SPECTROPHOTOMETRIC DETERMINATION OF THE STABILITY OF TIN(II) CHLORIDE COMPLEXES IN AQUEOUS SOLUTION UP TO 300°C

Abstract

The aim of this study was to gain insight into the aqueous chemistry of tin at elevated temperatures and pressures. For this purpose, the spectra of chloridotin(II) complexes in the ultraviolet region were measured in acid chloride solutions containing 1.00 x 10-4 m tin, 1.00 x 10-2 m HCl and varying NaCl concentrations up to 2.936 m from 25 to 300°C and at the equilibrium saturated vapor pressure of the system. The thermodynamic cumulative and stepwise formation constants of the individual tin(II) chloride species were calculated from the spectrophotometric data using a nonlinear least squares routine. At 25°C, a scheme including five species of SnCln2-n (0 =< n =< 4) best fitted the available data. At 300°C however, SnCl20 and SnCl3- are the predominant species at total chloride concentrations >0.1 m. The presence of SnCl42- in concentrated (up to 3 m) chloride solutions was not detected above 150°C. With increasing temperature, chloridotin(II) complex stability is characterized by quite large endothermic enthalpies and large positive entropies of formation. It was concluded that tin(II) chloride complexes could be important in the transport and deposition of tin by hydrothermal ore solutions of moderate to high salinity.