Potassium nitrate
Potassium nitrate is a chemical compound with a sharp, salty, bitter taste and the chemical formula. It is a potassium salt of nitric acid. This salt consists of potassium cations and nitrate anions, and is therefore an alkali metal nitrate. It occurs in nature as a mineral, niter. It is a source of nitrogen, and nitrogen was named after niter. Potassium nitrate is one of several nitrogen-containing compounds collectively referred to as saltpetre.
Major uses of potassium nitrate are in fertilizers, tree stump removal, rocket propellants and fireworks. It is one of the major constituents of traditional gunpowder. In processed meats, potassium nitrate reacts with hemoglobin and myoglobin generating a red color.
Etymology
Nitre, or potassium nitrate, because of its early and global use and production, has many names.As for nitrate, Egyptian and Hebrew words for it had the consonants n-t-r, indicating likely cognation in the Greek nitron, which was Latinised to nitrum or nitrium. Thence Old French had niter and Middle English nitre. By the 15th century, Europeans referred to it as saltpetre, specifically Indian saltpetre and later as nitrate of potash, as the chemistry of the compound was more fully understood.
The Arabs called it "Chinese snow" as well as bārūd, a term of uncertain origin that later came to mean gunpowder. It was called "Chinese salt" by the Iranians/Persians or "salt from Chinese salt marshes". The Tiangong Kaiwu, published in the 17th century by members of the Qing dynasty, detailed the production of gunpowder and other useful products from nature.
Historical production
From mineral sources
In Mauryan India saltpeter manufacturers formed the Nuniya & Labanacaste. Saltpeter finds mention in Kautilya's Arthashastra, which mentions using its poisonous smoke as a weapon of war, although its use for propulsion did not appear until medieval times.
A purification process for potassium nitrate was outlined in 1270 by the chemist and engineer Hasan al-Rammah of Syria in his book al-Furusiyya wa al-Manasib al-Harbiyya. In this book, al-Rammah describes first the purification of barud by boiling it with minimal water and using only the hot solution, then the use of potassium carbonate to remove calcium and magnesium by precipitation of their carbonates from this solution, leaving a solution of purified potassium nitrate, which could then be dried. This was used for the manufacture of gunpowder and explosive devices. The terminology used by al-Rammah indicated the gunpowder he wrote about originated in China.
At least as far back as 1845, nitratite deposits were exploited in Chile and California.
From caves
Major natural sources of potassium nitrate were the deposits crystallizing from cave walls and the accumulations of bat guano in caves. Extraction is accomplished by immersing the guano in water for a day, filtering, and harvesting the crystals in the filtered water. Traditionally, guano was the source used in Laos for the manufacture of gunpowder for Bang Fai rockets.Calcium nitrate, or lime saltpetre, was discovered on the walls of stables, from the urine of barnyard animals.
Nitraries
Potassium nitrate was produced in a nitrary or "saltpetre works". The process involved burial of excrements in a field beside the nitraries, watering them and waiting until leaching allowed saltpeter to migrate to the surface by efflorescence. Operators then gathered the resulting powder and transported it to be concentrated by ebullition in the boiler plant.Besides "Montepellusanus", during the thirteenth century the only supply of saltpeter across Christian Europe was "found in Spain in Aragon in a certain mountain near the sea".
In 1561, Elizabeth I, Queen of England and Ireland, who was at war with Philip II of Spain, became unable to import saltpeter, and had to pay "300 pounds gold" to the German captain Gerrard Honrik for the manual "Instructions for making saltpeter to growe".
Nitre bed
A nitre bed is a similar process used to produce nitrate from excrement. Unlike the leaching-based process of the nitrary, however, one mixes the excrements with soil and waits for soil microbes to convert amino-nitrogen into nitrates by nitrification. The nitrates are extracted from soil with water and then purified into saltpeter by adding wood ash. The process was discovered in the early 15th century and was very widely used until the Chilean mineral deposits were found.The Confederate side of the American Civil War had a significant shortage of saltpeter. As a result, the Nitre and Mining Bureau was set up to encourage local production, including by nitre beds and by providing excrement to government nitraries. On November 13, 1862, the government advertised in the Charleston Daily Courier for 20 or 30 "able bodied Negro men" to work in the new nitre beds at Ashley Ferry, S.C. The nitre beds were large rectangles of rotted manure and straw, moistened weekly with urine, "dung water", and liquid from privies, cesspools and drains, and turned over regularly. The National Archives published payroll records that account for more than 29,000 people compelled to such labor in the state of Virginia. The South was so desperate for saltpeter for gunpowder that one Alabama official reportedly placed a newspaper ad asking that the contents of chamber pots be saved for collection. In South Carolina, in April 1864, the Confederate government forced 31 enslaved people to work at the Ashley Ferry Nitre Works, outside Charleston.
Perhaps the most exhaustive discussion of the niter-bed production is the 1862 LeConte text. He was writing with the express purpose of increasing production in the Confederate States to support their needs during the American Civil War. Since he was calling for the assistance of rural farming communities, the descriptions and instructions are both simple and explicit. He details the "French Method", along with several variations, as well as a "Swiss method".
French method
and Lavoisier created the Régie des Poudres et Salpêtres a few years before the French Revolution. 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 high, wide, and long. The heap was usually under a cover from the rain, kept moist with urine, turned often to accelerate the decomposition, then finally leached with water after approximately one year, to remove the soluble calcium nitrate which was then converted to potassium nitrate by filtering through potash.Swiss method
describes a process using only urine and not dung, referring to it as the Swiss method. Urine is collected directly, in a sandpit under a stable. The sand itself is dug out and leached for nitrates which are then converted to potassium nitrate using potash, as above.From nitric acid
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 using the Birkeland–Eyde process, which used an electric arc to oxidize nitrogen from the air. During World War I the newly industrialized Haber process 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.Modern production
Potassium nitrate can be made by combining ammonium nitrate and potassium hydroxide.An alternative way of producing potassium nitrate without a by-product of ammonia is to combine ammonium nitrate, found in instant ice packs, and potassium chloride, easily obtained as a sodium-free salt substitute.
Potassium nitrate can also be produced by neutralizing nitric acid with potassium hydroxide. This reaction is highly exothermic.
On industrial scale it is prepared by the double displacement reaction between sodium nitrate and potassium chloride.
Properties
Potassium nitrate has an orthorhombic crystal structure at room temperature, which transforms to a trigonal system at. On cooling from, another trigonal phase forms between and.Sodium nitrate is isomorphous with calcite, the most stable form of calcium carbonate, whereas room-temperature potassium nitrate is isomorphous with aragonite, a slightly less stable polymorph of calcium carbonate. The difference is attributed to the similarity in size between nitrate and carbonate ions and the fact that the potassium ion is larger than sodium and calcium ions.
In the room-temperature structure of potassium nitrate, each potassium ion is surrounded by 6 nitrate ions. In turn, each nitrate ion is surrounded by 6 potassium ions.
| Unit cell | Potassium coordination | Nitrate coordination |
Potassium nitrate is moderately soluble in water, but its solubility increases with temperature. The aqueous solution is almost neutral, exhibiting pH 6.2 at 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.