Arsenous acid ionisation in aqueous solutions from 25 to 300°C

Structures and acidity constants of arsenite and thioarsenite species in hydrothermal solutions

“…The pK as are 6.7 and 6.0 at 473 K and 573 K, respectively, which is in agreement with the measurements within 1.1 pK units. Overall, our results show a decreasing trend with temperature, which supports the experiment of Zakaznova-Herzog et al (2006).…”

“…The molecular structures have been successfully reproduced, but due to the oversimplification of solvent effects, the calculated pK a results can be far from the experimental results. For example, 11.5 and 5.0 are obtained for the pK a1 s of H 3 AsO 3 and H 3 AsS 3 (Zimmermann and Tossell, 2009), which are close to the experimental results of 9.2 and 3.77, respectively (Zakaznova-Herzog et al, 2006;Zakaznova-Herzog and Seward, 2012). However, the calculated pK a2 and pK a3 of H 3 AsS 3 are 13.8 and 18.5, respectively, which are 7 pK units higher than the experimental results (Zakaznova-Herzog and Seward, 2012).…”

“…H 3 AsO 3 (arsenous acid) and H 3 AsS 3 (thioarsenous acid) containing As(III) are both weak acids; therefore, the acidity constants (pK a s) are the key thermodynamic quantities for determining the protonation states, significantly influencing the transport and fixation of the moieties in sediments and soils (Watts et al, 2014). The acidity constant of arsenous acid has been well constrained at ambient conditions (pK a1 is within 9.2-9.3) (Zakaznova-Herzog et al, 2006), but the measurements at elevated temperatures are very limited. The SUPCRT92 (Johnson et al, 1992) prediction shows that the pK a s first decrease to 8.2 at 473 K and then increase back to 8.5 at 573 K. In other words, the changes in pK a s are unobvious and limited in only 1.0 pK unit up to 573 K. In contrast, by using an ultraviolet spectrophotometric technique, Zakaznova-Herzog et al found that the pK a s monotonically decrease to 7.0 as temperature increases to 573 K (Zakaznova-Herzog et al, 2006).…”

“…Quantum chemical methods have been applied to investigate arsenic species (Tossell, 1997(Tossell, , 2003Zakaznova-Herzog et al, 2006;Helz and Tossell, 2008;Tossell and Zimmermann, 2008;Zimmermann and Tossell, 2009). In those studies, the molecular structures and pK a s of oxy/thio acids of As(III) and As(IV) were calculated by using the geometry optimization approach with continuum solvent models.…”

Arsenic Chemistry

“…Furthermore, like pH, pK a values are often sensitive to temperature. In hydrothermal waters, for example, the pK a value of Reaction 2.43 declines with increasing temperature so that the value approaches 7.11 at 300 • C (Zakaznova-Herzog, Seward and Suleimenov, 2006), 1936; Table 2.10. Dissociation constants for weak acids may be measured by titrating salts of the acids with strong acids, such as hydrochloric acid (HCl).…”