Determination of ionic surfactants based on ion association in an aqueous medium

Abstract

The determination of ionic surfactants was carried out on the basis of ion association between ionic dyes and ionic surfactants in an aqueous medium. Three kinds of dyes (tetrabromophenolphthalein ethyl ester(TBPE), methyl orange analogues and nitrophenylazophenol analogues) were used as novel ion-association reagents. The ion-association constants between cationic surfactants (Q+) with dyes (D-) and anionic surfactants (A-) were obtained. It was found that the hydrophobic interaction plays an important role in the formation of stable ion associates. On the basis of the ion association accompanying a color change, the analytical methods for the determination of ionic surfactants, such as a batchwise spectrophotometric method, a flow-injection method and a photometric titration method, were developed in the presence of a small amount of nonionic surfactant in an aqueous medium. In particular, the photometric titration method was examined in detail. Titration curves could be simulated by using the ion-association constants of both ionic surfactants and indicators with a titrant, quaternary ammonium ion. By using the simulation method of the titration curves, an evaluation of the end-points and selection of the desirable titration conditions were possible. The ionic surfactants at concentrations of the 1 × 10-6 M level can be determined by the photometric titration method. The proposed titration method was applied to several practical samples; the results were in good agreement with both the results of the JIS titration method (Epton method) and a solvent-extraction method with ethyl violet.