Evaluation of dissociation constants of ammonium ions in aqueous ammonium chloride and potassium chloride solutions and of pertinent higher-order parameters according to the Pitzer approach

Abstract

Dissociation constants of NH⁺₄ at 298.15 K in NH₄Cl solutions, calculated in terms of Pitzer's equations, have been compared with observed ones. The values in up to 3 mol kg^–1 solutions of NH₄Cl, calculated with reported values for the higher-order terms θ_{H N H4}(both ionic strength-dependent and -independent) and ψ_{H N H4Cl}, agreed with the observed ones within experimental uncertainties, while the calculated values showed large deviations from the observed ones in solutions of NH₄Cl more concentrated than 3 mol kg^–1. Thus, the values for the higher-order terms were re-evaluated by fitting the dissociation constants observed in those solutions to the values calculated with Pitzer's equations. Values for higher-order terms θ_{N H4K} and ψ_{N H4KCl} pertaining to the dissociation of NH⁺₄ in KCl solutions were determined by the same procedures. Activity coefficients of NH₃ in NH₄Cl and KCl solutions at 298.15 K, which were required for the calculation of the dissociation constants, were obtained by a transpiration method.