Potassium

Potassium salts have been used since ancient times for various applications. The poisonous smoke from saltpeter (potassium nitrate, KNO₃) was used in ancient India as a weapon. Saltpeter was later used to make gunpowder, starting in China during the first millennium CE. Alum (potassium aluminum sulfate, KAl(SO₄)₂), or some variant of it, was likely used in ancient Egypt and Rome. It was later used in the Middle Ages for dyeing, having been produced in Africa and exported to Europe. And potash (potassium carbonate, K₂CO₃) was used in the bronze age (3300–1200 BCE) for bleaching textiles and making glass, ceramics, and soap.

Potassium itself was discovered in 1807 by English chemist Sir Humphrey Davy. He used an electric current on “caustic potash” (potassium hydroxide, KOH) to isolate the potassium. This method of using electricity to drive a chemical separation is called electrolysis, and potassium was the first of many elements to be discovered with it. Davy noted that his potassium formed metal globules, which, when dropped in water, would float and catch fire. Dancing around the water’s surface, they burned with a purple flame.

Potassium is a soft metal that’s easily cut with a knife. Pure potassium is silver in color, but it oxidizes (tarnishes) in air within minutes. Therefore, it must be kept in a non-oxygen-based liquid, such as mineral oil.

It is one of the most reactive metals and one of the most electropositive (meaning its more likely to give its electrons when bonding with other elements). It is also the second-lightest metal after lithium, light enough to float in water. As with other alkali metals (those in the first column of the periodic table), it decomposes in water. In potassium’s case, it spontaneously catches fire in water.

Potassium is essential for plant growth, and natural soils usually contain potassium. However, in farming, plants are completely harvested each year, so the potassium must be replaced by adding fertilizer. Ninety-five percent of potassium production is for fertilizer.

Important potassium salts include its chloride (for fertilizer and medicine), nitrate (for fertilizer and gunpowder), hydroxide (for soaps and detergents), and carbonate (for glassmaking). Others are used as food additives, inks and dyes, and compounds for processing certain metals, among other uses.

An alloy of sodium and potassium (NaK) is used as a heat-transfer medium.

Potassium is the eighth most abundant element in the earth’s crust. It is plentiful, though not easily extractable, in rocks such as feldspar and granite. Potassium and its salts are more easily obtained in minerals found in large deposits in ancient lakebeds and seabeds. These minerals include large deposits of sylvite (potassium chloride, KCl), as well as carnallite, langbeinite, and polyhalite.

Potash is found and mined all over the world. The largest known deposit is found at a depth of 3000 ft in Saskatchewan, Canada.

Potassium metal is isolated by electrolysis of its hydroxide (Davy’s technique) and by heated reactions between its compounds and other elements, such as carbon, silicon, and sodium.