Nitrate and nitrite are both polyatomic ions formed from nitrogen and oxygen: essentially, nitrate molecules contain one nitrogen and three oxygen atoms, but nitrite molecules each contain one nitrogen and only two oxygen atoms. Despite the similarities there are substantial differences in properties and uses of these chemicals.
In both cases, note that references to nitrate and nitrite are commonly although certainly not always actually referring to covalent compounds which include nitrate and nitrite, plus some other atoms or molecules. For example, nitrous acid is made up of a nitrite ion and a hydrogen atom, while sodium nitrite is made up of a nitrite ion and a sodium atom. In the same way, ammonium nitrate, discussed below, is a highly common nitrate.
– Sources –
Both nitrates and nitrites are commonly found in nature, both in water as well as in common soil. (In fact, nitrogen itself is the most common atom in the Earth’s atmosphere, which is 80% nitrogen.) Nitrate forms a vital part of the natural food cycle: atmospheric nitrogen is turned into nitrate by bacteria, the nitrate is then absorbed by plants, animals eat these plants and then pass the nitrate in their feces, and this nitrate is then used by plants as a nutrient. (This is why early fertilizers consisted simply of livestock fecal matter, before industrial chemical processes allowed more efficient fertilizer production.)
Bacteria also exist which both produce nitrites from nitrates (by separating an oxygen atom), and then separate the nitrites to produce free nitrogen and oxygen, released back into the atmosphere.
– Uses and Applications –
Nitrates are most commonly found in fertilizers and explosives, which are both based on a chemical called ammonium nitrate. (Incidentally, ammonium is another polyatomic ion containing nitrogen: one nitrogen atom and four hydrogen atoms.) Nitrites, in contrast, are not useful for explosives, but are a common component of food preservatives: sodium nitrite, for example, is used to preserve meat.
Nitrates, especially, are not toxic to humans in normal levels (this can change at very high levels of exposure): after all, they are naturally a part of all of our foods. Some of the nitrates taken in are converted to nitrites by bacteria, as described above, on its way to being freed as nitrogen gas. Nitrites are also relatively harmless, although at very high levels, too much nitrites in the blood can lead to reduced effectiveness of hemoglobin at carrying oxygen, and, as a result, side effects ranging from blue skin to coma and death.
The same is not necessarily true for natural life systems. In the normal levels of the natural nitrogen cycle, plants and animals have adapted to incorporate both nitrates and nitrites without harm. However, human pollution can cause unnatural spikes in both chemicals in the environment. High nitrate runoffs (especially downstream from farms), for example, can cause dangerous increases in the nitrate level in the water, killing fish and causing massive algae growths. Algae in turn removes more oxygen from the water, starving marine life.