Spectrophotometric investigations on immobilised acid-base indicators

Diffuse reflectance spectra of four acid-base indicators immobilized on Amberlite XAD-2 resin are reported as a function of pH. Adsorption modifies the acid-base properties of the indicator by broadening the pH intervals over which the dyes change colour and by shifting these intervals to higher pH values. The different behaviour in the liquid and immobilized phases may be explained by multilayered adsorption of some indicators. The occurrence of hysteresis during a pH cycle is reported.