Potassium nitrate

Did You Know?

• In Mauryan India, saltpeter manufacturers formed the Nuniya and Labana caste.
• Potassium nitrate was once thought to induce impotence, but there is no scientific evidence for such properties.
• In 1800s Corn Belt farming lore, drought-killed corn in manured fields could accumulate saltpeter that would fall as a fine powder when the stalk was opened.

Etymology

The substance has accumulated many names across cultures and centuries. The consonants n-t-r appear in ancient Egyptian and Hebrew words for it, likely related to the Greek nitron, which became Latin nitrum or nitrium, then Old French niter and Middle English nitre. By the 15th century, Europeans called it saltpetre, specifically Indian saltpetre (Chilean saltpetre being sodium nitrate), and later nitrate of potash as chemistry advanced. Arabs knew it as 'Chinese snow' (thalj al-ṣīn) and bārūd, a term of uncertain origin that later came to mean gunpowder. Iranians/Persians called it 'Chinese salt' or 'salt from Chinese salt marshes' (namak shūra chīnī). The Tiangong Kaiwu, a 17th-century Qing dynasty work, detailed its use in gunpowder production.

Modern production

Potassium nitrate forms when ammonium nitrate reacts with potassium hydroxide. Another method uses ammonium nitrate from instant ice packs and potassium chloride, a common salt substitute, which avoids ammonia as a by-product. Neutralizing nitric acid with potassium hydroxide also yields it, releasing significant heat. Industrially, a double displacement reaction between sodium nitrate and potassium chloride is used.

Properties

At room temperature, potassium nitrate has an orthorhombic crystal structure, transforming to a trigonal system at 128 °C. On cooling from 200 °C, another trigonal phase forms between 124 °C and 100 °C. It is isomorphous with aragonite, a less stable polymorph of calcium carbonate, while sodium nitrate is isomorphous with calcite. Each potassium ion is surrounded by 6 nitrate ions, and each nitrate ion by 6 potassium ions. It is moderately soluble in water, with solubility increasing with temperature; a 10% solution at 14 °C has pH 6.2. It is not very hygroscopic, insoluble in alcohol, and not poisonous, but can react explosively with reducing agents. Between 550–790 °C it reaches an equilibrium with potassium nitrite.

Uses

The most famous use of potassium nitrate is as the oxidizer in blackpowder, which powered firearms from ancient times until the late 1880s, when cordite began to replace it. Blackpowder remains in use in black powder rocket motors, and potassium nitrate is combined with sugars in 'rocket candy' for amateur rocketry. It is used in fireworks such as smoke bombs, added to cigarettes for an even burn, and used in paper cartridges for cap and ball revolvers. It can be heated for niter bluing of steel firearm parts. In food, it has been a common ingredient in salted meat since antiquity or the Middle Ages, used in salami, dry-cured ham, charcuterie, and corned beef brine. In the Shetland Islands it is used to cure reestit mutton. As a food additive in the European Union it is referred to as E252, and it is approved in the United States, Australia, and New Zealand under INS number 252. Since October 2015, the WHO classifies processed meat as Group 1 carcinogen.