Activity coefficients of CaCl2 in (serine or proline + water) mixtures at T = 298.15 K

Activity coefficients for the (CaCl2 + amino acid + water) system were determined at a temperature of 298.15 K using ion-selective electrodes. The range of molalities of CaCl2 is (0.01 to 0.20) mol · kg−1, and that of amino acids is (0.10 to 0.40) mol · kg−1. The activity coefficients obtained from the Debye–Hückel extended equation and the Pitzer equation are in good agreement with each other. Results show that the interactions between CaCl2 and amino acid are controlled mainly by the electrostatic interactions (attraction). Gibbs free energy interaction parameters (gEA) and salting constants (kS) are positive, indicating that these amino acids are salted out by CaCl2. These results are discussed based on group additivity model.