Isopiestic determination of the osmotic and activity coefficients of ZnSO4(aq) at T=298.15 K, and the standard potential of the electrochemical cell ZnHgx(two phase)vertical bar ZnSO4(aq)vertical bar PbSO4(s)vertical bar PbHgx(two phase)

Isopiestic vapor-pressure measurements were made for ZnSO4(aq) at T = 298.1 5 K, both at Texas Christian University and at Lawrence Livermore National Laboratory, from molality m = 0.1004 mol . kg(-1) to the supersaturated m = 4.3092 mol . kg(-1), and included solubility determinations. These results were combined with critically assessed e.m.f. results from m = 6.7 . 10(-4 ) to m = 3.4413 mol . kg(-1) to yield recommended values of the osmotic coe fficients, water activities, and mean activity coefficients of ZnSO4(aq), a long with the thermodynamic solubility product and a CODATA compatible valu e for the Gibbs free energy of formation of goslarite: K-s{ZnSO4 . 7H(2)O(c r, orthorhombic)} = (0.01119 +/- 0.00033) and Delta (f)G(m)(o),(ZnSO4 . 7H( 2)O, cr, orthorhombic, 298.15 K) = -(2562.32 +/- 0.57) kJ . mol(-1). An equ ation of the type used by Pitzer and Mayorga (J. Solution Chem. 1974, 3, 53 9-546) accurately represents the experimental osmotic coefficients and e.m. f.s provided it is modified by making the third virial coefficient ionic-st rength dependent and by adding a fourth virial term. However, the mean acti vity coefficients of ZnSO4(aq) undergo a change from negative deviations fr om the Debye-Huckel limiting law below m similar to 0.01 mol . kg(-1) to po sitive deviations at higher molalities as described by Malatesta and Zambon i (J. Solution Chem. 1997, 26, 791-815), and our recommended values were ad justed for this effect. The standard potential of the reversible electroche mical cell: ZnHgx(two phase)\ZnSO4(aq)\PbSO4(cr)\PbHgx(two phase) was evaluated, and the resulting value was used for evaluating the standard potentials of the PbSO4(cr)\PbHgx(two phase) and Hg2SO4(cr)\Hg(1) electrod es. (C) 2000 Academic Press.