EPA method states that unpreserved samples analyzed for nitrite and nitrate must be analyzed within 48 h. If samples are preserved with sulfuric acid at pH < 2, then nitrite and nitrate are allowed a 28-day holding time. Early indications suggested that nitrite was not stable under acidic solutions and was in fact converted to nitrate within the holding time set by the methodology. Owing to this discrepency, an investigation was made of the effects of preservation on the stability and equilibrium of nitrite and nitrate in reagent-grade water, groundwater, leachate and surface-type waters. The results showed that when reagent-grade water is spiked with nitrite and nitrate, both anions are stable for > 30 days if simply kept refrigerated at 4-degrees-C, but when acidified to pH < 2 with sulfuric acid nitrite is rapidly converted to nitrate. When nitrite and nitrate were added to the matrices found in the environmental samples and held at 4-degrees-C, conversion of nitrite to nitrate only occurred in the surface water sample. This conversion is believed to have been caused by the presence of nitrogen-fixing bacteria. As it was believed that acid preservation would oxidize nitrite to nitrate, and bacteria could also cause conversion of nitrite and nitrate, we spiked these samples under highly basic conditions (preservation with sodium hydroxide at pH 12). It was hoped that these basic conditions would destroy the nitrogen-fixing bacteria and stabilize nitrite. The results indicated no conversion of nitrite during the 37-day test period in any of the test matrices when samples were preserved in base.
