Potassium nitrate

Potassium nitrate[1]
Other names Saltpetre Nitrate of potash Vesta powder
Identifiers
CAS number	7757-79-1 Y
PubChem	24434
ChemSpider	22843
UN number	1486
RTECS number	TT3700000
Properties
Molecular formula	KNO3
Molar mass	101.103 g/mol
Appearance	white solid
Odor	odorless
Density	2.109 g/cm3 (16 °C)
Melting point	334 °C
Boiling point	400 °C decomp.
Solubility in water	133 g/L (0 °C) 360 g/L (25 °C) 2470 g/L (100 °C)
Solubility	slightly soluble in ethanol soluble in glycerol, ammonia
Structure
Crystal structure	Orthorhombic, Aragonite
Hazards
MSDS	ICSC 0184
EU Index	Not listed
EU classification	Oxidant (O)
R-phrases	R8 R22 R36 R37 R38
S-phrases	S7 S16 S17 S26 S36 S41
NFPA 704	0 1 0 OX
Flash point	Non-flammable
LD50	3750 mg/kg
Related compounds
Other anions	Potassium nitrite
Other cations	Lithium nitrate Sodium nitrate Rubidium nitrate Caesium nitrate
Related compounds	Potassium sulfate Potassium chloride
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Potassium nitrate is a chemical compound with the formula KNO₃. It occurs as a mineral niter and is a natural solid source of nitrogen. Its common names include saltpetre (saltpeter in American English), from Medieval Latin sal petræ: "stone salt" or possibly "Salt of Petra" and nitrate of potash. The name Peru saltpetre or Chile saltpetre (American Peru saltpeter or Chile saltpeter) refers not to potassium nitrate but to a similar chemical sodium nitrate. Major uses of potassium nitrate are in fertilizers, rocket propellants and fireworks. When used as a food additive in the European Union, the compound is referred to as E252.

Contents 1 History of production 2 Production 3 Properties 4 Uses 5 Pharmacology 6 See also 7 References 8 Bibliography 9 External links

History of production

The earliest known complete purification process for potassium nitrate was outlined in 1270 by the Arab 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'), where he first described the use of potassium carbonate (in the form of wood ashes).^[2]

Into the 19th century, 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 1.5×2×5 meters in size.^[3] 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. Dung-heaps were a particularly common source: ammonia from the decomposition of urea and other nitrogenous materials would undergo bacterial oxidation to produce various nitrates, primarily calcium nitrate, which could be converted to potassium nitrate by the addition of potash from wood ashes.

A variation on this process, using only urine, straw and wood ash, is described by LeConte: Stale urine is placed in a container of straw hay and is allowed to sour for many months, after which water is used to wash the resulting chemical salts from the straw. The process is completed by filtering the liquid through wood ashes and air-drying in the sun.^[3]

During this period, the major natural sources of potassium nitrate were the deposits crystallizing from cave walls and the accumulations of bat guano in caves. Traditionally guano was the source 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 by the Birkeland–Eyde process. Today practically all nitrates are produced from the oxidation of ammonia made by the Haber process.

Production

Potassium nitrate can be made by combining ammonium nitrate, obtainable from instant cold packs, and potassium hydroxide.

The reaction is shown here: NH₄NO₃ (aq) + KOH (aq) → NH₃ (g) + KNO₃ (aq) + H₂O (l)

As toxic ammonia gas is produced, you should not do this indoors.

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)

Properties

Potassium nitrate has an orthorhombic crystal structure at room temperature, which transforms to a trigonal system at 129 °C. Upon heating to temperatures above 560 °C, it decomposes into potassium nitrite, generating oxygen:

2 KNO₃ → 2 KNO₂ + O₂

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 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.^[4]

Uses

Potassium nitrate is mainly used in fertilizers, as a source of nitrogen and potassium – two of the macro nutrients for plants. When used by itself, it has an NPK rating of 13-0-44. Potassium nitrate is also one of the three components of black powder, along with powdered charcoal (substantially carbon) and sulfur, where it acts as an oxidizer.

In the process of food preservation, potassium nitrate has been a common ingredient of salted meat since the Middle Ages,^[5] but its use has been mostly discontinued due to inconsistent results compared to more modern nitrate and nitrite compounds. Even so, saltpetre is still used in some food applications, such as charcuterie and the brine used to make corned beef.^[6] Sodium nitrate (and nitrite) have mostly supplanted potassium nitrate's culinary usage, as they are more reliable in preventing bacterial infection than saltpetre. All three give cured salami and corned beef their characteristic pink hue.

Potassium nitrate is an efficient oxidizer, which produces a lilac flame upon burning due to the presence of potassium. It is therefore used in amateur rocket propellants and in several fireworks such as smoke bombs.^[7] It is also added to pre-rolled cigarettes to maintain an even burn of the tobacco.^[8]

Potassium nitrate is the main component (usually about 98%) of tree stump remover, as it accelerates the natural decomposition of the stump.^[9] It is also commonly used in the heat treatment of metals as a solvent in the post-wash. The oxidizing, water solubility and low cost make it an ideal short-term rust inhibitor.

Pharmacology

Potassium nitrate can be found in some toothpastes for sensitive teeth.^[10] Recently, the use of potassium nitrate in toothpastes for treating sensitive teeth (dentine hypersensitivity) has increased dramatically, even though studies to this effect have been inconclusive.^[11].^[12]

Potassium nitrate successfully combats high blood pressure and was once used as a hypotensive. Other nitrates and nitrites such as glyceryl trinitrate (nitroglycerin), amyl nitrite and isosorbide derivatives are still used to relieve angina.

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.^[13][14]

See also

References

Bibliography

• Dennis W. Barnum. (2003). "Some History of Nitrates." Journal of Chemical Education. v. 80, p. 1393-. link.
• Alan Williams: The production of saltpeter in the Middle Ages, Ambix, 22 (1975), pp. 125–33. Maney Publishing, ISSN 0002-6980.

External links

• International Chemical Safety Card 018402216
• Confederate Gunpowder - women contributed urine to the war effort.

Potassium compounds KBr · KBrO3 · KCN · KCNO · KCl · KClO3 · KClO4 · KF · KH · KHCO2 · KHCO3 · KHF2 · KHSO3 · KHSO4 · KH2AsO4 · KI · KIO3 · KIO4 · KMnO4 · KNO2 · KNO3 · KOCN · KOH · KO2 · KPF6 · KSCN · K2CO3 · K2CrO4 · K2Cr2O7 · K2FeO4 · K2MnO4 · K2O · K2O2 · K2PtCl4 · K2PtCl6 · K2S · K2SO3 · K2SO4 · K2SO5 · K2S2O5 · K2S2O7 · K2S2O8 · K2SiO3 · K3[Fe(CN)6] · K3[Fe(C2O4)3] · K4[Fe(CN)6] · K3PO4 · K4MnO4 · K4Mo2Cl8