Nitrates and Nitrites
Nitrates are compounds containing the anion NO3-. Nitrites are compounds containing NO2-. Nitrates are salts derived from nitric acid and nitrites are salts or esters of nitrous acid.
All nitrates are readily soluble in water. Many nitrates are ionic in nature, but heavy metal nitrates and anhydrous nitrates have covalently bonded nitrate groups. Nitrates are formed from reacting nitric acid with metal oxides, hydroxides, or carbonates. Decomposition of nitrates on heating will give the metal oxide, nitrogen dioxide, and oxygen. The presence of nitrates in a compound can be shown by the following test.Iron (II) sulphate is added to the test solution and then concentrated sulfuric acid is carefully added down the side of the test tube. If a nitrate is present, a brown ring will form where the two liquids meet.
When ammonia is reacted with nitric acid, ammonium nitrate is produced. This is the most important nitrogenous fertilizer. Because of the high solubility of nitrates in water, it is very easy for nitrate fertilizer to be washed from the area of application into local rivers and water supplies. This is a problem for at least two reasons. First, the nitrate acts as a food supply for algae. This leads to a massive increase in numbers of algae called an algal bloom. As the algae die, their decomposition removes oxygen from the water, which in turn means that other organisms, such as fish, cannot survive in the polluted water. Second, when nitrates enter the drinking water supply, they can pose a threat to human health. Nitrates in drinking water have been linked to blue baby syndrome, a condition where the oxygen- carrying capacity of an infant's blood is impaired. Ammonium nitrate is also a major constituent of explosives.
Nitrites and nitrates are an important part of the nitrogen cycle. Nitrites are found in the soil where they are produced from ammonia by nitrifying bacteria such as Nitrosomonas. These nitrites are in turn transformed into nitrates that can be assimilated by plants through the action of other bacteria, such as Nitrobacter.
Nitrates and nitrites also are used as preservatives. They are added to cured meats such as bacon, hot dogs, and ham to retard the growth of bacteria (particularly the causative organism for botulism food poisoning) and to preserve the flavor and color of the meat. There is still some concern over the use these food additives because the nitrites react with the stomach acids to produce nitrous acid. The nitrous acid can then react with amino acids to produce nitrosamines which have been shown to cause cancer in laboratory animals.
In the 1970s the Food and Drug Administration (FDA) responded to public concerns by dramatically cutting back on the quantity of nitrates and nitrites that could be added to foods. By 1981, however, a thorough study of the issue by the National Academy of Sciences showed that nitrates and nitrites are only a minor source of nitrosamine compared to smoking, drinking water, cosmetics, and industrial chemicals. Based on this study, FDA finally decided in January 1983 that nitrates and nitrites are safe to use in foods.
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