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.

[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: The residue was dissolved in hot water, filtered, and evaporated to a cuticle.

[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.

Natural resources of potassium sulfate are minerals abundant in the Stassfurt salt.

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.

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