chemical element of Group Ia of the periodic table, the alkali metal group, indispensable for life. Potassium was the first metal to be isolated by electrolysis, by the English chemist Sir Humphry Davy, when he obtained the element (1807) by decomposing molten potassium hydroxide (KOH) with a voltaic battery.

Properties, occurrence, and uses.

Potassium metal is soft and white with a silvery lustre, has a low melting point, and is a good conductor of heat and electricity. Potassium imparts a lavender colour to a flame, and its vapour is green. It is the seventh most abundant element in the Earth's crust, constituting 2.6 percent of its material. Most potassium is present in minerals such as muscovite and orthoclase feldspar that are insoluble in water, making potassium difficult to obtain, but it can be prepared commercially by electrolysis from some refinable minerals, such as carnallite and polyhalite.

There is little commercial demand for potassium metal itself, though it is used for preparing potassium superoxide, KO₂, which refreshens exhaled air by liberating oxygen and removing carbon dioxide and water vapour, and for alloying with sodium as a liquid metallic heat-transfer medium. Potassium reacts vigorously with water, liberating hydrogen (which ignites) and forming a solution of potassium hydroxide, KOH. In plant metabolism, potassium compounds are absorbed from soil in the form of tartrates and oxalates, which may be converted to potassium carbonate (potash) when the plants are burned. In higher animals potassium ions (K⁺) together with sodium ions act at cell membranes in transmitting electrochemical impulses in nerve and muscle fibres and in balancing the activity of food intake and waste removal from cells. Too little or too much potassium in the body is fatal, but potassium in the soil ensures the presence of this indispensable element in food.

Natural potassium consists of three isotopes: potassium-39 (93.26 percent), potassium-41 (6.73 percent), and radioactive potassium-40 (about 0.01 percent); several artificial isotopes have also been prepared. Potassium easily loses the single 4s electron, so it has a valence of one in all its compounds.

Principal compounds.

Potassium compounds are very important in agriculture and to lesser extent in the manufacture of explosives. Potassium chloride, KCl, is a naturally occurring potassium salt that is used as fertilizer and as a raw material for the production of other important potassium compounds. Electrolysis of potassium chloride yields potassium hydroxide (also called caustic potash), which readily absorbs moisture and is employed in making liquid soaps and detergents and in preparing many potassium salts. Reaction of iodine and potassium hydroxide produces potassium iodide, KI, which is added to table salt and animal feed to protect against iodine deficiency.

Other potassium compounds of economic value include potassium nitrate, also known as saltpetre, or nitre, KNO₃, which has wide use as a fertilizer and in fireworks and explosives and serves as a food preservative; potassium chromate, K₂CrO₄, which is employed in tanning leather and dyeing textiles; and potassium sulfate, K₂SO₄, which is used in the production of fertilizers and potassium alums.