Spectrophotometric and potentiometric determination of acidic constants of oxo-phenyl pyridinium monoxime and dioxime

Abstract

The UV absorption spectra of 1-(1-hydroxyimino-2-oxo-2-phenyl) pyridinium chloride (compound I) and 1-(1-hydroxyimino-2-oxo-2-phenyl)-4-hydroxyiminomethyl pyridinium chloride (compound II) in water solution at different pH values have been measured. The spectral changes, with changing pH, in aqueous solutions are attributed to the dissociation of individual functional groups of the compounds. The mixed acidic constants (pK′a) of the investigated monoxime and dioxime, have been determined spectrophotometrically in the series of Britton-Robinson's buffer solutions in the pH range 3.0-5.19 and 7.70-9.90 (t=25±0.5°C, I=0.2). The following pK′a values have been obtained for monoxime pK′a1=4.30 and for dioxime pK′a1=4.28, pK′a2=8.36. Thermodynamic acidic constants (pKa) have been determined on the basis of potentiometric titrations and they have been found to be pKa1=4.32 for compound I and pKa1=4.27, pKa2=8.51 for compound II. The values obtained by transferring pK′a into pKa are in good agreement with the values obtained potentiometrically.