Determination of cationic surfactants in water by spectrophotometry

A spectrophotometric method for determination of trace content of cationic surfactant in water was established based upon the phenomenon that in a buffer solution of NaOH with pH=12.8,color developing agent 2,6-dibromo-4-nitrophenyldiazoamino-4-phenyl-2-thiazole(DBNPDAPT) can react with N-cetylpyridinium chloride(CPC) or N-cetylpyridinium bromide(CPB) forming blue purple colored associated ions.Laboratory test results show that,the associated ions exhibit maximum absorption at 635 nm with molar absorptivity of 2.30×104 and 1.73×104 L·mol-1·cm-1 respectively.For CPC and CPB in the concentration range 0-1.5×10-5 mol·L-1,Beer's law will be followed.The method has been applied to the determination of trace content of cationic surfactant(CPC) in household waste water samples with satisfactory results.