Anion-Specific Effects on Activity Coefficients in Aqueous Solutions of Sodium Salts: Modeling with the Extended Debye-Huckel Theory

The extended Debye-Huckel theory, which allows for concentration variation of electrolyte solution static permittivity, is employed to predict activity coefficients in aqueous solutions of sodium salts with various univalent anions (NaCl, NaBr, NaI, NaNO3, NaClO4 and NaSCN) at ambient conditions. Calculations without empirical adjustments reproduced the activity coefficients for NaI in the concentration range up to 6molkg(-1) and for NaSCN up to 2molkg(-1). In the case of other solutions, calculations underestimate water activity coefficients and overestimate mean ionic activity coefficients at concentrations beyond 0.5molkg(-1). In order to improve the representations, the model was extended to include ion pairing, which resulted in a better agreement between calculated activity coefficients and experimental data, especially for NaNO3. The ion pairing equilibrium constants were estimated and compared with available literature values. The extent of ion pairing was found to increase in the sequence NaI < NaSCN < NaBr < NaCl < NaClO4 < NaNO3, with violation of the Collins rule in the case of polyatomic oxygen-containing anions.