Potassium

Category: alkali metal Atomic number: 19 Colour: silvery grey Melting point: 63°C (146°F) Boiling point: 759°C (1,398°F) First identified: 1807

Many centuries ago, people discovered that potash was a useful fertilizer that could be prepared from a variety of plants. One eighteenth-century account of its production reads ‘Potas or Pot-ashes, is brought yearly by the Merchant’s Ships in great abundance from Courland [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.’ The ashes were boiled in water, then the liquor at the top was poured into large copper pots and reboiled to reduce it to a salt. A similar account from the seventeenth century describes the same process being applied to a herb called ‘Kali’ (whose Latin name is Salsola kali, which is better known as saltwort).

Both methods produce a mix of potassium carbonate and sodium carbonate. (Potash can also be used to describe any of potassium chloride, potassium sulphate or potassium nitrate.) The kali method, which produces more of the sodium compound, is the origin of the word ‘alkaline’, via ‘al-kali’ where ‘al’ is the Arabic definite article.

This is relevant, because potassium was the first of the alkali metals to be isolated: Humphry Davy succeeded in doing this in 1807 by electrolysing molten potassium hydroxide. Reportedly, when he 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’.

Minding your ‘P’s and ‘Q’s

The brilliant Swedish chemist Jöns Jacob Berzelius invented the system of chemical notation we use today, using abbreviations such as O for oxygen. The only difference is that Berzelius used a superscript (H²O) for numbers of atoms rather than a subscript (H₂O). You may have noticed that some of the elements have abbreviations that seem to have nothing to do with the name – this is usually down to historic disagreements between various European chemists; for instance, sodium (Na) was known as natrium or natronium in Germanic languages. Davy named potassium after potash, but Berzelius preferred ‘kalium’ after the plant kali, thus the chemical symbol K.

Potassium is light enough to float on water – although if you try this, you’ll find it explodes almost immediately as it is so reactive – in fact it will even burn a hole through ice. Its main industrial use today remains as a fertilizer, as plant cells need potassium (and it is a crucial part of our diet). It is also used in the manufacture of glass, liquid soap, pharmaceuticals and saline drips.