How are “droplet tests” for nitrate affected by nitrite?

It is often reported that seawater nitrate droplet tests are influenced by nitrite. From our measurements, we know that nitrite occurs in almost all seawater tanks in a concentration of 0.05-0.1 mg/l. – Reason enough to take a closer look at this situation.

We used standard solutions for nitrate and nitrite for our test, as well as a commercially available, frequently used nitrate test kit. Different samples were prepared containing only nitrite (0.05, 0.1, 1 mg/l) or only nitrate (2, 5, 10 mg/l), as well as a mixed sample with nitrite and nitrate.

You can see the result of our test here:

The result is quite clear, 0.05 mg/l nitrite in this nitrate test simulates a value of 2 mg/l, 0.1 mg/l nitrite indicates a false nitrate value of 5 mg/l. This is particularly interesting for aquarists who keep their tanks low in nutrients but still want to keep nitrate in the detectable range.

Why nitrate tests react to nitrite (only for chemistry freaks)?: Nitrate is difficult to measure “directly”, so the test sets make a detour: The nitrate is (partially) converted to nitrite (reduced). The nitrite then forms a diazonium salt in an acidic environment, which undergoes an azo coupling with a suitable coupling partner. This creates an azo dye that causes the pink color. Existing nitrite can thus react immediately to the azo dye and simulates a higher nitrate value.

We measure nitrite and nitrate using ion chromatography in our laboratory: The anions (including nitrate and nitrite as well as fluoride, chloride, bromide and sulfate) are separated on an anion separation column. Thereafter, these analytes migrate “singularly” past sensitive detectors (we use a conductivity detector and, in addition, a UV detector, because only then can nitrite and nitrate with a low detection limit be determined). This allows an unaffected nitrate measurement, even if a lot of nitrite should be in the water.