Potassium K

Discovery

Potash, the residue left after soaking wood ashes in water and boiling the solution down in iron pots, was used for centuries to make soap and glass. Chemists could not break it down, and by the late eighteenth century some suspected it was the compound of an unknown metal.

In October 1807 Humphry Davy passed a current from a powerful voltaic battery through slightly moistened caustic potash (potassium hydroxide). Small globules of metal formed at the negative pole; some burst into flame as soon as they appeared. It was the first metal ever isolated by electrolysis. According to his assistant's account, Davy was so delighted that he danced around the laboratory. Days later he isolated sodium by the same method.

Origin of the name

Davy named the new metal after potash, the source compound, a word that literally referred to ashes collected in pots. The symbol K comes from kalium, the name used in German and many other languages, derived from the Arabic al-qali, 'plant ashes', which also gave us the word alkali.

Main uses

• Fertilisers: the vast majority of mined potassium, mostly as potassium chloride, is spread on fields to support crop growth.
• Chemicals: potassium hydroxide is used in soft soaps, alkaline batteries and biodiesel production.
• Food: potassium chloride is used as a salt substitute, and potassium compounds serve as additives and preservatives.
• Explosives and fireworks: potassium nitrate is a component of gunpowder, and potassium perchlorate is an oxidiser in fireworks.
• Glass: potassium compounds help make special glasses, including some used for smartphone screens that are toughened by exchanging sodium ions for potassium.

Isotopes

Natural potassium contains two stable isotopes, potassium-39 (about 93.3 percent) and potassium-41 (about 6.7 percent), plus a trace of radioactive potassium-40 (about 0.012 percent). Potassium-40 has a half-life of about 1.25 billion years and decays partly to argon-40, the basis of potassium–argon dating of rocks. Because potassium is essential, every human body contains some potassium-40, making it one of the main sources of the natural radioactivity inside us.

In everyday life

Potassium ions are concentrated inside cells, and the difference between potassium inside and sodium outside allows nerves to fire and the heart to beat regularly. Potatoes, beans, leafy greens, bananas and dried fruit are rich sources. Both too little and too much potassium in the blood can disturb heart rhythm, so people with kidney disease often need to watch their intake.

Potassium metal is stored under oil; it ignites on contact with water.