Potassium sulfate

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.[4][5]

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.[6]

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.[6]

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.

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.[7] 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

Two crystalline forms are known. Orthorhombic β-K₂SO₄ is the common form, but it converts to α-K₂SO₄ above 583 °C.[7] These structures are complex, although the sulfate adopts the typical tetrahedral geometry.[8]

• 
  Structure of β-K₂SO₄.
• 
  Coordination sphere of one of two types of K⁺ site.
• 
  SO4 environment in β-K₂SO₄.

It does not form a hydrate, unlike sodium sulfate. The salt crystallizes 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.[9]

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.

It is sometimes used as an alternative blast media similar to soda in soda blasting as it is harder and similarly water-soluble.[10]

Potassium sulfate can also be used in pyrotechnics in combination with potassium nitrate to generate a purple flame.

A 5% solution of potassium sulfate was used in the beginning of the 20th century as a topical mosquito repellent.

• mindat.org: Arcanite
• webmineral: Arcanite