Aspooky element. That’s what phosphorus, the fifteenth element of the periodic table, is for many. In the early days after its discovery, some even looked at it as ‘the Devil’s element’. And for good reason.
Consider this. When bonded to four oxygen atoms, the element phosphorus forms phosphates that are the vital components holding our DNA and RNA molecules together. Meaning, without phosphorus it would be hard for any form of life to exist. But, strip the oxygen atoms away, and we are left with a dreadfully powerful element that is poisonous, glows in the dark with an eerie green light, and is flammable – catching fire on its own when left exposed to air. Besides, white phosphorus can cause painful diseases and even kill in a number of ways. One of the diseases eats away bones and causes them to glow. Other factors that added to the ‘dark’ character of phosphorus included its discovery through the evaporation of human urine and its earlier large-scale manufacture through dissolving bones in sulphuric acid.
On one hand, phosphorus is essential for life, but on the other, it has been used as a rat poison for several decades. In modern times, chemical warfare agents – nerve agents such as Sarin and VX – contain phosphorus compounds. Sarin is over twenty times deadlier than potassium cyanide. Last year, the ex-Russian spy and his daughter who were found in a critical condition in Salisbury, England, were believed to have been poisoned by spiking their food with Sarin.
Phosphorus is therefore a fine example of an element occurring in more than one form with varying appearances and starkly different properties.
Pure phosphorus has three major, very different forms, depending on the way the atoms are arranged: White phosphorus (also called yellow phosphorus); red phosphorus; and black phosphorus. Red phosphorus is obtained by heating white phosphorus in a closed vessel, thus changing it from a form that is flammable, toxic, and glows in the dark, to one that has none of these properties.
Being highly reactive, phosphorus does not occur on earth in the free state, always occurring as phosphate salts. Although around 550 minerals have been found to contain phosphorus, the main source is the apatite series.
Discovery by chance
Much like the young Arab shepherds who went out hunting for a lost goat near Jericho and ended up discovering a secret cave and the Dead Sea Scrolls, Hennig Brandt, a 17th-century German alchemist accidentally discovered phosphorus while boiling and evaporating his own urine in search of the ‘philosopher’s stone’ (that would turn ordinary metals into gold). It may sound bizarre, but Brandt believed he would find the legendary stone in urine, one of the reasons being urine had a colour similar to that of gold.
After boiling a large quantity – around sixty buckets, no less – of stinky, stagnant urine for days, Hennig isolated a white, waxy solid that had a horrible smell. That could have come as a disappointment. But he could have surely been enthused by the sight of the new substance glowing in the darkness of the night.
Hennig’s discovery in 1669 raised hopes of phosphorus being a safer substitute for candles for lighting up homes after sunset. But white phosphorus proved to be most unsuitable, for two reasons. It had a repulsive odour. And it was dangerously flammable.
Phosphorus became more widely available across Europe when it was found that bones contain calcium phosphate. It then began to be manufactured by heating the phosphoric acid obtained through dissolving bone in sulphuric acid with charcoal. Eventually phosphorus came to be mass produced from mineral phosphates like fluoroapatite (Ca5(PO4)3F.
Around fifty years after Brandt discovered phosphorus, Johann Thomas Hensing, a German professor of medicine showed that the new element was also present in the human brain. (The fact that it was present in a number of minerals was discovered much later.) Perhaps believing that phosphorus being present in our brain indicated it must be good for us, medicines containing elemental phosphorus started being sold in the early 18th century. The result was that many people ended up suffering from phosphorus poisoning.
Phossy Jaw
While using white phosphorus for indoor lighting was quickly ruled out, scientists discovered another use for it. It began to be used on matchstick heads so that a flame could be produced with the slightest heat from friction. Billions of such matchsticks were manufactured in the 19th century. The people employed in match factories for dipping the matchstick heads in a mixture of chemicals containing white phosphorus were called ‘dippers’. Unfortunately, these dippers who worked up to fourteen hours a day in poorly ventilated factories ended up continuously breathing in phosphorus fumes while at work. As a result, phosphorus would get deposited in their body, starting with the jaw.
Beginning with a tooth ache and teeth falling out, foul-smelling pus would ooze out of holes caused by abscesses in the jaw through which the dead bone could be seen underneath. Sometimes the bone glowed hideously in the dark due to the accumulated phosphorus. Such people had only two options. Stop working in such factories and lose their means of livelihood, or get the affected jaw, called phossy jaw, removed to prevent the phosphorus from reaching and damaging the liver, and other organs and killing them. Those who survived phossy jaw would be permanently disfigured.
Fortunately, in 1910, Britain and then other countries banned the use of white phosphorus in matches. Instead, the much safer red phosphorus that is used till today came to be applied to the side of match-boxes.
Uses
The most important use of elemental phosphorus is for manufacturing phosphoric acid. This acid in turn is the main feedstock from which other useful compounds are made, such as phosphate fertilisers, animal feed supplements, dishwasher powders, rust removers and preventers, and food additives.
A smaller amount of pure phosphorus is used for manufacturing fireworks, tracers, incendiaries, and rodenticides. Some of it is converted to sulphides and used in the manufacture of matches, insecticides, and oil additives, and some is converted to halides or oxides to be used for synthesising organic phosphorus compounds.
Along with nitrogen and potassium, phosphorus forms one of the three essential nutrients for plants. Following the introduction of phosphate fertilisers, global food production increased greatly, considerably reducing malnourishment.
Ultra-refined phosphorus – around 99.9999% pure – is used for making metal phosphides, such as those of gallium and indium, used in light emitting diodes.
Biological Significance
Besides being a crucial component of DNA and RNA, phosphorus is also critical for the development of healthy bones and teeth and for carrying out fundamental chemical reactions within our cells.
However, swallowing even a few milligrams of elemental white phosphorus could cause severe diarrhoea with loss of blood, damage to the liver, kidneys, stomach, intestines, and circulatory system, and even cause death. Just handling white phosphorus could be dangerous too, as this could result in severe skin burns.
Peak Phosphorus
Many scientists believe Earth may be approaching the ‘peak phosphorus’ stage, meaning the maximum possible global production of phosphorus. Potassium being a finite resource, some experts say mineral reserves of phosphorus could last for just around thirty years more, others insist it could be hundreds.
When it comes to the three essential plant nutrients, nitrogen is obtained from the air and potassium from mineral reserves of which we have enough to last several centuries. But there’s a big question mark regarding phosphorus reserves. In the 20th century, the use of N-P-K fertilisers dramatically increased agricultural production and led to the world’s population growing around six-fold.
Continued population growth would cause a rising demand for food and consequently phosphate fertilisers as well. Meaning we have to be prepared for the current readily available supplies of phosphate minerals running out someday.
Concerns and solutions
Human activities are responsible for phosphorus polluting freshwater basins and causing serious environmental problems. Today, excessive use of phosphate fertilisers and manure is leading to a higher concentration of phosphorus in the run-off from agricultural land. This excess phosphorus and that from livestock waste tends to collect in freshwater bodies causing the growth of toxic algae on their surface. Such toxic blooms make the water unfit for human use and activities like swimming and also suffocate marine life. There is also a lot of wastage of phosphorus at the sites where it is mined.
Innovative measures are therefore urgently needed to curb the irresponsible waste of phosphorus. We need more efficient extraction of phosphates at source and agricultural reforms for reducing the demand for phosphate fertilisers. Farmers need to be encouraged to use such fertilisers more judiciously and also to switch to using organic fertilisers and composting methods.
At the same time, we also need newer and better ways of recovering and reusing phosphorus from organic waste and wastewater streams.
References
1. Royal Society of Chemistry: Periodic Table – Phosphorus, http://www.rsc.org/periodic-table/element/15/phosphorus
2. Chemistry Explained: Phosphorus, http://www.chemistryexplained.com/elements/L-P/Phosphorus.html
3. Stephanie Pappas: Facts about Phosphorus, Live Science, 5 January, 2015, https://www.livescience.com/28932-phosphorus.html
4. Arno Rosemarin, Gert de Bruijne, Ian Caldwell: Peak Phosphorus, The Broker, August 04, 2009, http://www.thebrokeronline.eu/Articles/Peak-phosphorus
5. Kathryn Harkup: ‘The Devil’s element’: the dark side of phosphorus, The Guardian, 31 October, 2017, https://www.theguardian.com/science/blog/2017/oct/31/the-devils-element-the-dark-side-of-phosphorus
6. Petr Kilian, University of St. Andrews: Phosphorus: 350 years after its discovery, this vital element is running out, The Conversation, https://theconversation.com/phosphorus-350-years-after-its-discovery-this-vital-element-is-running-out-109535
7. Vera Thoss, Bangor University: Phosphorus is vital for life on Earth – and we’re running low, The Conversation, https://theconversation.com/phosphorus-is-vital-for-life-on-earth-and-were-running-low-74316.
