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 θH N H4(both ionic strength-dependent and -independent) and ψH 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 θN H4K and ψN 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.