SPECTROPHOTOMETRIC INVESTIGATION OF THE FORMATION OF ASSOCIATION COMPLEXES BETWEEN ALIZARIN RED-S AND BETA-CYCLODEXTRIN

The acid-base equilibria of Alizarin Red S (ARS) were investigated spectrophotometrically at 25-degrees-C and mu = 0.2 in the presence of beta-cyclodextrin (beta-CD). In the presence of beta-CD, the dissociation of the first proton (beta-OH) of ARS is slightly enhanced and that of the second one (alpha-OH) is significantly suppressed. All the ionic species of ARS interact with beta-CD to form 1:1 associated complexes, the stabilities of which decrease in the order; HL-HA > H2L-HA > L-HA > L-A, where L and A denote the fully deprotonated form of ARS and the deprotonated form of beta-CD, respectively. The distribution curves for all the species present in the ARS-beta-CD system are given as a function of the pH of the solution.