographic n-value will be n1 while the coulometric value will be 2n 1 . When the electrode process is accompanied by dimerization, dissociation or other 18 - type of antecedent, subsequent or parallel chemical reactions, initial concentration_ of electroactive substances, the rate of stirring, temperature etc. may effectively change the n-value. The effect of side reactions on the n-value may be well illustrated by the following reactions in organometallic chemistry. Controlled potential coulometry at the plateau of the first wave of diphenylthallium (III) cation49 gave

app value of 1.20. The electrode process involved has been reported as

R2T11- + Hg + e --->R2T1Hg ---->R2Hg + Tl with side reactions accounting for the deviation of napp > 1. In general, when n-value is integral and inde-. pendent of initial concentration. and rate of stirring and if log i vs. t plot is linear it can be concluded that n-value corresponds to the same overall process as that for polarographic wave45.

Microcoulometr •

The determination of n-value by measuring the quantity of electricity required to reduce completely a known amount of substances using a large working electrode in efficiently stirred solution has been_ discussed in the preceeding section. In this section the determination of n-value by simply following the

decrease in the limiting current during polarographic electrolysis at a DME will be discussed. This simple method was first employed by Gilbert and Rideal51 and by Meites et a152. On the basis of the following relationship

dc • • • • • • .1) i = K.c = -nFV dt ... (1 where c is initial concentration of the depolarizer in moles/lit and V is the volume in litre a mathematical expression for the rate of decrease of limiting current has been derived. It follows from the above equation_ that

-dc 11 • • • • • • • • • ... (1.2) dt nFV

During electrolysis both c and it decrease so that

i (i ) 1 1 o • • • • • • • • • ... (1.3) u=o where i1 and c are the limiting current and concentration at time, t and (i1)0 and ct=0 are those at the start of electrolysis. Combining equation 1.2 and 1.3 and after suitable mathematical operation the following equation can be obtained.

i.ot t = log ii log i 2.303 nFvCt=0 .t

Plot of log it vs. t gives a straight line, from whose slope n-value can be evaluated since all the other quantities are known. Although simple, this method has some inherent drawbacks, the most important one being caused by the fact that due to depletion of the solution around the capillary tip the concentration of the depolarizer in the vicinity of the electrode is always less than that in the bulk of the solution. Also, like controlled potential coulometry, it will yield misleading results whenever cyclic or competing reactions are involved.

1.4. Reversibility of the polarographic wave and identification of the intermediates.

Since polarographic data reflects the kinetics of electrode process, the conditions for reversibility must be defined rigorously. Delahay53 has shown that if the forward rate constant, k1 , for the electrode process

k4 + ne R

-2 is greater than 2.10 cm/sec, the rate of the electrochemical step is sufficiently high so that the rate of reaction is diffusion-limited and the process is reversible. In such cases the electrode potential and current follows Nernst equation. 21

(CR)s o RT E E nF In (C0)s j

where (CR)s and (C0 )8 are the concentrations of the product and the depolarizer respectively at the electrode surface. With decreasing value of k1 the wave shape changes and becomes more "drawn-out: The wave becomes totally irreversible for

k1 ‘. 3.10-5 cm/sec.

In general, the polarographically reversible processes must satisfy the following conditions:

(i) The shape of the polarographic wave must correspond to the theoretical equation derived for the particular electrode process.

(ii) The half-wave potentials of the oxidized and reduced forms must be identical and practically the same as_the reversible potential, E0.

(iii) The shape of the i-t curve must correspond to the theoretically predicted shape for the electrode process concerned.

Logarithmic analysis of the polarographic wave by means of Heyrovsky -Ilkovic equation although often used is quite inadequate54 for establishing the reversibility of the polarographic wave. In fact oscillographic polarography and cyclic voltammetry are often used for this purpose. Other methods55 such as a.c.polarography, commutator method, and 22

anodic stripping technique are also used. The shape of the instantaneous current-time curve on the rising portion of the polarographic wave also gives useful information56 regarding the nature of the rate- determining step.

Oscillographic polarography and cyclic voltammetry.

Oscillographic polarography and cyclic voltammetry have been used in this investigation for determining the reversibility of the polarographic steps and for verifying the mechanisms of electrode process. Both the techniques are fast potential-sweep techniques and were first introduced by Matheson and Nichols57 and later improved by Randles58 and by Sevnk59. A stationary electrode ( most often a HDME) is generally used for monitoring current, the potential of the electrode being varied either linearly or sinusoidally with time. When a DME having long drop-time and a cathode-ray oscilloscope for recording current-potential curve are used the method is referred to as oscillographic polarography. Fast potential-sweep technique is of two types: the single sweep technique and the multiple-sweep technique. In the first method, a single current- potential curve is recorded and the system is allowed to attain the original condition before a second sweep is applied. In the multiple-sweep method several consentive i-E curves are recorded so that 23

concentration gradient of the depolarizer changes from one cycle to another, but eventually reaches equilibrium and very reproducible cyclic current- potential profile is obtained. A typical cyclic voltammogram obtained by the application of a triangular impulse of potential is shown in figure 1.1. When a potential ramp is applied the reactant present in the vicinity of the electrode is reduced immediately and a concentration gradient is established.

Fig. 1.1. Typical cyclic voltammogram of a diffusion-controlled redox process; i dl' double layer charging current. 24

The simultaneous decrease of reactant concentration and the exponential increase of the rate with increasing potential leads to a peak current (Ip ) in the voltammogram. The current then decreases toward a diffusion-limited value. During the reverse sweep the current suddenly decreases. The magnitude of this rapid decrease equals twice the double-layer charging current. The current continues to decrease until the potential becomes sufficiently anodic and oxidation of the products of the forward step occurs and an anodic peak is obtained. Valuable information regarding the electrode process can be obtained from the peak heights and peak potentials and their change with voltage sweep rate. For a reversible process the relation between the half-wave potential and the peak potential is given by

- 1.1 (-1-2-) Ep = 2 nF and the peak seperation potential is given by

AE - 0.56 V at 250c.

Hence a direct estimate of the reversibility of the electrode process or the rate constant of the electron-transfer is provided by this technique. However if the product of electrode process undergoes chemical reaction subsequently large difference may sometimes

be observed between the peak potentials, although the electron-transfer step itself is fast. For irreversible systems the peak seperation potential is larger than the corresponding reversible process or the reverse peak may not be observed at all. The peak current for a reversible process is given_ by60

3 2 7 i =kn / ADo C o V2

where A is the area of the electrode, V is the rate of potential sweep in volt/sec and k is a constant known as Randles-Sev61k constant. The equation_ for peak current for a totally irreversible process61 is given by

1 1 i = k .n (0<1a)7A D7o C o

where 0( is the transfer coefficient and na is the number of electrons transferred to the activated complex ( usually 1.0). The expression for the peak potential for an irreversible process is

E ( RT 5 in (anaFV = E ) 0.77 - In ks + In DT + 0. s atFa () RT

where E is the start potential and ks is the rate constant for the electrode process at that potential. It is clear from the above equations that the peak potential for an irreversible process will become 26

more 'cathodic' with increasing sweep-rate although the shift is small, whereas that for the reversible process is independent of. the sweep rate. Recent developments62-66 in the theory of cyclic voltammetry have led to methods for quantitative resolution of electrochemical step and the accompanied chemical reactions. Nicholson and Shain have made a ta-ough study of different types of electrochemical processes and evaluated current functions numerically from which they obtained current-potential relationships. These relationships are of great value for comparing the observed experimental behaviour in a given system with that calculated for the various reactions mechanisms.

A.C. Polarography

A.c. polarographic technique is sometimes used for determining the reversibility of an electrochemical process. The method67-78 involves the superimposition of a small amplitude sinusoidal alternating potential on the d.c. voltage ramp under polarographic conditions and measurement of the impedance of the electrolysis cell. A.c. polarography is characterized by summit potential and a.c. peak current. For small amplitude of the superimposed alternating voltage the peak current is given by

2 2 i = k. n.F .D 2 . na3 .t3 .w2 . C. n.F.E / 2RT s o 27 where k is a constant, w is the frequency, E0 is the amplitude of the superimposed voltage and all the other terms have their usual significance. For an irreversible process, the peak current is smaller by 4-1-1.20 (wt) -0.22 j Hence a reversible process can be distinguished from an irreversible one by comparing their peak currents. Moreover, it is linearly dependent on the amplitude or on the square- root of the frequency w2 for small amplitudes.

1.5. Polarography as a trace analytical technique Two offs,pots of the classical d.c. polarographic technique viz. pulse and differential pulse polarography 79'80 have established polarography as one of the most important trace analysis techniques because of their high sensitivity and resolution.

Pulse Polarography

In the normal pulse polarographic technique, potential pulses of successively increasing amplitude are applied from a fixed initial potential to the working electrode at a fixed time during the drop-life. The pulses are usually 50-60 msec. in duration and the current is measured (sampled) at some fixed time (40 msec) after pulse application (Figure 1.2). Thus by sampling the current toward the end of the pulse application seperation of faradaic and capacitance currents is achieved. Furthermore the faradaic current tr.:4•gz

euitif Ant 16 ail II, /6 m•S

0-5- to 5 5e -c .

time

Fig. 1.2. (a) Series of increasing height potential pulses. (b) Current time relationship during pulse application to a DME.

Fig. 1.3. Excitation potential wave-form for differential pulse polarography.

is maximised by applying the potential causing the reaction for a minimum length of time before current- sampling takes place. The pulse polarogram is very similar to a d.c. polarogram. The current-potential relationship for a reversible system in which an amalgam is formed is given by

0.059 E = E-52- n log id and the diffusion current is given by the Cottrell equation

D id = nFA D Co

82,83 which is strictly valid for a planar electrode .

Differential Pulse Polarography.

The differential pulse polarographic technique79 consists of superimposing fixed-height potential pulse at regular intervals on the slowly varying potential associated with d.c. polarography (Figure 5.3). The current flowing into the working electrode is sampled twice during each time period, once just before the application of the potential pulse, and then again just before the end of the timing period. The difference between these two currents is determined electro- nically, and a voltage proportional to these currents difference is made available to the recorder until 30 the sequence is repeated during the next drop period. The theoretical relationships between the peak current, and the pulse modulation amplitude, AE, for this technique have been presented. For the case in which the pulse modulation is less than RT/nF the peak current is given by

2 = n2F2A C° D 6E p 4 RT t

For finite pulse modulation

C i = nFA D o( 6-1 ) t • 6 +1 where 6= exp ( AE/2) nF/RT, and the other terms have their usual significance. For a reversible process and for a very small 4 E the peak current potential occurs at the normal d.c. half-wave potential. With finite modulation, however, the peak current potential is related to El by

E = El - 4 E/2

Differential pulse polarographic technique is one of the most sensitive analytical techniques and permits analysis of solution concentration of the order of -8 10 M or even less. For example, analytical determi- 86 nation of certain organotin compounds can be carried out down to 1.10-9M concentration by this 31 technique. Moreover, this method is not limited to use with a DME but is extremely useful with stationary electrodes such as Glassy carbon, Carbon Paste and platinum electrode; these permit much greater convenience with better sensitivity in systems to which 81 they are applicable. 32

1.6. FACTORS INFLUENCING THE ETECTRODE PROCESS

The electrical double layer and the role of adsorption.

An electrode in solution provides an interface for electron- transfer and other accompanied chemical processes. The double-layer at the electrode-solution interface consists of a layer of electrons, a layer of specifically adsorbed ions/molecules (Inner Helmholtz layer), a layer of oppositely charged ions and solvent dipoles (outer Helmholtz layer), and a diffuse layer whose ionic atmosphere contains ions of one sign in excess of their normal bulk concentration and of the 319 other in defect

V"'

1.1 Y'o

Y".;=-0 --- — —. —...,... ■...... --..— — —■—a—L•••■—■—■—.....— ----

t-itilte1-p a-0,aer* *-----01466.44.- 1144' kleloKkoli5.lAget.

Fig. 1.4. Schematic representation of the electrical double layer.. 33

A schematic representation of the electrical double layer is shown in Figure 1.4 . The potential difference between the electrode and bulk of the solution (YK) together with a constant factor equals the potential of the electrode. The diffuse layer potential (Y's) is the potential at the site where the depolarizer particles must be transferred to be able to react with the electrode and its value depends on the electrode potential or more correctly on (E - E max). It also depends on the nature, charge and concentration of the electrolyte present. According to modern theories8889 , the depolarizer in the vicinity of the electrode surface acquires thermal energy randomly according to Boltzmann distribution and its solvation and conformation are modified in the activated state. The activated molecules then accept electron in a so-called adiabatic electron-transfer process and the resulting species, after further exchange of energy, form the products usually a radical or a radical-ion, which can then undergo further chemical/electrochemical reactions. All these processes — from activation to electron transfer generally take place in the double layer. The potential difference between the electrode surface and the plane of closert approach (Tna - yo ) determines the rate of electron-transfer90-92 When surface-active molecules are adsorbed into the double layer ionic constituents are displaced from this layer, 34

causing change in the charge distribution and capacity of the double layer and shifting the plane o'f closest approach away from the interface. The adsorbed layer can also hinder the approach of the reactive species to the electrode surface, the extent of inhibition being dependent primarily on the structure of the adsorbed film and on the degree of surface coverage93-98 Adsorption and orientation of the reactant molecules are therefore very important in connection with electron-transfer process and any subsequent reactions of the resulting intermediates. The following interactions are mainly responsible for adsorption at electrode-solution interface.

(1) Hydrophobic or Lyophobic interactions: These forces lead to expulsion of the reactant from the solution at the interface.

(ii) Field-dipole interactions of the electrode with functional groups of the reactant in relation to the corresponding interactions with solvent molecules.

(iii) Chemisorption of some groupings such as - S, - SH, -N etc. with the electrode surface or of radicals resulting from coupled electron-transfer and solvent protonation processes.

(iv) Dissociative chemisorption which cause bond fracture during adsorption. Valuable information on the structure of the doable layer and on the capillary activity of the 35

substances in solution can be obtained,from electrocapillary curves which can be constructed very simply by measuring drop-time as a function of its electrode potential. The interfacial tension vs. potential curve would be symmetrical and parabolic if there is no specific adsorption at the mercury surface. But if there is any specific adsorption interfacial tension is lowered. The onset of adsorption/desorption is also indicated by differential capacity measurements which show sharp peaks on both sides of the electrocapillary maximum in the potential region where molecules are absorbed. The adsorption equilibrium at the interface reflects a steady state of adsorption/desorption of the molecules and ions at the interface. The adsorption equilibrium can be described most simply by a Langmuir isotherm.

0 = where e' is the surface concentration for maximum coverage, C is the bulk concentration of the adsorbate, and fi is a constant characteristic for the particular adsorbate. The adsorption coefficient, p , depends on the characteristics of the adsorbate, the solution phase, on the electrode material and its potential. Langmuir equation was derived without consider- ing the interactions of adsorbed molecules. This 36 condition, as was shown by Frumkin102 is very often not fulfilled, specially with large organic molecules containing polar groups. More complex adsorption 99-102 isotherms have therefore been introduced . But dispite all these efforts little substantiated information about the adsorption of electroactive species during electrochemical reduction is available. It has been shown by Langmuir 103and Volmer104 that polar molecules can be initially adsorbed as a two-dimensional gas. In this state the adsorbed molecules lie flat on the electrode surface. When a sufficient concentration of organic molecules is accumulated at the electrode surface the adsorbate may acquire a definite structure and may markedly hinder free exchange of electrons. Besides surface coverage, polarity and other sterical factors also play important role in determining the inhibitive properties of the film105-109 Adsorbed materials may sometimes decelerate chemical reactions such as dehydration of the reacting species or detachment of complex ligands etc. which take place in the vicinity of the electrode surface, prior to the discharge reaction110-112. In the presence of adsorbed species, the usual 'free' electrode surface decreases, with a resulting increase in the effective current density. As a consequence electrode reactions which normally proceed at rates controlled by diffusion would decrease in the presence of a surface film 37 and may become kinetically controlled. Adsorption at the interface can also decelerate electron-transfer rate and consequently increase the irreversibility of electrode reaction. As a result the current at a given potential would decrease and the polarographic wave would become more 'drawn out'. However for an origi- nally diffusion-controlled process, on increasing the potential, the current rises by a reacceleration of the irreversible electron-transfer, and eventually a diffusion current may be observed before the desorp-. tion potential is reached. A change in the local potential drop in the ionic fraction of the ddUble layer may in some instances shift the entire wave to more negative potentials without affecting the shape of the wave113 . In many cases, it has been found that the electrolysis product is surface active or insoluble. Such products accumulate on the electrode surface and give rise to some unusual phenomena such as limiting current diminution, appearance of two waves, sway-backed polarographic waves114-126. The activity of the reduction product is lower in the adsorbed state than that in the solution. Reduction is therefore facilitated by the adsorption of the reduction product and 'adsorption prewave' appears at less negative potential. At higher concentration of the depolarizer, two waves may be observed. The first is the adsorption wave, whose height is independent of concentration above 38 that necessary to completely cover the electrode surface. The second wave is the 'normal' reduction wave, representing the reduction to a dissolved product. The sum of the two wave-heights is generally proportional to the concentration of the depolarizer. The adsorption of the reduction product occasionally hinder the reduction to a dissolved product and may distort the second wave. In some cases, the reactant may be adsorbed resulting in the appearance of a post- wave114'116,118-120,123,124,127-132

If the electrolysis product is insoluble similar effects may be observed. For example a single wave may be splitted into two at higher concentration. But in this case this may be due to inhibited electrode reaction, the inhibition or passivation being caused by the deposition of the insoluble product at the electrode surface. The reduction of sodium vanadate in 1.0M NH C1 + 6.0M NH is a typical example134, 4 3 Distortion of diffusion current due to adsorbed film formation is in some cases attributed to a slow penetration of the reducible species through the com- pact layer of adsorbed material rendering it kinetic in nature. Such kinetic currents have been observed in some cases'86 135,136 39

The Solvent Effect

Change of solvent from aqvus medium can appreciably modify the kinetics and in some cases the mechanism of reduction primarily due to the changing adsorption condition at the electrode-solution interfac137-139 Change of solvent may modify the partial molar free energy of the reactant, the activated complex, or the product, or may modify all these quantities. Many substances containing hydrophobic non-polar groups such as alkyl or aryl groups have a tendency to be expelled from the water phase at the electrode-solution interface. Change to a less polar solvent diminishes this effect and usually decreases the free energy of adsorption. The equilibrium adsorption process is characterized formally by the condition

itt j ,b for an organic substance j at the interface (s) and in the bulk (b). In terms of molefractions at the interface and in the bulk, X.jts andXj ,b this equation may be written as

fr".j,s + RT In Xj,S = A4-1,13 + RT In Xj,b

where, ( ) is the standard free J ,s energy of adsorption. 40

In the case of appreciable specific adsorption, change of solvent mainly modify At i,10. Better solvents for j will lower the energy ittj,b and diminish the magnitude of the standard free energy of adsorption140. Unless the adsorbate has very strong and specific interactions with a surface, its adsorbability will be determined by its solvation in the bulk in respect to that in the interphase. The effect of organic solvents is stronger if the depolarizer adsorptivity at the electrode surface is larger. For this reason the maximum effect of a change in the composition of the solution, can be observed close to the potential of electrocapillary Zero.

For electrode process without prior proton transfer, an increase in the organic solvent content often results in the shift of the wave towards more negative potentials, this shift being the greater, the less negative is the half-wave potential value of the wave141-145. The shift of El may be considered to be the result of a decrease in the activity of the depolarizer at the interface with increasing organic solvent content. At low organic solvent concentrations the shift of E i observed in many cases is larger if the surface activity of the solvent increases144,146 Apparently, at very high organic solvent concentrations, where practically complete desorption of the depolarizer occurs, the nature of the solvent has hardly any effect on the half-wave potential. 41

For electrode processes with preprotonation, an increase in the organic solvent content results in a rise of the pH of acidic solutions146,147 and in an increase in the dissociation constant of the protonated species145; both these factors sharply lower the preprotonation rate and consequently, lead to a shift of El of the reduction wave towards more negative poten- t tials146,148-153. A decrease in the limiting current for such cases is also expected. However, these effects are caused not only by the change in pH and in pKA of the protonated depolarizer, but also by the decrease in the surface concentration of the depolarizer154.

Adsorptivity of organic molecules at the electrode surface increases with increasing size of the molecules. Therefore, in the case of electrode processes resulting in the formation of dimers, a higher adsorptivity of the products compared to the adsorptivity of the initial substances should be observed154. Half-wave potential values are also sometimes affected on changing the solvent composition because of changing specific interactions of the depolarizer if any with the solvent. This seems to be the case when hydrogen bonds are formed between the depolarizer and the solvent155 or when ion-pair

formation56-158i takes place. 42

The pH-effect

The half-wave potential, the limiting current as well as the shape of the polarographic wave of organic/organometallic compounds usually change with pH of the medium. The mechanism of the electrode process often depends on the position and complexity of the protonation or any other pH-dependent chemical reactions. Thdifferent cases can be recognised:

(i) Protonation or any other pH-dependent chemical reactions preceeding the electrode process proper.

(ii) That interposed between two electron-transfer steps ( ECE type).

(iii) That consecutive to electron transfer step.

Preceeding Protonation

This type of process may be schematically represented as

+n e R. 1 products, where 0 is the depolarizer and R is the protonated depolarizer.

For this type of process if the protonation step is fast one single wave is observed, the height of which corresponds to the number of electrons involved, n1, and it does not change with pH. The half-wave potential for an irreversible process of this type 43 is given by159

Ei = Const. + RT In [11+3 m nF • K 411 -11 m

It is seen from this equation that if [1.14-]>K, E1 becomes almost independent of pH, but for [H+] << K,

El = Const. 2.303 RT pH T dnF

A typical example of this type of process is the pH-dependence of El of methylbutylphenacyl sulfonium 7 salt160. If however the protonation step is not quite fast but is comparably slow two waves may appear -- one corresponding to the reduction of the protonated form and the other corresponding to the unproto- nated form. If the number of electrons involved in the two processes are equal, the limiting current remains constant but the ratio iR / io depends on pH. With increasing pH, the height of the protonated reduction wave (iR) decreases in the form of a dissociation curve. This type of behaviour is quite common in organic polarography. Some typical examples are 161 recombination of c(-Keto acids , pyridine carboxylic acids162,163 or reduction of carbonyl groups164 etc.

Two waves can also be observed if the protonation 165-167 step is slow But the limiting currents for such cases are linearly dependent on the concentration of the two forms present and pH-dependence of the 44 limiting current followos the equation

(iR io) - (Ka+ [e])

ECE-type Protonation

This type of process may be represented as

+n1 e +H+‘ +n2e 0 ------4 R at E2> Products at E1 /v

For this type of process the establishment of acid-base equilibria can be either fast or comparable to the electron-transfer rate. A typical example of the former type is the reduction of aromatic and conjugated hydro- carbons168 while reduction of carbonyl compounds in alkaline media and of phthalimide are typical examples of the later type169'170. Sometimes it is found that

E2 at which second reduction takes place is compar- able171 to or even more positive than E1 . In such cases, only one wave corresponding to (n1 + n2) electro4s is observed.

Protonation Consecutive to Electron-transfer

This type of process may be represented by

+n e 0 R 45

Protonation/chemical reactions consecutive to the reversible electrode process can be detected by comparing the half-wave potential of the polarographic wave with the reversible potential determined poten- tiometrically. In such cases El value is different 2 from Er value. Anodic waves of ascorbic acid and other enediols are typical examples of this type of processes.

The Influence of the Electrode Material

Only during the last two decades serious attempt has been made to enlarge the range of electrode mate- ria1s60,175-178. The rate and mechanism of many electrode processes are dependent on the material from which the electrode is constructed. The electrode material can influence the electrode process through adsorption in several ways.

(i) Through specific interactions with the reactant, a product or an intermediate. Formation of metal alkyls at mercury179 or Zinc electrodes, chemisorption of S- and N- functions at mercury, interaction of olefinic and some aromatic compounds with certain electrodes are typical examples.

(ii) Through the production of electroactive intermediates by dissociative chemisorption of the reactant molecule. Formation of phenylmercury free radicals during electroreduction of aromatic lead compounds is a typical example. 46

180-182 (iii) Through electrostatic effects by which it can determine orientation of the reactant molecules, its surface coverage and adsorptivity.

(iv) Through coadsorption of solvent and supporting electrolytes such as halide ions or tetraalkyl ammonium ions etc140

Effect of Temperature

The diffusion current increases by about 1.8% deg 1 with increasing temperature. Limiting currents increasing by more than 3% deg-1 can be attributed to a kinetic current controlled by the rate of preceeding chemical reaction. However all kinetic currents do not show high temperature coefficient.

Adsorption currents show temperature coefficients that can be smaller, equal or greater than 1.8% deg 1 . Hence temperature-dependence of limiting current is of little use for establishing the nature of the limiting current. Effect of temperature on El is difficult to 2 interprete.

Effect of Background Electrolyte.

Buffer System:

Choice of suitable buffer system is absolutely essential in organic polarography. Because, occasionally interaction of buffer component with depolarizer 47 has undesireable effect on the polarographic wave183. For example, boric acid is usually an undesirable constituent due to its tendency to react with some depolarizer components184,185 which often results in the removal of waves.

In addition, the buffering system must have sufficient capacity and speed of reaction to keep the hydrogen ion concentration at the electrode/solution interface constant. Slowness of dissociation and association of the components may cause appearance of several waves.

Ionic Strength and Neutral Salts.

Diffusion currents are usually little affected by changes in ionic strength or neutral salts. Kinetic currents are however sensitive to these changes. Kinetic limiting currents can be changed both by the volume and surface reaction which can result from primary and secondary salt effects, changes in double- layer composition, salting-out effects etc. Some types of catalytic hydrogen evolution waves are specifically affected by the presence of certain cations186-188 2+ .2+ e.g., Co , NI , As3+ or Li. + .

Surface Active Agents.

Maximum suppressors and other surface active agents have marked and significant influence on the 48 polarographic behaviour of organic/organometallic compounds. Adsorption of these substances, even in trace quantities, may decrease the limiting current, shift the half-wave potential, and in some cases obliterate the entire wave .Sometimes, they can cause minima or deceiving split waves113. Hence it should be used with the utmost of care in suppressing polarographic maxima. It has been found that trouble- some maxima can often be removed by decreasing the concentration of the electroactive species; such decrease in concentration is far preferable to the use of maximum suppressors. 49

CHAPTER 2. ETTICTROCHEMICAL BEHAVIOUR OF BIS (TRI-n- BUTYLTIN) OXIDE AND TRIPHENYLTIN COMPOUNDS AND THEIR ANALYTICAL DETERMINATIONS BOTH AT FORMULATION AND AT TRACE LEVELS.

2.1. INTRODUCTION

In recent years organotin compounds have found widespread use as stabilizers for PVC plastics, as rubber antioxidants, as Zeigler-type catalyst in the polymerization of olefins, as active ingredients in certain veterinary medicines, as wood preservatives, as fungistats in paper, textile and polyvinyl paint manufacture and as fungicides in agriculture189-211 Bis (tri-n-butyltin) oxide (TBTO) has highly favour- able properties as a wood preservative, it is active against both fungi and insects which attack wood195-198 Furthermore it does not increase inflammability or impart undesireable colours or odours and is highly sensitive to leaching192-194. TBTO is also being extensively used as a highly effective marine antifouling agent201-203 and as a fungi preventing agent in painting optical and electronic instruments204 Tributyl- and triphenyl-tin compounds are used for impregnating cellulose and woolen fabrics to provide resistance against fungal attack and destruction by moths. Triphenyltin compounds are being widely used in agriculture as fungicide and pesticide and are playing a vital role in protecting foodstuffs205-208 50

Negligible mammalian toxicity of these compounds makes them particularly suitable for this purpose207-208 Organotin fungicides are degraded on weathering to give eventually non-toxic inorganic forms of tin so that, unlike the copper and mercury fungicides, there is no long-term environmental hazard in their use209-212 Methods of analysis of organotin compounds have mostly been based on the degradation of the tin compound and its subsequent determination as inorganic tin2 06,211-219 although some direct methods are also known220-224. This type of procedure, however, is nonspecific for fungicide analysis. Organotin compounds have been found to be reduced directly at the dropping mercury electrode226-236. This phenomenon has been successfully utilised in determining trace amounts of these compounds by anodic stripping 232 methods . The present work describes polarographic and related electroanalytical methods for direct determination of trialkyl and triaryltin compounds over a wide range of concentration down to the submicrogram level.

2.2. EXPERIMENTAL TECHNIQUE

Apparatus: Direct current polarograms were recorded with a Radelkis polarograph type OH-102 (Metrimpex, Hungary). A Kalousek cell with a seperated saturated calomel electrode was used. Pulse and differential 51 pulse polarograms were recorded with a PARTM Model 174 Polarographic Analyzer (Princeton Applied Research, Inc., Princeton, N.J.) in conjunction with a Sarvoscribe Potentiometric Recorder ( RE 511.20 VENTURE). Capillary characteristic were: t = 3.40 sec, m = 1.78 mg/sec in distilled water at h = 66 cm. Cyclic voltammetric experiments were carried out with a Chemtrix Polarograph (Model SSP3, Beaverton, Oregon), using a three electrode-cell with a hanging mercury drop electrode.

For microcoulometric measurements, controlled potential electrolysis were carried out with a PAR Polarographic Analyzer as Potentiostat using a modified H-cell for electrolysis. Controlled potential coulometric experiments were carried out with a PARTM Model 173 Potentiostat in conjunction with a PARTM Model 179 Digital Coulometer. The coulometry cell consisted of a stirred mercury pool working electrode with platinum counter and SCE reference electrode.

pH measurements were carried out with a digital pH meter Model 7040 (Electronic Instruments, Limited, Surrey).

Materials:

Samples of triphenyl stannic acetate ( 98 fo purity), Fentin acetate tech., Duter and Brestan were supplied by the Ministry of Agriculture, Fisheries and food, Plant Pathology Laboratory, Harpenden, 52

Herts and the Ministry of Technology, Laboratory of the Government Chemist. A TBTO containing sample ( Irgarol B1540) was supplied by Ciba-Geigy ( Man- chester) Ltd. All reagents used were of A.R. grade.

Procedure: A stock solution of organotin compound was prepared in absolute ethanol. Working solutions were made up,in constant ionic-strength buffer medium consisting of a 50% (v/v) ethanolic solution that was 0.1M in acetic acid and ammonia and containing 0.002% of Triton x-100. The apparent pH was 7.0. Solutions were deoxygenated with a stream of purified nitrogen gas for five minutes prior to measurements and all the experiments were carried out in nitrogen atmosphere. Stock solutions of formulation samples were prepared by dissolving a weighed amount of the sample to a definite volume of absolute ethanol and subsequent filtration. For carrying out controlled potential coulometric experiments about 10 ml. of triply distilled mercury and 10 ml. of acetate buffer (pH 7.0) containing 50% (v/v) ethanol was taken in the coulometry cell. The solution was thoroughly deaerated by passing purified nitrogen gas through it for at least 15 minutes and stirred smo°,hly and constantly with a magnetic stirrer. After de-aeration the supporting electrolyte was electrolyzed at the potential to be 53 used for the analyte. When the pre-electrolysis current has decreased to a very low level, the background compensation was set as required to give zero accumulated charge. The integrator was then reset, then "opened" and a definite volume of analyte was added with the help of a micropipette. The solution was then electrolyzed until the current decreased to the resi- dual level.

Repititive analyses were carried out by adding additional portions of analyte, electrolyzing, and resetting the integrator after each electrolysis. All the experiments were carried out in nitrogen atmosphere.

2.3. RESULTS AND DISCUSSION.

General Polarographic Behaviour

Dessy et a1230 have previously studied the polarographic behaviour of triphenyltin chloride in dimethoxyethane and observed two well-defined polarographic waves. The first wave was attributed to an one-electron reduction process to form triphenyltin radical which subsequently demerizes to give hexaphenylditin. The second wave was attributed to a two-electron reduction of the dimer to form the triphenyltin anion, Ph Sn. The reduction of triphenyltin 3 231 fluoride , however, in aqueous ethanol has been claimed to involve a two-electron reduction wave. These authors observed the adsorption of the electrolysis product and its subsequent dimerization to form

hexaphenylditin. in a subsequent study Booth and Fleet232 'had shon that polarogrdphic reduction of ' triphenyltin involves two steps and is preceded:by the appearance of an adsorption prewave. The following mechanism was postulated for the electrode process.

+e Ph3Sn v______Ph3Sn +e Ph Sn -e 3 4 Ph3Sn SnPh +e 3 Ph SnH 3 -2e,

The first step involves the formation of a radical which is further reduced in the second step to triphenyltin anion. The free radical also undergoes a competing side reaction to form the dimer, Ph SnSnPh 3 3' The appearance of a prewave indicates strong adsorption of the radical at the mercury surface.

Polarographic behaviour of trialkyltin compounds has been studied by several workers226-229,234-238 Tyurin et al234 showed that in aqueous ethanol solution tributyltin halides give two waves at low concentration but at higher concentration the first wave splits to yield prewave. Shkorbatova et al235 observed two adsorption prewaves for the reduction_ of TBTO in Britton-Robinson buffer. Two well-defined a.c. peaks were also observed at pH 7.11. Kochkin_ et a1236 observed two reduction waves for TBTO in 55 aqueous ethanol over a wide pH range. Devaud229 studied the polarographic behaviour of triethyltin halides in aqueous solution and observed that according to the pH, the reduction occurs in one or two steps of IP-each. The first step has beeh found to be very much 'disturbed' by adsorption phenomena. The mechanism of reduction put forward by Devaud is very much similar to that proposed by Booth and Fleet for triphenyltin compound. Mehner et al228, however, proposes a somewhat different pathway for the second reduction wave involving carbanion extrusion.

Present investigation_ has shown that TBTO solution gives four well-defined waves (Figure 2.1) and four differential pulse polarographic peaks (Figure 2.2 ). The height of the first wave has been found to be linearly dependent on concentration upto 1.4 x 10-4M, beyond which it is independent of concentration of TBTO. The second wave is only present above 1.8 x 10-4M and is linearly dependent on concentration. The third wave has been found to be independent of concentration within the concentration range studied while the fourth wave has been found to appear only above 0.92 x 10-4M and is linearly dependent on concentration. All the waves were found to be time-independent at pH 7.0, which was found to be optimum and was therefore chosen for further study.

E / V vs SCE

Fig. 2.1. Drop time curves. (0) 0.1M acetic acid/0.1M ammonia buffer (pH 7.0) in 50% (v/v) ethanol; (4) 2.76 x10-4M TBTO solution in the same buffer. Polarographic current-potential curves of the same TBTO solution. Start potential, -0.2V.

E/ V vs SCE

Fig. 2.2. Differential pulse polarogram of a 2.30 x 10-4M solution of TBTO in 50% (v/v) ethanol at pH 7.0. Start potential, -0.5V. 57

Limiting current corresponding to first wave has been found to be independent of pH while other waves have been found to decrease in height with increasing pH. A minium is observed at about -1.8 v vs. SCE which has been found to become pronounced in the alkaline region. Half-wave potential of all the waves have been found to shift towards more negative values with increasing pH (Figure 2.3). The first and third wave show linear relationship between limiting current and mercury reservoir height typical of adsorption process. The adsorption character of these two waves was further substantiated by the characteristic shape of their i-t curves. Limiting current corresponding to the second wave has been found to be independent of the height of the mercury reservoir. This phenomenon together with the characteristic shape of i-t curve as well as the slope of the log i - log t plot confirm second wave to be due to a rate controlled process.

It can also be seen from electrocapillary curve that there is considerable lowering of surface tension which is characteristic of adsorption process over the potential range - 0.85 V to - 1.80 V vs. SCE, thus confirming the earlier conclusion. The number of electrons involved in the first step has been found to be one by logarithmic analysis of the first wave. 58

--7•40.1

1.5

1.3

0.9

0.7

2 4 6 8 10 12 14

Fig. 2.3. Dependence of half-wave potentials of TBTO waves on the pH of the medium. 59

Controlled Potential Coulometry

Controlled potential coulometry at -1.25 V vs. SCE which corresponds to the .plateau of wave II showed that the quantity of electricity required for complete electrolysis is 2F/mole. Because of large background current no reliable information could be obtained about the number of electrons involved in the 3rd/4th step by controlled potential coulometry experiments.

Table 1. Coulometrically determined number of electrons consumed in the reduction of TBTO in 0.1M acetic acid / 0.1M ammonia buffer ( pH 7.0) containing 50% (v/v) ethanol.

Working electrode Potential of the n-value working electrode

Mercury pool -1.25 v vs. SCE 1.98 1.97 2.01 1.985

Thin Layer Chromatography

A drop of TBTC solution in absolute ethanol was applied to a thin layer chromatographic plate 60 coated with a silica gel as the stationary phase and eluted with chloroform-acetone mixture. After drying, the stationary phase was sprayed with a 0.17M solution dithizone in chloroform followed by 0.02N EDTA solution. One yellow and one orange-yellow spot having

RF values of 0.244 and 0.445 respectively were observed (Figure 2.4). This shows the presence of TBTO and tributyltin hydroxide in the system thereby confirming its hydrolysis1 Hydrolysis of TBTO in aqueous . solution is also reported in literature241

Solvent front

Base line

Fig. 2.4. Thin layer chromatogram of TBTO. Stationary phase, Silica Gel; Eluent, Chloroform-acetone mixture. 61

Mechanism of reduction

It appears that the second wave in the polarogram is the normal reduction wave involving the reduction of tributyltin cation to tributyltin free radical, which is adsorbed onto the electrode surface giving rise to the adsorption prewave. The kinetic character of the second wave can be accounted for by postulating that TBTO is not reduced at the same potential at which tributyltin hydroxide is reduced. The hydrolysis of TBTO which produces the reducible species Bu SnOH is actually the slow process contro- 3 lling the kinetics of the electrode process. The equilibrium amount of Bu SnOH is however, sufficiently 3 small to render the second wave almost completely a kinetic wave. The third wave is an adsorption wave and may be due to adsorption of electrolysis product onto the mercury drop surface, the fourth wave being the normal reduction wave and corresponds to the reduction of Bu Sri to Bu Sn. Probably electro- 3 3 chemical reduction of TBTO molecules takes place simultaneously at the potential region at which tributyltin free radicals are reduced. The following side reactions may also take place concurrently.

Bu3Sn' + Bu3Sn' ----> Bu6Sn2

Bu Sn- + Bu Sn+ Bu6Sn2 3 3

Bu Sn- + B 3 Bu3SnH + Bu3SnOH Bu6Sn2 + 62

The formation of hexaalkyl/hexaaryl-ditin during electroreduction of trisubstituted tin compounds has already been confirmed. Exhaustive electrolysis of trialkyltinhalides and of triphenyltin acetate at potentials, corresponding to the plateau of their respective 3rd/4th wave and subsequent isolation_ and characterisation of the products confirmed the formation of trisubstituted hydride. The mechanism of TBTO may be summarised in the following table.

+H2 0 + (Bu3Sn)20 cnU onyx].„ 2Bu Sn + 20H 3 ^ -----1T0E 2 ) 3

Bu Sn+ -----+e 3Sn7 .___+e Bu3Sn:- 3 c------e Bu -e

Bu3Sn.SnBu4 3 1 +H+

Bu3SnH -2e, -HI- +4e, H2O (Bu3Sn)20 2Bu3Sn- + 20H

Cyclic Voltammetry

The above mechanism of reduction has been confirmed by cyclic voltammetric experiments, CV behaviour of triphenyltin compounds has already been described. A steady state voltamagram with 2.30.10-4M TBTO solution and a scan rate of 200 my/sec shows four cathodic and five anodic peaks (Figure 2.5). 63

Peaks Ia and Ib correspond to the adsorption/ desorption phenomena. Peaks IIa and IIb correspond to the normal reduction of Bu Sn+ cation to the free 3 radical Bu Sri'. Peaks IIIa and Mb correspond to 3 adsorption/desorption while peak IVa and IVb correspond to the reduction of tribytultin free radical to tributyltin anion and its subsequent oxidation respectively. The anodic peak V corresponds to the two-electron oxidation of tributyltin hydride to cation + Bu Sn . 3

E I V (vs.SCE)

Fig. 2.5. Cyclic Voltammogram of 2.30.10-4M TBTO in 50% (v/v) ethanol at pH 7.0 and 0.002% Triton x-100. Start potential,-0.0V. Scan rate, 200 mV/sec. 64

Pulse and differential pulse polarography.

Pulse polarograms of triphenyltin acetate in 50% (v/v) ethanol at pH 7.0 shows two waves at lower concentration. Height of both the waves have been found to be linearly dependent on concentration. It was found that above 3.2.10-4M concentration the first wave splits to give prewaves (Figure 2.6), the height of which has been found to be independent of concentration. Employing the same solution conditions as for d.c. and pulse polarography, differential pulse polarograms were recorded for all the compounds. Three peaks at -0.78v, -1.12v and at -1.53 vs SCE were observed in the differential pulse polarograms of triphenyltin acetate (Figure 2.7). The first peak corresponds to the adsorption of the free radical product formed in the first reduction process. The second and third peaks correspond to the reduction of triphenyltin cation and triphenyltin free radical respectively. The second peak exists only at and above 4 2.0 10- M and is linearly dependent on concentration (Figure 2.8). Below this concentration height of the first peak shows linear dependence on concentration down to 2.5 10-7M. The third peak exists over the whole concentration range and is concentration dependent. TBTO, however, shows four peaks at -0.84v, -1.12v, -1.52v and at -1.73v vs SCE the second peak disappearing at and below 1.8 10-4M above which the first peak becomes independent of concentration 65

11., A

0.211A 1

,0,4v

E vs.Hg pool El ,/ vs. Hg pool

_Fig. 2.6. Pulse polarogram of a 5.0.10-4M triphenyltin acetate solution in acetate buffer. (pH 7.0) containing 50% (v/v) ethanol. Start potential, -0.2V.

Fig. 2.7. Differential pulse polarogram of a 5.0.10-4M triphenyltin acetate solution in acetate buffer (pH 7.0) containing 50% (v/v) ethanol. Start potential, -0.2V.

5.0

4.0

130

2.0 •

10 4 M 2004 XI 0- 4 Concentration/14

Fig. 2.8. Dependence of differential pulse polarogiaphic peak current on the concentration of triphenyltin acetate in acetate buffer (pH 7.0) containing 50% (v/v) ethanol. (0) Peak I, (0) peak II. 66

(Figure 2.8). The second and fourth peaks have been found to be linearly dependent on concentration (Figure 2.9). Lower limit of detection of the first adsorption peak has been found to be 1.10-7M. (Figure 2.10).

2.4. ANALYTICAL DETERMINATIONS Analytical determination_ of Fentin residues in Pesticide formulations

The preliminary investigation of the electroanalytical behaviour of organotin compounds shows the optimum condition for their analytical determination. Differential pulse polarographic method seems to be the most suitable method for their determination at concentration down to 10.-7M. This method was therefore used for the determination of fentin residues in three pesticide formulations. A calibration curve is shown in Fig. 2.11 and the results are shown in Table 1.

Table 1. Organotin Content of the Pesticide formulation

Pesticide Formulation Organotin Content (%)

Fentin acetate techn. 76.9 % Brestan 45.4 % Duter 45.4 %

al0

5.0

25 3.5 5.0 50 E / V vs SCE 104 x concentration / M

Fig. 2.9. Dependence of differential pulse polarographic peak current on the concentration of TBTO ; 0, peak II ; 9, peak IV.

Fig. 2.10. Differential pulse polarogram of a 4.6.10-7M TBTO solution in 50% (v/v) ethanol at pH 7.0. Start potential, -0.65V.

011

<0.09

007

5.1(57610 510 Concentration / M

Fig. 2.11. Dependence of differential pulse polarographic peak current (I) on concentration of triphenyltin acetate in acetate buffer (pH 7.0) containing 50% (v/v) ethanol. 68

CHAPTER 3 ELECTROCHEMICAL BEHAVIOUR OF DIBUTYLTIN DILAUREATE AND DIBUTYLTIN DIMALEATE AND THEIR ANALYTICAL DETERMINATION.

3.1. INTRODUCTION

Dialkyltin carboxylates constitute an important class of organotin compounds with the main area of application as the active ingradients in some ethical veterinary formulations189-192. Dibutyltin dilaureate (DBTL2) and dibutyltin dimaleate (DBTM2) are specially used for the controll of helminthic and protozoal 205 242 infections in poultry ' Dibutyltin_ dimaleate is extensively used for stabilisation of PVC plastics and dibutyltin dilaureate is used as catalyst for polyurethane foams243. Methods of analysis for these compounds have mostly been based on the determination of inorganic tin after breakdown of the complex although some direct analytical methods have also been reported. Organotin compounds are reduced at the dropping mercury ..electrode and this offered the possibility of a direct procedure230'232,244-254 A general survey of the electrochemistry of dialkyltin compounds however shows that authors disagree as to the nature of the electrochemical reduction process244-254. Polarographic behaviour of diethyltin dichloride was first investigated by Riccoboni and Popoff244, who claimed to have

observed one two-electron reduction wave. Toropova and Saikina245 studied similar compounds in 40% aqueous-ethanol at varying pH and found that the reduction became more difficult in the substituent series ethyl-propyl-butyl. Geyer and Seidlitz246 also carried out similar studies. Dessy et a1230 found that in dimethoxy ethane, dibutyltin dichloride gives a two-step wave and they proposed the following mechanism of reduction.

+e +2e Bu2SnCl ---__ Bu Sn'Cl ›Bu2Sn-SnBu2 ---.÷(Bu2Sn).n 2 -e 2 I 1 01 01 + Cl -

Morris 248studied the polarographic behaviour of dialkyltin compounds in aqueous acid solution_and claimed to have observed a reversible two-electron step distorted by formation of an insoluble polymer product

(R2Sn)n on the electrode surface. At concentration higher than 10-4M the wave has been found to be split into more steps. Similarly, a two-electron reduction wave for dialkyltin compounds in aqueous methanolic solution was also claimed249

xn 2+ +2e (R2SnOn R2Sn R Sn: -2e 2

At higher concentration of the dipolarizer, the wave has been found to be split.114, into three steps - an 70 adsorption prewave, a reduction wave which is limited by adsorption of polymer product and a desorption wave. The polymer product was isolated251 and its composition determined as (R2Sn)n. Flerov et. al234'252 observed a reversible one-electron wave for

Bu2SnC12 in water and in aqueous-ethanol solution at ca 10-4M. At higher concentration the wave has been found to be splitted into two and the process became irreversible. At still higher concentration the limiting currents for both the waves have been found to be same and independent of depolarizer concentration. These authors postulated the formation of adsorbed 2+ R2Sn radicals which do not dimerize. Subsequently, a second layer of the same radicals is superimposed and these dimerize with the radicals of the first layer to form a monolayer of the dimer (R2Sn.SnR2)2+ or (R Sn-SnR ) both in adsorbed state. In a recent 261 Cl 2 paper, Zezula and Markuova 253 claimed to have observed four reduction waves during the reduction of dimethyltin di-chloride at DME. These authors ascribed the first wave to an adsorption process and the second wave to a reversible two-electron reduction process.

2+ +2e +e (CI)) 2Sn (CH-)) 2Sn: unidentified c------2e or reduction products ) (CH3 2Sn(OH)2 [(CH ) 3 2Sia:] n 71

The third wave has been ascribed to a two—electron reduction of the compound (CH3)2Sn(OH)2 formed by hydrolysis of dimethyltin di—chloride and fourth wave 250 to reduction of (CH3)2Sn. Devaud et a1 studied the polarographic behaviour of dialkyltindichloride in detail and showed that the nature of the radical, R., profoundly influences the electrochemical behaviour of these compounds. These authors proposed two one—electron reduction processes for Bu2SnCl2. The present state of electrochemical reduction mechanism of dialkyltin compounds thus seems to be very confu- sing and ambiguous. In view of the importance of these compounds it was felt that a more detailed investigation of the electrode process was justified, primarily with the aim of developing a very sensitive method of analysis.

3.2. EXPERIMENTAL TECHNIQUE

Apparatus:

The apparatus used in this investigation has already been described in the preceeding chapter.

Materials:

Pure samples of Dibutyltin dilaureate and Dibutyltin dimaleate were supplied by Ciba—Geigy (Manchester) Ltd. All reagents were of A.R.grade. 72

Procedure: A 5.0 10-3M stock solution was prepared in absolute ethanol. Working solutions were made up in constant ionic strength buffer medium consisting of a 80% (v/v) ethanolic solution, unless otherwise stated, that was 0.1M in HAC and NH3. A 0.002% of Triton x-100 was used during recording of the third wave of DBTM2 for suppressing its maxima. The apparant pH of the medium was 7.0. Solutions were deoxygenated with a stream of purified nitrogen gas, presaturated with ethanol-water (4:1) vapour for at least five minutes before each measurement.

For carrying out microcoulometric experiments 1.0.ml of the acetate buffer solution (pH 7.0) containing 80% (v/v) ethanol was placed in the microcoulometry cell. Purified nitrogen gas, presaturated with the background solvent vapour was bubbled through the solution for about 10 minutes. 100,A-1 of the working solution was then added to the buffer and polarographed immediately. The solution was then subjected to polarographic electrolysis at a fixed potential which corresponded to a point on the plateau of the wave and polarograms recorded after every hour. Microcoulo- metric experiments were carried out in nitrogen atmosphere. 73

3.3. RESULTS AND DISCUSSION

General Polarographic Behaviour

Both DBTL2 and DBTM2 give three reduction waves in ethanol-water (4:1) over a wide pH range (Figures 3.1 and 3.2). The first two waves show no maxima while the third wave is accompanied by a large maximum. A typical d.c. polarogram of DBTL2 in 80% (v/v) ethanol at pH 7.0 is shown in the Figure 3.1. The heights of the first two waves are virtually independent of pH while height of the third wave decreases with increasing pH. All the waves have been found to be relatively time-independent at pH 7.0 which was chosen for further study for elucidating the mechanism of electrode process and for developing analytical methods. Figure 3.3 shows the variation of limiting current with concentration of DBTL2. The limiting current of the first wave rises linearly with DBTL2 concentration upto ca 3.0 10-4M. It then rises slowly upto ca 3.5 10-4M beyond which it increases rather slowly with further increase in the depolarizer concentration. The second wave is only present at and above ca 1.5 10-4M. Height of the second wave also rises linearly with concentration upto ca 3.0 10-4M, beyond which it rises slowly and finally reaches a limiting value at ca 3.5 10-4M. Beyond -4 3.5 10 M concentration limiting currents for both the waves are equal. Similar behaviour is also observed 74

4.0

3.0

E t V vs SCE

Fig. 3.1. Drop time curves. (0) 0.1M acetic acid/0.1M ammonia buffer (pH 7.0) in 80% (v/v) ethanol ; (I) 1.55.10-4M dibutyltin dilaureate in the same buffer. Polarographic current-potential curves of the same DBTL2 solution. Start potential, -0.2V-.

E / V vs SCE

Fig. 3.2. Drop time curves. (0) 0.1M acetic acid/0.1M ammonia buffer (pH 7.0) in 80% (v/v) ethanol; (0) 3.35.10-4M dibutyltin dimaleate solution in the same buffer. Polarographic current-potential curve of the same DBTM2 solution. Start potential, -0.2V.

013

0.4

1.5 2.0 3.0 3.5 104 x concent ration / M

Fig. 3.3. Dependence of d.c. limiting current on concentration of dibutyltin dilaureate. (1) First wave, (2) second wave.

180

1.7 0

Cf.

16 0

0 1 2 3 4 5 6

1/ ha r

Fig. 3.4. Dependence of the logarithim of the limiting current (wave I) of DBTL2 on time of electrolysis during microcoulometric experiment. 76 for DBTM2. This type of behaviour is expected when a film of insulating material is formed as the product of the electrode process. The third wave of DBTL2 is present only above ca 1.5 10-4M and is concentration dependent. The third wave of DBTM2, however, is present throughout the concentration range studied and is concentration-dependent, but the limiting current vs. concentration plot is concave upward. The first wave has been found to be diffusion controlled by the linear variation of its limiting current with square root of the height of the mercury reservoir. Limiting current of the second wave has been found to be independent of the height of the Hg reservoir indicating it to be a rate-controlled current. Diffusion character of the first wave and kinetic character of the second was further substantiated by the characteristic shape of their i-t curves and slopes of their logi-logt plot, the value of which for the first wave has been found to be 0.633. Half-wave potentials of the waves have been found to shift towards more negative values with increasing pH of the medium. It has been found that El- changes slowly with pH around pH 7.0 but rapidly in more acidic or more alkaline solution. An electrocapillary curve is shown in Figure 3.1. It is seen from the figure that there is no lowering of surface tension indicating no adsorption at the surface of the electrode. 77

The effect of varying the percentage of ethanol showed that with decreasing ethanol content the height of the second wave decreases and becomes rather poorly defined. Finally in 50% ethanol solution the second wave disappears completely, the third wave for DBTL2 also disappears in 50% (v/v) ethanol but not that for

DBTM2. This shows that the product of the first step, a radical ion, is stable in 80% (v/v) ethanol, its life period decreasing with decreasing ethanol content and that the radical ion is unstable in 50% (v/v) ethanol. It is also clear that the third step

DBTM of DBTL2 and that for 2 are not due to identical electrode processes. For a 1.55 10-4M solution of

DBTL -4 DBTM 2 and for a 2.76 10 M solution of 2 in acetate buffer (pH 7.0) containing 80% (v/v) ethanol plots of Edme vs. log id-i for the first wave have been found to be linear with slopes of 42.5 mV and 40 mV respectively. These values are intermediate between those for one and two-electron processes. Probably some disproportionation of the radical ion, + + + 2+ Bu2Sn , like Bu2Sn + Bu2Sn Bu2Sn + Bu2Sn: and consequent* regeneration of the depolarizer may be the cause of this irregularity.

Microcoulometry

Prolonged electrolysis of a 3.8 10-4M solution of DBTL using a DME 2 at -0.900V vs. SCE and at. -1.15V vs. SCE for six hours showed only slight 78 decrease of the limiting current of the first wave while the height of the second wave has been found to decrease considerably during the first three hours but afterwords it has been found to decrease very slowly with time (Figure 3.4). Probably the intermediates formed are reoxidized to give the original depolarizer by the same disproportion mechanism as above. Similar behaviour were also observed for

DBTM2 solutions.

Controlled Potential Coulometry

In order to determine the number of electrons involved in the reduction process, exhaustive controlled potential electrolysis of a 1.89 10-4M solu- . tion of DBTIJ2 was carried out at -0.900V vs. SCE and at -1.15V vs. SCE complete electrolysis required about one hour. Controlled potential coulometry at -0.900V vs. SCE gave n-value of 0.98 while that at -1.150V vs. SCE gave n-value of 2.1. This shows that both the first and the second step are due to one-electron processes. Contrary to previously published work on diethyltin compounds248 no irregularities were observed in these experiments except that the background current was large. Controlled potential coulometry at -1.50V vs. SCE gave a total n-value of three for DBTL2 and 6.3 for DBTM2. 79

Table 3.1. Coulometrically determined number of electrons consumed in the reduction of dibutyltin dilaureate in 0.1M acetic acid / 0.1M ammonia buffer (pH 7.0) containing 80% (v/v) ethanol.

Working electrode Potential of the n-value working electrode

Mercury pool -0.900V vs. SCE 0.980 0.975 0.985 -1.15V vs. SCE 2.12 2.11 2.10 -1.50V vs. SCE 3.1

The n-value of one for third step of DBTL2 can be accounted for by suggesting that a considerable part of Bu2Sn: formed in step 2 has been polymerized while the rest which is about 50% of the original participates in the electrode process giving n-value considerably lower than that for the straight reduction without competing polymer formation. In aqueous-ethanol medium the olefinic double bonds of DBTM2 are reduced concurrently with Bu2Sn: at about -1.50V vs. SCE. The reduction involves 4 electrons and is irreversible as is shown by CV experiments.

+4e,4114- Bu2Sn(00C.CH.CHC00Et)2 Bu2Sn(00C.CH2CH2C00Et)2

The reduction of olefinic double bond of maleate at -1.50V vs SCE in 80% (v/v) ethanol at pH 7.0 was confirmed by carrying out a seperate polarographic experiment with ethyl-maleate under the same experimental condition as for DBTM2.

Mechanism of reduction

From the experimental evidences obtained the following mechanism of reduction can be postulated. The one-electron reduction of the first step has been confirmed by coulometric experiments. The one-electron nature of the second step was also confirmed by coulometric experiments.

4-2e,2H20 Bu2on , 2+ ----- +e nu Dn.+ +e, 2 nU2on. > Bu SnH A -e 2 2 n (Bu2Sn.SnBu2)21- (Bu2Sn)n

—4e, 2H+

The irreversible nature of the second step was confirmed by cyclic voltametry. Bu2Sn"- ion-racial is stable in 80% (v/v) ethanol solution but is unstable in 50% (v/v) ethanolic medium as is shown by the 81 appearance of the second wave in 80% (v/v) ethanol and its disappearance in 50% (v/v) ethanol. The kinetic nature of the second step arises because of the following fact. During electroreduction Bugn: species is produced which polymerises to (Bu2Sn)n. This polymer formed an insulating shield around the mercury drop. Further reduction of Bu2Sn'+ ion-radical is then only possible after their penetration through the insulating film to reach the electrode surface, slow penetrating being the rate-controlling step in the electrode process. That the actual electron-transfer is a fast process can be seen from the well-developed anodic and cathodic CV peaks. This type of rate-controlled limiting current is already known in polarography and is generally called 'penetration current'. Formation of insulating film has also influence on the concentration-dependence of the limiting current at higher depolarizer concentration. It has already been stated that both the first and the second wave are linearly dependent on concentration up to a certain critical concentration beyond which it rises slowly, if at all, with increasing concentration of the depolarizer. Probably at this critical concentration the mercury drop is completely covered with the insulating film. 82

Cyclic Voltammetry

The proposed mechanism of reduction was confirmed by CV experiments. A steady state voltamagram with a depolarizer concentration of 1.55 10-4M and a scan rate of 200 mV/sec shows three cathodic and three anodic peaks (Figure 3.5). Peaks Ia and Ib show a peak seperation of 60 mV which corresponds to a reversible one-electron process. Peak IIa corresponds to the irreversible reduction of dibutyltin ion-radical, Bu2Sn'l- and peak IIb corresponds 2+ to the oxidation of Bu2Sn:Sn to Bu2Sn . Peaks IIIa and IIIb correspond to the two-electron reduction of Bu2Sn: to the hydride Bu2SnH2 during the cathodic sweep and the subsequent oxidation of Bu2SnH2 during during the reverse sweep respectively. This interpretation of our results, however, differs considerably from that proposed by Morris for the reduction of aquodiethyltin cation.

3.4. ANALYTICAL DETERMINATIONS

From preliminary investigation of the reduction mechanism of DBTL2 and DBTM2 the optimum condition for the analytical determination of these compounds can be established.

DC Polarography

General polarographic behaviour shows that in 50% (v/v) ethanol solution containing 0.1M buffer 83

E / V vs SCE

Fig. 3.5. Cyclic voltammogram of a 1.55 10-4M DBTL2 solution in 80% (v/v) ethanol at pH 7.0. Start potential, -0.1V; scan rate, 200 mV/sec. 84

(0.1M HAC, 0.1M NH ), both DBTL and DBTM give only 3 2 2 one wave, while in 80% (v/v) ethanol water medium they give three waves. The first wave shows a linear dependence on concentration over the range 8.0 10-7 to 3.0 10-4M. Hence this wave can be utilized for analytical purpose over this range of concentration. Wave II exists only above 1.55 10-4M and shows linear dependence of wave height on concentration upto ca 2.55 10-4M and is therefore not suitable for analytical purpose.

' Differential Pulse Polarography

Under the same experimental condition as in d.c. polarography DBTL2 gives three peaks (Figure 3.7). The first peak shows a rectilinear dependence on concentration of depolarizer upto ca 3.0 10-4M (Figure 3.8). It then increases slowly with increasing concentration, reaches a maximum and then decreases again. It is seen in Figure 3.7 that there is no differential pulse polarographic peak corresponding to the 2nd d.c. wave. This is because differential pulse polarographic experiments were carried out with a Hg drop time of 1 -sec, at which the second d.c. wave does not appear. Heights of the second and third peak are very small compared to that of the first peak. However, beyond ca 3.2 10-4M the second peak increases appreciably with increasing concen- 85

E / V vs SCE

Fig. 3.6. D.C. polarogram of a 6.0 10-7M DBTL2 solution in 80% (v/v) ethanol at pH 7.0. Start potential, -0.7V, (1) DBTL2 solution, (2) supporting electrolyte. 86

2 WA

0.2V

E / V vs SCE

Fig. 3.7. Differential pulse polarogram of a 1.55 10-4M DBLA2 solution in 80% (v/v) ethanol at pH 7.0. . 87

20 4 1 5.10 M concentration 3.5-10,4M

Fig. 3.8. Dependence of differential pulse polarographic peak current on concentration of DBT1,2. 88

concentration. At still higher concentration (>5.0 10-4M) two more new peaks appear. Similar

DBTM behaviour was also observed for 2 with the excep- tion that height of the third peak of DBTM2 is, under- standibly, much greater compared to the first one. The lower limit of detection of the first peak of

DBTL DBTM -9 2 and of 2 is surprisingly low, ca 5.0.10 M. A differential pulse polarogram of a 6.0.10-9M

DBTL solution of 2 is shown in Figure 3.9. It is clear, therefore, that the first peak may be utilized for analytical determination of dibutyltin compounds over the concentration range 5.0.10-4M to 5.0.10-9M. 89

0.1V

E / V. vs SCE

Fig. 3.9. Differential pulse polarogram of a 6.0 10-9M DBTL2 solution in 80% (v/v) ethanol solution at pH 7.0. 1, nTL2 solution, 2, supporting electrolyte. 90

CHAPTER 4. ETRCTROCHEMICAL BEHAVIOUR OF ORGANOTIN SULPHUR COMPOUNDS AND THEIR ANALYTICAL DETERMINATIONS.

4.1. INTRODUCTION

Organotin sulphur compounds are widely used in plastics industry as effective stabilizers for poly- vinylchloride compositions189-192,204,243. Polyvinyl chloride resin has a tendency to degrade on heating or on prolonged exposure to UV light. Moreover this degradation is accompanied by darkening of the polymer PVC turns black when as little as 0.1% of the polymer has decomposed. Fortunately as little as 0.5 to 2% of diorganotin compounds in the polymer is sufficient to maintain clarity and impart strength to the polymer255. The most important compounds used for polymer stabilization are mercaptides, mercap- toesters and mercaptocarboxylates based on dialkyltin groups, R2Sn< , specially where R is butyl or octyl. A scheme of analysis of this class of compounds has been described256. Quantitative determination of organotin stabilizers by direct procedures have recently been described221,224,257. Indirect determinations by coulometric titration for sulphur258, and by polarographic methods based on conversion of organotin to inorganic tin compound214 ,215 are also 91 known. Electrochemical behaviour of dialkyltin compounds containing no sulphur and methods of their direct determination down to submicrogram level have been described in the preceeding chapter. In the present chapter the results of a detailed investigation of the electrochemical behaviour of organotin marcaptocarboxylate stabilizers are described. Based on these observations polarographic methods of analysis have been developed suitable for application to formulation or trace analysis.

4.2. EXPERIMENTAL TECHNIQUE

Apparatus: The apparatus and experimental method used in this investigation have already been described in the preceeding chapters,.

Materials:

Pure samples containing di-n-octyltin dithioglycolic acid isooctyl ester (Irgastab 17MOK) and two other samples - Irgastab 17M and Irgastab 15MOK containing dialkyltin marcaptides were supplied by Ciba-Geigy (Manchester) Ltd. All reagents used were of A.R. grade.

4.3. RESULTS AND DISCUSSIONS

General Polarographic behaviour of three organotin marcaptides have been studied. All of them show three well-defined polarograms in 80% (v/v) 92 ethanol solution at pH 7.0. Two/three more waves, which are very drawn out and rather illdefined were also observed at more negative potentials (Figure 4.1). It has been found that the first two waves are anodic while the rest are cathodic in nature. In the following section polarographic behaviour of one of these compounds viz. di-n-octyltin dithioglycolic acid isooctyl ester (DOTDTG) will be described in detail. Preliminary experiments showed that DOTDTG is hydrolised in 50% (v/v) ethanol solution with seperation of white sulphur ppt. It was, therefore, decided to carry out the experiment in 80% (v/v) ethanol, which has been found to be most suitable for the purpose. Preliminary experiments also showed that all the waves were time-independent. Heights of the waves were found to be practically independent of pH but El. values have been found to shift towards more negative potentials with increasing pH of the medium. Waves II and III have been found to be diffusion-controlled from their dependence on square- root of the height of the mercury reservoir and from the slope of their log i - log t plot. As all the other waves are rather illdefined and very drawn out no attempt was made to ascertain their nature from these studies. It is seen from electrocapillary curves (Figure 4.1) that there is considerable lowering of surface tension indicating strong adsorption at DME over the potential range of 93

E/ V vs SCE Fig. 4.1. Electrocapillary curves: 0.1M Acetic acid/0.1M Ammonia buffer (pH 7.0) in 80% .(v/v) ethanol, 1.85.10-4M Di-n7octyltin dithio glycollic acid -iso octyl ester in the same buffer. Polarographic current-potential curves of the same DOTDTG solution, Start Potential, -0c0V. 94

0.0V to -1.2V vs. SCE. Logarithmic analysis of wave III gives a okn value of 0.84. Both waves II and III show linear dependence of concentration up to ca 3.5.104M beyond which the limiting current increases slowly, if at all, with increasing concentration.

Differential pulse polarography

A typical differential pulse polarogram of DOTDTG in 80% (v/v) ethanol at pH 7.0 is shown in Figure 4.2. It shows three well-defined peaks and four rather illdefined peaks at more negative potentials. Peaks I and II correspond to two anodic d.c. waves while the rest correspond to the cathodic waves of the d.c. polarogram. Peak I is presumably due to an adsorption process while peak II corresponds to the oxidation of marcaptide into dioctyltin cation according to

-2e 2+ Oct2Sn(SR)2 > Oct2Sn + Hg(SR)2. (Hg)

It is seen from Figure 4.2 that the height of peak III is approximately half the height of peak II indicating that the number of electrons involved in step III is half the number of electrons involved in step II. In other words, the first cathodic step (corresponding to peak III) is an one-electron reduction of DOTDTG i

E / V vs SCE

Fig. 4.2. Differential pulse polarogram of a 1.85.10 4M DOTDTG in 80% (v/v) ethanol at pH 7.0. Start potential, -0.0V.

into DOTDTG free-radical according to the process

+e, H+ Oct2Sn(SR)2 > Oct2Sn'R + RSH..

Peaks II and III show linear dependence on concentration up to ca 3.5.104M (Figure 4.3). Slow *linear rise of peak height for peaks I, IV and V were observed with increasing concentration of depolarizer. Peak IV may be due to one-electron reduction of the DOTDTG free-radical into dicotyltin and peak V may correspond to further reduction of dicotyltin to the corresponding hydride.

Coulometry

Controlled potential coulometry for DOTDTG at -0.900V vs. SCE shows n-value of 1.0 in two seperate determinations which confirms previous postulations that peak III correspond to a 1-electron process. Exhaustive electrolysis at -0.100V vs. SCE shows n-value of 2.

Mechanism of Electrode Process

From the evidences obtained and by analogy with the mechanism of reduction of dibutyltin compounds it is possible to postulate the mechanism of the electrode process in the following scheme:

2.0

1.6

0.4

4 2 4.10 M concentration

Fig. 4.3. Dependence of differential pulse polarographic peak current on the concentration of DOTDTG, 0, cathodic peak, 0 ,anodic peak. 98

+e +e +2e,2114- Oct2Sn(SR)2 Oct2Sn'SR Oct2Sn: >Oct2SnH2 -2e (Hg) Oct2Sn.SnOct2 (Oct2Sn)n Oct2Sn2+ + .SR SR SR SR

Dioctyltin dimarcaptide takes up one electron to give the dioctyltin marcaptide free radical which may suffer further reduction to give dioctyltin, or it may undergo dimerization. Probably both the processes occur simultaneously. Reversibility of the first step has been tested by cyclic voltammetry, which showed two coupled cathodic/anodic peaks but the peak separations have been found to be considerably greater than the theoretical value for one electron process. It is also known that dialkyltin undergoes rapid polymerisation251 . It may also undergo further electrochemical reduction giving the hydride. At less negative potential dioctyltin di-marcaptide undergoes oxidation producing dioctyltin dication and mercury mercaptide. There is considerable adsorption of the mercaptides at the DME as can be seen by the depression of the electrocapillary curve in the potential region 0.0V - 0.6V vs. SCE.

Cyclic Voltammetry

The above mechanism of the electrode process has been confirmed by cv experiments. A typical 99

5 P)A

1— z w cc Cr (_)

E / V vs SCE

Fig. 4.4. Cyclic voltammogram of a 1.85 10-4M Di-n-Octyltin dithioglycollic acid -iso-octyl ester in 80% (v/v) ethanol at pH 7.0. Start potential -0.0V, scan rate, 200 mV/sec. 100 steady state cyclic voltamogram of a 5.0.10-5M solution of DOTDTG in 80% (v/v) ethanol at pH 7.0 is shown in Figure 4.4. Peaks Ia and Ib correspond to adsorption/desorption phenomena and peaks IIa and IIb correspond to oxidation/reduction involving mercury marcaptide formation. The cathodic peaks IIIa, IVa and V correspond to three successive reduction steps while the two anodic peaks IIIb and IVb correspond to the oxidation of the products of steps IIIa and IVa respectively.

Analytical determination

It has already been shown that d.c. waves II and III are well-defined and linearly dependent on concentration up ca 4.0.10-4M. Hence either of the waves can be utilized for analytical determination of the organotin marcaptides. It has also been shown that differential pulse polarographic peaks II and III are linearly dependent on-concentration over the range 4.10-4m to 5.0.10-7M. Hence these peaks can be utilized for analytical determination of these compounds using 80% (v/v) ethanol medium. It has been found that Irgastab 17M and Irgastab 15 MOK behaves similarly (cf. Figures 4.6 and 4.7). Hence utilizing the first cathodic peak of DOTDTG the organotin content of these two samples were determined and found that both of them contain about 99% organotin. 101

E / V vs SCE

Fig. 4.5. 1, Differential pulse polarographic current potential curve of 1. 10-6M DOTDTG in 80% (v/v) ethanol. 2, Background electrolyte. Start potential, -0.5V. 102

CHAPTER 5. ELECTROCHEMICAL BEHAVIOUR OF TRIPHENYL-

. TREAD ACETATE AND ITS ANALYTICAL DETERMINATION.

5.1. INTRODUCTION

The principal commercial application of organolead compounds is their use as antiknock agent, as alkylating agent for the production of organomercury fungicide and as polymerisation cata- lysts259-262. More recently biological effects of organolead compounds are being widely investigated because of their possible use as insecticides and fungicides and as possible chemotherapeutic agent in the treatment of cancer 259-271. Trialkyllead acetate has been found to be a very effective rodent repellant for jute263,264. Triphenyllead compounds which are used as temporary sternutator towards human beings272-275 have great potentiality for treatment of allargies, asthma and rheumatoid arthritis276 . These compounds can also be used as highly effective antifouling agent277-279 . After the first synthesis of organolead compound by Lowig in 1853380-282 organolead chemistry has developed into one of the largest areas of organometallic chemistry. Despite the extensive research work done in other areas of organolead compounds, there has been scant attention to the electrochemical behaviour of organolead com-

103

283 compounds. Hein and Klein were the first to prepare hexaethyldilead by reducing triethyllead 285 bromide on a lead cathode. Costa showed that trialkyllead halides are reduced in two steps of one electron each in 30% isopropanol. Korshunov and Malyugina286 also observed two one-electron waves for triethyllead hydroxide. Dessy et al carried out electrochemical investigation on di- and triaryllead compounds in dimethoxyethane and postulated the following mechanism of reduction:

+e Ph Pb' Hg Ph Hg + Pb 3 2 Ph Pb+ 3 +2e Ph3 Pb

Triphenyllead cation takes up one electron to give Triphenyllead free radical which is incorporated into the electrode producing diphenylmercury. At higher negative potential the cation has been assumed to take up two electrons giving triphenyllead anion. Polarographic determination of triorganolead compou- 288 nds has also been carried out by Kochkin et a1 . More recently Colliard and Devaud289 carried out thorough investigation on the electrochemical behaviour of di- and triphenyllead compounds in aptus- alcohol medium. These authors observed some anomalies in the polarogram and to explain their results

104

postulated the following mechanism of reduction.

+e Ph P Ph Hg + Pb 3 2 +e H at -0.4V Ph2Hg + Pb Ph Pb+ 3 +4e,3114- > 3C6H6 + Pb +2e H + Ph2Hg + Pb

and -2e 2+ Pb El =-0.4V > Pb 2

All the reduction waves have been assumed to be due to direct reduction of triphenyllead cation resulting in the formation of the triphenyllead free radical at El of -0.1V vs. SCE and benzene at more nega- 2 tive potentials. The wave having half-wave potential of -0.45V has been found to be distorted by oxidation of metallic lead. It thus appears that there is disagreement amongst the authors about the mechanism of reduction of organolead compounds. Hence a thorough investigation of the electrochemical behaviour of these compounds was justified, primarily with the aim of developing a very sensitive specific method of analysis. 105

5.2. EXPERIMENTAL TECHNIQUE

Materials:

Triphenyllead acetate sample was supplied by Alpha Inorganics, Ventron, Beverly, Mass. Buffer solutions were prepared with A.R. grade chemicals. Triply distilled mercury was used for all polarographic measurements.

Apparatus:

All the polarographic measurements were carried TM out with a PAR Model 174 Polarographic Analyzer (Princeton Applied Research Inc., Princeton, N.J.). Capillary characteristics of the DME were: t = 3.31sec, m = 1.69 mg/sec in distilled water at a mercury reservoir height of 60 cm. The apparatus used for cyclic voltammetric experiments and for controlled potential coulometric experiments as well as the respective experimental methods adopted have already been described in the preceeding chapters.

5.3. RESULTS AND DISCUSSIONS

General Polarographic behaviour

A typical d.c. polarogram of triphenyllead acetate in 50% (v/v) ethanol and acetate buffer (pH 7.0) is shown in Figure 5.1. It is seen that there are two well-defined waves ( I and II ) 4.5

E ca. 0 0

4.0

0.2 V 3.5

E/V vs. SCE

Fig. 5.1. Drop-time curves; (0) 0.1M acetic acid/0.1M ammonia buffer (pH 7.0) in 50% (v/v) ethanol; (0) 1.68 10-4M triphenyllead acetate in the same buffer. Polarographic current-potential curves of the same TPLA solution. Start potential, -0.1V. 107 preceeded by two more waves one anodic and one cathodic, forming a S-shaped portion ( abc) on the i-E curve. Preliminary experiments showed that the waves were independent of time for at least two hours. Half-wave potentials of both the waves I and II have been found to shift towards more negative values with increasing pH but limiting currents of the waves have been found to be practically independent of pH (Table 5.1).

Table 5.1. Effect of pH on the half-wave potential and limiting currents of the polarographic waves of a 1.76.10-4M TPILA solution in 50% (v/v) ethanol.

Wave I Wave II pH of the El (Volts il(MA) El (Volts medium vs. SCE) vs. SCE)

1.42 0.3507 0.613 0.850V 0.950

4.10 0.425V 0.600 1.050V 0.900

7.0 0.425V 0.625 1.075V 0.900

9.3 0.400V 0.625 1.200V 0.925

13.4 0.575V 0.625 1,.200V 0.925 108

Table 5.2. Effect of solvent composition on the limiting current and half-wave potential of the polarographic waves of a 1.76.10-4M solution of TPLA in acetate buffer (pH 7.0).

Wave I Wave II

Ethanol content -El(Volts il(pA) -El(Volts il(PA) per cent by vol. vs. SCE) vs. SCE)

40 00.425 0.625 1.085 0.963

50 0.440 0.600 1.075 0.913

60 0.450 0.625 1.050 0.963

70 0.450 0.650 1.050 0.950

80 0.475 0.675 1.025 1.025

Wave I is a mixed oxidation/reduction wave which is seperated into an anodic and a cathodic wave in the alkaline medium ( Fig. 5.2). The limiting currents corresponding to wave I and wave II have been found to be linearly dependent on the square-root of the height of the mercury reservoir indicating that /*IA- are diffusion-controlled waves. Diffusion character of wave II was further substantiated by the characteristic shape of its i-t curve. Limiting current of the 109

Fig. 5.2. Polarographic current-potential curve of a 1.76 10-4M TPLA solution in 0.1M NaOH solution (pH 13.4). 110 mixed wave I has been found to increase with increasing drop-time. Height of the mercury reservoir has no significant effect on the S-shaped portion of i-E curve. Ethanol content of the medium has been found to have little or no significant effect on the polarographic waves (Table 5.2). Both wave I and wave II have been found to depend linearly on depolarizer concentration (Figure 5.3). The plot of E vs. log i/(id-i) has been found to be linear for wave II with a slope of 94 mV indicating that it is an irreversible wave. Irreversibility of this wave was also confirmed by cyclic voltammetric experiments.

Pulse and Differential Pulse Polarography

Typical pulse and differential pulse .polarograms of TPLA in acetate buffer (pH 7.0) containing 50% (v/v) ethanol are shown in the figures 5.4 and 5.5. It is seen from figure 5.4 that there are two well-defined pulse polarographic waves (I and II). Besides there is one rather unusual portion in the polarogram (abc) in the less negative potential region consisting of probably a cathodic and an anodic wave merged together and forming a cathodic and an anodic 'peak'. Pulse polarographic limiting currents of wave I and wave II have been found to depend linearly on concentration, but the anodic 'peak' height (Ia) has been found to be practically independent of 111

1.0 1.5 2.0 4 10 X concentration/ M

Fig. 5.3. Dependence of d.c. limiting current on concentration of TPLA. (1) wave I, (2) wave II. 112

0.2 V

E/ V vs. SCE

Fig. 5.4. Pulse polarogram of a 8.8 10-5M TPLA solution in 50% (tv/v) ethanol and acetate buffer (pH 7.0). Fig. 5.5. Differential pulse polarogram of the same TPLA solution. 113 concentration. There are three well-defined differential pulse polarographic peaks (Figure 5.5), one of which is anodic in nature. All the peak heights have been found to change little with changing concentration of TPLA, above ca. 1.0.10-4M but below this concentration peak II has been found to depend strictly linearly on concentration (Figure 5.6) while the peak height corresponding to peak I is linearly dependent on concentration below ca. 6.0.10-5M.

Controlled Potential Coulometry

Controlled potential coulometry at potentials -0.600V and at -1.200V vs. SCE using mercury pool cathode showed n-value of one and four respectively in three seperate determinations. Controlled potential coulometry at -0.325V vs. SCE under identical condition as before showed that electrolysis current increased cathodically at first, then decreased rapidly through zero to an anodic value which corresponds to the n-value of 0.5 only. It appears from this observation that the product of the primary reduction step undergoes subsequent chemical reaction/decomposition and electrochemical oxidation, the magnitude of the oxidation process being much greater than that of the primary reduction process. 114

2.5

2.0

1.5

1.0

0.5

5 10 15 105 X concentration/ M

Fig. 5.6. Dependence of pulse polarographic limiting current and differential pulse polarographic peak current on concentration of TPLA. (1) Pulse wave I, (2) differential pulse peak I. 115

Table 5.3. Coulometrically determined number of electrons consumed in the reduction of triphenyllead acetate in 0.1M acetic acid / 0.1M ammonia buffer (pH 7.0) containing 50% (v/v) ethanol.

Working electrode Potential of the n-value working electrode

Mercury pool -0.600V vs. SCE 1.05 0.99 1.09

-1.200V vs. SCE 3.99 4.01 4.04

Cyclic Voltammetry

A' typical cyclic voltammogram of a 1.76.10-4M solution of TPLA in acetate buffer (pH 7.0) containing 50% (v/v) ethanol is shown in Figure 5.7. It is seen that there are two well-defined cathodic peaks (I and II) and one well-defined anodic peak (III). Besides in the less negative potential region there is another cathodic peak (IV) pulled into the anodic region by the interfering larger anodic current.

116

The occurence of rather unusual portion (abc) in the cyclic voltammogram during the anodic sweep is not understood. Peak I is presumably due to the one-electron irreversible reduction of TPL+ cation producing triphenyllead free radicals.

Ph Pb+ +e > Ph Pio' 3 3

E/ V vs.SCE

Fig. 5.7. Cyclic voltammogram of a 8.8 10-5M TPLA solution in acetate buffer (pH 7.0) containing 50% (v/v) ethanol. Start potential, -0.0V; scan rate 200 mV/sec. 1 1 7

The free radicals produced at this step may undergo further reduction directly or they may react with mercury of the DME forming phenylmercury free radicals which may undergo further reduction to phenylmercury anions or phenyl carbanion and mercury metal286 in a subsequent step. Cyclic voltammetric peaks III and IV correspond to the d.c. polarographic waves Ia and Ib and may be due to the following 290 processto .

PA3 P-b+- +42- P24,3 Ph. ,,,e6 ov". PhHg+ +e PhHg* -e

The phenylmercury cations are produced initially by reaction with mercury according to the reaction

Ph Pb+ + Hg > Ph Hg + PhHg+ + Pb 3 2

Voltammetry at Glassy Carbon Electrode

A typical cyclic voltammogram of a 8.8.10-5M solution of TPLA in acetate buffer (pH 7.0) containing 50% (v/v) ethanol is shown in Figure 5.8. It is seen that there are two well-defined peaks one cathodic and one anodic. The peak ratio, i (cath)/ i (anod), being exactly 0.5. The peak heights have been found to depend on sweep rates. Heights of both the peaks have been found to decrease with increasing sweep rate. The slope of i / V vs. V plot has been 118

E/ V vs SCE

Fig. 5.8. Cyclic voltammog/am of a 8.8 10-5M TPLA solution in acetate buffer (pH 7.0) at a glassy carbon electrode. Start potential, -0.0V. Scan rate 200 mV/sec.

119 found to be negative which shows that the process is irreversible. The cathodic peak is therefore due to the irreversible reduction of TPL cation to triphe-. nyllead free radical.

+ +e Ph Pb > Ph Pb* Pb + 3C6H; 3 3

+H+ Ph6Pb or Ph Pb + Pb 3C 4 6H6

The triphenyllead free radical is not stable and decomposes to phenyl radical and lead metal, which is oxidized during the anodic sweep giving rise to peak II.

-2e Pb > Pb2+

A typical current-potential curve of TPLA in acetate buffer (pH 7.0) containing 50% (v/v) ethanol is shown in Figure 5.9. The small wave I in the d.c. voltammogram is probably an adsorption wave or the reduction wave of triphenyllead cation to the triphenyllead radical.

Mechanism of electrode process

The mechanism of reduction of TPLA in aq45us ethanol medium at a DME may be summared in the 120

Fig. 5.9. Current-potential curve of a 8.8 10-5M TPLA solution in acetate buffer (pH 7.0) at a glassy carbon electrode. (1) Forward scan, start potential, -0.0V; (2) reverse scan.

1 21

following scheme.

+Hg 1)Ph Pb+ -±2 4 Ph3Pb(ads)---- —)3PhHg-•+e >3P1;+ Hg 3 -Pb 13.05V

Ph Pb Ph Hg + Hg 3C H 6 2 2 6 6 and/or Ph Pb + Pb 4

2) Ph Pb+ + Hg > Ph Hg + PhHg+ + Pb 3 2

3) PhHg+ +e PhHg* -e

+2e 4) Pb2+ ` Pb(Hg) -2e

From coulometric evidences and from the pH-indepen- dence of the limiting current of wave I it is clear that wave I which is the normal reduction wave of TPLA corresponds to the process

Ph3Pb+ +e > Ph Pio' 3

From the drop time curve (Figure 5.1) it is known that Ph Pio' radicals are strongly adsorbed onto the 3 DME. Voltammetric experiments using glassy carbon electrode shows Ph Pip' radicals are unstable and 3 decomposes immediately after their formation. It is most likely that in presence of mercury it reacts with it 'producing phenyl-mercury free radicals 122 which eventually may undergo disproportionation into

Ph2Hg and metallic mercury or it may undergo further reduction giving rise to wave II of the d.c. polaro- abkkat ev, oti(Lut naL-atts: gram. Probably both the processesArun parallel, pi,3 p-b• 1".4- ^rmS Wave la is the adsorption prewave of the normal wave I. Wave Ia is greatly distorted by the oxidation wave of lead formed by the decomposition of the triphenyllead radical giving rise to the S-shaped portion portion in the i-E curve. The height of wave II has been found to be independent of pH in this investigation. This observation is contrary to that of 285 Colliard and Devaud . Furthermore, comparison of the results of the concentration dependence of the limiting current of wave II with that for wave I shows that this step is unlikely to involve four electrons as is postulated by these authors. Cyclic voltammetric experiments with an ordinary DME shows no appearence of wave II. But it appears with a HDME or with a DME having long drop time. This fact further disproves the postullation by Colliard and Devaud that it is a simple direct four-electron reduction process and leads us to postulate that it is due to the reduction of the products of a secondary chemical reaction which, in this case, is the formation of phenylmercury radical. The presence of Pb,Pb2 , Ph2Hg and PhHg+ species in the system has already been confirmed285 . 123

5.4. ANALYTICAL DETERMINATIONS

It has already been stated that both the d.c. waves I and II of triphenyllead acetate are linearly dependent on concentration over the range ca 10-3M to ca 1.0.10-6M (Figure 5.3). Hence these waves may be utilized for direct analytical determination of this compound. Pulse polarographic waves I and II are also linearly dependent on concentration over the range ca 2.0.10-4M to ca 5.10-7M (Figure 5.6). Hence either of these waves may be utilized for the analytical purpose. However it was observed.that the starting potential for recording the pulse polarographic waves has some definite influence on the shape of wave I as can be seen from Figure 5.10. It is clear from Figure 5.10 that for analytical purposes the best pulse polarograms can be recorded if and only if the initial starting potential is set at any point within the range, -0.22V to -0.304V vs. SCE under the present experimental conditions. The differential pulse polarographic peak II is linearly dependent on concentration of triphenyllead acetate over the range ca 1.0.10-4M to ca 1.0.10-7M while peak I is linearly dependent on concentration below ca 6.0.10-5M. A typical differential pulse polarogram of a 1.61.10-7M TPLA solution is shown in Figure 5.11. Initial reaction of triphenyllead acetate 124

1.011A

0.1 V

1 E/ V vs. SCE

Fig. 5.10. Pulse polarograms of a 1.61.10 M TPLA solution in acetate buffer (pH 7.0). Start potential (1) -0.30V, (2) -0.22V, (3) -0.14V. 125 with mercury has been found to have no significant effect on the accuracy of the determinations by the above methods.

Fig. 5.11. Differential pulse polarogram of a 1.61 10-7M TPLA solution in acetate buffer (pH 7.0) containing 50% (v/v) ethanol. (1) TPLA solution, (2) supporting electrolyte. 126

CHAPTER 6. ELECTROCHEMICAL BEHAVIOUR OF DIBUTYLLEAD DIACETATE AND ITS ANALYTICAL DETERMINATION AT ORDINARY AND AT SUBMICROMOLAR LEVEL.

6.1. INTRODUCTION

During recent years there has been considerable interest in the disubstituted organolead compounds because of their possible use as very effective insecticides, fungicide and bactericide 260,291-294 and as polymerization catalyst261,262. Both the aryl and the alkyl substituted lead compounds have been tested successfully for these purposes277,278,295. Dibutyllead diacetate has been shown_ to impart bacteriostatic and rotproofing properties to cotton fabrics296-298. It is also effective in killing such parasites as 'Hyme- nolepsis fraterna' and Raillietina Cesticillus' in 279 mice and chickens respectively . It is also suggested that dialkyl and diaryl lead chelates of EDTA are useful as metal sources in electroplating and in insoluble dental enamel300 .

Although other areas of organolead compounds have been studied rather throughly very little is known about the electrochemical behaviour of di substituted 230 lead compounds. Dessy et a1 studied the polarographic behaviour of diphenyllead salts in dimethoxyethane rather thoroughly and postulated the following 127

mechanism of reduction.

Ph2Pb (0Ac)2-1-2.--> Ph2Pb(OAc)-±-> Ph2Pb. Hg> Ph2Hg + Pb°

,287 Morris however claimed to have observed a two electron reduction wave of aquodiethyllead ion in aquous ethanol medium giving diethyllead which decayed rapidly by disproportionation to tetramethyllead and lead metal and by transmetallation to diethylmercury.

2+ +2e R2Pb aq R2Pb:

2R2Pb: > R Pb + Pb° 4

R2Pb: + Hg > R2Hg + Pb°

In a recent paper Colliard and Devaud289 showed that diphenyllead diacetate gives three or four polarographic waves in aquous methanol medium and the behaviour is quite complicated because of its reaction_ with mercury. It is therefore seen that electrochemical behaviour of diorganolead compounds is not quite clear. The present investigation was therefore undertaken to study the mechanism of the electroreduction of a typical lead compound, dibutyllead diacetate, primarily with the aim of finding out a suitable specific and direct electrochemical method of analysis of these compounds. 128

6.2. EXPERIMENTAL TECHNIQUE

Materials:

Dibutyllead diacetate was supplied by Alpha Inorganics, Ventron, Beverly, Mass. Buffer solution used in this investigation were prepared with A.R. grade chemicals.

Apparatus:

The apparatus used in this investigation and the respective experimental methods adopted have already been described in the preceeding chapters.

6.3. RESULTS AND DISCUSSION

General Polarographic Behaviour

Dibutyllead diacetate gives three well-defined waves in acetate buffer (pH 7.0) containing 50% (v/v) ethanol (Figure 6.1). Besides there is one rather ill-defined mixed cathodic/anodic wave in the less negative potential region. All the waves have been found to be practically time independent. Wave III has been found to be pH dependent wave height for the wave decreases with increasing pH of the medium. Limiting current of wave I has been found to change little with pH around pH 7.0 but in highly acidic (such as pH 1.42) or in highly alkaline (such as pH 13.4) medium it is splitted into two waves (Figure 6.2). All the waves have also been found to 129 it

4.5

4.0

3.5

E /V vs. SCE

Fig. 6.1. Drop time curves. (0) 0.1M acetic acid/0.1M ammonia buffer (pH 7.0) in 50% (v/v) ethanol; (0) 1.084 10-4M dibutyllead diacetate in the same buffer. Folarographic current-potential curves of the same DBLA2 solution. Start potential, -0.1V. 130

6 E SC E/ V vs.

Fig. 6.2. D.c. polarogram of 1.084.10 M DBLA2 solution in: 1) 0.1M HiSO/i(pH 1.42), 2) acetate buffer (PH 4.1), 3) acetate buffer (pH 7.0), 4) 0.1M NaOH (pH 13.4), 5) borate buffer (pH 9.3), Start pot., -0.3V.. 131

diminish greatly in borate buffer of pH 9.0. Half-wave potentials of these waves have been found to shift towards more negative potential region with increasing pH of the medium (Figure 6.2). Limiting current of wave I has been found to be linearly dependent on the square root of the height of the mercury reservoir indicating that it is a diffusion wave. Height of wave II is also linearly dependent on the square root of the height of the mercury reservoir. Dependence of limiting current of wave III on the height of the mercury reservoir could not be established properly. Diffusion character of waves I and II was further substantiated by the characteristic shape of their i-t curves.

Droptime curve of a 1.084 10-4M solution of dibutyllead diacetate in acetate buffer (pH 7.0) containing 50% (v/v) ethanol and that of a background solution is shown in Figure 6.1. It is seen that there is no indication of any adsorption onto the electrode surface

Height of wave I has been found to be linearly dependent on concentration over the concentration range studied while those of the other two waves ( II and III ) increase slowly with increasing concentration of the depolarizer (Figure 6.3). Effect of solvent composition on the waves showed that ethanol content has little effect on the shapes of wave II and III but height of wave II has been found to

132

0.6

__.

0.4

0.2

L I 1

1.1 1.5 2.0 104. concentrati on/ M

Fig. 6.3. Dependence of d.c.' limiting current on concentration of DELA2 solution. (1) Wave I, (2) wave II, (3) wave III. 133

increase slowly with increasing ethanol concentration of the medium. Wave I has been found to split into two in the medium containing more than 60% (v/v) ethanol (Figure 6.4). Height of the splitted wave is however very small compared to the parent wave.

Plot of E vs. log idi for wave I has been found to be linear with a slope of 62 mV indicating that it is probably an one-electron reversible wave. The reversibility of this step was further confirmed by cyclic viltammetric experiments. Controlled potential coulometry at potentials -0.600V and at -1.390V vs. SCE which correspond to the plateau of wave I and II respectively, showed n-value of unity for both the steps in three seperate determinations.

Cyclic Voltammetry

A typical cyclic voltammogram of a 1.084.10-4M solution of DBLA2 in acetate buffer (pH 7.0) containing 50% (v/v) ethanol at a HDEE is shown in Figure 6.5. It is seen that there are three well-defined cathodic peaks (I, II. and III) and two well-defined anodic peaks (Ia and I'a). Besides, there are two more anodic peaks (IV and V) which are rather very broad and not clearly defined. Peak I has been found to shift toward more negative potential region with successive sweeps for the first 3/4 sweeps. Probably dibutyllead ion reacts with mercury of the electrode giving some • 134

t.< 0

Fig. 6.4. Polarographic current- potential curves of a 1.084.104 M DBLA2 solution. Ethanol content, 1) 90%, 2) 80%, 3) 70%, 4) 60%, 5) 50% (v/v). Start potential -003V. 135 butylmercury cations, BuHg+, which are reduced during the first few sweeps (a-c). When reduction of this butylmercury ions is complete fresh dibutyllead ions diffuse towards the electrode surface and is reduced directly. The resulting ion-radicals are oxidized during the reverse sweep giving rise to the anodic peak Ia. Anodic peak I'a is probably due to oxidation elk 2, eb 14 2_ 2-+ , of the . . Peak III may be due to the reduction of Bu2Pb to Bu2PbHLand the anodic peaks ti IV aRmi.= a-e likely to correspond to the oxidation of Bm-t529itdE3t9.5.9.c=an-A of Bu2Pb to Bu2Pbt, ion-radicals. ras4pectively.

•

Fig. 6.5. Cyclic voltammogram of a 1.084.10-4M DBLA solution in acetate buffer (pH 7.0) containing 50% 2 (v/v) ethanol. Start potential, -0.OV; scan rate, 200 mV/sec.

136

Voltammetry and Cyclic Voltammetry at a glassy Carbon Electrode.

A typical i—E curve of a 1.08 10-4M solution of dibutyllead diacetate in acetate buffer (pH 7.0) containing 50% (v/v) ethanol is shown in Figure 6.6. It is seen that these are two well—defined waves. The reverse scan i—E curve shows two anodic peaks (III and IV). The anodic peaks were also observed when scan was reversed from —1.0V vs. SCE. Cyclic voltammetry at a glassy carbon electrode also shows two cathodic and two anodic peaks (Figure 6.7). The anodic peak III is clearly due tk.2, ebt 1:01-1,41-;LAh pla.,,K a .-, to oxidation ofAt4e metallic lead, Pb , which has been formed due to the decomposition of the dibutyllead + ion—radical, Bu2Pb or due to disproportionation of Bu 2 Pio* 'I A 1k " • Z (1„Lp.Bu2 Pb: to Bu2Pb2+. Cathodic peaks I and II are due 2+ to successive reduction of dibutyllead ion, Bu2Pb to dibutyllead, Bu2Pb:. Hence the mechanism of reduction of dibutyllead diacetate at glassy carbon electrode may be represented as

2+ +e + +e +n Bu2Pb Bu2Pb Bu2Pb ( Bu2Pb )n

(Bu2Pb.PbBu2) 2+ Bu Pb + Pb° 4 137

2.5/4A

0.1 V

Fig. 6.6. current-potential curve of a 1.08.10-4M MLA2 solution in acetate buffer (pH 7.0) containing 50% (v/v) ethanol at a G.C.E., (1) forward scan, start potential, -0.0V, (2) reverse scan. 138

10 PA

E / V vs SCE

0.3.0

0.2.0

0.10

1.0 1.5 2.0 104. concentration/M

Fig. 6.7. Cyclic voltammogram of a 1.08.10-4M DBLA2 soln. in acetate buffer (pH 7.0) containing 50% (v/v) ethanol at a G.C.E., 1) forward scan, start potential, -0.0V, scan rate, 200 mV/sec. Fig. 6.8. Concentration dependence of the pulse polarographic limiting'currents of DBLA2. 1) Wave Ia, 2) wave I, 3) wave II. 139

Mechanism of reduction. at the DME

From the above discussion it is possible to postulate the following mechanism of reduction of dibutyllead compounds at the dropping mercury electrode.

Bu Pb2+ +e Bu Pb+ Bu Pb. 114-'e Bu PbH 2 -e 2 2 2 2Hg + Pb° (Bu2PbPbBu2)24- Bu4Pb + Pb°

BuHg++e ; Bu4Pb + Pb2+

Dibutyllead cation takes up one electron and is +. reduced reversibly into the ion-radical Bu2Pb . The one electron nature of this process has been established by logarithmic analysis of wave I and is confirmed by controlled potential coulometry. Loga- rithmic analysis as well as cyclic voltammetric experiments also confirm the reversibility of the reduction process. The radical-ion produced in step I takes up another electron giving rise to d.c. wave II in an irreversible process. The resulting dibutyllead is then disproportionated into Bu2Pb and metallic lead, Pb or is incorporated into the DDT forming dibutyl- 287 mercury and metallic lead The diradical may also undergo further reduction into Bu2PbH giving rise to wave III of d.c. polarogram. All the last three 140 processes may occur concurrently in parallel.

The small splitted wave observed in highly acidic/ alkaline medium is probably due to the reduction of unionized dibutyllead compounds or some dibutyllead complexes. The great diminution of the wave heights in borate buffer (pH 9.3) are probably due to some interactions of these compounds with the buffer components, --- the borate buffer is known to be undesireable and sometimes notorious for its reaction with the depolarizer183.

6.4. ANALYTICAL DETERMINATION

D.C. Polarography

It has already been stated that limiting current of the d.c. wave I is linearly dependent on concentration and it can therefore be utilized for analytical determinations over the concentration range 2.5 10-4M down to ca 1.10-6M. The other two d.c. waves do not show appreciable change with change in concentration of depolarizer and hence are not suitable for analytical purpose.

Pulse and Differential Pulse Polarography

Pulse polarogram of a 1.08 10-4M solution of dibutyllead diacetate in acetate buffer (pH 7.0) and in 50% (v/v) ethanol shows three waves. Both the 141 second and third pulse polarographic waves are followed by minima which are pronounced in concentrated solution. Limiting current of wave III is also small compared to those of the other two. But all the waves are linearly dependent on concentration (Figure 6.8) over the con- - - centration region of 2.5 10 4M to 5.10 7M.

Dibutyllead diacetate also shows two well-defined differential pulse polarographic peaks (Figure 6.9) both of which are also linearly dependent on concentration below ca. 1.9.10-4M (Figure 6.10). Hence either of these peaks can be utilized for analytical determination. The lower limit of detection of this compound by differential pulse polarographic method has been - found to be 1.0.10 7M (Figure 6.11).

It has been found that initial reaction of dibutyllead diacetate with mercury of the DMB has no significant effect on the accuracy of analytical determinations. 142

1.01?

0.2 V

E / V vs.SCE

Fig. 6.9. Differential pulse polarogram of a 1.08.10-4M DBLA2 solution in acetate buffer (pH 7.0) containing 50% (v/v) ethanol. Start potential, -0.2V. 143

0 1 1 1.0 1.5 2.0 104 concentration/ M

Fig. 6.10. Concentration dependence of the differential differential pulse polarographic peak currents of DELA2 ;• (1) peak I, (2) peak II. 144

E / V vs. SCE

Fig. 6.11. Differential pulse polarogram of a -7 1.0.10 M DELA2 solution (peak I) in acetate buffer (pH 7.0) containing 50% (v/v) ethanol. Start potential, -0.3V. 145

CHAPTER 7. ETECTROCHEMICAL BEHAVIOUR OF TRIPHENYL- GERMANIUM BROMIDE AND ITS ANALYTICAL DETERMINATION.

7.1. INTRODUCTION

During recent years, interest in organogermanium chemistry has been increasing rapidly301-303, parallel to the fast-growing importance of organosilicon, organo-tin and organo-lead compounds, because of their 304-306, possible use as PVC-stabilizers as active ingredients in hydrolysis-proof coatings307, in photo- conductive layers308, in the treatment of cancer 309 cells and in synthetic organic chemistry310. It has been found that some organogermanium compounds, when added in a very small proportion to the filaments of polyamide, polyacrylonitrile or polyethylenetereph- ' thalate, increase their heat stability and reduce the collection of electrostatic charges by the fibres311 Some organogermanium compounds have also been found to be successful in promoting the growth of rice312 .

Although electrochemical behaviour of other organometallic compounds of group IVA have been 86'185,232,313 studied rather extensively there are only few publications in organogermanium electro- chemistry230,314-318

The first electrochemical investigation of organogermanium compounds was carried out by Foster. 146

and Hooper314 who produced triphenyl-germane and hexaphenyldigermane by electrolyzing triphenylgermylsodium in liquid ammonia at a mercury/platinum anode. Further electrochemical investigation of organogermanium compounds were also carried out by Allred et a1315,316 230 Dessy and co-workers carried out the most comprehensive study of organogermanium compounds in 0.1M tetrabutyl ammonium Perchlorate/1,2 dimethoxy-ethane • and suggested the following mechanism of reduction of triphenyl-germanium halides

Ph GeC1 Ph3Ge* sole. Ph3GeH 3

Tyurin et a1317 studied the electrochemical reduction of triethylgermanium bromide and observed a diffusion controlled wave in 0.1M lithium chloride/dimethyl- sulfoxide while in 0.1M lithium chloride/dimethylformamide they also observed a wave which had some kinetic complications.

In a recent paper Boczkowski and Bottei318 have described results of their thorough investigation of the electrochemical behaviour of triphenylgermanium halides in 1,2 dimethylformamide. These authors found that the reduction of triphenylgermanium fluoride is a pseudo-reversible process involving the transfer •of only one-half a Faraday per mole and postulated the following mechanism of reduction. 147

Ph3GeF Ph GeF (Ph3GeF) (Ph3G7-GePh3)- 3 —a F F

Addition of water or alcohol to the system caused the appearance of a new polarographic wave, which they consider to involve the intermediate. The electrochemical behaviour of triphenylgermanium chloride and triphenylgermanium bromide has been found to be very much the same and the following reduction mechanism was proposed for both the compounds.

Ph3GeX ----÷+e rn3u-e. DME Ph3GeH

V Ph GeGePh 3 3

The first step is an irreversible one-electron transfer resulting in the formation of triphenylgermyl radical,

Ph3Ge* which has not been found to give any oxidation peak in its cyclic voltammogram. It has been suggested that the radical has undergone rapid chemical reaction. The triphenylgermanium iodide reduction exhibited both a kinetically controlled and an irreversible diffusion controlled process. These authors made no attempt to explain the'rather unusual polarographic wave shape of the kinetic process'. They however, proposed a tentative mechanism of reduction for triphenylgermanium iodide. 148

Normal reduction: Ph GeI-112-4 Ph Ges + I 3 3

Kinetic Process: Ph3GeI + H2O(Ph3GeIH)-1- + 0H

(Ph3GeIH)+ Ph3Ge* + I + H2O

The last cathodic process has been assumed to be the normal reduction process, while a protonated germanium species was suggested as the species reduced during the first step. These authors also observed 'consistently greater value' of the number of Faradays required per mole during controlled potential coulometry. Anodic processes were also observed for all the compounds. These were interpreted as being mercury dissolution process resulting in the formation of a mercurous halide salt and the germanium ion, Ph3Ge'.

In view of the present state of electrochemistry of organogermanium compounds it was thought proper to undertake a thorough investigation of the electrochemistry of organogermanium compounds.

7.2. EXPERIMENTAL TECHNIQUE

Materials:

Triphenylgermanium bromide has been supplied by Alpha Inorganics, Ventron,Beverly, Massachusetts, and was used without further purification. All other reagents and chemicals used were of A.R. grade. 149

Apparatus:

All the apparatus used in this investigation and the experimental methods involved have already been described in the preceeding chapters.

7.3. RESULTS AND DISCUSSIONS

General Polarographic Behaviour

Triphenylgermanium bromide has been found to give two reduction waves in acetate buffer at pH 7.0 (Figure 7.1). The second wave has a rather unusual shape having a peak which could not be suppressed by surface active agents such as gelatin or triton X-100. Both the waves have been found to be time-independent. Limiting current of d.c. wave I has been found to be linearly dependent on concentration (Figure 7.2). Half-wave potential values of this wave has been found to shift consistently towards negative values with increasing pH. A limiting current (wave I) vs. pH curve is shown in Figure 7.3. The second wave has been found to disappear in alkaline medium while it has been found to rise rapidly with decreasing pH in acidic medium. Effect of ethanol concentration on the limiting current of wave I is shown in Figure 7.4. Half-wave potential values of wave I has also been found to shift towards more negative values with increasing ethanol concentration in the medium (Figure 7.5). Wave II has been found to decrease rapidly with 150

4.0

3.5

E/V vs. SCE

Fig. 7.1. D.c polarogram of a 4.90.10-4M triphenylgermanium bromide in acetate buffer (pH 7.0) containing 50% (v/v) ethanol solution. Start potential, -0.4V.0) Drop time curves of the same solution and of the supporting electrolyte (0). 151

3.0 5 4.0 5.0 10 . concentration / M

Fig. 7.2. Concentration dependence of the first d.c. polarographic wave. 152

1.5

1.0

2 6 pH 10 14

Fig. 7.3. pH-dependence of the limiting current of wave I. 153

40 6 0 so

Fig. 7.4. Dependence of the limiting current of wave I on the ethanol content of the medium. 154

120

al (..) Li) vi).

1.10

(NI Lir 1

100

30 50 70

Fig. 7.5. Dependence of the half—wave potential of wave I on the ethanol content of the medium. 155

increasing ethanol concentration and finally diminish to a very small wave in solutions > 80% ethanol. Only one small rather drawn out wave was observed in other non-aquous solvents such as dimethylsulfonide, acetonitrile etc. An electrocapillary curve is shown in Figure 7.1. It is seen that there is considerable lowering of surface tension in the potential range of -0.10 V to -1.5V vs. SCE indicating strong adsorption. The adsorption character of wave I was further confirmed by the linear dependence of its limiting current on height of the mercury reservoir and the characteristic shape of its i-t curves. Plot of E vs. log id/ (id-i) has been found to be linear for wave I yielding a value of 60 mV which corresponds to a reversible one-electron process.

Controlled Potential Coulometry

In order to determine the number of electrons involved in step I exhaustive controlled potential electrolysis of a 2.0.10-4M solution of triphenylgermanium bromide was carried out using a Hg pool cathode in acetate buffer (pH 7.0) at -1.20V vs. SCE which was well on the limiting current plateau of polarographic wave I in this medium. But contrary to previously 318 published report we observed that the electrolysis current decreases to a limiting value which is well above the background current. We allowed to continue electrolysis for about two hours (normal electrolysis 1 56

time, 30 minutes) and then polarographed the resulting solution and compared the polarogram with that of an unelectrolysed solution. It was found that wave I diminishes but it does not disappear completely after prolonged electrolysis, which indicates regeneration of the depolarizer from the products of reduction. We also noticed large increase in magnitude of wave II after prolonged electrolysis. The approximate n-value for the first step determined by coulometric experiments has been found to be 1.1. •

Cyclic Voltammetry

Cyclic voltammetric experiments were carried out to determine the reversibility of the electrochemical processes. A typical cyclic voltammogram of a 2.45.10-3M solution of triphenylgermanium bromide in 50% (v/v) ethanol at pH 7.0 is shown in Figure 7.6. It is seen that it gives two reduction peaks during the cathodic sweep and two anodic peaks during the anodic sweep. Peak I corresponds to the reduction of triphenylgermanium bromide to triphenylgermyl radical, Ph Ge. 3 and peak II probably corresponds to the simultaneous reduction of the protonated species, Ph3Gee,'to triphenylgermane and to the normal reduction of triphenylgermanium bromide. Peak III and peak IV probably correspond to the oxidation of triphenylgermane to hexaphenyldigermane and to triphenylgermanium 157 cation respectively.

E / V vs. SCE

Fig. 7.6. Cyclic voltammogram of a 2.45.10-5M triphenylgermanium bromide solution in acetate buffer (pH 7.0) containing 50% (v/v) ethanol solution. Start potential, —0.0V, scan rate, 200 mV/sec. 158

Mechanism of reduction

It has been stated that only one small wave of triphenylgermanium bromide is observed in the nonaquous solvents. The wave corresponds to the reduction process

Ph GeBr +e > Ph3Ge' + Br 3

In presence of water, however, an adsorption prewave (2, appears which shows that in aqupus-organic media the free-radical, Ph Ge', is strongly adsorbed onto the 3 electrode surface.

Logarithmic analysis of wave I shows that it probably correspondsto a reversible one-electron reduction process. However cyclic voltammetric experiments show no sign of reversibility. This may be due to the fact that the free radical has undergone rapid chemical reaction. The free radical, Ph Ge', is strongly 3 adsorbed onto the electrode surface and rapidly takes up a proton in the adsorbed state or it may undergo rapid dimerization. The pH dependence and the water dependence of limiting current of wave I are shown in Figures 7.3 and 7.4 respectively. These curves resemble the characteristic curves for processes in which subsequent reaction takes place with the formation of an electroactive product319. It is therefore concluded that the free radicals formed in step I rapidly combines with a proton according to 1 59

Ge') ads + 3GeH)+ ads. (Ph3 pH 7.0 (Ph

or, (Ph3Ge.) ads + H2O > (Ph3GeH)-1- ads.

the protonated species being reduced at more negative potentials at which normal reduction of triphenylgermanium bromide takes place. The shape of the second wave is typical of catalytic hydrogen evolution wave319 The catalytic character of this wave was further substantiated by its dependence of droptime -- it decreases rapidly with decreasing drop-time and disappears completely at drop-time of 0.5 sec. It can be concluded from this observation that the second step is mainly due to the reduction of the protonated species according to

Ph3GeH+ +e Ph3GeH which is actually a catalytic hydrogen-evolution process, the catalyst being the radical, Ph3Ge'.

Besides these reduction processes an anodic wave is observed in the polarogram as well as in the cyclic voltammagram (peaks Va and Vb) and this corresponds to the mercury dissolution process.

e + Ph3GeBr Hg Ph3Ge + T Hg2Br2 160

The mechanism of electrode processes may be summarized in the following scheme:

protogenic// aquOus -alkaline Ph Ge+ solvent medium 3 or, Ph3GeBrj-±! Ph3 G6 + +H > Ph GeH+ +e Ph GeH pH 3 3 -e +Hg Ph3GeGePh3

I -e, 4.1-1g2Br2 + Ph3Ge+ -2e, -H+

The compounds triphenylgermane, hexaphenyldigermane and triphenylgermanium hydroxide have been isolated and identified by infrared spectroscopy after exhaustive controlled potential experiments318.

7.4. ANALYTICAL DETERMINATION

D.C. Wave I may be utilized for analytical determinations over the concentration range 0.50.10-3M down to 5.0.10-6M. Limiting current of the pulse polarographic wave I also depends linearly on concentration (Figure 7.7). Hence this wave may be utilized for analytical determinations. The lower limit of detection of the pulse polarographic wave I has been found to be 1.0.10-7m . Differential pulse 161

polarogram of triphenylgermanium bromide shows two peaks. Both the peaks increase with increasing concentration of depolarizer. Unfortunately, the concentration dependence of the peaks is not linear and hence are not suitable for analytical determination (Figure 7.8).

0.20

0.15

0.10

0.0 5

2.5 3.5 4.5 10 5 X concentration /M

Fig. 7.7. Concentration dependence of the pulse polarographic limiting current (I). 162

0.40

0.30

0.20

0.10

2.5 3,5 4.5 10 5 X concentration/ M

Fig. 7.8. Concentration dependence of the differential pulse polarographic peak current. (1) Peak I, (2) peak II. 163

CHAPTER 8. THE ANALYSIS OF ORGANOTIN COMPOUNDS BY HIGH PRESSURE LIQUID CHROMATOGRAPHY USING A VOLTAMMETRIC DETECTOR SYSTEM.

8.1. INTRODUCTION

Analysis of organotin compounds by paper chromato- graphy320/ Reversed phase chromatography321 by Thin Layer Chromatography322-324 and by Gas-liquid Chromato- graphy325 have been reported. Paper Chromatography is based on the partition between the solvent and adsorbed water phase within the cellulose matrix. Low possible loadings, longer time of analysis, collection of seperated components and difficulty in quantitization are the main disadvantages of this technique326. The same is true for thin layer chromatography using silica, kieselguhr or cellulose as the supporting material327 Gas-liquid chromatography, although faster and much more convenient, has some inherent disadvantages with the requirement for formation of volatile derivatives because many organometallic compounds are insuffi- 00-1- ciently volatile or arn,61- decomposed under the conditions 328,329. of GLC seperations Modern (high-pressure) high-speed liquid chromatography, however, offers possibility for rapid seperation yielding both qualitative and accurate quantitative data. There are several potential advantages of the HPLC technique compared 164 with other chromatographic techniques. It provides higher speed, greater seperating power, and convenience than classical LC methods. Compared with TLC it permits measurements with higher reproducibility and resolution and with better sensitivity. Compounds are rarely degraded by this method as there is no necessity for prepa- ring volatile derivatives. Moreover there is additional advantage of availability of two chromatographic phases for the selective interactions with sample components, as compared with only one in GLC. A greater variety of seperation methods does exist in LC (such as Liquid- Liquid, Liquid-Solid, ion-exchange, and exclusion chromatography) which permit a much wider variation of selective interactions and a greater possibility of achieving the high resolution33°-333 power. It also offers the use of a number of unique detectors which have found limited or no application in GLC. In the present investigation an attempt will be made to seperate some organotin compounds by HPLC method using a novel electrochemical detector the glassy Carbon Wall-jet electrode assembly recently developed in this Laboratory18.

8.2. THEORETICAL PRINCIPLES

During the last five years there has been considerable advances in the theory of modern high-performance liquid chromatography. This has led to better understanding of the basic principles involved and 165

better optimisation of the various parameters resulting in highly efficient seperations. Several factors such as column resolution, chromatographic sampling technique, detection, analytical calibration procedure, which can be experimentally controlled, have a significant effect on the sensitivity , accuracy and reproducibility of LC analyses.

Column resolution.

The resolution (Rs) obtained in LC is expressed by the equation

1 R (a - 1) N7 s 1 + k

where, 0( is the seperation selectivity, N is the number of theoretical plates and k' is the capacity factor. All these parameters are effectively independent and can be- therefore be varried and optimised seperately. The seperation selectivity, c( , is varied by changing the composition of the moving and/or stationary phases. The number of theoretical plates, N, which measures the seperation efficiency is normally varied by changing the column length, L, particle size, dp, or mobile phase velocity, u. The capacity factors, k', can be varied by changing the eluting power of the mobile phase. By appropriate manipulation of the above parameters ( a, k' and N) it is possible to enhance the 166 sensitivity of the measurement, while still maintaining adequate resolution. Unfortunately, the determination of the conditions for a desireable change in o( is often difficult and a trial and error approach is necessary. Rules for predicting changes in 0( for LC have not yet been fully systematised, although some useful guidelines are available.

The sensitivity of HPLC analysis is influenced by the number of theoretical plates of the column more efficient columns, that is, columns with a larger number of theoretical plates, mean sharp peaks. Increa- sed N also results in greater resolution. Highly efficient columns of less than 10/um particles are particularly useful in providing both high resolution and larger peak heights334-336. The number of theoretical plates of the LC column is affected by the mobile phase velocity, u. Sensitivity of analyses therefore depends on the mobile phase velocity. It has been shown that peak height (h') varies with the velocity of the mobile phase according to the following equation,

h' = Z' a-u/2

Lower mobile phase velocity should therefore be used for high sensitivity.

Optimisation of k': The sensitivity of HPLC analyses is very much affected by the capacity factor k' of the solute peak. Generally k' values of about 1 to 10 are 167

optimum in terms of resolution, seperation time and band detection. Seperations involving k' > 10 produce long seperation times with excessive band broadening and decreasing peak height. Therefore, k' should be adjusted to obtain maximum peak height with adequate resolution. This is usually done by changing the eluting power of the mobile phase.

Sampling technique

Ideally, sample injection should be as a plug or sharp band which displaces the mobile phase at the column inlet. Unwanted mixing between the sample and the mobile phase prior to delivery of the sample to the head of the column should be avoided by using good injection system such as micro-sampling valves or by using proper column-switching techniques. Mixing results in decreased concentration. and reduced peak heights. Again if too large a sample mass is injected at the column inlet, the front of the column can be overloaded. This results in lower column efficiency and resolution, with resultant decrease in sensitivity.

Detection of resolved components:

For accurate quantitative HPLC analysis particu- larely at trace lavel it is first necessary to identify the expected component in a sample. This can be done by comparing the chromatographic retention times of observed peaks with those for authentic standards of expected 168

components. However absolute identification can only be done by trapping the fraction giving rise to the peak of interest and examining it by suitable supple- mentary techniques.

Detector sensitivity and selectivity are very important for developing sensitive and accurate analytical method. Detector sensitivity is dependent on the absolute sensitivity for a component as well as on the base-line noise level under the operating condition, the resolution of the chromatographic system and on other variables such as k', N, etc. Sample components which overlap or mask the component of interest can be ignored by certain selective detectors including polarographic detectors which do permit the selective detection of a variety of chemical species at very high sensitivity.

8.3. POLAROGRAPHIC AND OTHER VOLTAMMETRIC DETECTORS

Kemula and coworkers337 were the first to use DNE for monitoring the LC column effluents. He termed this technique as chromato-polarography. According to this technique a constant voltage is applied to the detector and the current resulting from either the reduction or oxidation of the solute is monitored as a function of time to give a chromatogram. This device should therefore be more accurately called an 169

338 amperometric detector. More recently, Huber , and and Stillman and Ma339 have developed modified, mini- turised amperometric detector system for use with high resolution chromatography.

Despite the several advantages of DYE such as reproducibility of the electrode surface and large cathodic range this device has inherent mechanical limitations as a sensor for continuous analysis and also has a limited anodic potential range which pre- cludes its use as a sensor for electro-oxidizable organic compounds. Use of solid electrode sensors have therefore been tried and several detector design proposed and tested. Joynes and Maggs340 used carbon impregnated silicon rubber membrane electrode while Macdonald and Duke341 used three-pin gold electrode and also a dual wax-impregnated carbon cell. Adams et a1342 have described the design of a flow cell detector using carbon paste electrode and applied it to the analysis of biogenic amines at the pico-gram level. More recently, a novel design of flow cell detector using glassy carbon as the working electrode material has been reported343. The flow cell is based on the Wall-jet electrode principles first described by Yamada and Matsuda344. A brief description of the wall-jet principles, details of the electrode design and its unique characteristics as a highly sensitive high resolution chromatographic detector will be described in the following sections. 170

Wall-jet electrode principles

An wall-jet can be termed as the flow of any fluid due to a jet spreading out over a plane surface. either radially or in two dimensions. Glauert345 first investigated the fluid dynamics of the wall-jet and solved the boundary layer equation and the velocity distribution across the jet. The wall-jet principles was first applied in electrochemistry by Yamada and Matsuda by allowing a jet of solution coming out of a circular nozzle to impinge perpendicularly on a disc electrode and setting up the counter and the reference electrode at an appropriate position remote from the wall-jet. A schematic representation of the wall-jet situation and the velocity and concentration distribution in an wall-jet electrode at the limiting diffusion current is shown in Figure 8.1. For such an wall-jet cell system, Yamada and Matsuda has shown that the limiting diffusion current is given by

7 iv (1.60K) n F C° D3 - 5/12 Ili a i- R-- where iv, the limiting diffusion current. y = kinamatic viscosity, D = diffusion coefficient, U = volume flow-rate, a = diameter of the nozzle, R = radi s of the disc electrode, o C concentration of the electroactive species, K = a constant.

171

x Electrode

Fig. 8.1. Schematic representation of the velocity and the concentration distribution in the wall jet. 172

It is seen from this equation that the limiting diffusion current in an wall-jet electrode is directly proportional to the concentration of the electroactive species which forms the basis of its application to analytical determinations.

The Design of the Wall-jet Cell

A schematic diagram of the wall-jet cell is shown in Figure 8.2. In this design the inlet solution is introduced through nozzle (1) and it inpinges perpendicularly on the disc electrode plane. The solution then exits to waste via the tubular platinum counter electrode. zG.E.

t •••••,%,...1 G.C.E.

REF.

Fig. 8.2. Wall-jet electrode detector for HPLC. 1 73

A saturated calomel reference electrode is connected to channel (4) via a 'Vicor' porous glass plug. The body of the disc electrode is threaded so that it is possible to adjust suitably the distance (d) between the nozzle tip and electrode disc.

Merits of the Wall-jet Cell as a HPLC detector

It has been established that the wall-jet cell has some unique charateristics as a HPLC monitor. It is highly sensitive, highly selective and is capable of detecting a wide range of compounds down to picogram levels because of its wide operating voltage range particularly in the anodic region. The cell has a very low and easily adjustable cell volume (= 0.5/4-L) and is robust and capable of easy maintenance. It is also remerkably free from problems of non-reproducibility caused by adsorption effects, most commonly encountered with other solid electrode systems. In fact, WED seems to be the most promising universal HPLC detector system. 174

8.4. APPLICATION OF THE WJED TO THE ANALYSIS OF ORGANOTIN COMPOUNDS BY HPLC METHOD.

8.4.1. EXPERIMENTAL TECHNIQUES

Apparatus:

A schematic diagram of the experimental set up is shown in the Figure 8.3. The high pressure pump used is a Laboratory made piston pump based on the Haskell design. This is operated directly from a nitrogen cylinder and produced a completely pulse-free eluent-flow. The nitrogen gas, purified by passing it through activated BASF deoxygenator packed in a copper tube, was used to deoxygenate the eluent in the reservoir and also to purge the pump by manipulation of the two-way top.

The LC column, made of seamless stainless steel tubing was 30 cm. long having id of 3 mm. and was slurry-packed to a 10,000 psi silica (sedimented) of o . 5 ht,, diameter.

A 100M-L micro-syringe (S.G.E., Australia) was used for injecting all the samples.

The monitoring system consisted of the glassy carbon wall-jet cell already described, connected to a PAR Model 174 Polarograph. It was also connected to the exit of the LC column by a short teflon tubing. 175

Injector

deoxygenator

Eluent R eservoir olumn

Pump

DETECTOR

POLAROGRAPH

Fig. 8.3. A schematic diagram of the experimental set up for chromatography. 176

Materials and Method

All the solvents and reagents were analytical reagent grade and were used without further purification. Aquous organic solvents containing supporting electrolyte were always used as the eluent. Samples for analysis were dissolved in methanol. Between 10-50/u-L was injected into the top of the column packing through a septum for each elution.

8.4.2. RESULTS AND DISCUSSIONS

Choice of the Electrode potential

It has been reported earlier that tributyltin and dibutyltin compounds are reduced at the DME giving rise to well-defined polarographic waves under different experimental conditions. Preliminary experiments with glassy carbon electrode showed that these compounds also give well-defined reduction waves at the GCE (Figure 8.4). Hence it should be possible to monitor these compounds amperometrically by using the glassy carbon wall-jet electrode. From the current-potential curves of the organotin compounds a suitable potential (-1.0V vs. SCE under the present experimental condition) which corresponds to the plateau of the voltammographic wave of all the compounds under investigation was selected and held fixed during operation. 177

--I ---1 1

I,.. -4. .

I i I 0 i •0 -r- ! j f . f.-

4 1.-

1.0P-A If

0 ft)

1 iJT02 V 0

0 E/V vs. SCE

Fig. 8.4. Current potential curve of 1. 32.10-5M solution in 0.1M KNO DBTL2 3 containing 50% (v/v) methanol at a glassy carbon electrode. Start potential, -0.0V. 178

The mobile Phase

It has been reported that organotin compounds are best seperated by thin layer chromatography using methanol IN hydrochloric acid (3+1) and butanol acetic acid (97+3) eluent systems. Hence it is expected that the same can be achieved by HPIC but with greater accuracy, higher precision and shorter seperation time. These eluents were therefore first tested. But unfortunately it was found that the wall-jet electrode cell is not suitable for monitoring the compounds using these eluent systems as hydrogen gas evolution takes place at the potential of even as low as 70.4V. Moreover after a few minutes of operation it has been found that highly polished glassy carbon electrode surface becomes covered with some grey deposits probably of tin metal.

Methanol-Acetate buffer mixture of different pH was next tested. But preliminary experiments showed that GCE is also not suitable for use with these systems because of hydrogen evolution over the potential range at which organotin compounds give reduction waves. However, methanol-1M KNO (1:1) mixture has been found 3 to be a good eluent for these compounds.

Seperation of Organotin Compounds

Seperation of TBTO and dibutyltin laureates/ maleates was achieved by using 1:1 methanol-1M KNO3 179 solution mixture. Two well-defined peaks have been observed (Figure 8.5). The peaks were identified by comparing the chromatographic retention times of the peaks due to the pure components determined in separate experiments. It is seen from Figure 8.5 that TBTO is eluted before the dibutyltin compounds. In liquid- solid chromatography non-ionic components are eluted faster than the ionic components. In aq4us-methanol solution the dibutyltin compounds are ionized according to

2+ Bu,SnX,c Bu,Snc + 2X

While the TBTO molecules are hydrolysed to tributyltin hydroxide according to

(Bu3Sn)20 + H2O 2Bu3SnOH

Hydrolysis is then followed by ionization of the hydroxide

+ Bu3SnOH Bu Sn + OH 3

Formation of Bu Sn+ ions is thus preceeded by a hydro- 3 lysis step, which is a slow process, while formation 2+ Bu2Sn ions involves no such slow preliminary stage. Hence it is very likely that TBTO molecules would be eluted faster than the dibutyltin compounds.

It was however observed that DBTL 2 and DBTM2 180

cc 0

0 co

0 2 min

time /min

Fig. 8.5. Chromatographic seperation of TBTO and DBTL2. Column: Silica Eluent: Methanol- 1M KNO3 (1:1 ) Potential of the working electrode: Flow rate: 1.25 ml/min. 1 81

could not be seperated from the mixture by the HPLC method under the present experimental conditions. A typical chromatogram of a mixture of the two compounds and that of pure DBTL2 are shown in Figure 8.6. It is seen that the chromatograms are identical in shape. The second peak in the chromatogram may be due to the 'slow elution' of the ionic components of the organotin compounds.

Problem of Quantitation

It has already been stated that glassy carbon is not suitable for use as working electrode in acidic medium because of its low hydrogen overvoltage. This electrode can however be successfully used in neutral solutions at high negative potential region of even -1.5V vs. SCE. Any monitoring system to be used for quantitative purpose, must give highly reproducible peak height. From preliminary experiments it has been found that the volume of the WJED compartment has a significant effect on the reproducibility of the chromatographic peak height. Theoretically, only ultralow volume of eluent is desirable and should be sufficient for working with the WJED system. And this is one of the attractive features of the WJED cell. We therefore tried to record chromatograms using WJED under "thin layer" condition. This was done by pushing the glassy carbon disc electrode as far as possible into the interior of the cell and recording the chromatogram 182

DBTL2+ 13131-M2

2min

time

Fig. 8.6. Chromatogram of a mixture of DBTL2 and DETM2 and of pure DETL2. 183

as usual. Some of the typical chromatograms are shown in Figure 8.7. It is seen that under this condition peak heights are not reproducible—it diminishes with successive injections. Two factors may be responsible for this irregularity the adsorption of the reduced products at the electrode surface and secondly the obstruction to the depolarizes to reach the electrode surface by the 'outflowing' eluent which may be termed as the 'washing out effect'. Larger detector volume can however be used. But this is not sensitive enough to serve any useful purpose in highly sensitive high- performance liquid chromatography. A detector volume of about 0.5 ml. has however been found by trial and error method to be sufficiently sensitive and able to give reproducible peak heights under the present experimental condition. The reproducibility of the chromatograms is shown in Figure 8.8. It is to be noted that the chromatograms were recorded without polishing the electrode surface after every run. This shows that adsorption problem is minimun under this condition.

8.4.3. CONCLUSION

It is clear from the above discussions that the long-held view that glassy carbon is an ideal inert electrode, usable at sufficiently high negative poten-

tial region under any condition is no longer true That it "fails" miserably in acidic solutions like 184

Fig. 8.7. Chromatograms of DBTL2 after successive injections under "thin layer" condition. Fig. 8.8. Chromatograms of DBTL2 after successive injections. 186 other carbonaceous electrodes has been shown. Hence it is not suitable as electrode material for WJED cell to be used in conjunction with HPLC working in the acidic medium.

Secondly the problem of surface adsorption is also present sometimes causing serious trouble in quantitative chromatographic determinations. 187

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PUBLICATIONS.

1. Electrochemical behaviour of organotin compounds. Part I. Application to the determination of trialkyl- and triaryltin derivatives, B. Fleet and N.B.Fouzder, J. Electroanal. Chem., 63(1975)59-68

2. Electrochemical behaviour of organotin compounds. Part II. Dialkyltin dicarboxylates, B. Fleet and N.B.Fouzder, J. Electroanal. Chem., 63(1975)69-78.

3. Electrochemical behaviour of organotin compounds. Part III. Dialkyltin mercaptocarboxylates, B.Fleet and N.B.Fouzder, J. Electroanal. Chem., 63 (1975) 79-85.

4. Electrochemical behaviour of organolead compounds. in aqueous ethanol medium. Part I. Triphenyllead acetate, B. Fleet and N.B.Fouzder, J. Electroanal. Chem., in press.

Electrochemical behaviour of organolead compounds in aqueous ethanol medium. Part II. Dibutyllead diacetate, B.Fleet and N.B.Fouzder, J.Electroanal. Chem., in press.

6. Electrochemical behaviour of triphenylgermanium bromide., B.Fleet and N.B.Fouzder, J.Electroanal. Chem., in press.

7. Analysis of organotin compounds by high pressure liquid chromatography using a voltammetric detector system., B.Fleet and N.B.Fouzder, ready for publication.