Abstract: A new chromogenic reagent,1-azobenzene-3-(3-nitro-5-chloro-2-pyridyl)-triazene (abbreviated as ABNCPyT) was synthesized and its molecular structure was identified by elementary analysis and IR spectrometry.It was found that in a borate buffer medium of pH 10.5 and in the presence of surfactant Triton X-100,a red colored coordination complex was formed between ABNCPyT and Hg(Ⅱ) with mole ratio (R to Hg) of 4 to 1.Apparent molar absorptivity of the complex was found to be 1.92×105L·mol-1·cm-1,at the wavelength of its absorption maximum 525 nm.Beer′s law was kept in the range within 600 μg·L-1.The proposed method has been applied the determination of mercury in samples of waste water,giving results in consistency with the results obtained by AAS method.Values of RSD (n=5) of the method was found to be ≤2.5%.Recovery was also tested and values obtained were in the range of 97.5% to 103.0%.