Effects of Inorganic Salts on the Dissociation of a Complex of β-Cyclodextrin with an Azo Dye in an Aqueous Solution

The apparent dissociation constant (Kapp) of a complex between β-cyclodextrin and an azo dye, sodium p-(4-hydroxy-1-naphthylazo)benzenesulfonate, was determined by spectrophotometry in a phosphate buffer solution (pH 5.91) in the absence and in the presence of various inorganic salts at 25 °C. The cyclodextrin and the azo dye formed a 1 : 1 complex with the Kapp value of 2.44×10−3 M in a 0.1 M phosphate buffer. Kapp decreased with an increase in the concentration of the phosphate buffer and with the addition of some inorganic salts, such as Li2SO4, Na2SO4, K2SO4, LiIO3, NaIO3, KIO3, and KF. These results were explained mainly in terms of the decrease in the activity of water, which takes part in the dissociation reaction of the inclusion complex, with an increase in the concentration of the inorganic salts. On the other hand, Kapp increased with the addition of such salts as KCl, KBr, KI, KNO3, KSCN, and KClO4. These results were attributed mainly to the formation of the inclusion complexes of the cyclodextrin with the anions of these salts, competing with the azo dye for the cyclodextrin-binding site.