Activity Coefficients of the System {yKCl + (1 – y)KH₂PO₄}(aq) at T = 298.15 K Determined by Cell Potential Measurements

Zero-current cell potential measurements were used to determine the solution activity coefficient in a ternary system { y KCl + (1 – y ) KH 2 PO 4 }(aq) at temperature T = 298.15 K. The cell of the type K–ISE|KCl( m KCl ), KH 2 PO 4 ( m KH 2 PO 4 )|Ag|AgCl was used in the total ionic strength range, I m = 0.0886–1.0046 mol kg –1 . In order to generate a set of parameters that can be applied in a wide range of mixed solution ionic strengths, the Pitzer, Scatchard, and Clegg–Pitzer–Brimblecombe models were used to fit all available experimental data, including cell potential and isopiestic measurements from the literature. The experimental and calculated values of thermodynamic properties for the studied system are in excellent agreement. Potential interactions and solution structure were discussed by means of the excess free energy of mixing via potential pairs, triplets, or quads for the investigated solution using the Scatchard model mixing parameters.