Ionization of methyl orange in aqueous sodium chloride solutions

Ionization constants of sodium 4′-dimethylaminoazobenzene-4-sulphonate (methyl orange) were determined by means of spectrophotometric measurements in water and in aqueous sodium chloride solutions with molalities up to 2 mol·kg−1 at temperatures between 278.15 K and 333.15 K. The temperature dependence of the thermodynamic acidity constant shows a slight curvature in accordance with published data. The influence of sodium chloride on the methyl orange deprotonation was assessed by the measurement of stoichiometric acidity constants in this ionic medium. The Pitzer theory, widely used in the evaluation of the excess free energy of non-ideal electrolyte solutions, was applied to the computation of the activity coefficients of the chemical species involved in the equilibria and a good fit of those equations to the experimental data was observed, at all temperatures under consideration.

► Thermodynamic ionization constant of methyl orange in the temperature range (278.15 to 333.15 K).
► Influence of sodium chloride on the ionization of methyl orange.
► Pitzer interaction coefficients for methyl orange (sodium salt).
► Ionic strength dependence of the stoichiometric ionization constant.