POTASSIUM: HISTORY, STRUCTURE, PROPERTIES, REACTIONS, USES - CHEMISTRY - 2024

The potassium is an alkaline chemical symbol is K. Its atomic number is 19 and is located below sodium in the periodic table. It is a soft metal that can even be cut with a knife. In addition, it is quite light, and it can float on liquid water while reacting vigorously.

Freshly cut, it has a very bright silvery-white color, but when exposed to air it oxidizes quickly and loses its luster, turning grayish (almost bluish, like the image below).

Partially oxidized pieces of potassium stored in mineral oil. Source: 2 × 910

Potassium reacts explosively with water to form potassium hydroxide and hydrogen gas. It is precisely this gas that is responsible for the explosiveness of the reaction. When it burns in the lighter, its excited atoms dye the flame an intense lilac color; this is one of his qualitative tests.

It is the seventh most abundant metal in the earth's crust and represents 2.6% of its weight. It is found mainly in igneous rocks, shales and sediments, in addition to minerals such as sylvite (KCl). Unlike sodium, its concentration in seawater is low (0.39 g / L).

Potassium was isolated in 1807 by the English chemist Sir Humphrey Davy, by electrolysis of a solution of its hydroxide, KOH. This metal was the first to be isolated by electrolysis and Davy gave it the English name potassium.

In Germany, however, the name kalium was used to refer to the metal. Precisely from this last name comes the letter 'K', used as a chemical symbol for potassium.

The metal itself has little industrial use, but it does give rise to many useful compounds. Biologically, however, it is much more important, since it is one of the essential elements for our body.

In plants, for example, it favors photosynthesis, the process of osmosis. It also promotes protein synthesis, thereby favoring the growth of plants.

History

Potash

Since ancient times, man has used potash as a fertilizer, ignoring the existence of potassium, much less its relationship with potash. This was prepared from the ashes of the trunks and leaves of the trees, to which water was added, which was later evaporated.

Vegetables contain mostly potassium, sodium, and calcium. But calcium compounds are poorly soluble in water. For this reason, potash was a concentrate of potassium compounds. The word is derived from the contraction of the English words 'pot' and 'ash'.

In 1702, G. Ernst Stahl suggested a difference between the sodium and potassium salts; This suggestion was verified by Henry Duhamel du Monceau in 1736. As the exact composition of the salts was not known, Antoine Lavoiser (1789) decided not to include alkalis in the list of chemical elements.

Discovery

In 1797, the German chemist Martin Klaproth discovered potash in the minerals leucite and lepidolite, so he concluded that it was not just a product of plants.

In 1806, the English chemist Sir Humphrey Davy discovered that the bond between the elements of a compound was electrical in nature.

Davy then isolated potassium by electrolysis of potassium hydroxide, observing globules with a metallic luster that accumulated at the anode. He named the metal with the English etymology word potassium.

In 1809, Ludwig Wilhelm Gilbert proposed the name kalium (kalium) for Davy's potassium. Berzelius evoked the name kalium to assign potassium the chemical symbol "K".

Finally, Justus Liebig in 1840 discovered that potassium was a necessary element for plants.

Structure and electron configuration of potassium

Metallic potassium crystallizes under normal conditions in the body centered cubic (bcc) structure. This is characterized by being thin, which agrees with the properties of potassium. A K atom is surrounded by eight neighbors, right in the center of a cube and with the other K atoms located at the vertices.

This phase bcc is also designated as phase KI (the first). When pressure increases, the crystal structure compacts to the face centered cubic (fcc) phase. However, a pressure of 11 GPa is needed for this transition to occur spontaneously.

This denser fcc phase is known as K-II. At higher pressures (80 GPa), and lower temperatures (less than -120 ºC), potassium acquires a third phase: K-III. K-III is characterized by its ability to accommodate other atoms or molecules within its crystalline cavities.

There are also two other crystalline phases at even higher pressures: K-IV (54 GPa) and KV (90 GPa). At very cold temperatures, potassium even exhibits an amorphous phase (with disordered K atoms).

Oxidation number

The electron configuration of potassium is:

4s ¹

The 4s orbital is the outermost and therefore has the only valence electron. This in theory is responsible for the metallic bond that holds the K atoms together to define a crystal.

From the same electron configuration it is easy to understand why potassium usually always (or almost always) has an oxidation number of +1. When it loses one electron to form the K ⁺ cation, the noble gas argon, with its full valence octet, becomes isoelectronic.

In most of its derivative compounds, potassium is assumed to be K ⁺ (even if its bonds are not purely ionic).

On the other hand, although less likely, potassium can gain an electron, having two electrons in its 4s orbital. Thus, calcium metal becomes isoelectronic:

4s ²

It is then said that it gained an electron and has a negative oxidation number, -1. When this oxidation number is calculated in a compound, the existence of the potaside anion, K ^- is assumed.

Properties

Appearance

Glossy white silver metal.

Molar mass

39.0983 g / mol.

Melting point

83.5 ° C.

Boiling point

759 ° C.

Density

-0.862 g / cm ³, at room temperature.

-0.828 g / cm ³, at melting point (liquid).

Solubility

Reacts violently with water. Soluble in liquid ammonia, ethylenediamine and aniline. Soluble in other alkali metals to form alloys, and in mercury.

Vapor density

1.4 in relation to air taken as 1.

Vapor pressure

8 mmHg at 432 ° C.

Stability

Stable if it is protected from air and moisture.

Corrosiveness

It can be corrosive in contact with metals. On contact, it can cause skin and eye burns.

Surface tension

86 dynes / cm at 100 ° C.

Heat of fusion

2.33 kJ / mol.

Heat of vaporization

76.9 kJ / mol.

Molar heat capacity

29.6 J / (mol · K).

Electronegativity

0.82 on the Pauling scale.

Ionization energies

First ionization level: 418.8 kJ / mol.

Second ionization level: 3.052 kJ / mol.

Third level of ionization: 4,420 kJ / mol.

Atomic radio

227 pm.

Covalent radius

203 ± 12 pm.

Thermal expansion

83.3 µm / (m · K) at 25 ° C.

Thermal conductivity

102.5 W / (mK).

Electrical resistivity

72 nΩ · m (at 25 ° C).

Hardness

0.4 on the Mohs scale.

Natural isotopes

Potassium occurs as three main isotopes: ³⁹ K (93.258%), ⁴¹ K (6.73%) and ⁴⁰ K (0.012%, radioactive emission β)

Nomenclature

Potassium compounds have the oxidation number +1 by default (with very special exceptions). Therefore, in the stock nomenclature the (I) at the end of the names is omitted; and in traditional nomenclature, names end with the suffix -ico.

For example, KCl is potassium chloride, not potassium (I) chloride. Its traditional name is potassium chloride or potassium monochloride, according to the systematic nomenclature.

For the rest, unless they are very common names or minerals (such as silvin), the nomenclature around potassium is quite simple.

Shapes

Potassium is not found in nature in metallic form, but it can be obtained industrially in this form for certain uses. It is found mainly in living beings, in the ionic form (K ⁺). In general, it is the main intracellular cation.

Potassium is present in numerous compounds, such as potassium hydroxide, acetate or chloride, etc. It is also part of about 600 minerals, including sylvite, alunite, carnalite, etc.

Potassium forms alloys with other alkaline elements, such as sodium, cesium, and rubidium. It also forms ternary alloys with sodium and cesium, through so-called eutectic fusions.

Biological role

Plants

Potassium constitutes, together with nitrogen and phosphorus, the three main plant nutrients. Potassium is absorbed by the roots in ionic form: a process favored by the existence of adequate conditions of humidity, temperature and oxygenation.

Regulates the opening and closing of foliar stomata: activity that allows the uptake of carbon dioxide, which combines with water during photosynthesis to form glucose and oxygen; These are ATP generating agents that constitute the main energy source of living beings.

It facilitates the synthesis of some enzymes related to plant growth, in addition to starch, an energy reserve substance. It also intervenes in osmosis: a process necessary for the root absorption of water and minerals; and in the rise of the water through the xylem.

Chlorosis is a manifestation of a potassium deficiency in plants. It is characterized by the leaves losing their greenness and turning yellow, with burnt edges; and finally, defoliation occurs, with a delay in plant growth.

Animals

In animals, in general, potassium is the main intracellular cation with a concentration of 140 mmol / L; while the extracellular concentration varies between 3.8 and 5.0 mmol / L. 98% of the body's potassium is confined to the intracellular compartment.

Although potassium intake can vary between 40 and 200 mmol / day, its extracellular concentration is kept constant by its regulation of renal excretion. The hormone aldosterone, which regulates potassium secretion at the level of the collecting and distal tubules, is involved in this.

Potassium is centrally responsible for the maintenance of intracellular osmolarity and, therefore, it is responsible for the maintenance of cellular integrity.

Although the plasma membrane is relatively permeable to potassium, its intracellular concentration is maintained by the activity of the enzyme Na, ATPase (sodium and potassium pump) that removes three sodium atoms and introduces two potassium atoms.

Cell repolarization

Excitable cells, made up of neurons and striated and smooth muscle cells; and striated muscle cells, made up of skeletal and cardiac muscle cells, are all capable of forming action potentials.

The interior of excitable cells is negatively charged in relation to the exterior of the cell, but when properly stimulated, the permeability of the plasma membrane of cells to sodium increases. This cation penetrates through the plasma membrane and turns the cell interior positive.

The phenomenon that occurred is called action potential, which has a set of properties, among them, it is capable of propagating throughout the neuron. A command issued by the brain travels as action potentials to a given muscle to cause it to contract.

For a new action potential to occur, the cell interior must have a negative charge. To do this, there is an exit of potassium from the cell interior, returning it to its original negativity. This process is called repolarization, being a main function of potassium.

Therefore, the formation of action potentials and initiation of muscle contraction is said to be a shared responsibility of sodium and potassium.

Other functions

Potassium serves other functions in humans, such as vascular tone, control of systemic blood pressure, and gastrointestinal motility.

An increase in the plasma concentration of potassium (hyperkalemia) produces a series of symptoms such as anxiety, nausea, vomiting, abdominal pain and irregularities in the electrocardiogram. The T wave that is related to ventricular repolarization is tall and wide.

This record is explained because as the extracellular concentration of potassium increases, it leaves the cell exterior more slowly, so ventricular repolarization is slower.

A decrease in the plasma potassium concentration (hypocalcemia) presents, among others, the following symptoms: muscle weakness, decreased intestinal motility, decreased glomerular filtration, cardiac arrhythmia and flattening of the T wave of the electrocardiogram.

The T wave is shortened, since by decreasing the extracellular concentration of potassium, its exit towards the cell exterior is facilitated and the duration of repolarization decreases.

Where is potassium found and production

Silvite crystal, which consists practically of potassium chloride. Source: Rob Lavinsky, iRocks.com - CC-BY-SA-3.0

Potassium is found mainly in igneous rocks, shales, and sediments. Also, in minerals like muscovite and orthoclase, which are insoluble in water. Orthoclase is a mineral that usually occurs in igneous rocks and granite.

Potassium is also present in water-soluble mineral compounds, such as carnalite (KMgCl ₃ · 6H ₂ O), sylvite (KCl), and landbeinite, which are found in dry lake beds and on the seabed.

In addition, potassium is found in brines and as a product of the incineration of plant trunks and leaves in a process used for the production of potash. Although its concentration in seawater is low (0.39 g / L), it is also used to obtain potassium.

Potassium is present in large deposits, such as the one in Saskatchewan, Canada, rich in the mineral sylvite (KCl) and capable of producing 25% of the world's potassium consumption. Saline waste liquids can contain a significant amount of potassium, in the form of KCl.

Electrolysis

Potassium is produced by two methods: electrolysis and thermal. In electrolysis, the method used by Davy to isolate potassium has been followed without major modifications.

However, this method from the industrial point of view has not been efficient, since the high melting point of the molten potassium compounds must be lowered.

The method of electrolysis of potassium hydroxide was used industrially in the 1920s. The thermal method nevertheless supplanted it, and became the dominant method for the production of this metal after 1950.

Thermal method

In the thermal method, potassium is produced by reducing molten potassium chloride at 870 ºC. This is continuously fed to a distillation column packed with the salt. Meanwhile, the sodium vapor passes through the column to produce the reduction of potassium chloride.

Potassium is the most volatile component of the reaction and accumulates at the top of the distillation column, where it is collected continuously. The production of metallic potassium by the thermal method can be outlined in the following chemical equation:

Na (g) + KCl (l) => K (l) + NaCl (l)

The Griesheimer process, which uses the reaction of potassium fluoride with calcium carbide, is also used in potassium production:

2 KF + CaC ₂ => 2 K + CaF ₂ + 2 C

Reactions

Inorganic

Potassium is a highly reactive element that reacts rapidly with oxygen to form three oxides: potassium oxide (K ₂ O), peroxide (K ₂ O ₂), and superoxide (KO ₂).

Potassium is a strongly reducing element, which is why it oxidizes faster than most metals. It is used to reduce metal salts, replacing potassium to the metal in the salt. This method allows the obtaining of pure metals:

MgCl ₂ + 2 K => Mg + 2 KCl

Potassium reacts strongly with water to form potassium hydroxide and release explosive hydrogen gas (image below):

Metallic potassium reacting with an aqueous solution of phenolphthalein, which turns purple-red when the OH- ions are released into the medium. Note the formation of hydrogen gas. Source: Ozone aurora and Philip Evans via Wikipedia.

Potassium hydroxide can react with carbon dioxide to produce potassium carbonate.

Potassium reacts with carbon monoxide at a temperature of 60 ° C to produce an explosive carbonyl (K ₆ C ₆ O ₆). It also reacts with hydrogen at 350ºC, forming a hydride. It is also highly reactive with halogens, and explodes in contact with liquid bromine.

Explosions also occur when potassium reacts with halogenated acids, such as hydrochloric acid, and the mixture is hit or shaken strongly. Molten potassium further reacts with sulfur and hydrogen sulfide.

Organic

Reacts with organic compounds containing active groups, but is inert to aliphatic and aromatic hydrocarbons. Potassium reacts slowly with ammonia to form potasomine (KNH ₂).

Unlike sodium, potassium reacts with carbon in the form of graphite to form a series of interlaminar compounds. These compounds have carbon-potassium atomic ratios: 8, 16, 24, 36, 48, 60, or 1; ie KC ₆₀, for example.

Applications

Metallic potassium

There is not much industrial demand for metallic potassium. Most of it is converted to potassium superoxide, used in respirators, as it releases oxygen and removes carbon dioxide and water vapor.

NaK alloy has great heat absorption capacity, which is why it is used as a coolant in some nuclear reactors. Likewise, vaporized metal has been used in turbines.

Compounds

Chloride

KCl is used in agriculture as a fertilizer. It is also used as a raw material for the production of other potassium compounds, such as potassium hydroxide.

Hydroxide

Also known as caustic potash, KOH, it is used in the manufacture of soaps and detergents.

Its reaction with iodine produces potassium iodide. This salt is added to table salt (NaCl) and feed to protect it against iodine deficiency. Potassium hydroxide is used in the manufacture of alkaline batteries.

Nitrate

Also known as saltpeter, KNO ₃, it is used as a fertilizer. In addition, it is used in the elaboration of fireworks; as a food preservative, and in hardening glass.

Chromate

It is used in the production of fertilizer and potassium alum production.

Carbonate

It is used in the manufacture of glass, especially those used in the manufacture of televisions.

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