Potassium nitrate is a chemical compound with the formula KNO3. It occurs as a mineral niter and is a natural solid source of nitrogen. Its common names include saltpetre (saltpeter in American English), from Medieval Latin sal petræ: "stone salt" or possibly "Salt of Petra" and nitrate of potash. The mineral nitratite also named Peru saltpetre or Chile saltpetre (American Peru saltpeter or Chile saltpeter) refers not to potassium nitrate but to the natural mineral form of a similar chemical sodium nitrate. Major uses of potassium nitrate are in fertilizers, rocket propellants and fireworks; it is one of the constituents of gunpowder. When used as a food additive in the European Union, the compound is referred to as E252.
 History of production
The earliest known complete purification process for potassium nitrate was outlined in 1270 by the Arab chemist and engineer Hasan al-Rammah of Syria in his book al-Furusiyya wa al-Manasib al-Harbiyya ('The Book of Military Horsemanship and Ingenious War Devices'), where he first described the use of potassium carbonate (in the form of wood ashes).
Into the 20th century, niter-beds were prepared by mixing manure with either mortar or wood ashes, common earth and organic materials such as straw to give porosity to a compost pile typically 1.5×2×5 meters in size. The heap was usually under a cover from the rain, kept moist with urine, turned often to accelerate the decomposition and leached with water after approximately one year. Dung-heaps were a particularly common source: ammonia from the decomposition of urea and other nitrogenous materials would undergo bacterial oxidation to produce various nitrates, primarily calcium nitrate, which could be converted to potassium nitrate by the addition of potash from wood ashes.
A variation on this process, using only urine, straw and wood ash, is described by LeConte. Stale urine is placed in a container of straw hay and is allowed to sour for many months, after which water is used to wash the resulting chemical salts from the straw. The process is completed by filtering the liquid through wood ashes and air-drying in the sun.
During this period, the major natural sources of potassium nitrate were the deposits crystallizing from cave walls and the accumulations of bat guano in caves. Traditionally guano was the source used in Laos for the manufacture of gunpowder for Bang Fai rockets.
Potassium nitrates not only supplied the oxidant and much of the energy for gunpowder in the 19th century, but after 1889 small arms and large artillery increasingly began to depend on cordite, a smokeless powder which required in manufacture large quantities of nitric acid derived from mineral nitrates (either potassium nitrate, or increasingly sodium nitrate), and the basic industrial chemical sulfuric acid. These propellants, like all nitrated explosives (nitroglycerine, TNT, etc.) use the energy available when organic nitrates burn or explode and are converted to nitrogen gas, a process that releases large amounts of energy.
From 1903 until the World War I era, potassium nitrate for black powder and fertilizer was produced on an industrial scale from nitric acid produced via the Birkeland–Eyde process, which used an electric arc to oxidize nitrogen from the air. During World War I the newly industrialized Haber process (1913) was combined with the Ostwald process after 1915, allowing Germany to produce nitric acid for the war after being cut off from its supplies of mineral sodium nitrates from Chile (see nitratite). The Haber process catalyzes ammonia production from atmospheric nitrogen, and industrially-produced hydrogen. From the end of World War I until today, practically all organic nitrates have been produced from nitric acid from the oxidation of ammonia in this way. Some sodium nitrate is still mined industrially. Almost all potassium nitrate, now used only as a fine chemical, is produced from basic potassium salts and nitric acid.
- NH4NO3 (aq) + KOH (aq) → NH3 (g) + KNO3 (aq) + H2O (l)
- NH4NO3 (aq) + KCl (aq) → NH4Cl (aq) + KNO3 (aq)
- KOH (aq) + HNO3 → KNO3 (aq) + H2O (l)
Potassium nitrate has an orthorhombic crystal structure at room temperature, which transforms to a trigonal system at 129 °C. Upon heating to temperatures above 560 °C, it decomposes into potassium nitrite, generating oxygen:
- 2 KNO3 → 2 KNO2 + O2
Potassium nitrate is moderately soluble in water, but its solubility increases with temperature (see infobox). The aqueous solution is almost neutral, exhibiting pH 6.2 at 14 °C for a 10% solution of commercial powder. It is not very hygroscopic, absorbing about 0.03% water in 80% relative humidity over 50 days. It is insoluble in alcohol and is not poisonous; it can react explosively with reducing agents, but it is not explosive on its own.
Potassium nitrate is mainly used in fertilizers, as a source of nitrogen and potassium – two of the macro nutrients for plants. When used by itself, it has an NPK rating of 13-0-44. Potassium nitrate is also one of the three components of black powder, along with powdered charcoal (substantially carbon) and sulfur, where it acts as an oxidizer.
In the process of food preservation, potassium nitrate has been a common ingredient of salted meat since the Middle Ages, but its use has been mostly discontinued due to inconsistent results compared to more modern nitrate and nitrite compounds. Even so, saltpetre is still used in some food applications, such as charcuterie and the brine used to make corned beef. Sodium nitrate (and nitrite) have mostly supplanted potassium nitrate's culinary usage, as they are more reliable in preventing bacterial infection than saltpetre. All three give cured salami and corned beef their characteristic pink hue.
Potassium nitrate is an efficient oxidizer, which produces a lilac-colored flame upon burning due to the presence of potassium. It is therefore used in amateur rocket propellants and in several fireworks such as smoke bombs, made with a mixture of sucrose and potassium nitrate. It is also added to pre-rolled cigarettes to maintain an even burn of the tobacco.
Potassium nitrate is the main component (usually about 98%) of tree stump remover, as it accelerates the natural decomposition of the stump. It is also commonly used in the heat treatment of metals as a solvent in the post-wash. The oxidizing, water solubility and low cost make it an ideal short-term rust inhibitor.
Potassium nitrate can be found in some toothpastes for sensitive teeth. Recently, the use of potassium nitrate in toothpastes for treating sensitive teeth has increased and may be an effective treatment.
Potassium nitrate in some toothpastes has shown to relieve asthmatic symptoms in some people. It was used in centuries past to treat asthma as well as arthritis.
Potassium nitrate successfully combats high blood pressure and was once used as a hypotensive. Other nitrates and nitrites such as glyceryl trinitrate (nitroglycerin), amyl nitrite and isosorbide derivatives are still used to relieve angina.
Potassium nitrate was once thought to induce impotence, and is still falsely rumored to be in institutional food (such as military fare) as an anaphrodisiac; however, there is no scientific evidence for such properties.
 See also
- ^ Record of Potassium nitrate in the GESTIS Substance Database from the IFA, accessed on 2007-03-09.
- ^ Ahmad Y Hassan, Potassium Nitrate in Arabic and Latin Sources, History of Science and Technology in Islam.
- ^ a b LeConte, Joseph (1862). Instructions for the Manufacture of Saltpeter. Columbia, S.C.: South Carolina Military Department. p. 14. http://docsouth.unc.edu/imls/lecontesalt/leconte.html. Retrieved 2007-10-19.
- ^ B. J. Kosanke, B. Sturman, K. Kosanke, I. von Maltitz, T. Shimizu, M. A. Wilson, N. Kubota, C. Jennings-White, D. Chapman (2004). "2". Pyrotechnic Chemistry. Journal of Pyrotechnics. pp. 5–6. ISBN 1889526150. http://books.google.com/books?id=Q1yJNr92-YcC&pg=PA9.
- ^ "Meat Science", University of Wisconsin
- ^ Corned Beef, Food Network
- ^ Amthyst Galleries, Inc.
- ^ Inorganic Additives for the Improvement of Tobacco, TobaccoDocuments.org
- ^ Stump Remover MSDS
- ^ Kirst, W.J. (1983). Self Consuming Paper Cartridges for the Percussion Revolver. Minneapolis, Minnesota: Northwest Development Co..
- ^ "Sensodyne Toothpaste for Sensitive Teeth". 2008-08-03. http://us.sensodyne.com/products_freshmint.aspx. Retrieved 2008-08-03.
- ^ "The Effect of Potassium Nitrate and Silica Dentifrice in the Surface of Dentin". http://sciencelinks.jp/j-east/article/200315/000020031503A0361500.php. Retrieved 2010-07-16.
- ^ Managing dentin hypersensitivity, Robin Orchardson and David G. Gillam, J Am Dent Assoc, Vol 137, No 7, 990-998. 2006
- ^ "The Straight Dope: Does saltpeter suppress male ardor?". 1989-06-16. http://www.straightdope.com/classics/a3_221.html. Retrieved 2007-10-19.
- ^ Jones, Richard E.; Kristin H. López (2006). Human Reproductive Biology, Third Edition. Elsevier/Academic Press. p. 225. ISBN 0120884658. http://books.google.com/books?id=pfiZfui2XLIC&pg=PA225.
- Dennis W. Barnum. (2003). "Some History of Nitrates." Journal of Chemical Education. v. 80, p. 1393-. link.
- Alan Williams: The production of saltpeter in the Middle Ages, Ambix, 22 (1975), pp. 125–33. Maney Publishing, ISSN 0002-6980.