The spectrophotometric determination of cationic surfactants with picric acid

Abstract

Bromophenol blue and bromothymol blue have been used in the past for the spectrophotometric determination of cationic surfactants, by extraction into 1,2-dichloroethane of the blue complex formed between the dye and the surfactant. The method is shown to be unsatisfactory because the dyes can exist in both blue and yellow forms, and often an uncontrollable change of colour occurs in the organic phase. Picric acid, which produces a stable, intensely coloured complex with surfactants, is suggested as a substitute. The pH of the aqueous solutions is restricted to the range 4 to 12; high ionic strengths are not deleterious.