Neutral antimony hydroxocomplex may be the main antimony species in hydrothermal solutions in a wide range of pH, Eh, pressure and temperature. However the question does at increasing Sb concentrations at higher temperatures monomer Sb(OH)o3(aq) or dimer Sb2(OH)o6(aq) becom dominant still remain to be answered.
To settle the question a study of dependence between Sb and H2S concentrations in solutions equilibrated with Sb2S3 (stibnite) at 350oC and 500 bars was made.
Two processes have been studied in common pressure vessel in an orderly sequence by solubility method. Weighed Sb2O3 (senarmontite) crystals was placed to the bottom of a pressure vessels and Sb2S3 crystals - in a cup, fixed to pressure vessel's roof. Sb2O3 was taken in variable amounts ( but less than its equilibrium solubility in water at 2500C) and Sb2S3 - in excess. Distilled water was added. After being blown with Ar pressure vessel's are placed in to electric furnace ( controlled to ±20C). Once a Sb2O3 has been dissolved in water at 2500C, the temperature was elevated to 3500C, allowing the solution to be in contact with the stibnite charge. The pressure 500 bar was conferred by the degree of the pressure vessel's by H2O. When the equilibrium was attained, the pressure vessels were quenched in cold water. The cup with the residual solid phase was removed. Total concentrations Sb in solutions were determined by mass loss technique ( as summing Sb from Sb2O3 and Sb from Sb2S3) and controlled by atomic adsorption spectroscopy. Contents of H2S in solutions were calculated from amounts of Sb2S3 that are dissolved.
The dissolution and complexation of stibnite by H2O can be written according to the equation:
0.5 Sb2S3(s) + 3 H2O = Sb(OH)03(aq) + 1.5 H2S(aq) (1)
Sb2S3(s) + 6 H2O = Sb2(OH)06(aq) + 3 H2S(aq) (2)
These equations vary in a slope of curves in lg mSb - lg mH2S coordinates (1.5 or 3.0 accordingly).
The experimentally determined slope is equal to 1.55 ±0.16 (from 24 points). The results indicated that a monomeric species Sb(OH)03(aq) is dominant in solutions at Sb concentration up to 3.5*10-2 mol/kg H2O. Lg Ko(1) = -5.08 ± 0.02.
The thermodynamic properties of Sb(OH)o3(aq) in the framework of the HKF model have been obtained.