Novel indicator system for the photometric titration of ionic surfactants in an aqueous medium. Determination of anionic surfactants with distearyldimethylammonium chloride as titrant and tetrabromophenolphthalein ethyl ester as indicator

Abstract

A photometric titration method for anionic surfactants with tetrabromophenolphthalein ethyl ester (TBPE) as indicator was examined. In the presence of a non-ionic surfactant (Triton X-100), TBPE was dissolved in an acidic aqueous medium giving a yellow colour in the acidic form (TBPE·H). When a bulky cation (Q⁺) was added, TBPE·H reacted with Q⁺ to form an ion associate (Q⁺·TBPE–), and the colour changed from yellow to blue. Distearyldimethylammonium ion was the preferred titrant. The titrant was added to a solution of anionic surfactant at pH 3.2, and absorbance changes were monitored with a fibre-optic sensor with a 640 nm interference filter. Anionic surfactants at concentrations from 5 × 10^–6 to 2 × 10^–4 mol dm^–3 could be determined, and the standard deviations and the relative standard deviations for ten replicate titrations of 25 cm³ of 2 × 10^–4 mol dm^–3 of various anionic surfactants were 0.03–0.15 cm³ and 0.03–1.65%, respectively.