Physio-chemical studies on acidity functions in mixed solvents III Studies on the optical absorption characteristics of p-nitroaniline, phenol and thioacetamide in solutions of potassium hydroxide

ABSTRACT

The ultraviolet absorption properties of the four above indicators in solutions of potassium hydroxide of different concentrations in ethanol water mixture are studied. The absorption spectra so obtained disclose absorption peaks of situation dependent on alkali concentration and alcohol content of the medium. The peaks are slightly shifted towards higher wave lengths with rise in alkali concentrations. The results substantiate that the different indicators with the OH-ions present in the solution by proton loss rather than by hydroxide addition. Calculations are made of the molar extinction coefficients epsilon, epsilon A. and epsilon HA. of, respectively, the partially ionised, completely ionised, and unionized forms of each indicator. The values of these coefficients are then used for the computation of the indicator ratios I of the different indicators in the various solutions. In additions, the pka values of the indicators in questions are obtained by plotting [log I -log COH-] against COH[-] and extrapolating to zero concentration. These values of T and pKA are of importance in the evaluation of the acidity function H- of the different solutions