Nitrate is the most completely oxidized form of nitrogen. It is formed during the final stages of biological decomposition, either in wastewater treatment facilities or in natural water supplies. Inorganic nitrates such as ammonium nitrate are commonly used as fertilizers.

Low-level nitrate concentrations may be present in natural waters. However, a Maximum Contaminant Level of 10 ppm nitrate- nitrogen has been established for drinking water by the USEPA. Excessive nitrate concentrations may indicate the presence of heavy fertilizer runoff or excessive discharge from a waste treatment facility.

These methods are applicable to the monitoring of drinking and surface waters as well as domestic and industrial wastes.

The Colorimetric Methods.
Nitrate is reduced to nitrite using cadmium or zinc as the reducing agent. The resulting nitrite concentration is then determined colorimetrically. Nitrite will interfere with the cadmium reduction test.  Results are expressed as ppm (mg/L) NO₃-N or NO₃.