INTRODUCTION Cationic surfactants (CS) are widely used as fabric softener, frother, hair conditioner, fermicide and emulsifier in many industrial fields. In recent years, more and more waste water containing mass cationic surfactants was flowed into river and sea, which leaded to the eutrophication of water. It was very harmful to the health of human body and aquatic organisms. So it is significant to study on the determination method of cationic surfactants. Several techniques, such as spectrophoto- metry 1-5 , fluorometry 6 , liquid chromatography 7 and photometric titration 8 etc., have been reported for the determination of cationic surfactants. The reagents, e.g., orange II, disulfine blue, ethyl orange, patent blue V, tetrabromophenolphthalein ethyl ester have been reported for determination of cationic surfactants based on extraction of the ion associates between cationic surfactant and anionic dyes. These spectrophotometric methods have the disadvantages of tedious procedures and the use of toxic solvents. In this work, a new spectrophotometric method for the determination of cationic surfactant without extraction was proposed. The method was based on the colour reaction of cationic surfactant with titan yellow (TY). The determination was carried out directly in aqueous solution and was success- fully applied to the determination of cationic surfactant in environmental water samples. EXPERIMENTAL A 722-N spectrophotometer (Shanghai precision & Scientific Instrument Co. Ltd., China) with matched 1 cm glass cells was used for all the spectrophotometric determination. Spectrophotometric Determination of Cationic Surfactant with Titan Yellow YING-HONG LIU 1,* , HONG-WEI ZHAN 2 and WEI-XING MA 1 1 College of Chemical Engineering, Huaihai Institute of Technology, Lianyungang 222005, Jiangsu Province, P.R. China 2 Jiangsu Lvyuan Engineering Design Research Co. Ltd., Lianyungang 222000, Jiangsu Province, P.R. China *Corresponding author: Tel: +86 518 85857922; E-mail: liuyh9506@163.com; yinghong1978@yahoo.com.cn (Received: 20 March 2012; Accepted: 19 November 2012) AJC-12442 An accurate, simple and rapid method for the determination of cationic surfactant was developed. The method is based on the formation of ion association between anionic dye and cationic surfactant. In this study, titan yellow and cetylpyridinium chloride were selected as dye and reactant surfactant, respectively. The ion association exhibits an absorption maximum at 400 nm. Beer's law was obeyed for cetylpyridinium chloride in the concentration range of 1-28 μg mL -1 and the detection limit was 0.4 μg mL -1 . The proposed method was successfully applied to the determination of trace cationic surfactant in environmental water samples. Key Words: Cationic surfactant, Cetyltrimethylammonium bromide, Spectrophotometry, Titan yellow. The standard stock solution (1 g L -1 ) of cetylpyridinium chloride was prepared by dissolving 0.1000 g of cetylpyridinium chloride in 100 mL volumetric flask with water. The cetyl- pyridinium chloride standard working solutions were obtained by diluting the above stock solution. Titan yellow solution (0.5 g L -1 ) was prepared by dissolving 0.5000 g titan yellow in 1000 mL water. 0.01 mol L -1 NaOH solution was used. All chemicals used were of analytical grade and double distilled water was used throughout the study. Procedure: A suitable volume of cetylpyridinium chloride standard working solution or sample solution was transferred into a 10 mL colorimetric tube. 1.5 mL of NaOH solution and 1.5 mL of titan yellow solution was added. The solution was diluted to the mark with water and kept aside for 5 min. The reagent blank solution which was absence of cetylpyridinium chloride was prepared as above. The absorbance A of the solution was measured at 400 nm with 1 cm cells by using reagent blank solution as reference. RESULTS AND DISCUSSION Absorption spectra: The absorption spectra of the titan yellow and cetylpyridinium chloride-titan yellow ion asso- ciation complex were shown in Fig. 1. It could be found that the maximum absorbance of titan yellow was located at 405 nm. The addition of cetylpyridinium chloride to titan yellow resulted in the absorbance decreasing at 380-450 nm and increasing at 450-540 nm, which indicated the formation of ion association complex between cetylpyridinium chloride and titan yellow. The maximum absorption wavelength of the Asian Journal of Chemistry; Vol. 25, No. 5 (2013), 2736-2738 http://dx.doi.org/10.14233/ajchem.2013.13761