Thermodynamics of amalgam cells {M-amalgam vertical bar Cl-2 (m)vertical bar AgCl vertical bar Ag} (M = Sr, Ba) and primary medium effects in {methanol plus water} and {ethanol plus water} solvent mixtures

摘要

For the amalgam cell {M(x)Hg(1 -x)vertical bar MCl2 (m)vertical bar AgCl vertical bar Ag} (with M = Sr, Ba) the potential difference E has been measured as a function of the mole fraction x(M) of the metal M in amalgams and of the molality m of MCl2 in {methanol + water} and {ethanol + water} solvent mixtures Z = {A + W} with mass fractions of alcohol w(A) <= 0.50 of alcohol, at temperature 298.15 K. The respective molal-scale standard potential differences E, have been determined together with the relevant activity coefficients gamma+ as functions of the MCl2 molality. The E-m(o) dependence on the alcohol mole fraction in the solvent mixture within the ranges explored turns out to be linear for both of these two metals M in the amalgams studied. Of course, also the molal-scale standard Gibbs free energy change (Delta G(w -> z)(o))(m) = 2F(E-W(m)o - E-Z(m)o), which constitutes the "primary medium effect" upon transferring MCl2 from pure water to the (alcohol + water) mixture, is linear in x(A). In the same context, following Feakins and French's treatment, which implies volume fraction statistics, the relevant primary medium effects upon MCl2 on the mol (.) dm(-3) scale have been analysed in terms of the expected linear relation of (Delta G(w -> z)(o))(c) = 2F(E-W(c)o - E-Z(c)o) against the logarithm of water volume fraction, leading to primary hydration numbers for SrCl2 and BaCl2, respectively, in acceptable agreement with Bockris' data based on different methods. (c) 2004 Elsevier Ltd. All rights reserved.