Isopiestic determination of the osmotic and activity coefficients ofZnSO4(aq) atT = 298.15 K, and the standard potential of the electrochemical cell ZnHgx(two phase)| ZnSO4(aq)| PbSO4(s)| PbHgx(two phase)

Abstract

Isopiestic vapor-pressure measurements were made for ZnSO4(aq) at T= 298.15 K, both at Texas Christian University and at Lawrence Livermore National Laboratory, from molality m= 0.1004 mol · kg−1to 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 · 10−4to m= 3.4413 mol · kg−1to yield recommended values of the osmotic coefficients, water activities, and mean activity coefficients of ZnSO4(aq), along with the thermodynamic solubility product and a CODATA compatible value for the Gibbs free energy of formation of goslarite: Ks{ ZnSO4· 7H2O(cr, orthorhombic) } = (0.01119 ± 0.00033) and ΔfGmo(ZnSO4· 7H2O, cr, orthorhombic, 298.15 K) =−(2562.32 ± 0.57)kJ · mol−1. An equation of the type used by Pitzer and Mayorga (J. Solution Chem.1974, 3, 539–546) accurately represents the experimental osmotic coefficients and e.m.f.s provided it is modified by making the third virial coefficient ionic-strength dependent and by adding a fourth virial term. However, the mean activity coefficients of ZnSO4(aq) undergo a change from negative deviations from the Debye–Hückel limiting law below m∼ 0.01 mol · kg−1to positive deviations at higher molalities as described by Malatesta and Zamboni (J. Solution Chem.1997, 26, 791–815), and our recommended values were adjusted for this effect. The standard potential of the reversible electrochemical cell:ZnHgx(twophase)|ZnSO4(aq)|PbSO4(cr)|PbHgx(twophase)was evaluated, and the resulting value was used for evaluating the standard potentials of the PbSO4(cr)|PbHgx(two phase) and Hg2SO4(cr)|Hg(l) electrodes.