Analytical Sciences

Resonance Rayleigh Scattering Method for the Determination of Cationic Surfactants with Chromium(VI)-Iodide System

Shaopu LIU, Yan SHI, Zhongfang LIU, Hongqun LUO, and Ling KONG

School of Chemistry and Chemical Engineering, Southwest China University, Beibei, Chongqing 400715, P. R. China

A method for detecting and identifying cationic surfactant in some chemical samples for daily use that include Head & Shoulder Ampoule and Slek Shower Lotion has been developed. In an acid medium, chromium(VI) oxidizes I^- to produce I₂, I₂ binds excess of I^- to form I₃^-, and I₃^- can further react with a cationic surfactant (CS) (such as cetyldimethyl benzylammonium chloride (CDBAC), Zephiramine (Zeph), cetylpyridinium bromide (CPB), tetradecyl pyridinium bromide (TPB) and cetyltrimethylammonium bromide (CTAB)) to form ion-association complexes [CS][I₃]. This results in a significant enhancement of resonance Rayleigh scattering (RRS) and appearance of new RRS spectra. The RRS spectral characteristics of the ion-association complexes, the influencing factors and the optimum conditions of the reactions have been investigated. The intensities of RRS are directly proportional to the concentration of CS. CS in samples are collected using a treated anion exchange column and subsequently complexed by I₃^-; then the RRS intensities of CS complex are determined at 495 nm. The reactions have high sensitivities, and their detection limits are 7.05 - 9.62 ng/mL for different CS. The effects of foreign substances are investigated and the results show that the method has good selectivity.

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