Dissociation constants of methyl orange in aqueous alcohol solvents

The dissociation constants of methyl orange (H+In-) has been determined at 25 degrees 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 pK(a) values decrease with increasing composition of the co-solvent in the order: tert-butanol>iso-propanol>ethanol>methanol>water. A linear relationship between pK(a) 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+, Delta G(t)(0) (H+), and the relative values of Delta G(t)(0)(In-) and Delta G(t)(0)(H+In-) in all solvent systems. (C) 1998 Elsevier Science B.V.