Abstract
2-(1, 8-Dihydroxy-3, 6-disulfo-2-naphthylazo)-mandelic acid forms a stable violet complex with thorium ion at 1:1 molar ratio in an acidic solution. The acid dissociation constants of the reagent and the apparent stability constant of its thorium complex were estimated spectrophotometrically at 25°C and μ=0.1. It was found that the thorium complex formed by the present reagent was more stable than those of the other phenyl-azo-chromotropic acid derivatives reported in the preceding papers.