Spectrophotometric Investigation of the Formation of Association Complexes between Alizarin Red S and β-Cyclodextrin

The acid-base equilibria of Alizarin Red S(ARS) were investigated spectrophotometrically at 25 °C and μ=0.2 in the presence of β-cyclodextrin (β-CD). In the presence of β-CD, the dissociation of the first proton (β-OH) of ARS is slightly enhanced and that of the second one (α-OH) is significantly suppressed. All the ionic species of ARS interact with β-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 β-CD, respectively. The distribution curves for all the species present in the ARS-β-CD system are given as a function of the pH of the solution.