Stability and structure of aqueous Sb (III) chloride complexes from 25 °C to 250 °C at 25 MPa at high chloride ion concentrations

Abstract

Despite their importance in modelling the formation of antimony-containing hydrothermal ore deposits, only a few values for the thermodynamic properties of the antimony(III) chloride and sulfide complexes have been reported above ambient conditions. The existing thermochemical data, which extend to ∼250 °C, and the structures of the equilibrium antimony chloride and hydroxy chloride complexes, are largely based on solubility data and EXAFS studies. In the present study, polarized Raman spectroscopy with pressure-controlled, high-pressure fused-silica capillary cells was used to identify the complexes of antimony present in highly-concentrated aqueous lithium chloride solutions (>2 mol·kg⁻¹) from 25 to 250 °C at 25 MPa. Vibrational band assignments were made by comparison with computational studies using Gaussian 16 (B3LYP, IEFPCM solvation model), which showed that SbCl₆³⁻ (octahedral), SbCl₄⁻ (see-saw) and SbCl₃⁰ (trigonal pyramidal) are the predominant aqueous antimony species up to 250 °C. Quantitative speciation data from the solvent-subtracted, reduced isotropic spectra were used to determine the stepwise formation constants, K₃,₄ and K₄,₆, together with the Lindsay-Meissner-Tester activity coefficient model. Attempts to fit K₄,₆ to the ionic-strength suggested that the predominant species of SbCl₆³⁻ under these conditions is an ion pair, LiSbCl₆²⁻, whose formation constant is expressed as K₄,_6Li. The experimental equilibrium concentrations and the fitted values of K₃,₄ and K₄,_6Li show that the equilibria shift with increasing temperature, such that SbCl₄⁻ becomes the dominant species in concentrated solutions at 250 °C. This study includes new evidence based on Raman band assignments to resolve the controversy of whether the dominant species at high alkalinities and lower temperatures is SbCl₆³⁻ or SbCl₅²⁻.