Photometric Determination of Microamounts of Cationic Surfactants by the Color-Fading Reaction of Congo Red with Chitosan

In a B-R buffer medium of pH 10.5,or an ammoniacal buffer medium of pH 9.0,color-fading set off by the reaction of congo red with chitosan (system Ⅰ) or with carboxy-methylchitosan (system Ⅱ) in presence of microamounts of cationic surfactants of either the cetyltrimethyl ammonium bromide (CTMAB) or the cetylpyridinium chloride (CPC) was observed.Linear relationship between the magnitude of decrease in absorbance at 530 nm and the concentration of the cationic surfactants was obtained in the range of 0.2-3.6 mg·L-1 for system Ⅰ,and in the range of 0.08-3.2 mg·L-1 for system Ⅱ.Values of apparent molar absorptivity found were as follows: (1) for system Ⅰ,εCTMAB=2.08×104L·mol-1·cm-1,εCPC=2.52×104L·mol-1·cm-1;(2) for system Ⅱ,εCTMAB=2.22×104L·mol-1·cm-1,εCPC=2.55×104L·mol-1·cm-1.The proposed method was used in the determination of cationic surfactants in water samples,giving values of recovery ranged from 99.1% to 100.8%,and values of RSD′s (n=6) in the range of 1.2% to 3.0%.