Catalytic simultaneous spectrophotometric determination of nitrite and nitrate with a flow injection system

Abstract

A catalytic spectrophotometric flow injection method is proposed for the simultaneous determination of nitrite and nitrate based on the catalytic effect of nitrite on oxidation of naphthol green B (NGB) by potassium bromate in phosphoric acid medium and nitrate being reduced to nitrite by a cadmium-coated zinc reductor column. The redox reaction is monitored spectrophotometrically by measuring the decrease in the absorbance of NGB at the maximum absorption wavelength of 722 nm. A technique of double zone injection of the sample isolated by a reductor column is adopted with a single valve. The flow system produces two signals, one corresponds to the concentration of nitrite and the other corresponds to the sum of both the ions. The detection limit is 0.5 μg l −1 for nitrite and 2.5 μg l −1 for nitrate. Up to 30 samples can be analyzed per hour with a relative precision of ca. 2%. The procedure has been successfully applied for the determination of nitrite and nitrate in natural waters.