Abstract: A new azo reagent,7-(4-sulfophenylazo)-8-hydroxy-1-amino-3,6-naphthalene disulfonic acid (SPAHANS),was synthesized by the reaction of diazonium compound of 4-aminobenzene sulfonic with H acid in 0.25 mol·L-1 Na2CO3 solution of pH 9.0.Molecular composition and structure of SPAHANS were determined by FT-IRS,UV-VIS spectrophotometry and elementary analysis.It was found that in an acetate buffer medium of pH 4.5 and heated at 40-65 ℃ for 5 min,a stable coordination complex (Cu2+∶R=1∶2) was formed by the reaction of SPAHANS with cupric ion,having the absorption maximum at 510 nm.Linear relationship between values of absorbance and concentration of Cu2+-ion was obtained in the range of 0.22-3.0 mg·L-1.Detection limit (3s/k) found was 5.6×10-2mg·L-1.The molar absorptivity of the color system was found to be 1.6×105L·mol-1·cm-1.Test for precision was made with standard Cu2+ solution of 1.0 mg·L-1 for 9 determinations,giving value of RSD 0.17%.