Determination of nitrate plus nitrite in small volume marine water samples using vanadium(III)chloride as a reduction agent

In this contribution a method to analyse nitrate in marine pore water, seawater and freshwater is presented. The method serves to replace the well-known cadmium column method for the reduction of nitrate to nitrite. Instead, acidic vanadium(III)-solution is used for the reduction avoiding the toxic Cd metal. Both, the already present and the newly produced nitrite are quantified by the established Griess-Ilosvay reaction. Sample preparation is easy because only one reagent solution has to be added to the sample. Efficiency of the nitrate reduction is 100 ± 3% (n = 12) using either a reaction time of 60 min at 45 °C or 10-20 h at room temperature. Measurements can either be done by conventional UV-VIS spectrophotometry using 1-5 cm cuvettes, by discrete sequential analysers based on the loop flow technique, by continuous flow technique or, when only small sample volumes are available, by microtiter plate readers (MR) in absorbance mode. The latter method requires only 0.5 mL of a sample for nitrite and nitrate quantification and has a precision of 2%. The limit of detection of the MR technique is comparable to conventional methods using a 1 cm cuvette (0.4 μM for nitrate, 0.07 μM for nitrite). The method requires only a spectrophotometer, and is simple and cost-effective; sample preparation is rapid and a salt error or an interference by dissolved organic carbon is not evident. Interferences induced by naturally coloured samples, or hydrogen sulphide can be corrected or eliminated. A procedure for direct nitrate determination is given as well as a procedure to compensate for the small loss of nitrite by nitric oxide formation for the nitrite present prior to the addition of a reductant.