Determination of Nitrate in Freshwater and Sea water by a idrazine Reduction Method

Abstract

A procedure for the determination of nitrate in eutrophic freshwater and sea water is presented. Before the determination of nitrate in sea water, magnesium and calcium were removed by precipitation as hydroxides. The nitrite initially present in the sample solutions was decomposed with ammonium amidesulfate under acidic condition. Then nitrate was reduced to nitrite with hydrazine sulfate under an alkline condition (pH 12.3) catalyzed by Cu²⁺for 30 min at 33°C. Zn was also added to the catalyst solution at a concentration in excess of the Cu²⁺ (Cu²⁺ : Zn²⁺=1 : 200) to saturate the complexing capacity of any water sample and to minimize the complexing of cupric ions. Nitrite thus produced was measured spectrophotometrically following a diazotization and coupling reaction.
In the above conditions, the applicable range is 0.05-0.7 mg/l as nitrate nitrogen. Variations in salinity have no significant effect up to 40 parts per thousand. In natural water samples interferences of soluble organic substances were eliminated by addition of Zn²⁺ to the Cu²⁺ catalyst solution and the proposed method was compared favourably with the automated copperized cadmium column technique for freshwater and sea water samples containing 0.1-1.7 mg/l as nitrate nitrogen.