lNDIAN J. CHEM., VOL. 18,4,.,DECEMBER 1979

220

200

t80

160

,-140 I~ ,

'" I

~ ,I~

120 2J'0

s I., -0 ,E I

100 Ic: I., I•.. I0I..J 80II,

60 IIII

40 IIII,

I20 I

III

/,

258 268 308 328 348Temperrature (K)

Fig. 1- Plot of loss in mass (dw) against temperature ofPrF.XeF. (curve 1), NdFaXeF, (curve 2) and LaF.XeF.

(curve 3)

detected in the TG curve. In general, we observethat the thermal stability of 2: 1 and 3: 1 complexes.de-creases with rise of atomic number of the lanthanide,This appears to be in good agreement with thelanthanide contraction.

Similar pattern was observed in the case of al~alimetal fluoride-xenon hexafluoride complexes. Caesiumfluoride formed the most stable complex while sodiumfluoride complex was found to be ~he least. stable,

These adducts are extremely reactive and in bulktend to react violently with water. Upon hydroly~is,all the xenon is retained in the aqueous solutionas xenon trioxide. On exposing the complexes toair the samples have been found to lose HF. .

The author thankfully acknowledges the kindpermission of Prof. George H. Cady to. workin the fluorine laboratories, University of Washingtonand to the Office of Naval Research for providinga post-doctoral fellowship (1968-70).

References1. PEACOCK,R. D., SELIG, H. & SHEFf, I., Proc. chem.. Soc.

(1964), 285.2. PEACOCK, R. D., SELIG, H. & SHEFf, I., J. inorg. nucl.

Chem., 28 (1966), 2561.

532

I

3. GARD, G. L. & CADY, G. H., Inorg. Chem., 3 (1964),1745.

4. SELIG, H., Science, (N. Y.), 144 (1964), 537.5. PULLEN, K. E. & CADY, G. H., Inorg. Chem., 5 (1966),

2057.6. MOODY, G. L. & SELIG, H., Inorg. nucl. chem. Lett., 2

(1966), 319.7. SHEFf, 1., SPITTLER, T. M. & MARTIN, F. H., Science,

(N.Y.), 145 (1964), 701.8. MISRA, SUDHINDRAN., Indian J. Chem. (Communicated).9. MALM, J. G., SHEFf, 1. & CHERNIK, C. L., J. Am. Chern.

Soc., 85 (1963), 110.

Selective Solvation of Ag(l) Sulphate in EthyleneGlycol-Dimethyl Sulphoxide Mixtures

C. KALIDAS*& P. SIVAPRASADDepartment of Chemistry, Indian Institute of Technology,

Madras 600 036

Received 7 March 1979; accepted 21 May 1979

The solubility data of Ag2SO., the transfer free energy of thesalt, Ag+ and SO ~- ions and the solvent transport number ofDMSO in ethylene glycol-dimethyl sulphoxide mixtures are re-ported. The solubility of silver sulphate is found to decreasecontinuously with the addition of DMSO. The transfer freeenergy of Ag+ from glycol to the mixed solvent is nearly equal tozero upto XDMSO •.••.•.0.2 and subsequently decreases. The transferfree energy of sulphate ion increases with the addition of DMSO.The solvent transport number, 6,. of DMSO passes through amaximum at XDMSO •.••.•.0.6. It is shown that Ag.SO. is hetero-selectively solvated in these mixtures at- XDMSO > 0.2.

IN continuation of our earlier work! on theselective solvation behaviour of Ag(I) sulphate

in ethylene glycol (EG)-acetonitrile mixtures, theresults of selective solvation of Ag(I) sulphate in EG-,dimethylsulphoxide (DMSO) mixtures at 30° arereported in this note'.

EG and DMSO were purified by standardmethods=". Silver sulphate (E. Merck, LR) was suit-ably dried before use. Other details were the same asdescribed earlier'.

The solvent transport number A was calculatedfrom measurements on the concentration cell (A)with transference

XiEG

Ag··, -

·,,x; EG

Ag

x; DMSO

saturated with the salt

x; DMSO

saturated with the salt(A)

wherein the mole fraction difference X2" - X2' waskept at 0.1 throughout and the two half cells weresaturated with the salt. These measurements werecarried in U-shaped cell fitted with a G3 frit in themiddle. The emf data on cell (A) were obtainedwith a Keithley solid state (model 602) electrometer.

The silver electrodes required in these cells wereprepared electrolytically according to the methodof Carmody".

The solubility data and the transfer free energyof the salt from EG to EG + DMSO mixtures (molal

NOTES

scale) were calculated from Eq.(1), given in the earlierwork", and the data are given in Table 1.

The free energy of transfer of silver ion fromEG to EG-DMSO mixtures on the basis of ferrocenereference method was obtained from Eq. (2), givenin the earlier work 1 The transfer free energy of thesulphate ion was then obtained from Eq. (3)1.

All these data (molal scale), given in Table 1,show that the solubility of Ag2S04 decreases with the_addition of DMSO and thus the transfer free energyof the salt from EG to the solvent continuouslyincreases. It is pertinent to soint out at this juncturethat this behaviouris different from that in EG + ANand methanol+DMSO mixtures but is similar to thatin water + DMSO mixtures.

An examination of the transfer free energy dataof silver ion shows that AGtO(Ag+)is nearly equalto zero or slightly negative upto XDMSO :::; 0.2 andsubsequently it is continuously negative with further.addition of DMSO indicating that silver ion ispreferentially solvated by DMSO in EG-DMSOmixtures (XDMSO > 0.2) than in EG. The transferfree energy of sulphate ion AG~(SO!-), however,increases continuously with the addition of DMSO.It thus appears that at low mole fractions of DMSO(XDMSO < 0.2) the silver ion behaves as if it were ~npure EG and this accounts for the decrease III

'solubility of the salt in this region. At high mole

/

TABLE 1- SOLUBILITY AND TRANSFER FREE ENERGY DATA OFAg2S0~, Ag+ and SO!- IONS AT 300

XDMSO

Solubility*of

Ag.SO,x 10'

6G?(Ag2S0~) 6G?(Ag+) 6G~(SO!-)kJ g mol-l kJ g ion"! kJ g ion-I

(mol kg-I)0 19.2 0.0 0.0 0.0

0.1 18.3 0.1 -0.1 0.30.2 16.5 0.7 -0.6 1.90.3 15.4 1.0 -1.9 4.8·0.4 13.1 2.0 -3.2 8.40.5 11.1 3.0 -5.1 13.2·0.6 10.5 3.4 -6.6 16.60.7 8.7 4.6 -8.5 21.60.8 7.8 5.4 -10.1 25.60.9 5.7 7.5 -12.4 32.31.0 5.2 8.3 -16.1 40.5

*Accurate to ± 1%

TABLE 2 - EMF DATA (CELL A) AND THE SOLVENT TRANSPORTNUMBER 6' FOR Ag2S04 IN EG-DMSO MIXTURES AT 300

Mole fro E* 6'-of DMSO (X2) mY

0.05 -3 0.10.15 -8 0.40.25 -9 0.70.35 -12 1.10.45 -14 1.30.55 -18 1.7065 -21 1.80.75 -18 1.30.85 -24 1.20.95 -13 0.2

*Accurate to ± 1 mY

..

I

(

fractions of DMSO, the large increase of AGtO(SO!-)with the addition of DMSO has a dominating rolein bringing about a decrease in the solubility of thesalt although AGtO(Ag+)decreases. Similar obser-vations were made by Harakany and Schneider?on the solubility of silver sulphate in water-DMSOmixtures and they suggested that strong solvent-solvent interactions, i. e. the stabilisation of waterstructure by .DMSO molecules at low mole fractionsof DMS08, may play an important part. SuchStrong solvent-solvent interactions presumablyare responsible for the low value of AG?(Ag+) inthe region upto XDMSO>0.2 in the present case also.Thus in this system the heteroselective solvation ofsilver sulphate is markedly observed at XDMSO > 0.2.The strong solvation of the silver cation by DMSOas evidenced by the decrease of 6.G~(Ag+) with theaddition of DMSO may be explained on the basis ofthe specific back bonding interaction of this cationwith the 'It-orbitals of the sulfoxide group. Further,the electrostatic interactions between the silver cationand the negative charge on the oxygen of the sul-foxide linkage may be expected to contribute to thelarge of decrease of AGtO(Ag+).

The solvent transport number, A is related tothe cell (A) according t09

E = _ RT ( X~ -- Xz ) A (I + 0 In 12 )F X2(l - X2) 0 In X2

... (1)where X~, X~ represent the mole fractions of DMSOin the two half-cells X2 = (X; + X~)/2 and 12 isthe activity coefficient of DMSO in the solventmixtures. In view of the non-availability of the activitycoefficient data for these solvent mixtures, A' definedby Eq. (2)

A' = A (1 + ~Ilnf2 ) ... (2)v n X2

was calculated from Eq.(1) from the measured emfof the cell (A) at various X2 values. These resultsare presented in Table 2. It is seen that 6.' whichrepresents (in this case, approximately) the solventtransport number of DMSO passes through amaximum of 1.8 at XDMSO :::; 0.6. Thus there isan increase of 1.8 moles of DMSO per faraday inthe cathode compartment when a solution of the saltis electrolysed in EG-DMSO mixtures correspondingto the above composition. This higher positivevalue of A' in these mixtures is in agreement with theconclusions arrived from the transfer free energydata of the salt in these mixtures earlier. The trans-port of DMSO into the cathode compartment occurslargely through the silver ion while the SUlphate iontransports EG in the opposite direction. Both theseeffects add together and thus a heteroselective sol-vation is characterised by a large value of 6. aroundXDMSO :::; 0.6 as noted in this case. It thus seems thatin this system, as in the water-DMSO mixtures, thesolvation behaviour of an electrolyte is stronglyinfluenced by the solvent-solvent interactions.

References1. KALIDAS, C. & SIVAPRASAD, P., Indian J. Chem.,17A (1978),

79.2. KAUDAS, C. & PALIT, S. R., J. chem, Soc., (1961), 3991.

533

nINDIAN J. CHEM., VOL. 18A, DECEMBER 1979

3. MARICH, D. L. & HObGON, W. G., Analyt. Chem.,37(1965), 1562.

4. WAGNER, C., Advances in electlychemistry. & electro-chemical engineering, Vol. 4 (Wiley Interscience, NewYork), 1966, 1-46..

5. CARMODY,W. R., J. Am. chem, Soc., 51 (1929), 2901.6. ROOEHUSER, L. & SCHNEIDER,H., Z. phys.Chem. (NF),

10) (1976),119.7. EL HARAKANY, A. A. & SCHNEIDER, H., J. electroanal.

Chem., 46 (1973), 255.8. MARTIN, D. & HAUTHAL, H. G., Dimethyl sulfoxide

(Academic Verlag, Berlin). 1971, 107.9. SCHNEIDER,H., Solute-solvent interactions, edited by J. F.

Coetzee & C. D. Ritchie (Marcel Dekker, New York),1976, 155-218.

A Polarographic Study of Cadmium MalonateComplexes in Mixed Solvents

S. S. KELKAR& B. 1. NEMADE*Department of Chemistry, University of Bombay, Vidyanagari,

Santacruz (East), Bombay 400 098

Received 14 December 1978; revised 12 March 1979, accepted15 May 1979

Malonate complexes of cadmium have been studied polaro-graphically in 25 and 50 % aqueous mixtures of methanol, ethanoland acetonitrile. Though the electrode reaction is reversiblein water-methanol, a quasi-reversible reduction is observed inwater-ethanol and water-acetonitrile media at higher concentra-tions of the ligand. Stability constants have been evaluated inall the cases. The decrease in dielectric .constant is found tofavour greater association of ions resulting in higher values ofstability constants.

THE polarographic behaviour of a substancechanges with the . nature of. solvent due. to

the changes in the physical properties of th~ mediumlike viscosity and dielectric constant WhIChaffectthe diffusion coefficient of the ion. The change in thenature of electrode reaction may take place becauseof the alteration of the double layer structure at thedropping mercury electrod~ by the solvent. f\polarographic study of cadmmm-!TI~lonat~ system mwater-alcohol and water-acetonitrile mixtures wasundertaken with a view to studying these effects.

The current-potential curves were recorded on abalancing type manual polarograph using a saturatedcalomel electrode as the reference electrode. Tem-perature was maintained at 30 ± 0.10 and the i~:>nicstrength was 2.0M(NaCI04). Ethanol was purifiedby the method described by Vogel'. All the o~herchemicals used were of AR grade, No maximasuppressor was found to be necessary. The capillarycharacteristics were : m = 1.98 mg/sec and t = 3,6sec. The currents reported were corrected fordiffusion current.

A well-defined wave was obtained for cadmiumin 25 and 50 % methanol, ethanol and acetonitrile.The product i« "Yjl/2, whic~ was.expecte~ to be c~>ntanton the basis of Stokes-Einstein 2 relation, was mcon-sistent as also found by other workers", The half-wave potential of the. simple <;d2+ ion (0.558 'Y.vsSCE in aqueous medium-) shifted to more POSItIvevalues in non-aqueous mixtures as suggested bySchaap" and observed by other workers".

,I

TABLE 1 - VALUES OF STABILITYCONSTANTSIN DIFFERENT'MEDIA

Medium log (31 log (3. log (33 log (3,

Water 2.00 2.78 3.4325% Alcohol 2.28 3.52 4,5950% Alcohol 2,51 4.29 5.28 6.2225 % Acetonitrile 2.57 3.71 4,6350 % Acetonitrile 3.06 4.59 6.00 7.26.•...

Polarograms of cadmium were obtained withdifferent concentrations of malonic acid (pK 5.00)at pH f""oJ 6,7. The half-wave potentials were taken.from the zero intercept of the log-plots.

Methanol- The reduction in the case of 25 and50% methanol was found to be reversible. Theplot~ of El/2 vs pA ares mooth curves indicatingpresence of more than one complex whose stabilitiesdo not differ appreciably. The data were thereforeanalysed for stability constant by DeFord andHume's method".

Ethanol- In the case of ethanol the red-uction was found to be reversible at lower concentra-tions of ligand. However, at higher concentration,the log-plots were found to be of curved nature sug-gesting a quasi-reversible reduction. Hence Hale andParson's method" was adopted to calculate thereversible half-wave potential along with kineticparameters. " .

The shift in the formal potentials obtained inethanol-water mixture at different concentrationsof malonic acid was found to fit the smooth curvesplotted for methanol, in both the c~s~~. Hence. thecoordination numbers and the stabilities of varIOUScomplexes in both methanol and ethanol were takento be the same.

Acetonitrile - In the case of acetonitrileas well the reduction was reversible at lower con-centrations of malonic acid and quasi-reversible athigher concentrations. The data were thereforeanalysed for formal potentials by Hale and Pars~m'smethod. The stability constants were obtainedusing formal potentials in place of reversiblehalf-wave potentials,

Three complexes could be observed in the case c:f25 % ac. tonitrile and four in the case of 50 % acetoru-trile.

From the results obtained in all the three media(Table 1) it is evident that the stabilities of variouscomplex species are enhanced with the lowering ofthe dielectric constant ot the media as observed byoth rs", This can be explained on the basis of thefact that the force between the ions is inverselyproportional to the dielectric constant. Therefore,decrease in dielectric constant will favour greaterassociation of ions resulting in higher values of stabi-lity constants.

The authors thank Dr A. K. Sundaram and DrR. Sundaresan of Analytical-Chemistry Division,BARC for the valuable discussion and helpfulsuggestions.

References1. VOGEL, A. I., A textbook of practical organic chemistry

(London Group Ltd, London), 1971.