Simultaneous spectrophotometric determination of nitrite and nitrate in water samples by flow-injection analysis

An automatic direct spectrophotometric method for the simultaneous determination of nitrite and nitrate by flow-injection analysis has been developed. The method is based on the reaction of nitrite and nitrate with the spectrophotometric reagent N-phenylanthranilic acid in sulfuric acid medium (pH=0.4 similar to 0.6) to form the same color products while the absorbance is measured at 410 nm. This method has the advantage of direct determination of nitrite and nitrate and without reducing nitrate to nitrite as other reported methods. Various analytical parameters, such as flow rate, sample size, effect of acidity(pH), reagent concentration and interfering species were studied. The detection limit is 2.5 ng ml(-1) for nitrite and 12 ng ml(-1) for nitrate. Up to 35 samples per hour can be analyzed with a relative precision of ca. 0.1-2%.