Spectrophotometric determination of cationic surfactants with benzothiaxolyldiazoaminoazobenzene

A novel spectrophotometric method for the determination of cationic surfactants (CS) by using a new reagent benzothiaxolyldiazoaminoazobenzene (BTDAB) is developed. In 0.06-0.10 M sodium hydroxide, CS such as cetyltrimethylammonium bromide (CTMAB) and cetylpyrdinium chloride (CPC) react with BTDAB to form a violet-red 1:2 (CS:BTDAB) ion association complex in the presence of Triton X-100. This ion associate exhibits an absorption maximum at 580 nm with an apparent molar absorptivity of 4.1×10⁴ l mol⁻¹ cm⁻¹. Beer’s law is obeyed for CTMAB or CPC in the concentration range 0-100 μg per 25 ml of solution. The proposed method based on the above colour reaction is simple and rapid, and there is no use of toxic organic solvents. It has been applied to the determination of trace CS in industrial wastewater with satisfactory results.