Das, Asim K.

Kundu, Kiron K.

English

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Indian Journal of ChemistryVol. 15A, September 1977, pp. 771-774Thermodynamics of Ionization of Benzoic Acid in Urea + Water MixturesKIROC\ K. xuxnu & ASBI K. DASDepartment 01 Chemistry. Jadavpur University, Calcutta 700032Received 1S September 1976; accepted 19 March 1977Thermodynamic ionization constants (sKa) of benzoic acid (HBz) have been determined frome.m.f, studies of the cell: Pt, H2 (g, 1 atm)JHBz (m1), NaBz (m2), NaCI (m3), solventJAgCI-Ag inaqueous solutions containing 11'52,20'31, 29·64and 36·83wt % urea at 10°, 15°, 20°, 25c and 30=C.The related thermodynamic quantities, viz. AGo, ASc and ARc have been evaluated and thesolvent effects on these parameters discussed in the light of dielectric constant, specific solvationeffects of the different species and the involved structural changes of the solvents.INcourse of our studies on the relative structured-ness of aquo-organic solvents including urea+water mixtures, from the transfer energeticsof various electrolytes! and non-electrolytes", thoseof benzoic acid= (HBz) also proved fairly useful.Although HBz was considered to serve as an essen-tially non-electrolyte in aqueous mixtures ofmethanol, ethanol and isopropanol because of de-creasing dielectric constant, it could hardly betaken to do so in aqueous mixtures of urea for theincreased dielectric constant (<") resulting from theincreasing proportion of urea", is liekly to increasethe ionization of the acid. So, in order to accountfor the effect of enhanced ionization of HBz inaqueous urea solutions the knowledge of thermo-dynamics of ionization of HB" in urea-watermixtures is essential. We, therefore, report in thispaper the ionization constant (sKa) of the acid inaqueous solutions of urea containing 11'52, 20'31,29·64 and 36·83% (by wt) of urea at 10°, 15°, 20°, 25°and 30° and the related thermodynamic quantities.Materials and MethodsPreparation of the solvents and the electrodeshas been described earlier+, HBz and NaCl, bothof Merck, GR variety, were used after usualpretreatment. The stock buffer solutions in eachsolvent were prepared by adding appropriateamounts of NaCl and HBz solutions to knownamounts of standard NaOH solution. Concentra-tions in moles kg-1 (m) of all the components in thestock solutions were obtained by simple calculations.The stock solvents as well as the solutions werepreserved at low temperature (",:,S°C). Buffersolutions of different ionic strengths were preparedfrom the stock solutions by mass dilution.General experimental procedures were similarto those described earlierv-". Constancy of the e.m.I.readings to ± O· 2 mV for 1 hr was considered thecriterion of equilibrium. A Harned-Ehlers5 typecell (A) was used.Pt, H2(g. 1 atm)!HBz(ml), NaBz(1112LNaCl(m3).solventIAgCl-Ag(A)ResultsThe p(sKa) values of the acid in each of the sol-vents at different temperatures were determinedadopting Harned and Owen's method" for the ioniza-tion constants of the order of 10-4 or greater.Accordingly, the 'apparent' hydrogen ion concen-trations (mH+) were computed directly from thee.m.f.'s (E) corrected to 1 atm. H2 pressure, for therespective buffer solutions by Eq. (1).I ' E - (sE~)AgCI-Ag 1 25"- og ntH+ = 2.303 RTjF + og 1113- ,Vv... (1)where [.L'=(m2+ma)ds and Sj=1·824X 106 (ET)-3/2.The required values of the density (ds) of thesolvents and Sf were obtained from an earlier papers.The apparent ionization constants K~ correspondingto tnH+ values were obtained from the relation (2).I K-' -! mH+(1n2+mH+) 2S ( d \1/2 (2)og a - og , ._- - f ,v s, •••m1-mH+where v=m2+lIl3+111H+. The true p(sKa) valueat any temperature for a particular solvent wasthen obtained by extrapolating to [.L=O, the observedlinear plot of P(sKa) (vide, Fig. 1, for a typicalcase) against [J.. The required values of sE~gCl-Agat different temperatures for each of the solventsWere taken from an earlier papers. The values ofp(sKa) on molal scale thus obtained are presentedin Table 1.The P{sKa) values at different temperatures foreach of the solvents were fitted to Harned-Robinsontype expression? (3)P(.Ka)=AjT -B+CT •.. (3)by the method of least squares. The values of thecoefficients A, Band C for the solvents are givenin Table 2. The values obtained using Eq. (3)are reproducible within ± 0·005 unit of the observedvalues. The values of the thermodynamic quantities,viz. !1Go, !1So and !1Ho (molal scale) for the ioniza-tion of the acid in the respective solvents werecomputed from the constants of the Eq. (3) andare given in Table 2.771INDIAN J. CHEM., VOL. 15A, SEPTEMBER 1977In order to have a better understanding of thesolvent effect, the changes of standard thermo-dynamic quantities (Llxo=LlGo, LlSo or LlHO) accom-panying the ionization of the acid (HEz) in anysolvent (sLlXO) relative to that in pure water (wLlXO)have been computed at 25° on mole fraction scale(N)8 using Eq. (4).~(LlXO)N = (sLlX°),v - (wLlXO)N= (sX'Hf +sXBz- - sXIIB.)- (wX'ii+ +wXYlz- -wX'iiBz)= AX?(Hf)+M?(Bz_)-LlX[(HBz)where )(7 is the standard partial molal thermo-dynamic parameter for the species i in the solventconcerned and LlXf(i) represents the changes inthe thermodynamic parameter accompanying thetransfer of one mole of the species i from the standardstate in water to the standard state in the solventconcerned, both on mole fraction scale. Thesevalues are listed in Table 3 and their variations withsolvent composition are illustrated in Fig. 2.DiscussionAs expected, the ionization of the acid increaseswith increased proportion of urea in water. Butthe magnitude of increase is not in accordancewith Born's prediction" for the Pf{ values passthrough a minimum at 29·64 wt % urea, whilethe decrease in £-1 values is fairly monotonic".Also, as is usual for many uncharged acids in manyTABLE 1 - p(sKa} VALUES FOR THE IONIZATION OFBENZOIC ACID IN UREA-\VATER }1IXTURES AT DIFFERENTTEMPERATURES(Values in molal scale)Wt% p(sKa}* aturea10° 15° 20° 25° 30°11'52 3·93 3·93 3·92 3·91 3·9020·31 3·83 3-82 3-81 3·80 3·7829·64 3·59 3'58 3·57 3·56 3·5436·83 3·78 3·77 3·76 3·74 3·72·Uncertainties in the P(.K,,} values are of the order of±0·OO5 unit.solventsIO,ll, the variation of P(sKa) values of thisuncharged acid with temperature in any solventmixture (Table 1) does not exhibit any maximumwithin this temperature rang , which usually resultsfrom the opposing effects of increased thermalagitation and decreased dielectric constant withthe rise in temperature. Moreover, the monotonicnature of ,,(AGO)-composition profile (Fig. 2) reflectsthat the solvent effect on the ionization is an in-volved process as is expected from the fact that itis guided by the resultant effects of free energyU~3.~53.8010'C3.~03.60_0 IS'C'" 3.50'"s 20'C.3.6C~ 2~·C."t ~~.I wel ! ,o C.05 0.:0,(J.Fig. 1 - Typical extrapolation curves for evaluating p(sKa)values of benzoic acid in 29·64 wt % urea-water mixture atdifferent temperaturesTABLE 3 - VALUES OF 1l(~GO), TIl(~SO) AND 1l(~HO)AT 25° FOR THE IONIZATIO" OF BENZOIC ACID IN UREA +WATER MIXTURES[Values in kJ mol"! (mole fraction scaler]Wt % 1l(!iGO) TIl(~SO} 1l(~W) ~Gt(HBZ) ~Gt(H+) ~Gt(Br}urea11-52 -1·8820·31 -2·6829·64 -4·2336·83 -3-265·287·338·718-463·404·654·485·20-0,92-1·67-2·45-3·14-1·51-2·89-4·48-5·90-1·29-1·46-2'20-0'50TABLE 2 - COEFFICIENTS OF EQ. (3) AND ~Go, ~SO AND ~HO VALUES AT 25° FOR IONIZATION OFBENZOIC ACID IN UREA+ WATER MIXTURES(Values in molal scale)Wt% A B 102 C sc: ASo AHourea kJ mol-' JK-' mol" kJ mol-1oa 819·6 1·287 0·9190 23·97 -80,3 0·0311·52 -1432·4 -14·106 -1·8180 22·30 -62·6 3·6420·31 -1451·6 -14,424 -1·9308 21·67 -55'7 5·0629·64 -1452·9 -14-191 -1'9323 20·33 -51-1 5·1036·83 -2192·7 -19·483 -2·8127 21·34 -51·9 5·86(a) Ref. 8. . . .. .Assuming that the maximum uncertainty m P(.K,.} values III all the solvents IS of the order ± 0·005, the estimated[Please, N.W., Biochem, j.. 56 (1954). 196J maximum uncertainties in the values of ~Go. fl.Ho and fl.')o at 25° are of theorder: ±O'02, ±1 kJ mole'? and ±2 JK-l mol'? respectively.772KUNDU & DAS: IONIZATION OF BENZOIC ACID IN UREA-WATER MIXTURES~-----------------------------------.TI(ASO)/,.//. ~--------- .._---- ---- TlIS;( ••..•.•e.r"f1 -- :~:., """"'1-, IL ~,L- ~_-_-_-_-_-_-_-_-_-_-~~A~G~;-(H-+)-~ IC" 15-~ole per cent ureaFig. 2 - Variation of solvent effects on thermodynamicquantities accompanying ionization of benzoic acid andtransfer free energies of the species involved at 250 withmole % ureatransfer of the individual species involved in ioniza-tion [vide, Eq. (4)J. SO, for a real understanding <;>fthe solvent effects on ionization of the benzoicacid the knowledge of the behaviour of individualspecies is in order. Since AGt values of HBz h~v.erecently been obtained from the measured solubili-ties of the acid in the solvents=, and those of H+from reference electrolyte (Ph4AsBPh4) assumption-",these values were utilized to compute 6..G't valuesfor Bz: using Eq. (4). These values are liste~ inTable 3 along with those for H+ and .HBz. [heAGt (i)-compositon profiles for the species are alsoillustrated in Fig. 2.It is interesting to note that AGt (H+) valuesare increasingly negative as are expected from theeffects of increased basicitylC,4,13,14 as well asincreased dielectric constant of the solvents''. Also,the increased downward trend of 6..Gt (HBz)-composi-tion profile reflects increased solvation of the acid.As indicated earlierl", this possibly results from the'acid-base' type interactions of the zwitterionic+ ·bform NH2=C(NH2)-6- the actual contn utor tothe resonating structures of urea, with the -C~OHgroup of the acid, as there is little scope of dlspe~-sion type interactions between the hydrophobicPh-group and the zwitt.erionic form of ure~.Moreover the observed downward curve of AG1-profile of Bz- which lies above that for HB~, indicate:that the magnitude of increased solvation of ~zis less compared to HBz. This suggests .tha~ whilethe solvation of -COOH group of HBz IS dlctat~dby the 'acid-base' type interaction'P-P a~ sh<;>wnIIIstructure (I), that for -COO- o~ Bz- IS (hctat~dby the 'acid-base' type interaction as shown IIIstructure (II).The observed upward trend of AGt (Bz] beyond10 mole % is possibly due to 'structural interaction'17of the phenyl group at that structurally criticalregion of the mixed solvents1c,4,18.Solvent effects on the other thermodynamic para-meters To(llSO) and o(AHO), accompanying the ion-ization of the acid, as reflected in their respective pro-files shown in Fig. 2, are highly involved quantities.This is because both the parameters are dictatednot only by the resulting effects of the transferquantities of the individual species concerned,but each of which also is guided by the complexeffects of structural changes of the solvents concern-ed18. Moreover, unlike 6..G't values, ASt and AH'tvalues which are known for the neutral speciesHBz, those for H+ are not known. It is, therefore,difficult to separate the contribution of the indivi-dual species, without which any meaningful con-clusion is hardly feasible.However, interestingly enough, when T6..St(H++Bz") values obtained by subtracting the correspond-ing values of HBz2b from the values of T3(llSO)are plotted against composition, the resulting curve(Fig. 2) is found to be strikingly similar to that for(H++Cl-)4.As indicated earlier- in the case of (H+ +Ct-),the observed profile is dictated, partly by Borntype electrostatic effect and partly by chemicaleffect". The electrostatic effect when corrected atleast tentatively by Born equations, shifts thechemical part downward retaining the essentialfeatures almost unaltered. And the 'chemical part'has been shown to result from the effect of structuralchanges of the solvents', Although proper correc-tion for the electrostatic part of T(6..St) for (H+ +Bz"] is not feasible, it is expected that the chemicalcontribution will not be untowardly different fromthe total T3(llSO) or from the corresponding partfor (H+ -l-Cl"}. It may, therefore, be concluded thatthe contribution of H+ being common, the observeddifference in both the curves (Fig. 2) is likely toresult from the hydrophobic structure promotingeffect of Ph-group= of Bz, which is pronounced inpure water but less so in the mixed solvents.AcknowledgementThe authors thank the CSIR, New Delhi, forfinancial assistance and for a senior research fellow-ship (to A.K.D.).References1. (a) KUNDU, K. K., Indian J. cu«, 10 (I972), 303.(b) KUNDU, K. K., JANA, D. & DAS, M. N., Electrochim.Acta, 18 (1973), 95. (c) KUNDU, K. K. & lVIAZUMDAR,K., J. chem. Soc. Faraday (1), 71 (1975), 1422. (d)BOSE, K., DAS, A. K. & KUNDU, K. K., J. chem, Soc.Faraday (I), 71 (1975), 1838.2. (a) BOSE, K. & KUNDU, K. K., Can. J. Chem.,(in press). (b) DAS, K., DAS, A. K. & KUNDU,K. K., J. phys, Chem., (communicated).773INDIAN J. CHEM., VOL. ISA, SEPTEMBER 19773. WYMAN (Jr), J., J. Am. chem, Soc., 55 (1933), 4116.4. KUNDU, K. K. & MAZUMDAR,K., J. chem, Soc. Faraday(1),69 (1973), 806.5. HARNED, H. S. & EHLERS, R. W., J. chem. Soc., 54 (1932),1350.6. HARNED, H. S. & OWEN, B. B., J. Am. chem, Soc., 52(1930), 5079.7. HARNED, H. S. & ROBINSON,R. A., Trans. Faraday Soc.,36 (1941), 973.8. HARNED, H. S. & OWEN, B. B., Physical chemistry ofelectrolytic solutions (Reinhold, New York), 1957, 11.9. BATES, R. G., cited in Hydrogen-bonded solvent system,edited by A. K. Covington & P. Jones (Taylor & Francis,London), 1968,49.10. HARNED, H. S. & OWEN, B. B., Physical chemistry ofelectrolytic solutions (Reinhold, New York), 1957, 689.11. MANDEL, M. & DECROLY, P., Trans. Faraday Soc., 56(1960),27.77412. KUNDU, K. K. & DAS, A. K., J. Soln cu«, (communi-cated).13. HUNSTON, D. L. & KLOTZ, 1. M., J. phys. cu«;75 (1971), 2123.14. KUNDU, K. K. & DAS, A. K., J. 501n cis«, 6 (1976),431.15. FEAKINS, D., Physico-chemical processes in mixed aqueoussolvent systems, edited by F. Franks (Heinmann, London).1967, 71.16. KUNDU, K. K., RAKSHIT,A. K. & DAS, M. N., Electrochim.Acta, 17 (1972), 1923.17. DESROSIERS,N., PERRON,G., MATHIESON,J. G., CONWAY,B. E. & DESNOYERS, J. E., J. su« Chem., 3 (1974),789.18. DAS, A. K. & KUNDU, K. K., J. phys. cu«; 79 (1975),2604.19. ERNETT, E. M., BENTRUDE,W. G., BURKE, J. J. & MAC.DUGGLEBY,P., J. Am. chem, Soc., 87 (1965), 1541.

Thermodynamics of Ionization of Benzoic Acid in Urea + Water Mixtures

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Thermodynamics of Ionization of Benzoic Acid in Urea + Water Mixtures

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