Antimonous acid protonation/deprotonation equilibria in hydrothermal solutions to 300 °C

The ultraviolet spectra of dilute aqueous solutions of antimony (III) have been measured from 25 to 300 °C at the saturated vapour pressure. From these measurements, equilibrium constants were obtained for the following reactions:H3SbO30 ⇄ H+ + H2SbO3−for which pK1 (antimonous acid) decreases from 11.82 to 9.88 over a temperature range from 25 to 300 °C andH3SbO30 + H+ ⇄ H4SbO3+for which log Ka initially decreases from 1.38 at 22 °C with increasing temperature up to 100 °C but then increases until it reaches a value of log Ka = 1.8 at 300 °C. Unionised antimonous acid, H3SbO30, will be the dominant species responsible for antimony transport in low sulphur geothermal fluids in the Earth’s crust. In hydrothermal fluids having a high magmatic input, the low pH environment will also encourage the stability of the protonated H4SbO3+ species.