A Spectrophotometric Study on the Thorium Complex of o-Carboxyphenyl-azo-chromotropic Acid

Abstract
o-Carboxyphenyl-azo-chromotropic acid was synthesized as a colorimetric reagent for thorium. The acid dissociation constants of the reagent have been determined spectrophotometrically at 25°C and μ=0.1 (NaClO4). The pK1 (for the dissociation of the carboxyl group) and the pK2 (for the dissociation of the naphtholic hydroxyl groups) were 3.20 and 10.17 respectively. The reagent forms a violet 1 : 1 complex with thorium, showing an absorption maximum at 565 mμ in an acidic solution. The apparent stability constant of the thorium complex was measured spectrophotometrically.