Spectrophotometric method for determination content of cationic surfactants with eosin methylene blue

A sensitive spectrophotometric method is developed for determination of content of cationic surfactants(CSs) in water samples.In weak acidic HCl-NaAc buffer medium,some cationic surfactants reacts:cetylpyridinium bromide(CPB) and cetyltrimethylammonium bromide(CTAB) with eosin methylene blue(EMB) to form an ion-association complex,and the maximum Absorption wavelength is 660 nm and 658 nm respectively.At such maximum Absorption wavelength,the color intensity shows good linearity with the concentration of the CS.Within the range 0～2.10× 10-5mol/L and 0～1.98×10-5mol/L of CPB and CTAB respectively,the relationship between the color intensity and concentration of the CSs follows Beer's Law.The apparent molar Absorptivity e is 1.79×104 L/(mol·cm) and 1.38×104 L/(mol·cm) respectively and the detection limit is 8.27×10-7 mol/L and 9.88×10-7 mol/L respectively.Average recovery is 99.5%~102.6%.The method has high sensitivity and fairly good selectivity.This method can be applied for the determination of cationic surfactants in water samples.