Direct determination of nitrate in small volumes of natural surface waters using a simple spectrophotometric method

Abstract

A simple, sensitive and selective spectrophotometric method is proposed for rapid and direct determination of nitrate in small volumes of natural surface waters (maximum 2.0 ml), and optimal experimental conditions, along with other analytical parameters, have been evaluated. The method is based on the reaction between nitrate and chromotropic acid, in concentrated sulfuric acid media (95%), when a yellow colored product is obtained. The visible spectra of this reaction product, recorded against a blank solution, show a maximum at 412 nm, and the absorbance remains stable for at least 24 h. The method allows the nitrate determination over the range 0.50–11.00 mg l-1, with a molar absorptivity of 3.837×104 l mol-1 cm-1 and a detection limit of 0.12 ppm. The interferences caused by several common ions (K+, NH4+, Na+, Ca2+, Mg2+, Al3+, Cl-, NO2-, HCO3-, CO32-, H2PO4-, HPO42-) which are present in most natural surface waters were determined. The validation of the spectrophotometric method was done by comparing the nitrate content obtained by the proposed method with those obtained by using the standard method with sulphosalicylic acid, for all studied types of waters (drinking water, tap water, river water and sea water). The results show that the proposed method can be successfully used for direct determination of nitrate in small volumes of natural surface waters (maximum 2.0 ml). The main advantage of this method is that the colored reaction product appears immediately after mixing the reagents; other intermediary steps are not necessary.