The slightly different chemical and physical properties of potassium nitrate and sodium nitrate make them suitable for different practical applications. Sodium nitrate, often referred to as Chile peter or Peru saltpeter, preserves meats. Potassium nitrate, often referred to as saltpeter, also preserves meat, but finds its major use in gunpowder and fireworks, as well as a fertilizer. Both improve the burning rate of matches and tobacco. Their toxicity in humans relative to food consumption depends on reactions with other substances.
Chemical Properties
Sodium nitrate and potassium nitrate have similar, though not identical, chemistry. Sodium nitrate consists of a sodium or Na atom bonded to a nitrate group, while potassium nitrate consists of a potassium or K atom bonded to a nitrate group. Nitrate groups consist of one atom of nitrogen and three atoms of oxygen, making the chemical formulas for sodium nitrate and potassium nitrate NaNO3 and KNO3, respectively.
Physical Properties
Physically, both sodium and potassium nitrates exist as white, odorless crystals. Both have similar degrees of acidity, having a pH of 2.26 and 2.1, respectively. Sodium nitrate, however, is much more soluble in water; 81.5 g of sodium nitrate dissolves in 100 g of water, while only 36 g of potassium nitrate dissolves in that same amount of water. Boiling points and melting points differ by a few degrees; sodium nitrate boiling at a temperature 36 degrees F lower than potassium nitrate and melting at a temperature of 45 degrees F lower than potassium nitrate. Crystals of potassium nitrate exist as orthorhombic shapes, having a rectangular box shape with rectangular ends, while sodium nitrate crystal exist as rhombohedrons, having rhomboid shapes on each face.
Nitrates in Foods
Nitrates exist in soil, naturally and added as fertilizer. Vegetables such as spinach, beets, radishes, celery, and cabbages contain very high concentrations of nitrates. In addition, cured meats, such as ham, bacon, sausage, deli meats and artificially smoked fish contain added nitrates to help to retain their color and prevent spoilage by bacteria, particularly the bacterium responsible for botulism, and other microorganisms. Potassium and sodium nitrate may also be present in drinking water due to their use in fertilizers, which then leach into water supplies.
Toxicity
Excessive consumption of nitrates can cause death. The fatal dose of potassium nitrate for an average adult ranges between 30 g to 35 g consumed in a single dose. Nitrates reduce to nitrites by microorganisms existing in both foods you eat and in your gastrointestinal tract. Nitrites are also toxic. Sodium nitrate consumed in significant quantities to yield about 22 mg of sodium nitrite per kg of body weight causes death. However, the amount of cured meat to cause this problem is about 60 pounds. While acute doses appear rare, moderate consumption over extended periods of time can also cause problems. Low doses of sodium or potassium nitrate can cause acute methemoglobinemia, a condition where your blood can no longer carry oxygen, cutting off oxygen supplies to many parts of your body, including your brain and heart. According to the United Kingdom Food Guide, children should not consume either sodium or potassium nitrate.
Nitrosamines
When sodium nitrate and potassium nitrate convert to nitrites in your body, these nitrites in turn can combine with amines, the product of protein degradation, creating nitrosamines. You can also create nitrosamines in your food by cooking sodium nitrate and potassium nitrate preserved meats such as bacon at high temperatures for extended periods of time; for example, cooking bacon from medium well to well-done. Nitrosamines are well known carcinogens. Sodium nitrate and potassium nitrate are specifically designated by the U.S. FDA as "prior sanctioned." This means that they were approved by the FDA before the 1958 Food Additives Amendment. Though not specifically listed as GRAS or Generally Recognized As Safe, documented GRAS reports using meats containing high levels of nitrite showed no evidence of the nitrates, by themselves, causing cancer.
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