Determination of Nitrate in Natural Waters by Vanadium Reduction and the Griess Assay: Reassessment and Optimization

An accurate, precise, simple, and efficient spectrophotometric method has been developed for the determination of nitrate concentrations in natural waters by using premixed vanadium and the Griess reagents. We have systematically reassessed and optimized several key parameters influencing nitrate to nitrite reduction and pink azo dye formation, which include the order of reagent addition, reagent concentrations, heating temperatures, and cooling procedures. Samples added with the premixed reagents are heated gradually in a 50 °C water bath for 25–30 min followed by a quick cooling step to fix the final yield. Both nitrite and nitrate produce a stable and equal molar absorptivity of ∼50000 M–1 cm–1 under the optimized condition, without showing a significant salt effect. With a detection limit of 0.2 μM and a precision of 1% (relative standard deviation) for nitrate at 5–50 μM, this simple and reliable protocol provides an alternative to the traditional Cd–Cu reduction method for routine nitrate analysis in natural waters and other aqueous samples.