pyridine-3-(4-phenylazophenyl)-triazene(PYPAPT) reagent could react with anionic surfactant and cetylpyridinium bromide(CPB) to form a purplish blue ternary complex in NaOH-NaCl-CH_2(NH_2)COOH solutions. The maximum absorption wavelength of the complex is at 630 nm. The system was used to determine anionic surfactants by spectroscopy. The apparent molar absorptivities of sodium dodecyl benzene sulfonate(DBOSO_3Na), sodium dodecyl sulfonate(DOSO_3Na) and sodium dodecyl sulfate(DSO_4Na) are 1.61×10 4,1.44×10 4,1.76×10 4 L·mol -1 ·cm -1 , res...