Spectrophotometric determination of the dissociation constants of methyl yellow in mixed protic solvents

The concentration dissociation constants (pK(a)) of methyl yellow, MY (H+In ) in mixed aqueous solvents of methanol, ethanol, iso-propanol, tert-butano l have been accurately determined from spectrophotometric measurements at 2 5 degrees C and a constant ionic strength of 0.1 mol 1(-1). It has been sho wn that in these solvents, the pK(a) values decrease with increasing compos ition of the organic co-solvent. A linear relationship between pK(a) and th e mole fraction (x(2)) of the co-solvent was observed in a limited range of the compositions for each of the solvent systems. The results have been di scussed in the light of transfer thermodynamic properties of the species ex isting in the dissociation equilibrium, solvent basicity and solute-solvent interactions. Furthermore, it was also observed that with the change of th e solvents, the absorption spectra of MY shifted apparently and the color t ransition changed accordingly. The solvent effect on the spectra has been a ttributed to the isomerization equilibria of MY. A simple application of MY was also shown to the sodium acetate-hydrochloric acid titrations in the m ixed solvents. (C) 1999 Elsevier Science B.V. All rights reserved.