Flow injection analysis of nonionic surfactants with plasticized poly(vinyl chloride) membrane electrode detector

An electrode based on a poly(vinyl chloride) membrane plasticized with o-nitrophenyl octyl ether, which contains no ion-exchange site, was used as an indicator electrode in FIA of nonionic surfactants. The flow injection manifold was composed of a two-channel system. The sample solution (100 μl) was injected into a sample carrier stream (water), and merged with a stream of baseline potential supporting electrolyte solution (BLSS). The carrier and BLSS were pumped at a flow rate of 1.1 ml/min. The electrode responds to cationic complexes formed between metal ion in BLSS and nonionic surfactants in sample. The electrode detector showed the cationic response with a slope of 25 mV per decade for Triton X-100 from 10^-4 M to 10^-3 M, when the mixture of 0.1 M BaCl₂ or 0.1 M CaCl₂ and 10^-5 M sodium dodecyl sulfate (NaDS) was used as BLSS. However, the electrode detector showed the cationic potential response with a slope of 45 mV per decade for Triton X-100 from 10^-3 M to 10^-1 M, when the mixture of 0.1 M NaCl and 10^-5 M NaDS was used as BLSS. The response of the detector to other several nonionic surfactants such as poly (ethylene glycol) mono-p-nonylphenyl ether (n = 15, 18) and poly (ethylene glycol) dodecyl ether (n = 7, 25) was similar to that for Triton X-100, when the mixture of 0.1 M NaCl and 10^-5 M NaDS was used as BLSS. The detection limit for Triton X-100 became lower with increase of the concentration of NaDS in BLSS. Coexistence of common inorganic electrolytes less than 10^-2 M essentially gave no disturbance on the determination of nonionic surfactants. The ionic surfactants, however, gave serious interferences.