Spectrophotometric determination of the stability of thallium (I) chloride complexes in aqueous solution up to 200°C

The aim of this study has been to gain insight into the aqueous chemistry of thallium at elevated temperatures and pressures as a first step towards trying to understand the association of Tl with Au, As, and Sb, which is often found in both fossil and active hydrothermal systems. The complexing of Tl+ with chloride ligands was studied spectrophotometrically on solutions containing 0.20 mmol/dm(3) Tl, 1.0 mmol/dm(3) HCl and varying NaCl concentrations up to 3.00 mol/dm(3). The deconvolution of the spectra and computation of the equilibrium formation constants were performed using a nonlinear least squares routine. The scheme which best fitted the data at 25 degrees C contained the species, Tl+, TlCl degrees, TlCl2-, and with less certainty TlCl43-. With increasing temperature to 200 degrees C, increasing proportions of Tl an present as the free aquated Tl+ ion and the uncharged TlCl degrees. The formation of TlCl degrees with increasing temperature is associated with increasingly endothermic enthalpies and by large positive entropies. In ore-forming fluids at T greater than or equal to 200 degrees C, the dominant thallium(I) chloride complex will be the neutral species, TlCl degrees. Copyright (C) 1998 Elsevier Science Ltd.