ABSTRACT
The absorption spectra of 5 phenol derivatives indicators in alcohol water mixture disclose absorption peaks whose position is dependent on alkali concentration and alcohol content of the medium. The peaks are slightly shifted towards higher wave lengths with rise in alkali concentrations. Indicators with OH ions are ionized in the solution by proton loss rather than by hydroxide addition. The molar extinction coefficient epsilon, epsilon a and epsilon HA of the partially ionized, completely ionized and non-ionized forms of each indicator are calculated. 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 addition, the pKa-values of the indicators are obtained by plotting [log I-log COH] against COH and extrapolating to zero concentration. These values of I and pKa are of importance in the evaluation of the acidity function H of the different solutions