Indian Journal of ChemistryVol. 24A. August 1985. pp.688-690

Conductometric Studies on the DissociationConstant of Monochloroacetic Acid in

Ethanol +Water Mixtures

A PAL. S K MAlTY & s C LAHIRI*Department of Chemistry, University of Kalyani, Kalyani 741235

Received 3 August 1984; revised 3 December 1984; rerevised andaccepted 5 March 1985

The dissociation constant of monochloroacetic acid in ethanol+water mixtures has been determined using the titration techniquesuggested by Gelb and the Fuoss-Kraus method. The transfer freeenergy changes have been split into electrostatic part and non-electrostatic part. The results have been interpreted in terms of thesolvent basicity and the solute-solvent interactions.

The importance of the determination of thedissociation constants of weak acids in different mixedor non-aqueous solvents is well-known. However, thelimitations for such determinations have been amplystressed by Popovych 1 and Lahiri and coworkers 2 -6.

We prefer the conductometric method for thedetermination of the dissociation constants of weakacids in mixed solvents as it involves no seriousapproximations. Furthermore, the dissociationconstants can be determined for very low con-centrations and in the absence of neutral electrolytes,conditions specially favourable for determining themedium effects.

We report in this note the dissociation constants ofmonochloroacetic acid, determined conductornetri-cally in different concentrations of ethanol + watermixtures using the titration method suggested byGelb 7. The results have been further verified by Fuossand Kraus method":". Monochloroacetic acid is astronger acid than acetic acid and can be well utilisedfor the preparation of buffers in mixed solvents.

Preparation of conductivity water, purification ofethanol, determination of the weight percentages ofethanol in different mixed solvents and determinationof their dielectric constants have been described inearlier communications? -6. Conductometricmeasurements were carried out as describedpreviously? -6. The measurements were done at 298

. ±O.OI K.The dissociation constant (I<) for the reaction,

HA¢:H+ +A - (I)

(HA =monochloroacetic acid)in presence of HCI04 can be calculated from therelation,

688

K = «(XCHA + C HClO).rz±

(I - (X)... (2)

All the terms have their usual meanings.When the conductance of the mixture (lIR)

(containing HA and HCI04) and blank solution (lIR*)are equal, we have? -7,

C HClO •. A'HClO. + (XCHA.AHA = CfIClO •. AfIbo. ... (3)

(* indicates the quantities in blank solution) whereA 1 is the equivalent conductance of the completelydissociated electrolyte at any ionic strength and A' isthe equivalent conductance of the electrolyte at thationic strength i.e. Al = (XA' «(X= I in case of HCl04) inaqueous and mixed solvents.

In view of the closeness of the mobilities of theanions, we can assume A'HClO. = A'HA

Moreover, the concentrations of HCI04 in themixture and the blank solutions are nearly equal;therefore, we can assume

A'HClO. = A'JfClO •.

Thus, we have from Eq. (3),

(X= (CfIClO. - CHClO)ICHA ... (4)

The «-value thus obtained can be utilised tocalculate Kfrom Eq. (2). The 'titration method' can beimproved by calculating (X with p-correction using therelation,

(X= «(fIClO. - CHClO)1 PC HA

AHA AHAwhere f3 =-- ~--

A~CI04 AHClO•

The approximation is valid since the solutions arevery dilute and both the solutions have similar but lowionic strengths, the slight variations of A values willnot markedly affect the p-value.

The above expressions are valid for aqueous as wellas mixed solvents. However, we could not determineA0 -values of chloroacetic acids accurately using theusual relationship,

... (5)

Though A 0 HClO. and A" KClO. values in water andethanol + water mixtures are known, A 0 ClCH

2COOK

could not be determined as our attempts to prepareClCH2COOK by neutralising CICH2COOH withKOH solution or K2C03 did not succeed.

However, we tried to verify the results using the

more accurate method of Fuoss and Kraus":".Approximate values of A°CICH,COOH and K wereobtained from the plots of AC against IIA of a numberof dilute solutions from the relationship,

The A° values thus obtained were utilised tocalculate Z and CI.using Eqs (8) and (9).

Z =(eA °+ a)A 0-3/2 JAC

ACI.=---

AOF(Z)

where F(Z) is function of Z represented by the relation,

F(Z)= l-Z(I -Z(I _...)-112)-1/2)112

The values of the On sager constants a and e havebeen calculated as before.

The values of F(Z) were obtained from the Tablegiven by Fuoss". The values off ± were calculated fromthe equation -log f ± = A ;;C using the appropriatevalues of A in mixed solvents.

The accurate values of K and A° can be obtainedutilizing the Fuoss-Kraus equation+",

F(Z) 1 ACj2+ IA= KAo2' F(Z)- +N ... (10)

from the plot ofF(Z)/A against ACff:. IF(Z). Generally,two iterations are sufficient to have consistent values.However, some uncertainty exist in the extrapolatedAO-values.

The pK-values of mono chi oro acetic acid obtained indifferent percentages of alcohol using (i) the titrationmethod (without or with ,B-correction), (ii) the plots ofAC vs l/A and (iii) the Fuoss-Kraus method arerecorded in Table 1.

The precision of the results may be taken to varyfrom ± 0.03 pK-units in water to ± 0.06 pK units in

NOTES

(7)

higher percentages of ethanol. However, in view of theuncertainties in the extrapolated A°-values ofchloroacetic acid, we did not calculate the pK-valuesby titration method at higher percentages of alcohol.

The results show that the pK-values determined byus using conductometric methods agree well with thevalues reported by Grunwald and Berkowitz.'?potentiometrically using a Beckman Model pH -rneter.The cell used was: glass electrode/Hzv, NaA, NaCI04,

HCI04, KCI/AgCI-Ag. Acid dissociation constantswere derived from the rate of change of pH with addedstrong reagent at the equivalence points in the titrationof weak bases or weak acids under conditions wherethe uncertain potentials were constant. The accuracywas claimed to be 2-3%.

The pK-values increase with increasing percentageof ethanol as expected. Linear relationship is observedwhen the pK-values are plotted against mole fractionand lie well upto about 60-70 wt% of alcohol beyondwhich there is appreciable deviation. The resultsindicate that specific solute-solvent interactions are ofimportance. To comprehend the nature of solute-solvent interactions, we calculated the approximatevalues of non-electrostatic contributions, ~G~(nonel)using the relation,

(8)

(9)

The ~G~el) was calculated using the Bornequation 1I,

~G~(el)=4.184 x 166(*-0.0127)

... (11)

rw was taken to be equal to 0.86ft. as used by Sager etal.12 and Rochester et al. 13; rCICH 2COO - was taken

Table l-pK. Data of the Chloroacetic Acid in Water-Ethanol Media

Ethanol AO of CICH2COOH pK; from p K; from p K; from pK, after pK.(Wt~J (from Fuoss- AC vs If A Fuoss- titration p-corr- (Grunwald

Kraus Kraus (without p- ection et at.)method) method correction)

0.0 391.0 2.82 2.85 2.95 2.87 2.86(0.0)·8.0 344.0 2.92 3.06 3.16 3.08

16.4 272.8 3.12 3.20 3.32 3.1825.3 227.0 3.23 3.34 3.43 3.3234.4 181.8 3.44 3.56 3.64 3.57 3.57(35.0)44.0 148.1 3.58 3.72 3.80 3.6854.1 127.0 3.70 3.97 4.0464.7 111.1 4.28 4.32 3.95(50.1)76.0 88.9 4.58 4.73 4.41(65.1)87.6 75.9 4.97 5.22 4.98(79.9)

(* Values in brackets indicate wt% of ethanol)

689

INDIAN r. CHEM., VOL. 24A, AUGUST 198.5

Table 2--':The values of 6G?, 6G~el)' 6G~nonel)and [6G? (CLCH 2COO -) - 6G?(ClCH 2C02H)] in Different Percentage (wt%)of Ethanol

~G~(el)(kllmol) ~G~none1lkllmol)Ethanol (1M x 102 ~G~ -~G~(H+) a* [Eq. II] using(Wt%) (k.l/rnol) (kllmol) (kl/mol)

r; -=3.33A r; -=2.25A r-: -=3.33A rA - =2.25A

8.0 1.35 1.20 1.08 2.28 0.81 0.89 0.39 0.3116.4 1.42 2.00 1.83 3.83 1.52 1.67 0.48 0.3325.3 1.53 2.79 2.70 5.49 2.64 2.90 0.15 -0.1134.4 1.71 4.05 3.38 7.43 4.47 4.91 -0.42 -0.8644.0 1.90 4.96 4.52 9.48 6.40 7.03 -1.44 -2.0754.1 2.14 6.39 5.53 11.92 8.84 9.71 -2.45 -3.3264.7 2.44 8.38 5.63 14.01 11.89 13.06 -3.51 -4.6876.0 2.87 10.72 ;.53 16.25 16.26 17.86 -5.54 -7.1487.6 3.37 13.52 4.66 18.18 21.34 23.44 -7.82 -9.92

a* = [~G?(CICH2COO -) - ~G?(CICH2COOH)]

to be 3.33A (ref. 14) and 2.25A (ref. 15), the valuesreported for CH3COO - from X-ray and viscositymeasurements. The values 6G~ and the approximatevalues of 6G~(el)' 6G~nonel) are recorded in Table 2.

The results show that 6G~(none1)is positive at thebeginning but becomes increasingly negative from25 wt% of alcohol onwards. The results suggest thatthe mixed solvents become increasingly basic incharacter, though the 6G~(H +)16 values suggest thatthe basicity is maximum in the region 65 wt% ofalcohol. However, the 6G~(nonel) values are onlyapproximate and the term basicity is only relative.

These aspects can be better understood if we anal ysethe free energy of transfer of chloroacetic acid in theway given below.

6G~= 6G~(H +) + 6G~(CICH 2COO -)-6G~(CICH2COOH) .. (12)

or 6G~-6G~(H +)=6G~(CICH2 COO-)

-6G~(CI.CH2COOH) ... (13)

The values of [6G~(Cl.CH2COO-)-6G~(CICH2.COOH] are recorded in Table 2. It hasbeen observed that the 6G~(CI.CH2COO -) is in thehigher free energy state compared to6G~(Cl.CH2COOH). It is natural to expect that6G~(Cl.CH2COOH) would be increasingly negativewith increasing alcohol concentrations. Since, 6G~value of anions are generally increasingly positive, it isexpected that 6G~CICH,Coo-) would also follow theexpected pattern.

where 6G~(ncul)has been assumed to be equal to 6G~of the neutral analogue of the ion. Thus, we have,

6G~(eIXClCH,coo -)= 6G~CICH,coo -')- 6G~CICH,coo~

The [6G~C1CH.coo -)- 6G~CICH,CooHJ

690

thus gives the relative values of 6G~el~CI.CH2COO"

It is known that ions like H + and Cl.CH 2COO -destroy the solvent structure and preferential solvationof ions generally takes place. The greater - 6G~(H +)values show that H + ions are solvated more firmly bywater molecules in the mixed solvents whereas thereverse would be the case for the anions.

An analysis of the A 0-values shows that A 0-values ofCl.CH2COOH decrease with increasing percentage ofalcohol as expected. However, systematic studies arenecessary to explore the role of solvents on thedissociation constants and the various possibleinteractions.

The authors thank the UGC, New Delhi forawarding Junior Research Fellowships to two of them(A.P. and S.K.M.).

ReferencesI Popovych 0, Analyt Chern, 46 (1974) 2009.2 Mandai A K & Lahiri S C, J prakr Chern, 319 (1977) 377.3 Mandai A K & Lahiri S C, Indian J Chern, 15A(1977) 728, 928.4 Mandai A K & Lahiri S C, Bull chern Sac, Japan, 49 (1976) 1829.5 Mandai A K & Lahiri S C, J Indian chern Sac, 54 (1977) 894, 898.6 Bhattacharyya A, Mandai A K & Lahiri S C, Electrochim Acta,

25 (1980) 559; Indian J Chern, 19A (1980) 532.7 Gelb R I, Analyt Chern, 43 (1971) 1110.8 Fuoss R M & Kraus C, J Am chern Sac, 55 (1933) 476, 2390.9 Fuoss R M & Accascina F, Electrolytic conductance

(Interscience, New York) 1959.10 Grunwald E & Berkowitz B l. J Am chern Sac, 73 (1951) 4338:II Born M, Z Physik, 1 (1920) 45.12 Sager E E, Robinson R A & Bates R G, J Res Nat Bur Stand A, 68

(1964) 305.13 Parson G H & Rochester C H, J chern Sac Faraday I, 71 (1975)

1058.14 International critical tables, Vol I (McGraw Hill, New York,

London) 1928, 347.IS Moelwyn-Hughes E A, Physical chemistry, 2nd Edn (Pergamon

Press, Oxford) 19M. 859.16 Bhattacharyya A K, Sengupta D & Lahiri S C, Z Physikchem

(Leipzig), 265 (1984) 372.