Potassium (K)

• Element Symbol: K
• Atomic Number: 19
• Atomic Mass: 39.0983
• Group # in Periodic Table: 1
• Group Name: Alkali metals
• Period in Periodic Table: 4
• Block of Periodic Table: s-block
• Discovered by: Sir Humphry Davy (1807)

Potassium is an extremely light, silvery-white metal found in abundance in Earth’s crust. Its chemical symbol is K, and its atomic number is 19, making it a Group 1 alkali metal in the periodic table. Its atomic weight is 39.

Potassium oxidizes rapidly in air, and it has a violent reaction with water, producing potassium hydroxide and hydrogen gas. Because the resulting hydrogen gas burns spontaneously, it is necessary to store the potassium in a way that is safe. Therefore, a liquid such as kerosene is often used to form a physical barrier that separates the potassium from air, and in this way, the hydrogen gas will not spontaneously combust. Potassium will also tarnish in air, producing a dull silvery coating.

In small amounts, potassium is essential for life on Earth insofar as it is a constituent of all animal and plant tissues; it is also a key element that makes soil fertile. The U.S. Food and Drug Administration (FDA) lists tomatoes, orange juice, white beans, bananas, potatoes, and beet greens as some of the richest sources of dietary potassium.

It was English chemist Sir Humphry Davy who discovered and named the element potassium in 1807. He isolated it by electrolyzing potash (potassium hydroxide), thus making potassium the first metal to be isolated using this electrolysis. The name potassium came from the word potash, which was an alkali extracted from water in a pot containing ash of burned wood or leaves.

Physical Properties

In its pure form, potassium is a soft, silvery white metal with a melting point of 63.38°C. This alkali metal has a density of 0.856 grams per cubic centimeter and is so soft that it can be cut with a knife. It is the second lightest metal after lithium and can float on water. It has a Mohs hardness rating of 0.4. Its standard state at 298 K is solid. It has a boiling point of 759°C. When directly exposed to fire, it produces a lilac-colored flame. Potassium is colorless in water and cannot easily be precipitated.

Chemical Properties

Potassium is one of the most electropositive metals and easily combines with other elements to form compounds. The elements that potassium most often combines with are oxygen, chlorine, carbon, sulfur, and iodine. The properties of potassium are very similar to those of sodium; however, potassium is far more reactive than sodium. In contrast to sodium, which doesn’t react to graphite, potassium forms rich interlamellar compounds, the richest being KC8. Both NaK and liquid potassium are far more reactive with air and oxygen than liquid sodium is.

Potassium exists in three isotopic forms in nature, with mass numbers 39, 40, and 41. Potassium-39 is the most abundant form of these three natural forms; 93.25% of the potassium found in nature is potassium-39, 6.7% is potassium-41, and 0.01% is potassium-40. Potassium-40 is a radioactive isotope. Isotopes are chemical variants of an element that differ in the number of neutrons, but the isotopes in these elements have the same number of protons in each atom. Of these three naturally occurring isotopes, potassium-40 is the most unstable, with a half-life of about 1.248 × 10⁹ years. Twenty-one artificial isotopes of potassium have been prepared. Potassium isotopes are used for dating minerals by the K-Ar method. They are likewise used in weathering studies, and because these isotopes are utilized by all living tissues, they are also used in nutrient-cycle studies.

Applications

Potassium is the eighth most abundant element in Earth’s crust (1.6%), but it is too reactive to occur as a free element. It is obtained from minerals such as sylvite (KCl), langbeinite (K₂Mg₂(SO₄)₃), carnallite (KMgCl₃·6H₂O) and polyhalite (K₂Ca₂Mg(SO₄)₄·2H₂O). These minerals are mined from seabeds and ancient lakes. Caustic potash is obtained primarily from mines in Utah, New Mexico, and California (in the Unites States), and also from Germany.

In humans, potassium is the major intracellular cation (a negatively charged ion), and it is necessary for normal cell functioning. Potassium constitutes about 0.2% of body mass. Potassium is responsible for normal nerve transmission. A deficiency of potassium leads to a condition called hypokalemia, which can produce muscle weakness, intolerance to glucose, and a disruption in the cardiac rhythm. A moderate potassium deficit can lead to higher blood pressure, increased chances of kidney stones, higher salt sensitivity, and an increase in the breakdown of bone, leading to an increase in serum calcium levels. As part of a balanced diet, fresh fruits and vegetables serve as rich sources of potassium.

Potassium is used by plants to make proteins. Hence, it is a vital component of plant fertilizers, usually showing up in the form of potassium chloride (KCl) or potassium sulfate (K₂SO₄). Fertilizers consume about 95% of total global potassium production, of which 90% is supplied by KCl. Potassium chloride is also used as a substitute for common salt (that is, sodium chloride, NaCl).

Potassium is widely used in the chemical industry as well, and compounds of potassium are employed in various industries. Potassium hydroxide (KOH), for example, is used to manufacture soft soaps, detergents, and drain cleaners because it is a strong alkali. Serving as an electrolyte, potassium is used in the manufacture of alkaline batteries too. Potassium nitrate (KNO₃), also called saltpeter or nitre, is used to prepare toughened glass. (The glass is immersed in molten potassium nitrate to make it tougher). Also, potassium nitrate is the key ingredient that gives gunpowder its explosive properties, making it a vital constituent of fireworks and explosives. It is also used to pickle meat.

Potassium chromate (K₂CrO₄) and potassium bichromate (K₂CrO₇) are used to make match heads and pyrotechnics, for dyeing (in the textile industry), and for tanning leather. Yet another compound is potassium bromide (KBr), which is a combination of bromine and potassium. This compound is used in engraving, photography, and lithography. Potassium bromide is also used as a sedative drug.

Potassium iodide (KI) is used in photography, in the preparation of gelatin emulsion, and in medicine. It is used to treat rheumatism and an overactive thyroid gland (hyperthyroidism). Potassium permanganate (KMnO₄) is a widely used germicide and disinfectant that is used as an oxidizing agent in chemical reactions. Potassium hydrogen tartrate (known as cream of tartar) is used in baking powder and in medicine. Potassium sodium tartrate is used to silver mirrors and is the main constituent of baking powder. Potassium bromate is used in the food industry to produce dough that is stronger and that tends to rise higher.

Potassium bisulfate is a food preservative used in wine and beer and as a bleaching agent used in textiles and in the straw industry. It is also used to tan leather.

Bibliography

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Greenwood, N.N, and A. Earnshaw. Chemistry of the Elements. 2nd ed. Oxford: Butterworth-Heinemann, 1997. Print.

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<http://www.chemicool.com/elements/potassium.html>.