Dissociation constants of methyl orange in aqueous alcohol solvents

The dissociation constants of methyl orange (H+In−) has been determined at 25°C and an ionic strength of 0.1 mol l−1 in water and in mixed aqueous solutions of methanol (10–90 wt%), ethanol (10–70 wt%), iso-propanol (10–60 wt%) and tert-butanol (10–50 wt%) by spectrophotometric measurements. It has been shown that the solvents affect the acid–base equilibria, visible absorption spectra and color transition range of methyl orange to different extents. The pKa values decrease with increasing composition of the co-solvent in the order: tert-butanol>iso-propanol>ethanol>methanol>water. A linear relationship between pKa and the mole fraction 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 terms of the standard Gibbs energies of transfer of H+, ΔG0t (H+), and the relative values of ΔG0t(In−) and ΔG0t(H+In−) in all solvent systems.