Potassium: the essentials

Potassium is a metal and is the seventh most abundant and makes up about 1.5 % by weight of the earth's crust. Potassium is an essential constituent for plant growth and it is found in most soils. It is also a vital element in the human diet.

Potassium is never found free in nature, but is obtained by electrolysis of the chloride or hydroxide, much in the same manner as prepared by Davy. It is one of the most reactive and electropositive of metals and, apart from lithium, it is the least dense known metal. It is soft and easily cut with a knife. It is silvery in appearance immediately after a fresh surface is exposed.

It oxidises very rapidly in air and must be stored under argon or under a suitable mineral oil. As do all the other metals of the alkali group, it decomposes in water with the evolution of hydrogen. It usually catches fire during the reaction with water. Potassium and its salts impart a lilac colour to flames.

The picture above shows the colour arising from a burning mixture of potassium chlorate (KClO₃) and sucrose (only to be demonstrated by a professionally qualified chemist).

The chemistry of potassium is dominated by electron loss to form K⁺. Cartoon by Nick D Kim ([Science and Ink], used by permission).

Potassium: historical information

Until the 18th century no distinction was made between potassium and sodium. This was because early chemists did not recognise that "vegetable alkali" (K₂CO₃, potassium carbonate, coming from deposits in the earth) and "mineral alkali" (Na₂CO₃, sodium carbonate, derived from wood ashes) are distinct from each other. Eventually a distinction was made.

Well before potassium was recognized as an element, potassium carbonate was mixed with animal fat to make soap. The carbonate was made by extracting wood ash with water before concentration by boiling - hence the name "potash" for potassium salts.

Potassium was isolated in 1807 by Sir Humphry Davy, who obtained it through the electrolysis of very dry molten caustic potash (KOH, potassium hydroxide). Potassium collected at the cathode. Potassium was the first metal isolated by electrolysis. Davy isolated sodium by a similar procedure later in 1807.

Sometime prior to the autumn of 1803, the Englishman John Dalton was able to explain the results of some of his studies by assuming that matter is composed of atoms and that all samples of any given compound consist of the same combination of these atoms. Dalton also noted that in series of compounds, the ratios of the masses of the second element that combine with a given weight of the first element can be reduced to small whole numbers (the law of multiple proportions). This was further evidence for atoms. Dalton's theory of atoms was published by Thomas Thomson in the 3rd edition of his System of Chemistry in 1807 and in a paper about strontium oxalates published in the Philosophical Transactions. Dalton published these ideas himself in the following year in the New System of Chemical Philosophy. The symbol used by Dalton for potassium is shown below. [See History of Chemistry, Sir Edward Thorpe, volume 1, Watts & Co, London, 1914.]

Potassium around us Read more »

Potassium salts are essential for both animals and plants. The potassium cation (K⁺) is the major cation in intracellular (inside cells) fluids (sodium is the main extracellular cation). It is essential for nerve and heart function. A normal diet containing reasonable amounts of vegetables contains all the potassium necessary.

Potassium metal is far too reactive to be found uncombined in nature. The element is quite abundant and makes up about 1.5 % by weight of the earth's crust. Potassium is found extensively as potash (KOH). It is mined in Germany, USA, and elsewhere. Minerals such as sylvite (KCl), carnallite, and langbeinite, are found in ancient lake and sea beds. That as Saskatchewan in Canada contains about 10,000,000,000 tonnes of potassium chloride. Potassium is found in seawater, but in relatively small amounts compared to sodium.

Abundances for cobalt in a number of different environments. More abundance data »

Location	ppb by weight	ppb by atoms	Links
Universe	3000	100
Crustal rocks	15000000	7800000
Human	2000000 ppb by weight	320000 atoms relative to C = 1000000

Physical properties Read more »

• Density of solid: 856 kg m^-3
• Molar volume: |196| cm³
• Thermal conductivity: |206| W m^‑1 K^‑1

Heat properties Read more »

• Melting point: 336.53 [63.38 °C (146.08 °F)] K
• Boiling point: 1032 [759 °C (1398 °F)] K
• Enthalpy of fusion: |203| kJ mol^-1

Crystal structure Read more »

The solid state structure of potassium is: bcc (body-centred cubic).

Potassium: orbital properties Read more »

Potassium atoms have 19 electrons and the shell structure is 2.8.8.1. The ground state electronic configuration of neutral Potassium is [Ar].4s¹ and the term symbol of Potassium is ²S_1/2.

• Pauling electronegativity: 0.82 (Pauling units)
• First ionisation energy: 418.8 kJ mol^‑1
• Second ionisation energy: 3052 kJ mol^‑1

Isolation

Isolation: potassium would not normally be made in the laboratory as it is so readily available commercially. All syntheses require an electrolytic step as it is so difficult to add an electron to the poorly electronegative potassium ion K⁺.

Potassium is not made by the same method as sodium as might have been expected. This is because the potassium metal, once formed by electrolysis of liquid potassium chloride (KCl), is too soluble in the molten salt.

cathode: K⁺(l) + e^- → K (l)

anode: Cl^-(l) → ¹/₂Cl₂ (g) + e^-

Instead, it is made by the reaction of metallic sodium with molten potassium chloride at 850°C.

Na + KCl ⇌ K + NaCl

This is an equilibrium reaction and under these conditions the potassium is highly volatile and removed from the system in a form relatively free from sodium impurities, allowing the reaction to proceed.

Potassium isotopes Read more »

Although Potassium only has two stable isotopes (K-39 and K-41), the long-lived radioisotope K-40 is often also regarded as a stable isotope. Potassium isotopes, mainly K-40 and K-41, are used to study the impact of potassium on the growth of plants and of the human cardiovascular system.

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