Acid-base indicator properties of sulfonephthaleins and benzeins in acetonitrile.

Sulfonephthaleins and benzeins differ only in the presence of a sulfonic acid group in the former. In aqueous medium, both exhibit two color changes, one from yellow to the strongly colored acid form in strongly acid medium, and one from yellow to the strongly colored basic form. The indicator constants of the sulfonephthaleins are about the same as those of the corresponding benzeins. In the present paper the acid-base characteristics of both types of indicators in acetonitrile (AN) are described. The two benzeins studied in AN exhibit two color changes as in water, but the indicator constants in AN are quite different from those in water. In general, the sulfonephthaleins in AN exhibit three color changes which are in the order of decreasing pH: blue(red) to yellow, yellow to colorless, and colorless to red. The indicator changes corresponding to these color changes have been determined using the classical spectrophotometric method for evaluation of the ratio of concentrations of acid to basic forms of the indicators in mixtures of weak acids and their tetraalkylammonium salts. The paH of these mixtures was measured with a glass electrode. Denoting HI as the acid form of the indicator, I as the alkaline form, f' and JHI as their respective activity coefficients, the solvated proton as H+, and KdI as the dissociation constant of the acid form,