Abstract
The UV-visible spectra of o-methyl red were examined at 25°C in water at various acidities and the variations in the spectra were correlated with the structures of the several forms of the compound. The two acid-base equilibria that involve the monoprotonated form of o-methyl red were treated as overlapping equilibria. The thermodynamic acid dissociation constants (K a2 and K a3) of the diprotonated and the monoprotonated forms were found to be (4.16 ± 0.14) × 10 −3 and (1.40 ± 0.02 ) × 10 −5 mol dm −3, respectively. Complete extraction of o-methyl red from the aqueous phase into the CCl 4 phase was observed in the pH range 1.25–5.6. The significance of this result with respect to the structure of the monoprotonated form is discussed.
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