Potassium sulfate (US) or potassium sulphate (UK), also called sulphate of potash (SOP), arcanite, or archaically potash of sulfur, is the inorganic compound with formula K2SO4, a white water-soluble solid. It is commonly used in fertilizers, providing both potassium and sulfur. History 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 The residue was dissolved in hot water, filtered, and evaporated to a cuticle. It was then left to crystallise. It was used as a diuretic and sudorific. 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, and more. Natural resources 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, KMg(SO4)·Cl·3H2O 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. Kieserite, MgSO4·H2O, can be combined with a solution of potassium chloride to produce potassium sulfate. Production 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 occurs at room temperature: KCl + H2SO4 → HCl + KHSO4 The second step of the process is endothermic, requiring energy input: KCl + KHSO4 → HCl + K2SO4 Structure