Thermodynamics of amalgam cells {M-amalgam|MCl2 (m)|AgCl|Ag} (M = Sr, Ba) and primary medium effects in {methanol + water} and {ethanol + water} solvent mixtures

For the amalgam cell {MxHg1 − x|MCl2 (m)|AgCl |Ag} (with M = Sr, Ba) the potential difference E has been measured as a function of the mole fraction xM 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 wA ⩽ 0.50 of alcohol, at temperature 298.15 K. The respective molal-scale standard potential differences Em∘ have been determined together with the relevant activity coefficients γ± as functions of the MCl2 molality. The Em∘ 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 (ΔGW→Z∘)m=2F(WEm∘-ZEm∘), which constitutes the “primary medium effect” upon transferring MCl2 from pure water to the (alcohol + water) mixture, is linear in xA. 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 (ΔGW→Z∘)c=2F(WEc∘-ZEc∘) 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.