Refining the Use of Sodium Azide to Counteract Nitrite Interference in Dissolved Oxygen Analysis of Seawater

Abstract:

High nitrite concentrations are known to interfere with the analysis of dissolved oxygen in seawater samples, though the affected range has yet to be defined. This error can be counteracted by the addition of sodium azide to the hydroxide-iodide pickling reagent. The 2013 US GEOTRACES zonal transect included stations off the coast of Peru with nitrite values up to 10µmol/kg in the upper 400 meters of the water column. Low concentrations of dissolved oxygen were also present in the upper 800 meters, providing an opportunity to study the effect of high nitrite levels on dissolved oxygen analysis over a range of concentrations. Without the addition of azide, the error in dissolved oxygen measurement increased linearly with nitrite concentration. The interference was only significant in samples with nitrite concentrations higher than 1.5 µmol/kg, all of which also had low dissolved oxygen concentrations (<45µmol/kg). The unique combination of high nitrite and low dissolved oxygen is present in such well known and relatively small areas of the world’s oceans that the addition of azide is not necessary as a standard procedure for the vast majority of oceanographic measurements.