nitrate, chemical compound containing the nitrate (NO3) radical. Nitrates are salts or esters of nitric acid, HNO3, formed by replacing the hydrogen with a metal (e.g., sodium or potassium) or a radical (e.g., ammonium or ethyl). Some important inorganic nitrates are potassium nitrate (KNO3), sodium nitrate (NaNO3), silver nitrate (AgNO3), and ammonium nitrate (NH4NO3). Calcium nitrate is used in fertilizers; barium and strontium nitrates are used to color fireworks and signal flares; bismuth nitrate is used in making pharmaceuticals. Saltpeter (potassium nitrate), a diuretic, was once believed to be an anaphrodesiac. Nearly all metal nitrates are readily soluble in water; for this reason they are often used when a water soluble salt of a metal is needed. The presence of nitrates in the soil is of great importance, since it is from these compounds that plants obtain the nitrogen necessary for their growth. Nitrogen-fixing bacteria are important in keeping the soil supplied with nitrates. Because of the widespread use of artificial fertilizers containing nitrates, nitrates have contaminated both ground and surface waters in some agricultural areas. Organic nitrates are esters formed by reaction of nitric acid with the hydroxyl (-OH) group in an alcohol. Nitroglycerin is the trinitrate of glycerol; guncotton is a nitrate of cellulose. In chemical analysis, a test for nitrates involves the addition of a solution of ferrous sulfate to the substance to be tested, followed by the addition (without mixing) of a few drops of concentrated sulfuric acid; the presence of a nitrate is indicated by the formation of a brown ring—of Fe(NO)+2 complex ion—where the sulfuric acid contacts the test mixture.