A metal that floats and burns
Potassium is the lightest metal but one — only lithium is lighter — and among the softest: a fresh cut shows a silver surface that dulls to grey within seconds as the air attacks it. The single electron in its outer shell, 4s¹, is held so loosely that potassium gives it up to almost anything. Dropped into waterCompound: Water. Two hydrogens, one oxygen, and most of the living world it skitters across the surface, and the hydrogenElement: Hydrogen. The lightest element, and most of the universe it releases catches fire at once, burning with the lilac flame that also betrays potassium salts in a Bunsen burner. In air it forms the superoxide KO₂, an orange solid that gives up oxygen when it meets carbon dioxide and for that reason fills the emergency breathing sets of submarines and mines. Its ion, K⁺, is the opposite: colourless, stable and content, the state in which the element spends its existence.
From rock to fertiliser
About 2.1 % of the crust is potassium, held in feldspars and micas that weather slowly and release it to soils, rivers and the sea. Where ancient seas dried up they left beds of sylvite, KCl, and carnallite, a chloride of potassium and magnesium; the largest lie under Saskatchewan, the Urals and Belarus, and mining them yields tens of millions of tonnes of potash a year, over nine tenths of it for fertiliser. Potassium is one of the three nutrients plants need in bulk — the K of NPK — and every crop harvested is potassium carried off the field.
Very little becomes the metal, which is awkward to make: potassium dissolves in its own molten salts, so it cannot be won by electrolysis as sodium is. Potassium chloride is heated with sodium at about 850 °C and the potassium, the more volatile, is distilled off. A few hundred tonnes a year are enough.
Inside the cell
Every living cell keeps potassium in and sodium out. A pump in the membrane, the sodium–potassium ATPase that Jens Skou discovered in 1957, spends a large share of a resting animal's energy holding the potassium inside a cell some thirty times more concentrated than in the fluid outside. When a nerve fires, sodium rushes in and potassium rushes out; the pump restores the balance and the next impulse can follow. The heart's rhythm rests on the same gradient, which is why too little or too much potassium in the blood can stop it. An adult carries about 140 g and replaces a few grams a day from food — potatoes, beans, bananas, greens.
Potash itself was the first chemical of industry: leached from wood ash and boiled down in iron pots, it made soap and glass for centuries, and as saltpetre, potassium nitrate, it gave gunpowder its oxygen. Potassium cyanide dissolves goldElement: Gold. The yellow metal that never tarnishes out of crushed ore, the process that opened the mines of the Witwatersrand in the 1890s.
Discovery and name
Potash, a battery and a dance
Chemists had long told potash, the “vegetable alkali” of plant ashes, from soda, the “mineral alkali” of natron; both were suspected to be oxides of unknown metals, but no fire or charcoal would strip the oxygen away. On 6 October 1807 Humphry Davy, at the Royal Institution, passed the current of a large voltaic battery through a piece of slightly damp potash. At the negative wire small globules of metal appeared, bright as mercury; some burst into flame. By his cousin's account Davy danced about the laboratory in delight. Sodium followed within days, and the next year, by the same method, calcium, barium, strontium and magnesium. Davy called the new metal potassium, after potash. Berzelius preferred kalium, from kali, the name Klaproth had used for potash, itself from the Arabic al-qalī, the ashes, and gave it the symbol K, which it has kept.