| Discovery date | 1807 |
| Discovered by | Sir Humphry Davy |
| Origin of the name | The name is derived from the English word 'potash'. |
| Allotropes | - |
| Group | 1 | Melting point | 63.5 oC, 146.3 oF, 336.65 K |
| Period | 4 | Boiling point | 759 oC, 1398.2 oF, 1032.15 K |
| Block | s | Density (kg m-3) | 862 |
| Atomic number | 19 | Relative atomic mass | 39.098 |
| State at room temperature | Solid | Key isotopes | 39K |
| Electron configuration | [Ar] 4s1 | CAS number | 7440-09-7 |
| ChemSpider ID | 4575326 | ChemSpider is a free chemical structure database | |
| Atomic radius, non-bonded (Å) | 2.750 | Covalent radius (Å) | 2 |
| Electron affinity (kJ mol-1) | 48.367 |
Electronegativity (Pauling scale) |
0.820 |
|
Ionisation energies (kJ mol-1) |
1st
418.810
2nd
3051.829
3rd
4419.604
4th
5876.917
5th
7975.471
6th
9590.635
7th
11342.807
8th
14943.637
|
||
| Covalent bonds |
|
|
|
| Common oxidation states | 1 | ||||
| Isotopes | Isotope | Atomic mass | Natural abundance (%) | Half life | Mode of decay |
| 39K | 38.964 | 93.258 | - | - | |
| 40K | 39.964 | 0.012 | 1.248 x 109 y | β- | |
| - | β+ | ||||
| 41K | 40.962 | 6.73 | - | - | |
|
Molar heat capacity (J mol-1 K-1) |
29.6 | Young's modulus (GPa) | Unknown | |||||||||||
| Shear modulus (GPa) | Unknown | Bulk modulus (GPa) | 3.1 | |||||||||||
| Vapour pressure | ||||||||||||||
| Temperature (K) |
|
|||||||||||||
| Pressure (Pa) |
|
|||||||||||||
Potassium salts in the form of saltpetre (potassium nitrate, KNO3), alum (potassium aluminium sulfate, KAl(SO4)2), and potash (potassium carbonate, K2CO3) have been known for centuries. They were used in gunpowder, dyeing, and soap making. They were scraped from the walls of latrines, manufactured from clay and sulfuric acid, and collected as wood ash respectively. Reducing them to the element defeated the early chemists and potassium was classed as an ‘earth’ by Antoine Lavoisier. Then in 1807, Humphry Davy exposed moist potash to an electric current and observed the formation of metallic globules of a new metal, potassium. He noted that when they were dropped into water they skimmed around on the surface, burning with a lavender-coloured flame.
| Listen to Potassium Podcast |
|
Transcript :
Chemistry in Its Element - Potassium(Promo)
You're listening to Chemistry in its element brought to you by Chemistry World, the magazine of the Royal Society of Chemistry (End promo)
Chris Smith Hello, this week the story of the first alkaline metal ever isolated, why its 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 1807 by Humphry Davy after the compound from which he isolated the metal, potash, or potassium hydroxide. An extract from the 1730s 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.
Almost twenty years earlier, the Italian physician Luigi Galvani noticed that when he touched the central nerve of a dead frog with a metal knife the muscles in the frog's leg violently contracted. After further experiments, he found that combinations of different metals gave even stronger effects. His fellow countryman, Alessandro Volta, recognised the electrical origin of these observations, and went on to develop the so-called Voltaic pile. This [device consisted] of alternating plates of two different metals, such as copper and zinc, sandwiched with cloth soaked in a solution of brine.
Such a Voltaic pile had been used in spectacular fashion at the turn of the century by English chemists William Nicholson and Anthony Carlisle to decompose - or electrolyse - water into its constituent elements hydrogen and oxygen. Humphry's younger brother John Davy stated that this experiment "immediately impressed powerfully the mind of my brother". In his notebook in August 1800 Humphry records "I cannot close this notice without feeling grateful to Volta, Nicholson, and Carlisle, whose experience has placed such a wonderful and important instrument of analysis in my power".
It was to caustic potash that Humphry focussed his attention. It had been suspected for a while that this substance was not elemental, but its exact nature unknown. Lavoisier's "Elements of Chemistry" from which Humphry taught himself chemistry as a young lad made it quite clear that both potash and soda were compounds and likely to be decomposed into their true constituent elements in the future. Davy himself suspected potash to be composed of phosphorus or sulphur united to nitrogen.
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 caustic potash. This he did on the sixth of October, 1807 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 - he actually danced about the room in ecstatic delight; some little time was required for him to compose himself sufficiently to continue the experiment"
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 [from whence it came], 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
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.
Chemistry in its element is brought to you by the Royal Society of Chemistry and produced by thenakedscientists dot com. There's more information and other episodes of Chemistry in its element on our website at chemistryworld dot org forward slash elements.
(End promo) |
Mining and Sourcing data: British Geological Survey – natural environment research council.
Text: John Emsley Nature’s Building Blocks: An A-Z Guide to the Elements, Oxford University Press, 2nd Edition, 2011.
Additional information for platinum, gold, neodymium and dysprosium obtained from Material Value Consultancy Ltd www.matvalue.com
Data: CRC Handbook of Chemistry and Physics, CRC Press, 92nd Edition, 2011.
G. W. C. Kaye and T. H. Laby Tables of Physical and Chemical Constants, Longman, 16th Edition, 1995.
Members of the RSC can access these books through our library.