Potassium

Potassium: the essentials

 Brief description: 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.

Table: basic information about and classifications of potassium.

Name: Potassium Symbol: K Atomic number: 19 Atomic weight: 39.0983 (1) Standard state: solid at 298 K CAS Registry ID: 7440-09-7

Group in periodic table: 1 Group name: Alkali metal Period in periodic table: 4 Block in periodic table: s-block Colour: silvery white Classification: Metallic

The reaction between potassium metal and water (only to be demonstrated by a professionally qualified chemist).

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).

Cartoon by Nick D Kim (nearingzero.net, used by permission).

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.

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Potassium	Potassium
Kalium	Kalium
Potassio	Potasio
Kalium	Potássio