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 flowtechnique, by continuous flow technique or, when only small sample volumes are available, bymicrotiter
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 conventionalmethods
using a 1 cm cuvette (0.4 μMfornitrate,0.07μMfor nitrite). Themethod 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.