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

Abstract

Single ion activity coefficients of H+ and Br− in aqueous hydrobromic acid solutions, γH+ and γBr−, have been estimated potentiometrically at 25°C 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 γH+ and γBr− well agree with the literature values over the entire concentration ranges studied, 1.04 × 10−3 to 0.528molkg−1. Overall dependencies of γH+ and γBr− on ionic strength are similar to those found previously in aqueous HCl solutions, but the degree of the dependence is distinctively stronger in both γH+ and γBr−. Plots of γH+ and γBr− as a function of the ionic strength are nearly congruent with the predictions by Fraenkel's smaller-ion shell (SiS) model up to 0.05molkg−1. However, with a further increase in HBr concentration experimental γH+ and γBr− more rapidly increases and decreases, respectively, in comparison with the SiS predictions.