Potassium sulfate (in British English potassium sulphate (SOP), also called sulphate of potash, arcanite, or archaically known as potash of sulfur) is the inorganic compound with formula K2SO4. It is a white water-soluble solid. It is commonly used in fertilizers, providing both potassium and a source of sulfur.
3D model (JSmol)
|E number||E515(i) (acidity regulators, ...)|
CompTox Dashboard (EPA)
|Molar mass||174.259 g/mol|
|Melting point||1,069 °C (1,956 °F; 1,342 K)|
|Boiling point||1,689 °C (3,072 °F; 1,962 K)|
|111 g/L (20 °C) |
120 g/L (25 °C)
240 g/L (100 °C)
|Solubility||slightly soluble in glycerol |
insoluble in acetone, alcohol, CS2
Refractive index (nD)
|Safety data sheet||External MSDS|
|Lethal dose or concentration (LD, LC):|
LD50 (median dose)
|6600 mg/kg (oral, rat)|
|Potassium hydrogen sulfate|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Potassium sulfate (K2SO4) has been known since early in the 14th century. It was studied by Glauber, Boyle, and Tachenius. In the 17th century, it was named arcanuni or sal duplicatum, as it was a combination of an acid salt with an alkaline salt. It was also known as vitriolic tartar and Glaser's salt or sal polychrestum Glaseri after the pharmaceutical chemist Christopher Glaser who prepared it and used medicinally.
Known as arcanum duplicatum ("double secret") or panacea duplicata in pre-modern medicine, it was prepared from the residue (caput mortuum) left over from the production of aqua fortis (nitric acid, HNO3) from nitre (potassium nitrate, KNO3) and oil of vitriol (sulphuric acid, H2SO4) via Glauber's process:
- 2 KNO3 + H2SO4 → 2 HNO3 + K2SO4
According to Chambers's Cyclopedia, the recipe was purchased for five hundred thalers by Charles Frederick, Duke of Holstein-Gottorp. Schroder, the duke's physician, wrote wonders of its great uses in hypochondriacal cases, continued and intermitting fevers, stone, scurvy, etc.
The mineral form of potassium sulfate, arcanite, is relatively rare. Natural resources of potassium sulfate are minerals abundant in the Stassfurt salt. These are cocrystallizations of potassium sulfate and sulfates of magnesium calcium and sodium.
Relevant minerals are:
- Kainite, MgSO4·KCl·H2O
- Schönite (now known as picromerite), K2SO4·MgSO4·6H2O
- Leonite, K2SO4·MgSO4·4H2O
- Langbeinite, K2Mg2(SO4)3
- Aphthitalite (previously known as glaserite), K3Na(SO4)2
- Polyhalite, K2SO4·MgSO4·2CaSO4·2H2O
The potassium sulfate can be separated from some of these minerals, like kainite, because the corresponding salt is less soluble in water.
Approximately 1.5 million tons were produced in 1985, typically by the reaction of potassium chloride with sulfuric acid, analogous to the Mannheim process for producing sodium sulfate. The process involves intermediate formation of potassium bisulfate, an exothermic reaction that occur at room temperature:
- KCl + H2SO4 → HCl + KHSO4
The second step of the process is endothermic, requiring energy input:
- KCl + KHSO4 → HCl + K2SO4
Structure and properties
Two crystalline forms are known. Orthorhombic β-K2SO4 is the common form, but it converts to α-K2SO4 above 583 °C. These structures are complex, although the sulfate adopts the typical tetrahedral geometry.
It does not form a hydrate, unlike sodium sulfate. The salt crystallize as double six-sided pyramids, classified as rhombic. They are transparent, very hard and have a bitter, salty taste. The salt is soluble in water, but insoluble in solutions of potassium hydroxide (sp. gr. 1.35), or in absolute ethanol.
The dominant use of potassium sulfate is as a fertilizer. K2SO4 does not contain chloride, which can be harmful to some crops. Potassium sulfate is preferred for these crops, which include tobacco and some fruits and vegetables. Crops that are less sensitive may still require potassium sulfate for optimal growth if the soil accumulates chloride from irrigation water.
The crude salt is also used occasionally in the manufacture of glass. Potassium sulfate is also used as a flash reducer in artillery propellant charges. It reduces muzzle flash, flareback and blast overpressure.
Potassium hydrogen sulfate (also known as potassium bisulfate), KHSO4, is readily produced by reacting K2SO4 with sulfuric acid. It forms rhombic pyramids, which melt at 197 °C (387 °F). It dissolves in three parts of water at 0 °C (32 °F). The solution behaves much as if its two congeners, K2SO4 and H2SO4, were present side by side of each other uncombined; an excess of ethanol the precipitates normal sulfate (with little bisulfate) with excess acid remaining.
The behavior of the fused dry salt is similar when heated to several hundred degrees; it acts on silicates, titanates, etc., the same way as sulfuric acid that is heated beyond its natural boiling point does. Hence it is frequently used in analytical chemistry as a disintegrating agent. For information about other salts that contain sulfate, see sulfate.
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- Klooster, van (1959). "Three centuries of Rochelle salt". Journal of Chemical Education. 36 (7): 314. doi:10.1021/ed036p314.
Chambers, Ephraim, ed. (1728). "Arcanum duplicatum". Cyclopædia, or an Universal Dictionary of Arts and Sciences. 1 (first ed.). James and John Knapton, et al. p. *125.
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