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, KC, 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, pKca,
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.
Acid Dissociation Constants of 5-Substituted Tropolones and the Stability of their Iron Chelates