Photometric titration of small amounts of cationic surfactants in an aqueous medium

Abstract

A photometric titration method for cationic surfactants with tetrabromophenolphthalein ethyl ester (TBPE) as an indicator was developed. 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. This colour reaction was used for the detection of the end-point. Direct and indirect titration methods for the determination of cationic surfactants were examined. Dodecylbenzenesulfonate ion (DBS^–) was used as a titrant for cationic surfactants in the direct titration. The colour changed from blue to yellow at pH 3.6, and absorbance changes were monitored with a fibre-optic sensor with a 630 nm interference filter. Benzyldimethyltetradecylammonium (zephiramine) ion was found to be the preferred titrant in the indirect titration. The titrant was added to a solution of cationic surfactant containing a known excess amount of anionic surfactant DBS^– at pH 3.6. Cationic surfactants at concentrations from 5 × 10^–6 to 1 × 10^–3 mol dm^–3 could be determined.