A spectrophotometric study of the tautomeric and acid- base equilibria of methyl orange and methyl yellow in aqueous acidic solutions

Abstract

The UV-visible spectra of methyl orange and methyl yellow were examined in alkaline acidic and strongly acidic aqueous solutions at 25° C. In the alkaline medium methyl orange is present in the monomer form for concentrations ⩽1.2 × 10 −4 moldm −3 while methyl yellow shows significant deviations from Beer's law in the alkaline medium and time has a drastic effect on the spectra. In 0-032–0-46 mol dm −3 HCl both azo dyes are present as a tautomeric mixture. The position of the tautomeric equilibrium shifts to the side of the azonium tautomer for acidities > 0.5 mol dm −3 HCl. The tautomeric equilibrium constant (K t) is calculated at several HCl concentrations. The pK a values for methyl orange and methyl yellow are 3.37 ± 0.01 and 3.01 ± 0.01 respectively. The microscopic acid dissociation constants of both azo dyes were calculated. Methyl orange was not extractable into CCl 4 at any acidity while methyl yellow was completely extractable into CCl 4 at acidities ⩽ 0.3 mol dm −3 HCl.