Acid–base characteristics of naphthazarin and solution equilibria of yttrium(III) chelates

Abstract

The spectral absorption and acid–base characteristics of naphthazarin (5,8-dihydroxy-1,4-naphthoquinone; NAZA) in ethanol–water medium containing 50% v/v ethanol have been studied, and pK_a values have been determined. Two equilibria are established in solution at pH 2.5–11.5, based on the acid dissociation of the non-ionized form of the reagent. The existence of these equilibria is confirmed by potentiometric pH titrations, and the precision of the equilibrium study is improved. The composition, molar absorptivities and stability constants of the chelates of NAZA with yttrium(III) have been determined by spectrophotometry under the conditions encountered in this study. The complexation equilibria existing in solution are demonstrated and characterized. The Y(NAZA) mono-ligand chelate formed at pH 5.5 (ε= 1.15 × 10⁴ l mol^–1 cm^–1 at 595 nm) allows the determination of 1.95–8.86 µg ml^–1 of yttrium. Interfering species and their elimination have been studied.