Application of the Pitzer model to a weakly associated uni-univalent electrolyte in mixed solvents: Solubility of thallium(I) chloride in the system TlCl plus NaCl plus methanol plus water at 25 degrees C

The solubility of thallium(I) chloride has been measured in methanol + wate r mixtures (10, 20, 30, and 40%, w/w methanol) containing added sodium chlo ride at concentrations up to 1 mol-kg(-1) at 25 degrees C. The data were an alyzed using Fitter's equations, without making an explicit assumption of a ssociation of Tl+ and Cl- ions to ion pairs, by assuming that alpha is 3 in stead of 2, the value used by Pitzer for strong electrolytes in aqueous sol utions. It is found that reasonable fits of the experimental solubility dat a could be obtained without the need to introduce a formal association cons tant. This implies that the beta((1)) parameter plays the role of an associ ation constant. beta((0)) and beta((1)) values are derived in addition to t he mixture parameters, theta(Tl,Na) and psi(TlNaCl). Application of the hig her-order limiting-law correction for symmetrical mixing in the manner prop osed 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.