Thermodynamics of the amalgam cells {M-amalgam|MCl or MCl2(m)|AgCl|Ag} (M = Rb, Cs, Sr, Ba) and primary medium effects in (acetonitrile + water)

Abstract The potential difference E of the amalgam cell {M x Hg 1 −  x |MCl or MCl 2 ( m )| AgCl |Ag} (M=Rb, Cs, Sr, Ba) has been measured as a function of the mole fraction x M of M metal in amalgams and of the molality m of MCl (or MCl 2 ) in (acetonitrile [A] + water [W]) solvent mixtures containing up to acetonitrile mass fraction w A =0.50, at T =298.15 K. The respective molal-scale standard potential differences E ∘ m have been determined together with the relevant activity coefficients γ ± functions of the MCl (or MCl 2 ) molality. The E ∘ m dependence on the mole fraction x A of acetonitrile in the solvent mixture within the range explored turns out to be linear for all the four metals M in the amalgams considered. Of course, also the difference ([ E ∘ m ] W −[ E ∘ m ] A ), which is a measure of the primary medium effect upon transferring MCl (or MCl 2 ) from pure water [W] to the acetonitrile [A] mixture, is linear in x A . In this context, following Feakins and French's scheme, which implies volume fraction statistics, analysis of the relevant mol · dm −3 scale primary medium effects, i.e., ([ E ∘ c ] W −[ E ∘ c ] A ), upon MCl (or MCl 2 ), as a linear function of the logarithm of water volume fraction, would lead to primary hydration numbers of 4.2 for RbCl, 4.0 for CsCl, 10.7 for SrCl 2 , and 10.3 for BaCl 2 , respectively, in acceptable agreement with literature data by Bockris based on different methods.