Transcript :
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You're listening to Chemistry in its element brought to you by Chemistry World, the magazine of the Royal Society of Chemistry.
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Chris Smith
Hello, this week the story of the first alkaline metal ever isolated, why it's an alkaline metal at all and why its symbol begins with the letter K. Here's Peter Wothers.
Peter Wothers
Potassium - the only element named after a cooking utensil. It was named in by Humphry Davy after the compound from which he isolated the metal, potash, or potassium hydroxide.
An extract from the s by the Dutch chemist Herman Boerhaave describes how potash got its name:
"Potas or Pot-ashes is brought yearly by the Merchant's Ships in great abundance from Coerland (now part of Latvia and Lithuania), Russia, and Poland. It is prepared there from the Wood of green Fir, Pine, Oak, and the like, of which they make large piles in proper Trenches, and burn them till they are reduced to Ashes... These ashes are then dissolved in boiling Water, and when the Liquor at top, which contains the Salt, is depurated, i.e. freed from impurities, by standing quiet, it is poured off clear. This, then, is immediately put into large copper Pots, and is there boiled for the space of three days, by which means they procure the Salt they call Potas, (which signifies Pot-Ashes) on account of its being thus made in Pots.
Even earlier in the 16th Century, Conrad Gesner tells us that "Of the hearbe called Kali, doe certayne prepare a Salt"
He describes this plant, Kali whose Latin name is Salsola kali but is more commonly known as Saltwort:
"Kali is of two Cubites of heygth, hauing no prickles or thornes, & is sometymes very red, saltye in taste, with a certayne vngratefull smell, found & gathered in saltie places: out of which, the Salt of Alkali maye be purchased"
His method of production of this Salt of Alkali is pretty similar to that described by Boerhaave with both processes actually yielding an impure mixture of what we would now call potassium and sodium carbonate; the wood ash method yielding more potassium carbonate, potash, the salty herbs giving more sodium carbonate, soda. However, it is from the herb kali, that we owe the word that describes both - al-kali or alkali; the 'al' prefix simply being Arabic definite article 'the'.
The crude potash can be made more caustic or 'pure' by treating a solution of it with lime water, calcium hydroxide. The potassium carbonate and calcium hydroxide solutions react with a bit of chemical partner-swapping: insoluble calcium carbonate or chalk precipitates out, leaving a solution of potassium hydroxide. It was from this pure hydroxide that Davy first isolated the metal potassium. To do this he used the relatively new force of electricity.
After unsuccessfully trying to electrolyse aqueous solutions of potash, during which he only succeeded in breaking apart the water, he reasoned that he needed to do away with the water and try to electrolyse molten potassium hydroxide. This he did on the sixth of October, using the large Voltaic pile he had built at the Royal Institute in London. His younger cousin, Edmund Davy, was assisting Humphry at the time and he relates how when Humphry first saw "the minute globules of potassium burst through the crust of potash, and take fire as they entered the atmosphere, he could not contain his joy".
Davy had every right to be delighted with this amazing new metal: it looked just like other bright, shiny metals but its density was less than that of water. This meant the metal would float on water --at least, it would do if it didn't explode as soon as it came into contact with the water. Potassium is so reactive , it will even react and burn a hole through ice. This was the first alkali metal to be isolated, but Davy went on to isolate sodium, calcium, magnesium and barium.
Whilst Davy named his new metal potassium after the potash, Berzelius, the Swedish chemist who invented the international system of chemical symbols now used by chemists the world over, preferred the name kalium for the metal, better reflecting its true origins, he thought. Hence it is due a small salty herb that we now end up with the symbol K for the element pot-ash-ium, potassium.
Chris Smith
Cambridge chemist Peter Wothers. Next time beautiful but deadly is the name of the game.
Bea Perks
Link to shuntong
Arsenic gets its name from a Persian word for the yellow pigment now known as orpiment. For keen lexicographers apparently the Persian word in question Zarnikh was subsequently borrowed by the Greeks for their word arsenikon which means masculine or potent. On the pigment front, Napoleon's wallpaper just before his death is reported to have incorporated a so called Scheele's green which exuded an arsenic vapour when it got damp.
Chris Smith
So potent or not, licking the wallpaper in Napoleon's apartments is definitely off the menu. That's Bea Perks who will be with us next time to tell us the deadly tale of arsenic, I hope you can join us. I'm Chris Smith, thank you for listening and goodbye.
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Chemistry in its element is brought to you by the Royal Society of Chemistry and produced by thenakedscientists.com . There's more information and other episodes of Chemistry in its element on our website at chemistryworld.org/elements
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Potassium chloride is the principal commercial form of potash , and some potassium nitrate is also produced. About 90 percent of the production of these goes to fertilizers. For other purposes, the similar sodium salts are cheaper, but for a few special uses potassium has the advantage. Some ceramic uses require potassium, and potassium bicarbonate is more effective than sodium bicarbonate in extinguishing fires.
The simplest part of this diagram is the portion representing potassium. The element potassium is seventh in order of abundance in the Earth&#;s crust, about the same order as sodium, which it resembles very closely in its properties. Although sodium is readily available in the sodium chloride in the ocean, most of the potassium is contained in small proportions in a large number of mineral formations, from which it cannot be economically extracted. When the use of potassium salts as fertilizers began in the second half of the 19th century, it was believed that Germany had a monopoly with the deposits at Stassfurt, but many other workable deposits of potassium salts were later found in other parts of the world. World reserves are adequate for thousands of years, with large deposits in the Soviet Union , Canada (Saskatchewan), and Germany (East and West).
Phosphorus presents a more complicated picture. It has many uses other than in fertilizers. By far the largest source is phosphate rock, although some use is made of phosphatic iron ore, from which the phosphorus is obtained as a by-product from the slag. As with potassium, there are extensive reserves. The largest deposits are in North Africa (Morocco, Algeria, Tunisia), the United States (largely Florida), and the Soviet Union, but there are also sizable deposits in numerous other countries.
Phosphate rock is found in deposits of sedimentary origin, laid down originally in beds on the ocean floor. The rock consists largely of the insoluble tricalcium phosphate, together with some other materials, including some fluorine. To be used as a fertilizer, phosphate must be converted to a form that is soluble in water, even if only slightly so.
Phosphoric acid (H3PO4) has three hydrogen atoms, all of which are replaceable by a metal. Tricalcium phosphate, in which all three of the hydrogen atoms are replaced by calcium, must be converted to the soluble form, monocalcium phosphate, in which only one hydrogen atom is replaced by calcium. The conversion is done by sulfuric acid, which converts the phosphate rock to superphosphate, widely used as fertilizer. This operation requires large tonnages of sulfuric acid.
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The fertilizer industry is not only a matter of manufacturing the right chemical but also of distribution, getting the right material to the right place at the right time. Fertilizers are made centrally but must be distributed over a large agricultural area. A fertilizer factory is, typically, a large installation, characterized by enormous storage silos; the product is manufactured all the year round, but it requires considerable space to store it until the few weeks during which it is distributed on farmlands.
The weight of the superphosphate is greater than that of the original phosphate rock by the amount of the sulfuric acid added; the superphosphate also carries the dead weight of the calcium sulfate that is formed in the manufacturing process. This dead weight can be reduced by replacing sulfuric acid with phosphoric acid (itself obtained by the action of sulfuric acid on phosphate rock, followed by separating the products; or else by an electric furnace process). This process results in triple superphosphate, in which all the calcium originally in the phosphate rock appears as calcium monophosphate. The useful content of the fertilizer, expressed as the percent of phosphoric oxide, is increased from 20 percent in ordinary superphosphate to about 45 percent in the triple variety, resulting in a better than twofold reduction in the amount of material that must be distributed to provide a given amount of the useful oxide.
Instead of using either sulfuric or phosphoric acid to treat the phosphate rock, nitric acid can be employed. One of the resulting products, calcium nitrate, is itself a fertilizer, so what is obtained is one of the many varieties of mixed fertilizers. Instead of neutralizing phosphoric acid with calcium, which contributes nothing but dead weight, ammonia can be used, giving ammonium phosphate, in which both constituents contribute fertilizer elements. Such improvements in fertilizers are constantly being made.
Many other compounds of phosphorus are used. One group is composed of phosphoric acid and various phosphates derived from it. The acid itself is used in soft drinks for its pleasant taste when sweetened and its nutritive value. Other food applications include the use of disodium phosphate in processed cheese; and phosphates in baking powder, flameproofing, and the treatment of boiler water in steam plants. An important use of some of the phosphates is in detergents, discussed below.
World War IAllied troops making a night attack as a white phosphorus bomb explodes nearby, Gondrecourt, France, August 15, .
Elemental phosphorus exists in many allotropic forms. White phosphorus is used in rodent poison and by the military for smoke generation. Red phosphorus, comparatively harmless, is used in matches. Ferrophosphorus, a combination of phosphorus with iron, is used as an ingredient in high-strength low-alloy steel. In addition, the many organic compounds of phosphorus have varied uses, including those as additives for gasoline and lubricating oil, as plasticizers for plastics that otherwise would be inconveniently rigid, and, in some cases, as powerful insecticides, related to nerve poisons.
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