The Stabilities of Metal Chloride Complexes in Hydrothermal Solutions

Abstract

The stabilities of the ion pair and metal chloride complexes in hydrothermal solutions have been studied experimentally and theoretically. Solubilities of silver chloride in aqueous hydrochloric acid solutions have been determined from 100 up to 350°C. From these measurements, the ionization constant Kd of HCl has been evaluated up to 225°C. Evidence is presented to show that a protonated species, HAgCl°v2, exists at 275°C and above, which makes evaluation of K vd at these temperatures from the experimental data difficult. Available experimental data up Co 200°C have been fitted to Pitzer's equation to generate an algorithm to calculate stoichiometric activity and osmotic coefficients of HCl up to 350°C and concentrations up to at least 3.0 m. Using the present results and those of Wright et al. (1961) and Pearson et al. (1963) above 300°C, the dissociation constant of HCl as a function of temperature is described by the equation log10 K vd=-32030.6/T+1396.24+0.69749T-3•2206x10 v-4•T^2-593.026 log10^T (T in K) which is valid in the range 25-350°C. Calculated Gibbs free energy (∆G), enthalpy (∆H), entropy (∆S) and constant-pressure heat capacity change (∆Cp) functions for HCl° dissociation have been rationalised in terms of changing solute and solvent characteristics as temperature is raised. Solubilities of AgCl in aqueous Zn(II)-HCl solutions have been measured from 100 to 350°C at the saturated vapour pressure. From these measurements, the cumulative equilibrium formation constants of chlorozinc(II) complexes have been evaluated using a nonlinear least squares procedure. At 100°C, all the species ZnCl ^(2-n) vn (n = 0 to 4) exist, but as temperature rises, ZnC^- vl3 concentrations decrease and its stability field vanishes above 200°C. Chlorozinc(II) complex formation is characterized by large positive enthalphy and entropy changes reflecting the predominantly electrostatic interaction between the acceptor (zinc) and donor (chloride) species. Zinc transport is evaluated in two contrasting geothermal fluids and it is found that zinc chloride complexing is important in low pH, sulphur deficient hydrothermal solutions (Bacon-Manito, Philippines) of moderate to high salinity but not necessarily in near neutral to alkaline solutions (Broadlands, New Zealand). Sphalerite solubilities in chloride solutions have been calculated using the formation constants obtained in the present study. The optical absorption spectra of a series of NiClv2-HCl aqueous solutions have been obtained from room temperature up to 300°c. With increase in temperature, solutions low in chloride change in colour from pale green to yellow. Solutions which are high in chloride turn to yellow at intermediate temperatures but become blue at the highest temperatures. The following changes on the spectra are observed: (1) the positions of the absorption maxima shift to lower energies, (2) there is an increase in intensity and (3) some band boadening has taken place. By deconvoluting the spectra into individual bands, the formation constants of the nickel(II) chloride complexes have been obtained. The observed spectra have been rationalised and supported by crystal field calculations. It was found that The changes in the energy levels in hexaaquonickel(II) ion can be explained by a decrease in the the ligand field strength around the metal ion. Changes in intensity have been modeled by invoking vibronic coupling. The use of the isocoulombic approach has been shown to be valid in estimating instability constants of a large number of metal chloride complexes up to 300°C. Trends in the calculated K, ∆s and ∆H are consistent with changes in the thermodynamic and electrostatic properties of the solvent with temperature rise. Limitations of the method are also pointed out. Results have been applied in calculating metal speciation and solubilities of copper sulphides in hydrothermal solutions.