A study of the self-association of simple azo dyes using the potentiometric method

Abstract

The thermodynamic aspects of the self-association of three monoazo dyes, viz. C.I. Acid Red 88, C.I. Acid Orange 7 and C.I. Acid Orange 8, in aqueous solution have been studied by means of a potentiometric method. An ion-selective membrane electrode, selective to a dye onion, has been constructed. This electrode permits a direct determination of the dye monomer concentration. Measurements of e.m.f. versus dye concentration were carried out and the results show that the e.m.f. response is not linear over the whole measured concentration range. Because of dye association at higher concentrations the measured e.m.f. values start to deviate from the theoretical linear relationship. From these deviations the dye monomer concentration at a given total dye concentration was determined. Using an appropriate stepwise association model the dimerization constants and concentrations of the dimers and higher multimers were calculated. It is concluded that C.I. Acid Red 88 aggregates more readily than C.I. Acid Orange 7. C.I. Acid Orange 8, the structure of which includes a charged sulphonic acid group in the central part of the molecule, does not aggregate in the measured concentration range. The effects of temperature and added salt on the association of C.I. Acid Red 88 have also been investigated. From the results, it can be seen that the association of C.I. Acid Red 88 decreases with increasing temperature, and strongly increases with increasing salt concentration. Thermodynamic functions of dimerization of C.I. Acid Red 88 in aqueous solutions at different temperatures were also determined. It appears that the dimerization of C.I. Acid Red 88 in aqueous solution is governed mainly by dispersive interactions between the dye molecules.