A fast and direct spectrophotometric method for the sequential determination of nitrate and nitrite at low concentrations in small volumes

The use of vanadium (III) has been proposed recently as a suitable alternative to cadmium for the reduction of NO3− to NO2− during spectrophotometric analysis. However, the methods proposed suffer from decreased sensitivity and additional steps for the measurements of nitrite and nitrate. We have developed an improved fast and sequential protocol that permits the determination of low concentrations of nitrite and nitrate in marine and freshwater samples using small volumes. NO2− concentration is firstly determined by using the common Griess reaction. The subsequent addition of a 2% VCl3 solution in 6 N HCl in the same sample and the reaction at 60 °C for 25 min results in an efficient reduction of the NO3− to NO2− (> 95%), which is also detected by the already added Griess reagents. The method has a detection limit < 0.05 μM, a high precision (ranging from 0.2 to 11%) and accuracy (0.07 μM) for the determination of NO3− + NO2− concentrations lower than 30 μM. Comparison of the proposed method with the established Cd column method using samples from a variety of environments (fresh water reservoir, sediment freeze lysable pore water, estuarine water samples and samples from an acid mine drainage impacted reservoir) showed good agreement between the two methods, with a difference between methods of 0.073 ± 0.099 μM. The analysis can be performed in large batches (~ 60 samples) by using small sample volumes (≤ 1 mL) for the determination of both NO3− and NO2− in less than 1 h.