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

摘要

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-llosvay 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 ℃ 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 mi-crotiter 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 spectropho-tometer, 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.