Application of the Pitzer Model to a Weakly Associated Uni-univalent Electrolyte in Mixed Solvents: Solubility of Thallium(I) Chloride in the System TlCl + NaCl + Methanol + Water at 25°C

The solubility of thallium(I) chloride has been measured in methanol + water mixtures (10, 20, 30, and 40%, w/w methanol) containing added sodium chloride at concentrations up to 1 mol-kg−1 at 25°C. The data were analyzed using Pitzer's equations, without making an explicit assumption of association of Tl+ and Cl− ions to ion pairs, by assuming that α is 3 instead of 2, the value used by Pitzer for strong electrolytes in aqueous solutions. It is found that reasonable fits of the experimental solubility data could be obtained without the need to introduce a formal association constant. This implies that the β(1) parameter plays the role of an association constant. β(0) and β(1) values are derived in addition to the mixture parameters, θTl,Na and ψTlNaCl. Application of the higher-order limiting-law correction for symmetrical mixing in the manner proposed by Pitzer led to perceptibly, although insignificantly, worse fits of the experimental data, so that this correction in its present form appears to be more of theoretical than practical interest.