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

• Published in: Geochimica et cosmochimica acta 2006-01, Vol.70 (9), p.2298-2310
• Main Authors: Zakaznova-Herzog, V.P., Seward, T.M.
• Format: Article
• Language: English
• Subjects: Antimony; Chemical equilibrium; Fluid dynamics; Fluid flow; Fluids; Sulfur; Sulphur; Vapor pressure
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2006.01.029

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: H 3SbO 3 0 ⇄ H + + H 2SbO 3 − for which p K 1 (antimonous acid) decreases from 11.82 to 9.88 over a temperature range from 25 to 300 °C and H 3SbO 3 0 + H + ⇄ H 4SbO 3 + for which log K a 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 K a = 1.8 at 300 °C. Unionised antimonous acid, H 3SbO 3 0, 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 H 4SbO 3 + species.