Add time:08/07/2019         Source:sciencedirect.com

The dissociation constant of meta-arsenous acid (HAsO2) was accurately determined from spectrophotometric measurements, combined with volumetric methods. The measurement of pH of the analyte solution containing HAsO2, which was titrated with sodium hydroxide, was achieved by the spectrophotometric method using thymol blue as a pH indicator. Acetic acid and aqueous ammonia were used as primary standards. The pKa of HAsO2 was determined to an accuracy of ± 0.005 pKa units at temperatures between 25 and 65°C, and ionic strengths between 0 and 3 M. The standard enthalphy change of HAsO2, ΔH°, which was calculated from its dissociation constants, was 29.59 ± 0.78 kJ/mol. Determination of the pKa values is necessary for the characterization of As2O3 as a dual purpose acid-base and oxidation-reduction primary standard reagent under various conditions.

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