Indian Journal of ChemistryVol. 23A, May 1984, pp. 409-410

Dissociation Constant of Copper Sulphatein Aqueous Solution from e.rnJ. Data

S N PRASAD & J C GHOSH*Department of Chemistry, Patna University, Patna 80000S

Received 26 February 1983; revised 29 August ,19ID:accepted S December 1983

The pKs of CuSO. in aqueous solution determined at 278.IS,288.15, 298.IS and 308.IS K are 2.87, 2.';14,3.00 and 3.0S respectivelywhile L'.Gos at these temperatures are 15.3, 16.2, 17.1 and 18.0 kJrnol " respectively. fl..S and!1H over the temperature range haveconstant values of -89 J mol 1 K 1 and -9.6 kJ mol 1

respectively.

The literature values of the dissociation constant ofCuS04 in aqueous solution determined conducto-metrically are old and not reliable. In this note wereport the values of the dissociation constant and therelated changes in thermodynamic functions ofCuS04

in aq. solution in the temperature range of 278.15 to308.15 K.

The dissociation constant was measured from thee.m.f. data obtained from cells of the types (A) and (B),where C2/CI was equal to 5 and the stoichiometricmolarities were accurate upto a micromol dm -3

Pt IOHJH2S0,,(C1)I H~,,(C,) I Hi>O,,(cl) H92S0" I HgCuSO,,(c2) CuSO,,(C2) CuSO,,(C2)

(A)

Pt I OH, H~()"I H2~O,,\ H2~O" H92SO,,\ Hg

(8)

The ionic strength of the solutions was kept below 0.05mol dm-3 but the minimum concentration of H2S04used was 8 x 10-4 mol dm -3, since at concentrationslower than this Hg2S04 underwent hydrolysis",Hg2S04 was prepared electrolytically. H2S04 andCuS04 were of E. Merck (G.R.) quality. Copper.H2S04 and total sulphate content of stock solutionswere estimated by standard methods. A IIexperimentalsolutions were prepared in doubly distilled water byappropriately diluting the stock solutions strictly at thetemperature of the experiment and were deoxygenatedby passing presaturated N 2 gas overnight. Theelectrode vessels, bridge and the setting up of cell 2 andthe thermostat and potentiometer assembly were asdescribed earlier '. Duplicate cells were set up, thee.m.fs of which agreed within 0.1 m V. The recorded

e.m.fvalues(in absolute volt) were the mean of the twoe.m.f. values].

It has been previously shown in our laboratory thatEq. (I) fairly represents the activity coefficients.j, of anion in a dilute solution.

log/> - A7T.J It - hiP ... (I)where A is Debye-Hiickel constant in mol " ' 2 dm ' 2

and hi is an empirical constant which is additive andreasonably independent of the composition of ionicenvironment, provided the composition of the ionicenvironment is not drastically changed", and f1 is theionic strength. This means that at the same ionicstrength (Jl) the value of/M'.m. will be the same whetherwe use H2S04 alone (as in cell of the type-B) or amixture ofCuS04 and H2S04 (as in cell of the type-A).So the e.m.f. of cells (A) and (B) will be given by Eq. (2).

E=E _ 2.3026RTI [H]2[SO]2F og 4

2.3026 RT I /'2 r- 2F og H vso, .. .(2)

Combining Eqs (1) and (2) we get Eg. (3)

k -E=E' -2Iog[Hf[S04]+3kA .Jp-hJl

where, k = 2.3026 R T/F and b =(2hH + hso)

Values of EO and b have been determined 2 fromEq. (3) using e.m.f. values of cell (B).

Relation between second dissociation constant K2of H2S04, KA and u is given by Eq. (4)

... (3)

... (4)

where, KA = [H][S04Ji[HS04] and b, is a constantwhose values have been recorded". Likewise thesolubility product Kp of Hg2S04 is related to f1 by Eq.(5), where h2 is a constant whose values along with thevalues of K,p have been recorded".

log [Hg2][S04] = log K,p + 8 A .Jp - h2J1 ... (5)

In cell (A) H2S04 and CuS04 dissociate as

H2S04 + aq -.H "(aq) + HSO; (a g)CI CI ('1(1 -ct)

1tH "(aq) + SOi (aq)cia cia

CUS04+aq ~ Cu2+(aq)+SO~ "(aq).

tThe e.mJ. values and related data are available with the authors andcan be obtained from them on request.

409

INDIAN J. CHEM., VOL. 23A, MAY 1984

All the ionic species in solution are hydrated andcharged, but for the sake of convenience they ha ve notbeen shown.

Now assuming an arbitrary value of II, we find thevalue of [H]2[S04] from Eq.(3) and the value of[H][S04]/[HS04] from Eq. (4) and hence the value of[H][HS04] or cW _(X2). Since the value of C1 isknown this gives the value of (X arid [H], [HS04] and[S04h1soi Substituting these values in Eqs (4) and (5),[S04]' [Hg2] and [Cu] can be calculated since [Cu]= [S04]euso" [Hg2] = [S04]Hg,SO, and [S04] =[S04]H,SO. + [S04] Hg,SO.+ [S04]CuSO"

The ionic strength, J1 of the solution in cell (A) isgiven by Eq. (6).

Il=C1 +4[S04]-2[S04]H,SO, ... (6)

Substituting the values of [S04] and [S04h,so, inEq. (6) a fresh value 'of J1 is obtained. The iteration iscontinued till J1 and the concentrations of various ionicspecies correct to 10 micromol dm -3 are obtained.

Assuming feuso, = I in dilute solutions and puttingK.: = [Cu] [S04]/[CUS04]

... (7)logKc-8 A v II =logK - BII

where B=bcu+bso,.

Plots of log K; - 8A Y II against J1 at all thetemperatures were linear, the intercepts at 11=0 gavelog K and the slopes gave B. Knowledge of Band Eq.(I) enables one to find fcuIso, at any J1 upto 0.1 moldm :".

The slope of the linear plot of log K against I;T givesAH ; the values of AH over the temperature range are

410

Table l-e-Dissociation Constant and ThermodynamicQuantities Related to Dissociation of Copper Sulphate

Temp pA I\G -I\H -1\5(K) (kJ mol ') (kJ mol ')(J mol 'K')

278.15 2.87±0.023 15.3±0.12 9.6±0.23 89±2.9288.15 2.94±0'()68 16.2±0.38 9.6±0.23 89±4.2298.15 3.00±0.012 17.1±0.07 9.6±0.23 89±2.5308.15 3.05±0.020 18.0±0.12 9.6±0.23 89±2.8

found to be constant (Table I). The other thermody-namic parameters Ao and AS were calculated usingwell known equations. AG values at all thetemperatures are positive. This is clearly due toentropy effect. Negative values 01 AS suggest that theions formed on dissociation are hydrated . .1.S valuesremain constant over the temperature range. Perhapsthe hydrates formed by Cu2 + and Sol in the firsthydration zone remain stable over the temperaturerange while the second hydration zone is weak.

The· authors are thankful to Dr B Prasad for hisinterest and to the UGC, New Delhi for the grant ofateacher fellowship to one of them (S N Pl.

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1194.4 Sharma L. Ionic equilibria ill tri-bioaicnt Sa/I. Ph.D. Thesis. Patna

University. 1969.5 Sharma L & Prasad B. .I Indian chctn Soc. 46 (1969) 241.6 Sharma L & Prasad B. .I II"IiulI C/I<'III .\0(, 47 /1970) 193.