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

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 (Kt) is calculated at several HCl concentrations. The pKa 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 CCl4 at any acidity while methyl yellow was completely extractable into CCl4 at acidities ⩽ 0.3 mol dm−3 HCl.