Inside Potassium nitrate

Potassium nitrate is a chemical compound with the chemical formula KNO3. It is an ionic salt of potassium ions K+ and nitrate ions NO3−, and is therefore an alkali metal

nitrate.

It occurs as a mineral niter and is a natural solid source of nitrogen. Potassium nitrate is one of several nitrogen-containing compounds collectively referred to as saltpeter or saltpetre.

Major uses of potassium nitrate are in fertilizers, tree stump removal, rocket propellants and fireworks. It is one of the major constituents of gunpowder (blackpowder) and has been used since the Middle Ages as a food preservative.

Properties

Potassium nitrate has an orthorhombic crystal structure at room temperature, which transforms to a trigonal system at 129 °C (264 °F).

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 (57 °F) 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.[2]

Thermal decomposition[edit]

Between 550–790 °C (1,022–1,454 °F), potassium nitrate reaches a temperature dependent equilibrium with potassium nitrite:[13]

2 KNO3 ⇌ 2 KNO2 + O2

History of production

From mineral sources[edit]

The earliest known complete 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 (The Book of Military Horsemanship and Ingenious War Devices). In this book, al-Rammah describes first the purification of barud (crude saltpeter mineral) by boiling it with minimal water and using only the hot solution, then the use of potassium carbonate (in the form of wood ashes) 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.[14] This was used for the manufacture of gunpowder and explosive devices. The terminology used by al-Rammah indicated a Chinese origin for the gunpowder weapons about which he wrote.[15]

At least as far back as 1845, Chilean saltpeter deposits were exploited in Chile and California, USA.

From caves

A major natural source of potassium nitrate was the deposits crystallizing from cave walls and the accumulations of bat guano in caves.[16] 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.

LeConte

Perhaps the most exhaustive discussion of the production of this material 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". N.B. Many references have been made to a method using only straw and urine, but there is no such method in this work.

French method

Niter-beds are 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 4 feet (1.2 m) high, 6 feet (1.8 m) wide, and 15 feet (4.6 m) 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 the potash.

Swiss method

LeConte 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 were then converted to potassium nitrate via 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 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

Production

Potassium nitrate can be made by combining ammonium nitrate and potassium hydroxide.

NH4NO3 (aq) + KOH (aq) → NH3 (g) + KNO3 (aq) + H2O (l)

An alternative way of producing potassium nitrate without a by-product of ammonia is to combine ammonium nitrate and potassium chloride, easily obtained as a sodium-free salt substitute.

NH4NO3 (aq) + KCl (aq) → NH4Cl (aq) + KNO3 (aq)

Potassium nitrate can also be produced by neutralizing nitric acid with potassium hydroxide. This reaction is highly exothermic.

KOH (aq) + HNO3 → KNO3 (aq) + H2O (l)

On industrial scale it is prepared by the double displacement reaction between sodium nitrate and potassium chloride.

NaNO3 (aq) + KCl (aq) → NaCl (aq) + KNO3 (aq)

Uses

Potassium nitrate has a wide variety of uses, largely as a source of nitrate.

Nitric acid production

Historically, nitric acid was produced by combining sulfuric acid with nitrates such as saltpeter. In modern times this is reversed: nitrates are produced from nitric acid produced via the Ostwald process.

Oxidizer

The most famous use of potassium nitrate is probably as the oxidizer in blackpowder. From the most ancient times through the late 1880s, blackpowder provided the explosive power for all the world's firearms. 

After that time, small arms and large artillery increasingly began to depend on cordite, a smokeless powder. Blackpowder remains in use today in black powder rocket motors, but also in combination with other fuels like sugars in "rocket candy". It is also used in fireworks such as smoke bombs.[18] It is also added to cigarettes to maintain an even burn of the tobacco  and is used to ensure complete combustion of paper cartridges for cap and ball revolvers.

 It can also be heated to several hundred degrees to be used for niter bluing, which is less durable than other forms of protective oxidation, but allows for specific and often beautiful coloration of steel parts, such as screws, pins, and other small parts of firearms.

Food preservation

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 because of inconsistent results compared to more modern nitrate and nitrite compounds. Even so, saltpeter is still used in some food applications, such as charcuterie and the brine used to make corned beef.

 When used as a food additive in the European Union, the compound is referred to as E252; it is also approved for use as a food additive in the USA and Australia and New Zealand (where it is listed under its INS number 252). Although nitrate salts have been suspected of producing the carcinogen nitrosamine, both sodium and potassium nitrates and nitrites have been added to meats in the US since 1925, and nitrates and nitrites have not been removed from preserved meat products because nitrite and nitrate inhibits the germination of C. botulinum endospores, and thus prevents botulism from bacterial toxin that may otherwise be produced in certain preserved meat products.

Food preparation

In West African cuisine, potassium nitrate (saltpetre) is widely used as a thickening agent in soups and stews such as okra soup[28] and isi ewu. It is also used to soften food and reduce cooking time when boiling beans and tough meat. Saltpetre is also an essential ingredient in making special porridges, such as kunun kanwa. literally translated from the Hausa language as 'saltpetre porridge'. In the Shetland Islands (UK) it is used in the curing of mutton to make "reestit" mutton, a local delicacy.

Fertilizer

Potassium nitrate is used in fertilizers as a source of nitrogen and potassium – two of the macronutrients for plants. When used by itself, it has an NPK rating of 13-0-44.

Pharmacology

Used in some toothpastes for sensitive teeth.[32] Recently, the use of potassium nitrate in toothpastes for treating sensitive teeth has increased and it may be an effective treatment.

Used historically to treat asthma.

 Used in some toothpastes to relieve asthma symptoms.

Used in Thailand as main ingredient in kidney tablets to relieve the symptoms of cystitis, pyelitis and urethritis.

Combats high blood pressure and was once used as a hypotensive.

Other uses

Electrolyte in a salt bridge

Active ingredient of condensed aerosol fire suppression systems. When burned with the free radicals of a fire's flame, it produces potassium carbonate.

Works as an aluminium cleaner.

Component (usually about 98%) of some tree stump removal products. It accelerates the natural decomposition of the stump by supplying nitrogen for the fungi attacking the wood of the stump.

In heat treatment of metals as a medium temperature molten salt bath, usually in combination with sodium nitrite. A similar bath is used to produce a durable blue/black finish typically seen on firearms. Its oxidizing quality, water solubility, and low cost make it an ideal short-term rust inhibitor.

To induce flowering of mango trees in the Philippines.

Thermal storage medium in power generation systems. Sodium and potassium nitrate salts are stored in a molten state with the solar energy collected by the heliostats at the Gemasolar Thermosolar Plant. Ternary salts, with the addition of calcium nitrate or lithium nitrate, have been found to improve the heat storage capacity in the molten salts.

In folklore and popular culture

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.