DETERMINATION OF THE DISSOCIATION CONSTANTS OF SOME p-SUBSTITUTED AROMATIC HYDRAZONES

Abstract

<jats:p>A b s t r a c t: The acid-base behavior of five p-substituted aromatic hydrazones has been studied, using UV spectrophotometric method. The influence of the acidity of the medium on the absorption spectra is followed in aqueous sodium hydroxide solutions in pH region from 7 to 14. The measurements are performed at room temperature, and at ionic strength of 0.1, 0.25 and 0.5 mol dm–3. A batochromic shift of the absorption band that appears in neutral media is observed, when pH is up to 7. It suggests that the dissociation process of the amide and hydroxyl group takes place. Deprotonation enthalpies and total energy values are calculated by using the semiempirical methods AM1 and PM3. Using the changes in the UV spectra with pH of the solution, the determination of dissociation constants, pKBH, at three different ionic strengths, as well as, the thermodynamic dissociation constants at zero ionic strength, is performed. In order to obtain more precise results, the calculations are made from the absorbance values at four selected wavelengths. Furthermore, the pKBH values were determined graphically from the intercept of the dependence of logI on pH. The results showed that the numerically calculated pKBH values are identical to those graphically obtained.</jats:p>