Studies in ‘isodielectric’ media. Part I. Standard potentials of Ag–AgX (X = Cl or Br) electrodes in methanol–propylene glycol at 25 °C

Abstract

Standard potentials (_sE°) of the Ag–AgCl and Ag–AgBr electrodes have been determined in a series of approximately ‘isodielectric’ media, formed by mixing methanol (D_s= 32·63) and propylene glycol (D_s= 31·0) in various proportions at 25°, from the e.m.f. measurements of the cell: Pt, H₂(g, 1 atm)|HCl(m), solvent|AgCl–Ag and Pt, H₂(g,1 atm)|HOAc(m₁), NaOAc(m₂), NaX(m₃), solvent|AgX–Ag, where X = Br. The required p(_sK_a)_HOAc values for the latter were determined from the same type of cell with X = Cl. The molal activity coefficients (^s_sγ)_HCl of HCl, referred to the standard state in each solvent mixture, have been evaluated at different concentrations (0·005—0·07m), and are found to vary only slightly from solvent to solvent at any concentration, as expected from the small difference in dielectric constants of the solvents. The standard free-energy changes ΔG_t°(HX) accompanying the transfer of 1 mole of HX from methanol to the other solvent have been computed, and are found to decrease some–what sharply at first but less so as the proportion of propylene glycol increases. The relative magnitudes of ΔG_t°(HX) for both HCl and HBr indicate that anions are glycophilic and that H⁺ is methanophilic, suggesting that methanol is more ‘basic’ than propylene glycol.