Spectrophotometric determination of the dissociation constant (pKa) of arsenous acid

✍ Scribed by H. Yamazaki; R.P. Sperline; H. Freiser

• Publisher: Elsevier Science
• Year: 1993
• Tongue: English
• Weight: 497 KB
• Volume: 284
• Category: Article
• ISSN: 0003-2670
• DOI: 10.1016/0003-2670(93)85323-c

✦ Synopsis

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The dissociation constant of meta-arsenous acid (HAsO,) was accurately determined from spectrophotometric measurements, combined with volumetric methods. The measurement of pH of the analyte solution containing HAsO,, 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 pK, of HAsOz was determined to an accuracy of f 0.005 pK, units at temperatures between 25 and 65"C, and ionic strengths between 0 and 3 M. The standard enthalpy change of HAsO,, AH", which was calculated from its dissociation constants, was 29.59 f 0.78 kJ/mol. Determination of the pK, values is necessary for the characterization of As,O, as a dual purpose acid-base and oxidation-reduction primary standard reagent under various conditions.