Determination of single-ion activity coefficients of hydrogen and bromide ions in aqueous hydrobromic acid solutions based on an ionic liquid salt bridge

摘要

Single ion activity coefficients of H~+ and Br~- in aqueous hydrobromic acid solutions, γ_(H~+) and γ_(Br~-), have been estimated potentiometrically at 25℃ with a hydrogen electrode and a silver-silver bromide electrode, respectively, based on the assumption that the change in the liquid junction potential at the interface between a sample solution and an ionic liquid salt bridge is negligibly small. The mean activity coefficients of HBr calculated from experimentally obtained values of γ_(H~+) and γ_(Br~-) well agree with the literature values over the entire concentration ranges studied, 1.04 × 10~(-3) to 0.528 mol kg~(-1). Overall dependencies of γ_(H~+) and γ_(Br~-) on ionic strength are similar to those found previously in aqueous HCl solutions, but the degree of the dependence is distinctively stronger in both γ_(H~+) and γ_(Br~-). Plots of γ_(H~+) and γ_(Br~-) as a function of the ionic strength are nearly congruent with the predictions by Fraenkel's smaller-ion shell (SiS) model up to 0.05 mol kg~(-1). However, with a further increase in HBr concentration experimental γ_(H~+) and γ_(Br~-) more rapidly increases and decreases, respectively, in comparison with the SiS predictions.