Activity Coefficients of Potassium Dihydrogen Phosphate in Aqueous Solutions at 25°C and in Aqueous Mixtures of Urea and this Electrolyte in the Temperature Range 20–35°C

Abstract

Simple two-parameter Hückel and Pitzer equations were determined for the calculation of the activity and osmotic coefficients of aqueous solutions of potassium dihydrogen phosphate at 25°C up to a molality of the saturated solution (= 1.83 mol kg⁻¹). The isopiestic data measured by Stokes (1945) were used in the parameter estimations. The resulting parameter values were tested with all thermodynamic data found in the literature for this electrolyte at this temperature. In these tests it was observed, that the Hückel equation applies to the data within experimental error up to a molality of 1.0 mol kg⁻¹, and the Pitzer equation applies well to the data in the molality range 1.0–1.83 mol kg⁻¹ but not very well in dilute solutions. Therefore, also a three-parameter Pitzer equation was determined that applies to all data. The activity and osmotic coefficients calculated by these three models were compared to the values suggested by Robinson and Stokes (1959) and to those calculated by the equations of Hamer and Wu (1972) and of Pitzer and Mayorga (1973) for this electrolyte. Solubility measurements of KH₂PO₄ at 20, 25, 30 and 35°C were made in aqueous urea solutions, and the molality of urea varied in these measurements from 0 to 2.5 mol kg⁻¹. The thermodynamics of these studies were analyzed by using the Pitzer formalism. It was observed that only one interaction parameter between urea molecules and ions (this parameter is linearly dependent on the temperature) was needed to describe almost completely the new solubility data.