Does the cadmium reduction method for nitrate include nitrite? If so, why is nitrite listed as an interference? How can nitrite interference be accounted for?

The cadmium reduction method for nitrate will detect nitrite, but not quantitatively. Once the sample pretreatment described in the interference section of the method is performed, the method will quantitatively measure the sum of nitrate and nitrite.

Nitrate (NO ₃) in the sample is reduced to nitrite (NO ₂) by cadmium in the NitraVer® reagent. However, the reduction is not 100% efficient (not all of the nitrate is converted to nitrite). The inefficiency is accounted for in the calibration curve. The nitrate that has been converted into nitrite then reacts completely with the indicator and the absorbance is measured. Nitrite is considered an interference because any nitrite which was present in the original sample will react directly with the indicator and is 100% efficient. In contrast, not all of the nitrate will react with the indicator as the reduction is not 100% efficient. 

This means that testing a 10 mg/L (NO ₃ -N) standard of nitrate should result in a 10 mg/L (NO ₃ -N) reading. However, a 10 mg/L (NO ₂-N) standard of nitrite  could result in a larger value when tested in a nitrate method using NitraVer® reagents.

In most samples, the amount of nitrite is insignificant when compared to the nitrate. If the nitrite concentration is higher and therefore significant, it should be corrected for as described in the interference section of the cadmium reduction methods.

In the pretreatment procedure for nitrite interference, bromine and phenol solutions are added to the sample. The bromine addition oxidizes all of the nitrite into nitrate, then the phenol solution destroys any excess bromine. After the pretreatment, everything in the sample will be nitrate and will go through the partial reduction. The results are reported as combined Nitrate/Nitrite as nitrogen (NO ₃/NO ₂-N).

To determine just the nitrate, a separate nitrite test method must be used on the untreated sample and the result subtracted from the combined nitrate/nitrite results. An alternative would be the  use of the chromatropic acid method for testing nitrate as nitrite is not expected to interfere unless it is above 12 mg/L.