Study of complex formation equilibria of pamoate ion with copper(II) ion in a dioxane-water solution

Abstract

Protonation constants of the carboxylate groups in pamoic acid and stability constants of copper(II) complexes with pamoic acid have been determined from EMF data in a 50% (v/v) dioxane-water solution at 25°C and 0.1 M KNO 3, and protonation constants for the hydroxyl groups have been determined spectrophotometrically in 1 M KCl aqueous medium. The values obtained for the constants are: protonation constants for the carboxylate groups: log β 1 = 4.352±0.001 and log β 2 = 7.706 ± 0.001; for the hydroxyl groups: log K 1 = 14.80±0.24 and log K 2 = 13.05±0.065. For the complex-forming reactions Cu 11+ nH 2pam ⇌ Cu(pamH) n + nH ( n = 1 or 2), the equilibrium constants are: log β 11-1 = −1.96± 0.02 and log β 21-2 = −5.28±0.02. An insoluble 3:2 copper(II)-pamoate complex is identified.