Potassium nitrate



Potassium nitrate is a chemical compound with the chemical formula KNO3. A naturally occurring mineral source of nitrogen, KNO3 constitutes a critical oxidizing component of black powder gunpowder. In the past it was also used for several kinds of burning fuses, including slow matches. Since potassium nitrate readily precipitates, urine was a significant source, through various malodorous means, from the Late Middle Ages and Early Modern era through the 19th century.

Its common names include saltpetre, from Medieval Latin sal petrae: "stone salt" or possibly "Salt of Petra" (saltpeter in US English), Nitrate of potash, and nitre. The name Chile saltpeter is applied to sodium nitrate, a different nitrogen compound that is also used in explosives and fertilizers.

Description
Potassium nitrate is the oxidizing component of black powder. Before the large-scale industrial fixation of nitrogen through the Haber process, major sources of potassium nitrate were the deposits crystallizing from cave walls and the draining of decomposing organic material. Dung-heaps were a particularly common source: ammonia from the decomposition of urea and other nitrogenous materials would undergo bacterial oxidation to produce nitrate. It was and is also used as a component in some fertilizers. When used by itself as a fertilizer, it has an NPK rating of 13-0-38 (indicating 13.9%, 0%, and 38.7% of nitrogen, phosphorus, and potassium, by mass, respectively). Potassium nitrate was once thought to induce impotence, and is still rumored to be in institutional food (such as military fare) as an anaphrodisiac; these uses would be ineffective, since potassium nitrate has no such properties. However, potassium nitrate and other nitrates do successfully combat high blood pressure and are used medically to relieve angina.

Manufacture
Historically, nitre-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 meters high by 2 meters wide by 5 metres long. 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. The liquid containing various nitrates was then converted with wood ashes to potassium nitrates, crystallized and refined for use in gunpowder.

In more ruraltimes, urine was collected and used in the manufacture of gunpowder. Stale urine was filtered through a barrel full of straw and allowed to continue to sour for a year or more. After this period of time, water was used to wash the resulting chemical salts from the straw. This slurry was filtered through wood ashes and allowed to dry in the sun. Saltpeter crystals were then collected and added to brimstone and charcoal to create black powder.

Potassium nitrate could also be harvested from accumulations of bat guano in caves. This was the traditional method used in Laos for the manufacture of gunpowder for Bang Fai rockets.

During the 19th century and until around World War I, potassium nitrate was produced on an industrial scale, first by the Birkeland-Eyde process in 1905, and then later from ammonium produced by the much more efficient Haber process. The latter process came online during World War I, and supplied Germany with nitrates critical for the warfare that it otherwise had no access because the deposits of natural nitrate in Chile were in British hands. It is assumed that this prolonged World War I. Today practically all nitrates are produced by ammonia from the Haber process.

Applications
Saltpeter was used as an anaphrodisiac throughout history.

Potassium nitrate is also used as a fertilizer, in amateur rocket propellant, and in several fireworks such as smoke bombs, in which a mixture with sugar produces a smoke cloud of 600 times their own volume. The ratio for smoke bombs using sucrose (powdered sugar) and potassium nitrate is 40(C12H22O11):60(KNO3). It can be used as is,or consolidated into a lump by mixing with water to make a paste and allowing to dry overnight.

In the process of food preservation, potassium nitrate has been a common ingredient of salted meat, but its use has been deprecated. In the European Union, it is referred to as E252.

It is commonly used in pre-rolled cigarettes to maintain an even burn of the tobacco

Potassium nitrate is also the main component (usually about 98%) of tree stump remover; it accelerates the natural decomposition of the stump. 

It has also been used in the manufacture of ice cream and can be found in some toothpastes for sensitive teeth. Recently, the use of potassium nitrate in toothpastes for treating sensitive teeth has increased dramatically, despite the fact that it has not been conclusively shown to help dental hypersensitivity.

Potassium nitrate is also one of the three parts of black powder, along with powdered charcoal (substantially carbon) and sulfur, where it acts as an oxidizer.

Literature

 * Alan Williams: The production of saltpeter in the middle ages, Ambix, 22 (1975), p. 125-33. Maney Publishing, ISSN 0002-6980.