Interaction of hydrophobic anions with cationic dyes and its application to the spectrophotometric determination of anionic surfactants

The change in absorption spectra of Ethyl Violet solution on addition of anionic surfactants in an aqueous solution was used for the determination of anionic surfactants. This change is caused by the hydrophobic interaction between the bulky dye and the detergent. The contribution from the substituents of the cationic dyes and anionic surfactants to the hydrophobicity is discussed. The calibration graph is linear over the concentration range from 4 × 10–6 to 4 × 10–5 mol dm–3. The molar absorption coefficient for lauryl sulfate is 1.76 × 103 m2 mol–1 at 700 nm.