Potassium

Potassium was first isolated in 1807 by Humphry Davy, who electrolysed molten potash and watched droplets of a soft metal skitter and ignite on the surface, the first metal won by electricity. The English name comes from potash, the potassium carbonate leached from wood ashes in iron pots. The symbol K derives from the Latin kalium, itself from the Arabic al-qali, the calcined ashes, the same root that gives the word alkali. Potassium is essential to life, its ions carrying nerve impulses and regulating the water balance of cells, and the great bulk of the potash mined today, chiefly potassium chloride, is spread as fertiliser to supply this nutrient to crops. Potassium carries a small natural radioactivity: about one atom in ten thousand is the isotope potassium-40, present in every banana and in the human body itself, a fact popularised as the banana equivalent dose, a rough, informal unit sometimes used to put small radiation exposures in perspective.

Potash was also economically important early in American history: producing and exporting potash and pearl ash, refined from wood ash, was one of the young United States' first industrial exports, and in 1790 the first patent ever granted by the US government went to Samuel Hopkins for an improved method of making potash.

Potassium's names record an old craft: the leaching of wood and plant ashes to make potash, used for centuries in soapmaking, glass and bleaching. The Arabic al-qali, source of the symbol K and of the word alkali, ties the element to the vocabulary of early chemistry, where the alkalis stood opposite the acids as one of the great classes of matter. The metal itself belongs firmly to the nineteenth century and has no planetary or alchemical correspondence, but the substances it names, potash and the alkalis, were among the essential reagents of the pre-modern laboratory and workshop.

The acid-alkali distinction potassium's name preserves reaches back to the medieval Arabic alchemical tradition associated with Jabir ibn Hayyan, whose Latinised name, Geber, lent later European alchemy much of its technical vocabulary. Potassium nitrate, saltpetre, was in its own right the indispensable oxidiser of black powder, mined from cave deposits and manure heaps and fought over as a strategic resource for centuries before anyone understood it as a potassium salt; the element's alkaline and explosive faces, soap and gunpowder, both trace to the same humble ash and mineral chemistry of the pre-modern world.