A spectrophotometric study of the formation and deprotonation of thioarsenite species in aqueous solution at 22°C

The ionization constants of thioarsenous acid have been determined at 22 degrees C by measuring the ultraviolet and visible spectra of thioarsenite species in dilute, aqueous sulfide solutions having S/As = 4.18, As = 1.1 x 10(-4) mol/dm(3) and pH approximate to 6-10.4. No oxidized species such as arsenate, thioarsenate or polysulfides were detected in the experimental solutions. The equilibrium constants for the thioarsenous acid ionization reactions were obtained from a principal component analysis treatment of the spectra and are as follows: H3AsS3 = H+ + H2AsS3- for which pK(a1) = 3.77 (+/- 0.15) H2-AsS32- = H+ + H2AsS3- for which pK(a2) = 6.53 (+/- 0.08) HAsS32- = H+ + AsS33- for which pK(a3) = 9.29 (+/- 0.08) The pK(a)'s for the oxythioarsenous ionization reactions have also been estimated by analogy with those for the end member arsenous and thioarsenous acids. The data emphasize the important role of the simple arsenic(III) thioanions in defining the transport and redox chemistry of arsenic in sulfide-containing natural waters. (C) 2011 Elsevier Ltd. All rights reserved.