Standard potentials of Ag-AgBr and Ag-AgI electrodes in urea + water mixtures. Free energies and entropies of transfer of the hydrogen halides

Abstract

Standard potentials, E°, of Ag-AgBr and Ag-AgI electrodes in aqueous solution containing 11.52, 20.31, 29.64 and 36.83 % urea by weight have been determined by e.m.f. measurements on the cell: H₂(g, 1 atm)| HX (m), solvent | AgX-Ag, where X = Br or I, at seven different temperatures from 10 to 40°C at 5°C intervals. These E° values have been used to compute the free energies, ΔG^°_t, and entropies, ΔS^°_t, of transfer of HBr and HI from water to each of these solvents at 25°C. The ΔG^°_t values for HBr and HI as well as those for HCl obtained earlier, were used to determine ΔG^°_t(i) values for the individual ions by a method of “simultaneous extrapolation”. These values are dictated by increased dielectric constant, “acid-base” type ion-solvent interactions as well as by the enhanced “chemical affinity” of the mixed solvents towards H⁺. A detailed analysis of the TΔS^°_t against composition profiles of hydrogen halides exhibiting “roller coaster” type behaviour further indicates that the results conform to what is expected if increasing addition to urea shifts the equilibria: water (bulky)↔ water (dense)↔ urea–water clusters ↔ water (dense)+ urea, to the right.