Dihydroxyanthraquinone complexes of zirconium

Abstract

The nature of the zirconium complexes of alizarin red S (1,2-dihydroxy- anthraquinone-3-sulphonate), alizarin (1,2-dihydroxyanthraquinone), and hystazarin (2,3-dihydroxyanthraquinone) were studied using spectrophotometric, kinetic, and voltammetric techniques. In HClO₄- NaClO₄, 75% methanol media, these compounds form 1 : 1 complexes with zirconium. The equilibrium constants, K^C, for the reaction of zirconium with alizarin red S, alizarin, and hystazarin were found to be 2.8 x 10⁵, 3.0 x 10³, and 6.9 x 10², respectively, at 25° and μ = 1.0M. The concentration acid dissociation constants, pK^c_a, for the dihydroxyanthraquinones in 75% methanol were determined as: alizarin red S 6.71 and 11.35; alizarin 7.31 and 11.84; hystazarin 7.57 and 11.40; at 25° and μ 1.0M. Zirconium reacts slowly with alizarin red S and alizarin, and mainly via the hydrolysed Zr(OH)₂²⁺ species, whereas the reaction with hystazarin is rapid. Rate constants were determined for the first two reactions. The zirconium-alizarin red S complex includes two coordinated hydroxyl groups, the alizarin complex one, and the hystazarin complex contains none. It is suggested that these coordinated hydroxyl groups hydrogen-bond to the sulphonate and/or quinone groups and enhance both the stability and colour of the zirconium complexes.