The cationic dye, crystal violet, is applied to the indicator for the determination of cationic surfactants such as benzyldimethyltetradecylammonium chloride by photometric titration, using a potassium poly(vinyl sulfate) (PVSK) solution as a titrant. Before the titrant is added to the sample solution containing crystal violet, the absorbance of the crystal violet itself is observed because the crystal violet does not react with the cationic surfactants in the sample solution. Before the equivalence point, the absorbance of the sample solution decreased slightly as the amount of added titrant increases. The decreased absorbance may be due to that the cationic surfactant in the sample solution, preferentially forms ion associate with the titrant and, hence, the sample solution is diluted with the titrant. After the end-point, the absorbance of the sample solution decreases abruptly by abrupt decrease of the concentration of crystal violet caused by the association reaction between the crystal violet and the titrant. The end-point of the titration is detected as the break point of the titration curve. The linear relationships between the concentration of cationic surfactants and the end-point volume of the titrant existed in the concentration range from 0 to 2. 5 × 10-5 M for benzyldimethyltetradecylammonium chloride, cetylpyridinium chloride (CPC), and stearyltrimethylammonium chloride (STA).
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