C T IR 0 A CA CH E M I C A A C T A 41 (1969) 15

CCA-530 541.18.041 :546.42.226 Original Scientific Paper Precipitation of Sulphate in Aqueous and Mixed Media of Different Dielectric Constants* M. Slovene and B. TeZ:ak Laboratory of Physical Chemistry, Faculty of Science, University of Zagreb, Zagreb, Croatia, Yugoslavia

Received July 1, 1968 The influence of on the precipitation of strontium sul­ phate was [nvestigated in aqueous solutions in statu nascendi. Sol formation and kinetics of precipitation were investigated by the tyndaUometric technique, as well as by the electrophoresis method (determination of the charge sign by means of an ultramicroscope), and by measuring pH values. The results of investigations are given in from of general precipitation diagrams. Concentrational regiO'Ils as well as concentration ratio of Na2S04 to Sr(N03h of precipitat­ /non precipitation Limits show the limited area where the con­ stant donic product may be applied. Outside of this area there is presumably a formation of ionic associates. Lowering of the dielec­ tric constant and increase of the ethanol content results in (1) de­ crease of of solid phase and (2) change in kinetics of pre­ cipitation. INTRODUCTION

The mechanism and kinetics of p recipitation of slightly soluble salts can be considered as composed of several elementary processes: formation of com­ plex species, embryonation, nucleation, direct growth resulttng in formation of particles having their colloid individuality and, finally, secondary aggrega­ tion and/or crystal growth1• In this sense genotypical and phenotypical cha­ racteristics of strontium sulphate precipitation have been iinvestigated both in aqueous solutions and in water-ethanol mixtures. Previous studies on the in rather narrow concentration SrS04 precipitation have been mainly made ranges2-1o in the vicinity of the equivalence point. In the present paper the range of concentrations has been extended to determine the precipitation boundaries. EXPERIMENTAL All measurements were performed by the turbidimetric method using a Zeiss tyndallometer joined to a Pulfrich photometer. The method has been described earlier11• Chemicals used were of analytical reagent quality and were not purified further: (Carlo Erba S.A., Milano), sodium sulphate (E. Merck, Darmstadt) and ethanol abs. (Kemika, Zagreb). Investigated precipitation systems were prepared by mixing equal volumes of solutions of the precipitating components. The concen­ trations given refer to the final mixtures. The glass test tubes with samples were

* Contribution No. 143 from the Laboratory of Physical Chemistry Faculty of Science, Univ. of Zagreb 16 M. SLOVENC AND B. TEZAK kept in a constant temperature bath at 20.0±0.1° C. The pH values were measured with a glass electrode 24 hours afted mixing. The charge sign of the particles was determined in a microelectrophoresis apparatus similar to that described12•

RESULTS

Information on the rate of precipitation of SrS04 can be obtained from time tyndallograms like the one shown in Fig. la. The arrows indicate that after that particular period most of the particles have sedimented leaving a clear supernatant liquid. A more informative picture can be obtained from concentra­ tion tyndallograms where the turbidity values for given times after mixing are plotted. A typical concentration tyndallogram is shown in Fig. lb.

~ I 0.1 - ----, ----·- 1- 2min 2- Smin 3-10min

Ol36 ~----+-

UJ UJ ::> ::> . ...J ~o.b91~·---l~<'>++-f--+---+------j ~ 0001 - - - \---+t-i-----+-----t--1 > u u a: ~ ... UJ UJ ::;: ::;: 0 g ...J ~004~--t-~-i~-~--+--H< ­ ~ ...

-2 c-3 LOG. CONC (N)Sr(N03)2

Fig. 1 ab. Time and concentration tyndallograms of the system Sr(N03); -- Na2SO,

From such data the boundaries of the general precipitation body (PB) could be determined graphically. In order to obtain a general picture of the influence of ethanol on the SrS04 precipitation a series of experiments was made in which the ethanol content was varied (20, 30, 40. vol. 0/o) keeping the concentrations of Sr2+ and So/ - constant. In Fig. 2 a-d the boundaries of PB's are show,n for mixtures with varying ethanol content. There are two boundaries which can be distinguished from concentration tyndallograms. One of them is placed on the side of the optically clear solution. The other is characterized by a steep increase of turbidity. The systems precipitated in pure water or in solutions containing 30 and 406/o ethanol show these double boundaries. I:n the system with 20°/o ethanol the boundaries are fused into one distinct boundary. It can be seen that the precipitation boundaries move towards lower con­ centrations as the macroscopic dielectric constant becomes lower. Also, the lowering of the dielectric constant influences the kinetics of precipitation as can be seen from Fig. 3. PRECIPITATION OF STRONTIUM SULPHATE 17

-6 SrS04 -5- ---+------<----t- WATER -5 SrS04 1omin, zo'c WAl ER•ETHANOL (20 % vol) 3-4 ·-- 1omin, 20' c Ul ,.,_, N ~ l'h / ~ -3

% :}, u ~-2 / z iates 0 0 u u g -1 8-1 v / ...J ...J 0 I/ I/ 1 - -1 -a -5 -7 0 -1 -2 -3 -4 -5 -6 -7 LOG. CONC.~Sr(N03}2 LOG. CONC.(N) Sr (N03)2

--· ·- SrSb4 ' SrS04 -5 IWATER• ETHANQ 3-Jllt;ol) WATER•ETHANOL (40 f., VOL) 10min,20'C 10min, zo•c 64 r 3-41----l~:-'l~0±~-4>~::71--,----,-,--1 Ul sf ' =--. / Ul N " N ~-3 ""' ~-~-~+--+-~lfl"'"""Jl"--<'>l--l3.--l--+--t--~ % pre< ~~are~ ~~, ~-2 ~-21---l~--+---+'----r-&t--~--+---t---I 0 0 u ~p u 0-1 / g-1 1----ll-----''----+--+--<,;..e~-1----t----t-----j '3 ...J I 0 v 0f---~'---+---+--l----+---+---+---+--I I/ 0 -1 -2 -3 -4 -5 -6 7 -1 -3 -4 -5 -6 -7 LOG. CONG (N) Sr(NO)z LOG. CONC.(N) Sr~03lz Fig, 2a-d. Effect of media (water-ethanol) on PB of SrS04 (at 20° C, 10 min.) (a) water, (b) 20'/o ethanol in water (c) 30°/o ethanol in water (d) 40°/o ethanol in water

I 3 I i Sr(NOJ\2 3><16 N 3 't ~so4 3x16 N WATER+ ETHANOL O" -1 Qj36•--l----'>oq__ 1--·1 10"' -2 t I 20·1. -3 I ! 30ro -4 · I 40"-s

I I j_ -1----. -, w ;::) ..J ~ I L) ~0.049f---+-+1'1~ ­ w ~ 0 _J .1 (.; z >- 2 1- 2451020 60 1440 LOG. TIME IN MINUTES Fig, 3. Time tyndal!ogram of Srso. precipitation systems in water-ethanol solution with their different ratios 18 M. SLOVENC AND B . TEZAK SQ The SrS04 precipitates obtained by mixing equivalent amounts of Na2 4 and Sr(N03) 2 ~n water were observed microscopically, 5, 60 and 1440 minutes after mixing (Fig. 4.) During this period a continuous growth of SrS04 particles was observed. The particles are spindle-shaped, similar to those observed of barium sulphate1a,14.

() ~ ~ 0 '~ ~c() ~ ~ ~ 0 f@' ~~ (} ~~ co () ~ ~.3. ~ ~ 0 J 0 t:P ~ ~~ ·~ "$.It ~ a b c Fig. 4. Photomicrographs of Srso, particles formed at different reaction times (a) 5 min; (b) 60 min; (c) 1440 min. (concentration of reactants s.10-• N)

De2elic et al.15 have shown that the reciprocal of the dispersion quotient Q' should be a simple function of the particle size. Therefore, from the time dispersoidogram and tyndallogram of the systems containing ethanol (Fig. 5) it can be concluded that the particle size increases slowly with time. It should be mentioned that in pure aqueous systems slow growth of SrS04 particles could be observed in every case.

Sr(N0 ) 1x10-2N 3 2 2 - Na2S04 5x10 N w WATER+ETHANO (20~VOl.) 3 Q64 -r---r-t-,--~-7"'f--- Q018 ~ I CE ,_ w ~ +-f-·-+----l------1'0.009(5 PARTICLE SIZEJO ~ TYNDALLOM. \41L.- • ,_ _I_ _j_J_ _ 2451020 60 LOG. TIME IN MINUTES

:5' ig. 5. Plot of reciprocal relative dispersion quotient Q ' of strontium particles in dependence of time. PRECIPITATION OF STRONTIUM SULPHATE 19

DISCUSSION

The influence of various precipitation mechanisms on the form of the 1 general PB has been discussed by one of us • According to the postulates made in this paper the following conclusions can be drawn on the basis of the data at hand: Symmetrical ion pair formation is represented by the line normal to the equivalence line (see e.g. Fig. 2.). This part of the boundary is determined by the ionic product. The ionic product, as calculated from our data obtained with aqueus solutions was 7.53 X 10-4 (mole2/12) , which tis not in accordance with literature values16,17 ranging from 3.3 X 10-7 to 5.2 X 10-7 (mole2/l2). This discrepancy can be explained by the formation of supersaturated solutions in our precipitation experiments. The existence of boundari<;s parallel to the coordinate axes may be caused by the formation, of ion associates, whereas the concentration changes may also cause changes of ionic state. The lowering of the dielectric constant may affect several factors influenc­ ing the precipitation process. It was observed that the increase of ethanol content results in a decrease of solubility of the solid phase. Since the intensity of the scattered light depends on the number as well as on the size of the colloidal particles, the observed decrease of tyndallometric values in the sy­ stems containing ethanol (as compared with the aqueous systems) may be explained by the fact that the change of the dielectric constant changes the number and size of the particles by affecting the solubility. These changes are probably connected with the mechanism of nucleation as well as with the distribution of stabilizing . The presence of ethanol drastically alters the kinetics of precipitation so that the precipitation can be observed much earlier in ethanolic solutions than in aqueous ones (Fig. 3.). This fact is also in accordance with the above statement. For all systems the pH-values were nearly constant (6.6-7.4). The charge of the particles was observed in a ultramicroscope-electropho­ retic cell, and was positive or negative according strontium or sulphate ion was in excess in the precipitating medium.

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13. K. Taki yam a, Bull. Chem. Soc. Japan 31 (1958) 950. 14. J. Petre s, Gj. Dez el i c, and B. Tezak, Croat. Chem. Acta 38 (1966) 277. 15. Gj. D e z el i c, N. D e z el i c, and B. T e z a k, J. Colloi d Sci. 18 (1963) 888. 16. G. Ga 11 o, Ann. Chim. Appl. 25 (1935) 628. 17. H. S. Booth and E. F. Po 11 a rd, Ind. Eng. Chem. 40 (1948) 1986.

IZVOD

Precipitacija SrS04 u vodenim i mijesanim otapalima razlicitih dielektricnih konstanti M. Slovene i B. Tefok

Ispitivan je utjecaj etanola na precipitaciju SrS04 u vodenim o,topinama in statu nascendi. Stvaranje sola i kinetika precipitacije pracena je tindalometrijskom teh­ nikom i odredivanjE.m naboja elektroforetski na ultramikroskopu. Rezultati ispitivanja dati su na precipitacionom diagramu. 0 koncentracionim odnosima komponenata gdje vrijedi Sr(N03h i Na2S04 OViisi stvaranje asocijata ili izlucivanje uzduz granice, produkt taplj:ivosti. Smanjenjem dielektricke konstante s povecanjem koncentracije etanola (20, 30, 400/o vol) dovodi do (1) smanjenja top!jivosti cvrste faze, te (2) pro­ mjene u kinetici precipitacije. Fl.ZICKO-KEMIJSKI Z A VOD PRIRODOSLOVNO-MATEMATICKI F AKULTET Primljeno 1. srpnja 1968. SVEUCILISTE U ZAGREBU