The color reaction of new reagent 1 (2 hydroxy 3,5 dinitrophenyl) 3 (4 phenylazophenyl) triazene (HDNPAPT) with copper was studied. Results show that the color reagent reacts with Cu 2+ to form a stable red complex in the presence of emulsifier OP and Na 2B 4O 7 NaOH medium(pH=11.0). The composition of the complex is n Cu 2+ ∶ n HDNPAPT =1∶2. The maximum absorption of the complex is at 540 nm with apparent molar absorptivity of 1.73×10 5 L·mol -1 ·cm -1 . Beers law is obeyed in the range of 0~0.36 mg/L for copper. The method has been ap...