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

Abstract

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°C contained the species, Tl +, TlCl°, TlCl 2 −, and with less certainty TlCl 4 3−. With increasing temperature to 200°C, increasing proportions of Tl are present as the free aquated Tl + ion and the uncharged TlCl°. The formation of TlCl° with increasing temperature is associated with increasingly endothermic enthalpies and by large positive entropies. In ore-forming fluids at T ≥ 200°C, the dominant thallium(I) chloride complex will be the neutral species, TlCl°.