Potassium nitrate is a chemical compound with the formula KNO3. It is an ionic salt of potassium
ions K+ and nitrate ions NO3−. 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,
rocket propellants and fireworks. It is one of the major constituents of gunpowder and
has been used since the Middle Ages as a food preservative.
History of production From mineral sources
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. 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 a Chinese origin for the gunpowder weapons about which he wrote. While potassium
nitrate was called "Chinese snow" by Arabs, it was called "Chinese salt" by the Iranians/Persians.
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. 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 1.5×2×5 meters in size. 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 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. The Haber process
catalyzes ammonia production from atmospheric nitrogen, and industrially produced hydrogen.
From the end of World War I until today, practically all organic nitrates have been produced from
nitric acid from the oxidation of ammonia in this way. Some sodium nitrate is still
mined industrially. Almost all potassium nitrate, now used only as a fine chemical, is produced
from basic potassium salts and nitric acid. History of use
Potassium nitrates supplied the oxidant and much of the energy for gunpowder in the 19th
century, but after 1889, small arms and large artillery increasingly began to depend on
cordite, a smokeless powder which required in manufacture large quantities of nitric
acid derived from mineral nitrates, and the basic industrial chemical sulfuric acid. These
propellants, like all nitrated explosives use both parts of the nitrate ion: the oxygen
promotes rapid combustion, and the expansion of the previously solid nitrogen to N2 gas
provides kinetic energy. Production
Potassium nitrate can be made by combining ammonium nitrate and potassium hydroxide.
NH4NO3 + KOH → NH3 + KNO3 + H2O 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 + KCl → NH4Cl + KNO3 Potassium nitrate can also be produced by
neutralizing nitric acid with potassium hydroxide. This reaction is highly exothermic.
KOH + HNO3 → KNO3 + H2O On industrial scale it is prepared by the
double displacement reaction between sodium nitrate and potassium chloride.
NaNO3 + KCl → NaCl + KNO3 Properties
Potassium nitrate has an orthorhombic crystal structure at room temperature, which transforms
to a trigonal system at 129 °C. Upon heating to temperatures between 550 and 790 °C under
an oxygen atmosphere, it loses oxygen and reaches a temperature dependent equilibrium
with potassium nitrite: 2 KNO3 → 2 KNO2 + O2
Potassium nitrate is moderately soluble in water, but its solubility increases with temperature.
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.
Uses Potassium nitrate has a wide variety of uses,
largely as a source of nitrate. Fertilizer
Potassium nitrate is mainly 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.
Oxidizer Potassium nitrate is an efficient oxidizer,
producing a lilac-colored flame upon burning due to the presence of potassium. It is one
of the three components of black powder, along with powdered charcoal and sulfur, both of
which act as fuels in this composition. As such it is used 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, made with a mixture of sucrose and potassium nitrate.
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.
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 due to 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. Food preparation
In West African cuisine, potassium nitrate is widely used as a thickening agent in soups
and stews such as Okra soup and Isi ewu. It is also used to soften food and reduce cooking
time when boiling beans and tough meat. Salt petre is also an essential ingredient in making
special porridges such as kunun kanwa literally translated from the Hausa language as 'salt
petre porridge'. Pharmacology
Used in some toothpastes for sensitive teeth. 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. Component 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 as an anaphrodisiac;
however, there is no scientific evidence for such properties.
See also Nitrocellulose
Potassium perchlorate References
Bibliography Dennis W. Barnum.. "Some History of Nitrates."
Journal of Chemical Education. v. 80, p. 1393-. link.
David Cressy. Saltpeter: The Mother of Gunpowder 237 pp online review by Robert Tiegs
Alan Williams. "The production of saltpeter in the Middle Ages", Ambix, 22, pp. 125–33.
Maney Publishing, ISSN 0002-6980. External links
International Chemical Safety Card 018402216