Abstract: It was proposed in this paper that NO-3 ion in samples of surface water was preliminarily transformed into NO-2 ion by reduction with zinc metal in a solution of cadmium chloride.The conditions for the reduction reaction,including the concentration of sodium chloride and cadmium chloride in the solution,suitable pH-value of the solution,amount of zinc metal (in the form of rolls) used and time required for quantitative reaction,were studied and optimized.The nitrite ion (NO-2) obtained was then diazotized with sulfanilamide,and the diazotized product was further coupled with naphthalene ethylenediamine to give a red colored azo-compound with its absorption maxima at the wavelength of 543 nm.Amount of NO-3 ion was found indirectly from the standard calibration curve by the absorbance of the colored compound.Beer′s law was obeyed in the range of 0-0.400 mg·L-1 (as NO-3),with a detection limit (S/N=3) of 0.008 mg·L-1.Precision of the method was tested at the concentration level of 0.1 mg·L-1,and value of RSD′s (n=6) found was 2.1%.Test for recovery was performed at 3 different concentration levels,values of recovery obtained were in the range of 99%-103%.