title: Spectrophotometric determination of the dissociation constants of methyl yellow in mixed protic solvents.
abstract: The concentration dissociation constants (pK(a)) of methyl yellow, MY (H(+)In) in mixed aqueous solvents of methanol, ethanol, iso-propanol, tert-butanol have been accurately determined from spectrophotometric measurements at 25 degrees C and a constant ionic strength of 0.1 mol l(-1). It has been shown that in these solvents, the pK(a) values decrease with increasing composition of the organic co-solvent. A linear relationship between pK(a) and the 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 discussed in the light of transfer thermodynamic properties of the species existing in the dissociation equilibrium, solvent basicity and solute-solvent interactions. Furthermore, it was also observed that with the change of the solvents, the absorption spectra of MY shifted apparently and the color transition changed accordingly. The solvent effect on the spectra has been attributed to the isomerization equilibria of MY. A simple application of MY was also shown to the sodium acetate-hydrochloric acid titrations in the mixed solvents.
authors.affiliation: Department of Chemistry, Henan Normal University, Xinxiang, Henan, People's Republic of China. wjjly@public.zz.ha.cn
authors.forename: J
authors.initials: J
authors.lastname: Fan
authors.forename: X
authors.initials: X
authors.lastname: Shen
authors.forename: J
authors.initials: J
authors.lastname: Wang