Reactions of metallochromic indicators on micelles—I: General observations

Abstract

Purple-red alkaline solutions of Xylenol Orange are decolourised by the addition of cationic long-chain quaternary salts. A definite minimal ratio of the detergent to the indicator is necessary to bring about the full decolourising effect. The formation of micellar aggregates is supposed. In these one dye molecule is surrounded by at least five molecules of cetylpyridinium bromide and six molecules of cetyltrimethylammonium bromide, respectively. According to absorption spectra, the electronic structure of the Xylenol Orange in this state is that normally found at pH 6.4. Xylenol Orange, bounded in these aggregates, is capable of forming coloured complexes with a number of cations over a wider pH range than usual. Some ions, such as calcium and magnesium, not usually determined complexometrically with Xylenol Orange as an indicator may be successfully titrated using the modified indicator.