Potassium nitrate

The crystal structure of KNO₃

The chemical compound potassium nitrate is a naturally occurring mineral source of nitrogen. It is a nitrate with chemical formula K N O₃.

Its common names include saltpetre (from Medieval Latin sal petrae: "stone salt" or possibly "Salt of Petra"), American English saltpeter, Nitrate of potash and nitre. The name Chile saltpetre is also applied to sodium nitrate.

Description

Potassium nitrate is the oxidising component of black powder. Prior to the large-scale industrial fixation of nitrogen through the Haber process, a major source of Potassium nitrate was the deposits crystallising from cave walls or the drainings 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 fertilisers. When used by itself as a fertiliser, it has an NPK rating of 13-0-44 (indicating 13%, 0%, and 44% of nitrogen, phosphorus, and potassium, by mass, respectively). Potassium nitrate was once thought to suppress the male reproductive system, and was rumored to be added to institutional food to pacify the consumers. If true, the treatment was ineffective, as potassium nitrate has no such properties.

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 metres high by 2 metres 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, crystallised and refined for use in gunpowder. In more rural times, 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. Today, most potassium nitrate comes from the vast deposits of sodium nitrate (NaNO₃, nitratine) in the Chilean deserts. The sodium nitrate is purified and then reacted in solution with potassium chloride (KCl, sylvite), from which the less-soluble potassium nitrate is precipitated out.

In England, the privilege of manufacturing explosives was once in the hands of the family of John Evelyn, the celebrated diarist, as a crown monopoly since before 1588.

Applications

One of the most useful applications of potassium nitrate is in the production of nitric acid, by adding concentrated sulfuric acid to an aqueous solution of potassium nitrate, yielding nitric acid and potassium sulfate which are separated through fractional distillation.

Potassium nitrate is also used as a fertiliser, in model 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 carefully melted together using a hot plate.

In the process of food preservation, potassium nitrate is a common ingredient of salted meat, but there are theories indicating that using nitrates in meats can cause cancer. As a preservative it can be known as E252.

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 sensitive teeth has increased dramatically, despite the fact that it has not been conclusively shown to help dental hypersensitivity. [1]

A popular misconception is that potassium nitrate is an anaphrodisiac and was added to food in all-male institutions. In fact, potassium nitrate has no such effect in humans. [2]

Potassium nitrate is also one of the three parts of gunpowder, along with charcoal and sulfur. It conntributes largely to gunpowder's explosive properties, but by itself is very hard to light. If sufficient heat is applied, hoever, KNO3 will ignite and burn with a very hot pink-orange flame, producing lots of smoke Potassium nitrate can also be bought at an older drugstore as saltpetre.

References in culture and literature

• Potassium nitrate is referenced in Edgar Allan Poe's famous short story "The Cask of Amontillado" as nitre, which lines the crypt where Monstresor buries Fortunato alive.
• The "Book of Secrets" attributed to Albertus Magnus refers to this substance as "Stony Salt" ("salis petrosi" in Latin).
• In the film One Flew Over the Cuckoo's Nest, the main character relates his fear of being "slipped" the substance in a mental institution.
• In the novella Chronicle of a Death Foretold, the character Bayardo San Roman is described as having "skin slowly roasted by saltpeter."
• Salt Peter is an album by the singer Ruby.
• It is referenced in the Anarchist Cookbook as an ingredient in a smoke bomb.

See also

• Nitric acid
• Nitre
• Black powder
• Gunpowder
• Sodium nitrate
• Sodium nitrite
• Potassium nitrite

External links

• International Chemical Safety Card 0184
• Manufacturing Saltpetre 1862