Phosphorus

• 1. Introduction

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  Phosphorus, element number 15, first came to light in Hamburg in 1669, in the hands of an alchemist named Hennig Brand. Brand was chasing the fabled philosopher's stone. His chosen raw material was urine, which carries dissolved phosphates left over from ordinary metabolism.

  He let it rot, boiled it down to a paste, and distilled it at a high temperature. The vapours were led through water. What gathered was a white, waxy substance that glowed in the dark and burned brilliantly. Brand named it phosphorus mirabilis.

  It was the first element anyone had discovered that was not already known from ancient times. A strange question hangs over that glowing lump. Why does its glow fool even the words we use for light? How did a poison that rots the jaw end up in matches, farmland and weapons? And why does every living cell on Earth depend on it?

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• 2. Glow and Colours

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  The name phosphorus comes from the Greek word Φωσφόρος, the god of the morning star, which is also the planet Venus. Brand's lump earned the label because it shone faintly when it met the air. White phosphorus gives off a pale green and blue light as it oxidises. The reaction happens only at the surface of the solid or liquid, producing short-lived molecules that emit visible light. In pure oxygen the glow disappears entirely, because the oxidation works only within a certain range of partial pressures.

  The word phosphorescence grew out of this glow, and substances that shine in the dark are still loosely called phosphors. Strictly speaking, though, phosphorus is not phosphorescent. It is chemiluminescent. It glows because of a chemical reaction, not because it slowly releases light it absorbed earlier.

  White phosphorus is soft, waxy and insoluble in water. It is very toxic, and it ignites in air at about 30 C. The yellow phosphorus people often mention is not a separate form at all. It is white phosphorus that has aged or is impure, slowly degrading toward red under light and heat.

  Red phosphorus forms when white phosphorus is heated to 250 C without air, or when sunlight falls on it. Freshly made, it ignites at about 300 C. After long heating or storage it darkens and no longer catches fire by itself in air.

  In 1865 Johann Wilhelm Hittorf recrystallised phosphorus from molten lead and obtained a red-purple form. That is why violet phosphorus is sometimes called Hittorf's phosphorus. Black phosphorus is the least reactive form. It is black and flaky like graphite, conducts electricity, and takes a pressure of about 12000 atmospheres to make from white phosphorus. A single layer of it is called phosphorene, the counterpart of graphene.

  The calmer red form would soon matter to ordinary households far more than any of these exotic cousins.

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• 3. The Recipe Travels

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  Brand first tried to guard his method, but he later sold the recipe for 200 thalers to a buyer from Dresden. That man, Kraft, toured much of Europe with the secret, including London, where he met Robert Boyle. The crucial fact that the substance came from urine eventually leaked out. In 1678 Johann Kunckel managed to reproduce it in Sweden.

  In 1680 Boyle also made phosphorus, and he published how he did it. He was the first to use it to ignite wooden splints tipped with sulfur, ancestors of the modern match. He also improved the reaction by adding sand. His assistant Ambrose Godfrey-Hanckwitz later turned making phosphorus into a business.

  A different source appeared in 1769, when Johan Gottlieb Gahn and Carl Wilhelm Scheele showed that bones contain calcium phosphate. They proved it by extracting elemental phosphorus from bone ash. Antoine Lavoisier recognised phosphorus as a true element in 1777.

  Bone ash then became the main industrial source. Workers ground bones into tricalcium phosphate and treated it with sulfuric acid. They dehydrated the result, mixed it with ground coal or charcoal in an iron pot, and distilled phosphorus vapour out of a retort. Two-thirds of the phosphorus emerged as white phosphorus, while one-third stayed in the residue as calcium orthophosphate. The carbon monoxide produced was burnt off in a flare stack.

  That bone-based supply held its place as the primary source until the 1840s.

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• 4. Matches and Matchgirls

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  In 1830 Charles Sauria made the first striking match with a phosphorus head. Its head combined white phosphorus, a binder, and an oxygen-releasing compound such as potassium chlorate, lead dioxide or sometimes a nitrate. These matches were dangerous from every angle. They were toxic if swallowed, sensitive to how they were stored, and liable to flare if accidentally scraped on a rough surface.

  The workers who made them suffered most. Breathing the vapours caused severe necrosis of the jawbone, known as phossy jaw. The risks to match workers lay behind several notable early cases of industrial action, including the London Matchgirls' strike of 1888.

  Red phosphorus offered a way out. Matches built around it were much safer to use and to manufacture, and they gradually replaced white phosphorus. Around 1900 the French chemists Henri Sévène and Emile David Cahen invented the modern strike-anywhere match. They swapped the white phosphorus for phosphorus sesquisulfide, a compound that is non-toxic and non-pyrophoric but still ignites under friction. These matches were popular for a time, though the red phosphorus safety match eventually overtook them.

  Law followed chemistry. Several countries banned white phosphorus matches between 1872 and 1925. An international treaty to the same effect was signed after the Berne Convention of 1906.

  A modern safety match works by keeping its ingredients apart. The striker strip holds red phosphorus, and the match head holds potassium chlorate. Striking rubs tiny amounts of each together into Armstrong's mixture, a very touch-sensitive powder that ignites at once and sets the head alight.

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• 5. Fertile Ground

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  In 1609 Inca Garcilaso de la Vega published Comentarios Reales, describing how the Incas farmed before the Spaniards arrived. He recorded the use of guano as fertiliser, harvested by Incas living near the coast. The Moche people had mined it even earlier and carried it back to Peru by boat.

  In the early 1800s Alexander von Humboldt brought guano to Europe as a fertiliser, after finding it in exploitable quantities on islands off the coast of South America. On some islands the deposits were reported to be more than 30 metres deep. International trade in guano only began after 1840. By the start of the 20th century the supply was nearly exhausted, and superphosphate production had overtaken it.

  Phosphate rock was first used to make phosphorus in 1850. In 1888 James Burgess Readman introduced the submerged-arc furnace, patented the following year, and bone ash became obsolete. With guano running out around the same time, mineral phosphates became the major source of phosphate fertiliser. Rock production grew greatly after World War II.

  The reason for all this digging is that plants need phosphorus and soil rarely gives it up. After nitrogen, it is the nutrient most often in short supply. It supports energy transfer, strong roots and stems, photosynthesis, and the formation of seeds and flowers. Natural phosphorus compounds dissolve poorly and move little through soil, and even added phosphorus can become fixed there. The natural cycle is therefore too slow to sustain intensive farming.

  Farmers answer with superphosphate of lime, made by reacting calcium phosphate with sulfuric acid and water. This trade is so large that it is the main industrial market for sulfuric acid and the greatest industrial use of elemental sulfur. It leaves a heavy trail too. Every ton of phosphoric acid made by the wet process yields about five tons of phosphogypsum waste, and worldwide output is estimated at 100 to 280 million tons a year.

  Runoff from heavy fertiliser use has also overenriched lakes and rivers, a process called eutrophication.

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• 6. Rock and Reserves

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  The United States produced a record 54.4 million metric tons of phosphate rock in 1980. It was the world's largest producer from at least 1900 until 2006, when China overtook it. In 2019 the United States supplied 10 percent of the world's rock. The leading producers in 2024 were China, Morocco, the United States and Russia.

  Morocco holds the real prize. Its commercial reserves are estimated at 50 billion metric tons, compared with 3.2 billion for China and 2.8 billion for Egypt. Morocco alone accounts for two-thirds of the estimated exploitable reserves worldwide. Algeria, Syria, Brazil, Saudi Arabia, South Africa, Australia, the United States and Finland follow with 2.2 to 1.0 billion tons each. Mining is the only cost-effective way to get phosphorus, so a shortage or price spike could threaten food security.

  How long the rock will last is fiercely argued. In 2023 the United States Geological Survey estimated economically extractable reserves at 72 billion tons. World mining in 2022 was 220 million tons, which at zero growth would last around 300 years. That broadly matches a 2010 report from the International Fertilizer Development Center.

  Critics are not convinced. A 2014 review concluded that the report gave an inflated picture, particularly for Morocco, where largely hypothetical and inferred resources had simply been relabeled as reserves. Extensive new deposits found in Norway in 2023 complicate the picture further. Before them, affordable reserves were expected to run out in 50-100 years.

  Conservation offers a partial answer. No-till farming, terracing, contour tilling and windbreaks all slow the loss of phosphorus from farmland, and grass or forest strips beside rivers cut losses of phosphate. Sewage plants can recover it by incinerating sludge and collecting the ash, or by adding magnesium to waste to precipitate struvite. At current world prices, those technologies are not yet cost-effective.

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• 7. Fire and Poison

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  During World War I, white phosphorus went into incendiary ammunition, smoke screens and tracer rounds. The electric furnace had raised production to the point where this became possible. One special incendiary bullet was designed to hit the hydrogen-filled Zeppelins flying over Britain, since hydrogen burns so readily.

  In World War II, Britain distributed Molotov cocktails made of phosphorus dissolved in petrol to specially selected civilians, in case of invasion. The United States developed the M15 white-phosphorus hand grenade, a forerunner of the M34, and the British introduced the similar No 77 grenade. They were used mostly for signalling and smoke screens, though they also worked as anti-personnel weapons. Burning phosphorus is hard to put out and causes very severe burns, which had a strong psychological effect on enemy troops.

  Phosphorus incendiary bombs were used on a large scale to destroy Hamburg, the city where the miraculous bearer of light had first been found. Modern international law constrains white phosphorus munitions, yet they are still used for incendiary bombs, smoke and tracer ammunition.

  Other phosphorus chemistry is deadlier still. Fluorophosphate esters such as sarin are among the most potent neurotoxins known, and VX and Tabun are also organophosphorus nerve agents. Phosphorus can also reduce iodine to hydroiodic acid, a reagent that can turn ephedrine or pseudoephedrine into methamphetamine. For that reason red and white phosphorus are List I precursor chemicals under the United States Drug Enforcement Administration.

  Swallowing white phosphorus is lethal at around 1 mg/kg. It is used as a rodenticide, and children have eaten it mistaking it for toothpaste or because it was spread on bread to kill rodents. Symptoms come in three stages: a day of vomiting, one to three symptom-free days, then acute liver and kidney failure. There is no reliable diagnostic test and no known antidote. Breathing it over time causes jaw necrosis, and the United States sets a permissible exposure limit of 0.1 mg/m3 across an eight-hour workday.

  For burns, a 2% copper sulfate solution was once used to wash the skin. According to 2009 United States Navy guidelines, copper sulfate is itself toxic, so a bicarbonate solution is preferred. That guidance notes that fragments can be found in the dark, because they shine as luminescent spots.

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• 8. Life's Element

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  An average adult carries about 0.7 kg of phosphorus, and 85-90% of it sits in bones and teeth as apatite. The main component of bone is hydroxyapatite, along with amorphous calcium phosphate. Hydroxyapatite is also the main mineral of tooth enamel. Water fluoridation helps teeth resist decay by partly converting it to the harder fluorapatite.

  Inside cells, phosphate is part of the structural framework of DNA and RNA. Cells use it to carry energy in adenosine triphosphate, or ATP, which powers every energy-using process and drives phosphorylation, a key regulatory event. Every cell is also wrapped in a membrane built on a phospholipid matrix, where a phosphate group sits in place of one of glycerol's fatty acids.

  The daily requirement is modest. For adults aged 19 and over, the United States Institute of Medicine sets the estimated average requirement at 580 mg and the recommended allowance at 700 mg. The tolerable upper limit is 4000 mg. European guidance sets an adequate intake of 550 mg for people aged 15 and over. Milk, meat and soya all supply phosphorus, and a diet with enough protein and calcium usually covers the need.

  Too little causes hypophosphatemia, with neurological problems and damage to muscle and blood cells from lack of ATP. Too much can cause diarrhoea and harden organs and soft tissue. Phosphoric acid, listed as additive E338, gives colas and jams their tang and acts as a preservative.

  The element's origin lies in the stars. In 2013 astronomers detected phosphorus in Cassiopeia A, confirming that supernovae make it. Material from the remnant had a phosphorus-to-iron ratio up to 100 times higher than the Milky Way generally. In 2020 astronomers studying the star-forming region AFGL 5142 found phosphorus-bearing molecules that comets could have carried to the early Earth.

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