Determination of the thermodynamic acidity constant of 3-phenyl-4-benzoylisoxazol-5-one (HPBI) using the binding mean spherical approximation model

The acidic properties of 3-phenyl-4-benzoylisoxazol-5-one (HPBI), a well-known extractant of potential use in nuclear waste processing, have been studied by means of UV-Visible spectrophotometry. The effects of perchloric and hydrochloric acids on the absorption spectra of HPBI have been examined, from low to very high concentrations of these two additives. The HPBI absorption spectra are very similar in the presence of either acid up to 5 mol dm(-3). The mean spherical approximation model has been used in this range of concentration to describe the thermodynamic properties of the solutions, and thus to determine the pK(a) of the first acidity of HPBI. A comparison with other values given in the literature is made. Above 5 mol dm(-3), a clear difference appears between the absorption spectra obtained in aqueous solutions of HCl or HClO4, which may be ascribed to changes in the properties of the solvents.