Potassium sulfate (US) or potassium sulfate (UK), also called sulfate of potash (SOP), arcanite , or obsolete sulfur potash , is an inorganic compound with the formula K2 SO4 , a white water – soluble solid. It is commonly used in fertilizers , providing both potassium and sulphur .
Potassium sulfate (K 2 SO 4 ) has been known since the beginning of the 14th century. It was studied by Glauber, Boyle and Tachenius. In the 17th century, it was named Arcnuni or Sal duplicatum , because it was a combination of an acid salt with an alkaline salt. It was also called bitter tartar and Glaser’s salt or Sal polychrestum Glaseri after the pharmaceutical chemist Christopher Glaser who prepared it and used it for medicine.  
Known as Arcanum duplicatum (“double secret”) or Panacea duplicata in pre-modern medicine, it was prepared from residues ( Nisar mortuum ) left over from the production of aqua fortis (nitric acid, HNO 3 ) from saltpetre (potassium nitrate, KNO 3 ) and oil of vitriol (sulfuric acid, H 2 SO 4 ) through the process of Glauber :2 KNO 3 + H 2 SO 4 → 2 HNO 3 + K 2 SO 4
The residue was dissolved in hot water, filtered and evaporated into a cuticle. It was then left to crystallize. It was used as a diuretic and sudorific. 
According to Chambers’ Cyclopedia , the recipe was purchased by Charles Frederick, Duke of Holstein-Gottorp, for five hundred thalers. Duke’s physician, Schröder, continued to write miracles of its great uses in cases of hypochondriacal, fever, appendicitis, scurvy, and more. 
The mineral form of potassium sulfate, arkanite, is relatively rare. The natural resource of potassium sulfate is an abundant mineral in Stasfurt salt. These are cocrystallization of potassium sulfate and sulfates of magnesium, calcium and sodium.
Relevant minerals are:
- Kyanite, KMg(SO 4 ) Cl 3H 2 O
- Shonite (now known as picromerite), K 2 SO 4 MgSO 4 6 H 2 O
- Leonite, K 2 SO 4 MgSO 4 4 H 2 O
- Langbeinite, K 2 Mg 2 (SO 4 ) 3
- Aphthalite (formerly known as glacerite), K 3 Na (SO 4 ) 2
- Polyhalite , K 2 SO 4 MgSO 4 2CaSO 4 2H 2 O
Potassium sulfate can be distinguished from some minerals such as kainate, as the corresponding salt is less soluble in water.
Kieserite, MgSO 4 H 2 O, can be mixed with a solution of potassium chloride to form potassium sulfate.
About 1.5 million tons were produced in 1985, usually by the reaction of potassium chloride with sulfuric acid, analogous to the Mannheim process for the production of sodium sulfate.  This process involves the intermediate formation of potassium bisulfate, an exothermic reaction that occurs at room temperature:
KCl + H 2 SO 4 → HCl + KHSO 4
The second stage of the process is endothermic, requiring an energy input:
KCl + KHSO 4 → HCl + K 2 SO 4
Structure and Properties
Two crystalline forms are known. Orthorhombic β-K 2 SO 4 is the normal form, but it converts to α-K 2 SO 4 above 583 °C .  These structures are complex, although sulfate adopts the typical tetrahedral geometry.
It does not form a hydrate unlike sodium sulfate. The salt crystallizes as a double six-sided pyramid, classified as a rhombus. 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 absolute ethanol.
When potassium sulfate is heated in water and swirled in a beaker, the crystals form a multi-arm spiral structure when they are allowed to settle.
The main use of potassium sulphate is as a fertilizer. K 2 SO 4 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. Less sensitive crops may still require potassium sulfate for optimal growth if the soil accumulates chloride from irrigation water. 
Raw salt is also sometimes used 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.
It is sometimes used as a substitute media similar to blasting soda in blasting soda as it is hard and similarly soluble in water. [11 1]
Potassium sulfate can also be used in pyrotechnics with potassium nitrate to produce a purple flame.
Potassium hydrogen sulfate (also known as potassium bisulfate ), KHSO4 , is readily produced by reacting K2SO4 with sulfuric acid . It forms the rhombus pyramid, which melts at 197 °C (387 °F). It dissolves in three parts of water at 0 °C (32 °F). The solution behaves very much as if its two progenitors, K 2 SO 4 and H 2 SO 4 , were present side-by-side; Normal sulfate (with little bisulfate) precipitates out with the remaining excess acid from excess of ethanol.
The behavior of mixed dry salt is similar when heated to several hundred degrees; It acts on silicates, titanates, etc., in the same way as sulfuric acid that is heated beyond its natural boiling point. Therefore it is often used as a disintegrating agent in analytical chemistry. For information on other salts that contain sulfate, see Sulfate.