Potassium

• 1. Introduction

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  Potassium is a metal that can be sliced with a kitchen knife. Drop a piece into water and it bursts into a lilac-colored flame. Leave it in open air for a few seconds and it turns into a flaky white crust of potassium peroxide. This is element number 19 on the periodic table, symbol K, and almost none of what most people assume about metals applies to it.

  How did an element this reactive stay hidden for so long? Why does the body need it so urgently that a 70 kg person contains about 120 g of it at all times? And what happens when the supply runs out?

• 2. A Name With Two Homes

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  Humphry Davy named potassium after isolating the metal himself in 1807, drawing on the word "potash," which described the old practice of burning wood or leaves, soaking the ash in water, and evaporating the liquid in a pot to collect the salts left behind.

  But not everyone adopted Davy's name. In 1809, Ludwig Wilhelm Gilbert proposed calling the new element Kalium, after the Arabic root al-qalyah, meaning plant ashes. In 1814, the Swedish chemist Berzelius backed Kalium as well and pushed for K as the chemical symbol. English-speaking countries sided with Davy, along with French chemists Joseph Louis Gay-Lussac and Louis Jacques Thénard. The Germanic countries followed Gilbert and Klaproth. The International Union of Pure and Applied Chemistry eventually settled the matter by designating K as the official symbol in its reference document known as the Gold Book.

  The split was not simply a matter of linguistic preference. It traced a real fault line in scientific tradition between the French-British school and the German school, both of which had strong reasons to claim this newly isolated element as their own.

• 3. Davy's Electric Discovery

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  In 1807, Humphry Davy used a newly invented voltaic pile to run electricity through molten caustic potash, a compound now known as potassium hydroxide, or KOH. What appeared was a small quantity of pure metal. Potassium became the first metal ever isolated by electrolysis.

  Later that same year, Davy applied the same technique to caustic soda and extracted sodium, demonstrating that the two elements were genuinely distinct and not just variations of the same substance. Georg Ernst Stahl had suggested as much in 1702, and Henri Louis Duhamel du Monceau had offered experimental proof of their difference in 1736. But Antoine Lavoisier, writing his landmark list of chemical elements in 1789, had still not included the alkalis. Even after Davy's extraction, some time passed before the wider scientific community accepted that potassium and sodium were truly separate elements.

  The electrolysis process Davy pioneered was scaled up industrially in the 1920s. By the 1950s it had been largely replaced by a thermal method: reacting sodium with potassium chloride in a chemical equilibrium reaction written as Na + KCl producing NaCl + K.

• 4. Farming the Ashes

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  For centuries, potash was mainly useful for making glass, bleach, soap, and gunpowder as potassium nitrate. Potassium soaps made from animal fats and vegetable oils were valued for being soft and water-soluble, known in the trade as soft soaps.

  The decisive shift came in 1840, when Justus Liebig demonstrated that potassium is a necessary element for plant growth and that most soil types are deficient in it. Demand for potassium salts climbed sharply. Wood ash from fir trees served as an early fertilizer source, but that changed in 1868 with the discovery of mineral deposits containing potassium chloride near Staßfurt, Germany. Industrial-scale potassium fertilizer production began from those deposits, which span from Great Britain across Germany into Poland, laid down in the Zechstein geological formation during the Middle to Late Permian period.

  By the 1960s, Canada had become the dominant producer. Today, the principal potash mines in the Canadian province of Saskatchewan sit 1,000 m below the surface. Agricultural fertilizers account for 95% of global potassium chemical production, with about 90% supplied as potassium chloride. In 2005, about 93% of world potassium production was consumed by the fertilizer industry alone.

• 5. A Radioactive Neighbor

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  Potassium has 25 known isotopes, but only three occur in nature. The most common is potassium-39, at 93.3%. Potassium-41 makes up 6.7%. The third, potassium-40, appears in only 0.0117% of natural potassium, yet it carries significant consequences.

  Potassium-40 has a half-life of 1.25 billion years. It decays in two directions: 88.8% of the time it releases a beta particle and becomes stable calcium-40; the remaining 11.2% of the time it captures an electron or emits a positron and becomes stable argon-40. That steady accumulation of argon in rock is the foundation of potassium-argon dating, a widely used method for establishing the age of volcanic rocks and minerals including biotite, muscovite, and hornblende.

  In a 70 kg human body, about 4,400 nuclei of potassium-40 decay every second. Potassium-40 is the most abundant radioisotope in the human body, accounting for more radioactivity than any other natural source within us. Large bags of salt substitutes containing potassium are radioactive enough to serve as classroom demonstration sources.

• 6. The Body's Invisible Regulator

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  98% of the potassium in a human body sits inside individual cells. Normal plasma concentration is kept between 3.5 and 5.5 millimoles per liter, a range so narrow that deviating from it can disrupt heart rhythm, nerve transmission, and muscle function.

  The Na+/K+-ATPase pump, found in almost every cell, uses ATP to move three sodium ions out of the cell and two potassium ions in. This creates an electrochemical gradient that drives nerve signals, muscle contractions, and the heartbeat itself. Potassium deficiency, called hypokalemia, is observed in 20% of hospital patients. Severe deficiency produces muscle weakness, respiratory paralysis, and cardiac arrhythmia. Without any potassium intake at all, the body excretes roughly 200 mg per day; after about a week the plasma level drops to a mildly deficient range, and if deprivation continues, death eventually follows.

  The kidneys manage plasma potassium through three mechanisms: a reactive negative-feedback loop that increases excretion when plasma levels rise; a feed-forward loop triggered by gut receptors detecting ingested potassium even before plasma levels change; and a circadian system driven by an oscillator in the suprachiasmatic nucleus of the brain, which causes the kidneys to increase potassium secretion during typical mealtime hours regardless of actual food intake.

  In 2020, potassium was the 33rd most commonly prescribed medication in the United States, with more than 17 million prescriptions written that year.

• 7. Fire, Industry, and Dangerous Storage

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  Potassium metal has a density of 0.89 g per cubic centimeter, meaning burning potassium floats on water. That detail matters because water is ineffective against a potassium fire and can make it worse. Nitrogen, argon, dry table salt, dry soda ash, and dry sand are among the agents that work, by cutting off oxygen and cooling the metal.

  Because potassium reacts with water vapor in the air, it is stored under anhydrous mineral oil or kerosene. Unlike lithium and sodium, potassium should not be kept under oil for longer than six months unless held in an inert, oxygen-free atmosphere or under vacuum. Extended air exposure produces shock-sensitive peroxides that can detonate when a container lid is opened.

  Across industry, potassium hydroxide is used to neutralize acids, control pH, saponify fats and oils, and carry out hydrolysis reactions. Potassium nitrate, also called saltpeter, is the oxidant in gunpowder and a major agricultural fertilizer. Potassium cyanide dissolves copper, silver, and gold, and finds use in gold mining, electroplating, and organic synthesis. Potassium carbonate goes into glass, soap, color television tubes, fluorescent lamps, and textile dyes. One niche compound, potassium cobaltinitrite, is used by artists as the yellow pigment known as Aureolin or Cobalt Yellow. Another, potassium superoxide, is carried in respiration systems for mines, submarines, and spacecraft because it releases oxygen and absorbs carbon dioxide in a compact form.