In this project, a new rapid, sensitive and non-extraction batch and flow-injection spectrophotometric method for the determination of cationic surfactants (CS) is proposed. The method is based on the interaction of cationic surfactants with Eriochrome black T to form an ion-association complex. This complex has strong absorbance at 708 nm. The effects of chemical parameters and FIA variables on the determination of cationic surfactants were studied and in detail especially for cetyltrimethylammonium bromide (CTAB). Under the optimum conditions the working range for this example is linear from 3.0 × 10 -6 to 5.0 × 10 -3 mol L -1 , and 2.0 × 10 -6 to 2.0 × 10 -4 mol L -1 for the batch and flow injection spectrophotometric method, respectively. The sample throughput was 35±5 samples h -1 at room temperature. The relative standard deviations for 10 replicates of analysis of (2.0, 0.6 and 0.2) × 10 -4 mol L -1 CTAB were 1.2, 1.3 and 0.8 %, respectively. In addition, the influence of potential interfering substances on the determination of cationic surfactants was studied. The proposed method 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.
