Estimation of activity coefficients of individual ions in solutions with ionic strengths up to 0.3 mol dm−3

Abstract

SUMMARYFor chemical speciation calculations concerning soil solutions it is essential to convert ionic concentrations to activities. For dilute solutions with ionic strengths up to 0.3 mol dm−3, relations are given for calculating the single‐ion activity coefficients of 26 ions. Using the mean‐salt method, measured mean activity coefficients have been converted into single‐ion activity coefficients. For each ion, the variation in ion activity coefficient with ionic strength has been curve‐fitted with the first degree extension of the Debye‐Hückel equation. The ion‐dependent parameters in the extended Debye‐Hückel equation have been quantified.The derived set of relations can be incorporated in geochemical models, which makes calculating ion activities in soil solutions with variable ionic strengths up to 0.3 mol dm−3 simple and accurate. With an ion speciation model it is shown for various solutions with ionic strengths up to 0.3 mol dm−3 that the type of equation chosen for calculating single‐ion activity coefficients can lead to differences in computed ion speciation.