Potassium (19)

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 oxidizes 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 color to flames (very pretty).

•Name: Potassium
•Symbol: K
•Atomic number: 19
•Atomic weight: 39.0983
•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
•Color: silvery white
•Classification: Metallic

Historical information

Potassium was discovered by Sir Humphrey Davy at 1807 in England. Origin of name: from the English word "potash" (pot ashes) and the Arabic word "qali" meaning alkali (the origin of the symbol K comes from the Latin word "kalium").Until the 18th century no distinction was made between potassium and sodium. This was because early chemists did not recognize 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.

Physical properties

•Melting point: 336.53 [or 63.38 °C (146.08 °F)] K
•Boiling point: 1032 [or 759 °C (1398 °F)] K
•Density of solid: 856 kg m-3

Orbital properties

•Ground state electron configuration: [Ar].4s1
•Shell structure: 2.8.8.1
•Term symbol: 2S1/2

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.

Interesting Facts:

1.       Potassium is element number 19. This means the atomic number of potassium is 19 or each potassium atom has 19 protons.

2.       Potassium is one of the alkali metals, which means it is a highly reactive metal with a valence of 1.

3.       Because of its high reactivity, potassium is not found free in nature. It is formed by supernovas via the R-process and occurs on Earth dissolved in seawater and in ionic salts.

4.       Pure potassium is a lightweight silvery metal that is soft enough to cut with a knife.

5.       Pure potassium usually is stored under oil or kerosene because it oxidizes so readily in air and reacts in water to evolve hydrogen, which may be ignited from the heat of the reaction.

6.       The potassium ion is important for all living cells. Animals use sodium ions and potassium ions to generate electric potentials. This is vital for many cellular processes and is the basis for the conduction of nerve impulses.

7.       Potassium was first purified in 1807 by Sir Humphry Davy from caustic potash (KOH) via electrolysis. Potassium was the first metal to be isolated using electrolysis.

8.       Potassium compounds emit a lilac or violet flame color when burned.

9.       Potassium is used as a heat transfer medium. Its salts are used as a fertilizer, oxidizer, colorant, to form strong bases, as a salt substitute, and for many other applications.

10.   The name for potassium comes from the English word for potash. The symbol for potassium is K, which is derived from the Latin kalium and Arabic qali for alkali. Potash and alkali are two of the potassium compounds known to man since ancient times