Spectrophotometric determination of trace ionic and non-ionic surfactants based on a collection on a membrane filter as the ion associate of the surfactant with Erythrosine B

The spectrophotometric method for the determination of trace surfactants with Erythrosine B (EB) based on the aqueous reaction and the collection on a membrane filter by filtration was studied. Cationic surfactants (CS⁺), such as a quaternary ammonium ion, and polyoxyethylene non-ionic surfactants (NS) in the presence of potassium ion, containing a long-chain alkyl group associate with EB buffered at pH 5.5. CS⁺ associates with anionic surfactants (AS⁻). For the determination of CS⁺, four methods were employed: the collection of the ion associate of CS⁺ with EB on a mixed cellulose ester (MCE) or PTFE membrane filter, the collection of the ion associate of CS⁺ with AS⁻ on a PTFE membrane filter followed by the ion exchange of AS⁻ with EB, and the first collection of CS⁺ followed by the second collection of EB on a PTFE membrane filter. For the determination of AS⁻, the collection of the ion associate of AS⁻ with CS⁺ on a PTFE membrane filter followed by the ion exchange of AS⁻ with EB was done. For the determination of NS, the ion associate of NS with EB was collected on a MCE membrane filter. The MCE membrane filter with the analyte was dissolved in methyl cellosolve. The analyte on the PTFE membrane filter was eluted with ethanol. The CS⁺ up to 5×10⁻⁷ M can be determined by the absorbance at 542 nm of the methyl cellosolve solution or the absorbance at 535 nm of the ethanol solution. The AS⁻ up to 5×10⁻⁷ M can be determined by the absorbance at 536 nm of the ethanol solution. The NS up to 2.53×10⁻⁶ M can be determined by the absorbance at 537 nm of the methyl cellosolve solution. This is the sensitive method for the determination of 10⁻⁸ to 10⁻⁷ M order of ionic surfactants and 10⁻⁷ to 10⁻⁶ M order of NS without toxic organic solvents.