Potassium

What is potassium?

Potassium is a chemical element or silvery-white alkali metal of Group-1 or IA of the periodic table with the symbol K and atomic number 19. It was first isolated by Humphrey Davy in 1807 by the electrolysis of molten potassium hydroxide (KOH) with a voltaic battery. It is extensively involved in different types of biological processes in our life.

Many compounds of potassium have been used since the early days of human civilization. The large difference between the first and second ionization energy suggests the preferred oxidation number or state of the potassium will be +1.

Properties of potassium

The physical and chemical properties of potassium can be readily understood in terms of its outer electronic configuration. It has only one electron in the s-orbital over a noble gas core. Due to the very low first ionization energy, it loses one electron to form a K⁺ cation.

Potassium has the largest size among the period-3 elements. The large size and small charge of the cation favored the formation of ionic bonding in the compounds. The small electron affinity and electronegativity also lead to the formation of ionic compounds.

It gives a characteristic violet flame due to easy excitation of the outermost s-electron. This fact leads to the development of analytical chemistry for the estimation of metals by flame photometer. Name, symbol, discovery, and some important properties of the elements are given below the table.

Properties of Potassium
Name	Potassium
Symbol	K
Name derived from	The English word ‘potash’
Discovery	Humphry Davy in 1807
Atomic number	19
Atomic weight	39.0983
Common isotope	39K
Electron per shell	2, 8, 8, 1
Electronic configuration	[Ar] 4s1
Block	s-block
Period	period-4
Group	group-1
Melting point	63.5 °C, ​146.3 °F
Boiling point	​759 °C, ​1398 °F
Density	0.89 g/cm3
Molar heat capacity	29.6 J mol−1 K−1
Crystal structure	body-centered cubic crystal lattice (bcc)
Common oxidation state	+1
Electronegativity	0.82 (Pauling scale)
Ionization energy (kJ/mol)	1st	2nd	3rd
	418.8	3052	4420
CAS number	7440-09-7

Potassium on the periodic table

The valence shell electronic configuration of potassium [Ar] 4s¹. Therefore, it is placed on the s-block of the periodic table with group members of hydrogen, lithium, sodium, rubidium, and francium.

Where is potassium found?

All the alkali metals like Li, Na, K, Rb, and Cs are widely distributed in the earth’s crust of our nature. With the increasing atomic number, the elements become rare. Sylvite (KCl), sylvinite (KCl, NaCl), and Carnallite (KCl, MgCl₂, 6H₂O) are the most common ores of potassium metal.

Like sodium, K salts are also leached out from the silicon minerals by the action of the weather. The soluble minerals of the metal are called potash. A massive deposit of potash is found in many places of the world like Germany, the United Kingdom, and Canada.

Isotopes

There are 25 known isotopes of potassium but only three like ³⁹K, ⁴⁰K, and ⁴¹K are found in nature. The radioactive isotopes of K have been used as tracers in studies of weathering and medicine.

Production process

Commercial production of potassium by electrolysis of molten KCl is not favored because it is quite soluble in the molten electrolyte and does not float on the top. Commercially, it was prepared by reduction of molten KCl by sodium metal at 850 °C.

The reduction process is rather unusual since K is a better reducing agent than Na but at 850 °C, they are in chemical equilibrium. But higher volatility of K in the temperature range 850 °C to 880 °C, sifts the equilibrium towards the right. Therefore fractional distillation was carried out to obtain the metal.

Interesting facts about potassium

The large difference between the first and second ionization energy of elements suggests the preferred oxidation state of the alkali metals will be +1.

The first ionization energy of potassium is more than the compensated lattice energy for most ionic compounds. Therefore, the major chemistry of potassium is dominated in the form of K⁺ ions.

Chemical compounds

Potassium hydride

All the alkali metal hydrides are ionic in nature containing H⁻ ion. Potassium hydride is formed by the direct combination of hydrogen with metal at 350 to 600 °C. The cubic crystal lattice, KH is a colorless solid with a reasonably high melting point. On electrolysis of hydride, the metal is migrated to the positive electrode or cathode, and hydrogen is migrated to the negative electrode or anode.

Potassium oxide

Potassium reacts readily with oxygen to form a number of binary compounds like monoxide (K₂O), peroxide (K₂O₂), and superoxide (KO₂). The stability of the superoxides increases with the increasing atomic size of the cations. The peroxide is prepared by oxidation of metal in liquid ammonia by oxygen. It is used to restore oxygen in submarines and space capsules for driver breathing apparatus.

Potassium hydroxide

Potassium hydroxide (KOH) is a very strong base with a high pH scale. It is a white crystalline solid soluble in water and alcohol. The melting point of alkali hydroxides decreases with the increasing atomic number of alkali metals. Measurement of proton affinities of alkali hydroxides suggests that the base strength increases from LiOH to CsOH.

Potassium halide

Potassium halides are colorless, crystalline solids with high melting, boiling point, and high conductivity in the fused state. They all may be prepared by reacting KOH or K₂CO₃ with appropriate hydrogen halide. For a given cation the lattice energy decreases from lithium to cesium and for a given alkali metal the lattice energies decrease from fluorine to iodine. All the alkali metal halides crystalize as a face-centered cubic crystal lattice.

What is Potassium used for?

For most of the soil, K⁺ ion is an essential component of plant nutrition. Potassium chloride (KCl) is an important compound that is used in different types of fertilizers to grow the production of crops. Agricultural fertilizers consume 90 percent of the global production of metal in the form of KCl. It forms Na-K alloy which is used in the heat exchanger in breeder reactors.

Many compounds like KCl, KOH, K₂CO₃, KNO₃, KClO₃, KBr, and KMnO₄ are used in different types of chemical industries.

Potassium Compounds	Uses
KCl	used in important fertilizers like muriate of potash (MOP).
KOH	used in rubber processing, and preparation of potassium phosphate.
K2CO3	used in decorative glass, color TV tubes, textiles, dyes, and pigment industries.
KClO3	used in explosives and matches.
KBr	used in photography.
KMnO4	oxidizing agent or bleaching agent.

It is direct burning in the air or oxygen to form KO₂, which is used in oxygen supplements for submarines and space capsules.

Biological function

The function of sodium and potassium to penetrate the cell membrane differs due to their ionic size. The metals also differ in their transport mechanism and efficiency to activate enzymes. Potassium occurs at a high concentration inside the cell but sodium occurs at a lower concentration inside the cell.

Potassium is used mainly to activate many enzymes, the transmission of nerve signals, and participation in the oxidation of glucose to produce adenosine triphosphate (ATP).