Alkali metal
Potassium is a chemical element; it has symbol K (from Neo-Latin kalium) and atomic number 19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to form flaky white potassium peroxide in only seconds of exposure. It was first isolated from potash, the ashes of plants, from which its name derives. In the periodic table, potassium is one of the alkali metals, all of which have a single valence electron in the outer electron shell, which is easily removed to create an ion with a positive charge (which combines with anions to form salts). In nature, potassium occurs only in ionic salts. Elemental potassium reacts vigorously with water, generating sufficient heat to ignite hydrogen emitted in the reaction, and burning with a lilac-colored flame. It is found dissolved in seawater (which is 0.04% potassium by weight), and occurs in many minerals such as orthoclase, a common constituent of granites and other igneous rocks.
Potassium is chemically very similar to sodium, the previous element in group 1 of the periodic table. They have a similar first ionization energy, which allows for each atom to give up its sole outer electron. It was first suggested in 1702 that they were distinct elements that combine with the same anions to make similar salts, which was demonstrated in 1807 when elemental potassium was first isolated via electrolysis. …
Potassium metal was first isolated in 1807 by Humphry Davy, who derived it by electrolysis of molten caustic potash (KOH) with the newly discovered voltaic pile. Potassium was the first metal that was isolated by electrolysis. Later in the same year, Davy reported extraction of the metal sodium from a mineral derivative (caustic soda, NaOH, or lye) rather than a plant salt, by a similar technique, demonstrating that the elements, and thus the salts, are different. Although the production of potassium and sodium metal should have shown that both are elements, it took some time before this view was universally accepted.
Potassium is a soft silvery solid that is easily cut with a knife. Because of the sensitivity of potassium to water and air, air-free techniques are normally employed for handling the element. It is unreactive toward nitrogen and saturated hydrocarbons such as mineral oil or kerosene. It readily dissolves in liquid ammonia, up to 480 g per 1000 g of ammonia at 0 °C to form the electride [K(NH3)6]+e−, which features an electron as an anion.
Reflecting its low first ionization energy of 418.8 kJ/mol, potassium is a strong reducing agent, i.e., it readily releases an electron upon contact with other materials. With graphite, potassium metal forms graphite intercalation compounds. One such compound has the formula KC8, a gold colored solid that is described as a K+ salt of negatively charged graphite. Potassium can reduce many salts to the metal as illustrated by the Rieke method for making magnesium powder from magnesium chloride:
MgCl2 + 2 K → Mg + 2 KCl
Most potassium compounds are ionic. Owing to the high hydration energy of the K+ ion, these salts often exhibit excellent water solubility. The main species in water solution are the aquo complexes [K(H2O)n]+ where n = 6 and 7. Although typically insoluble in organic solvents, potassium salts dissolve in polar organic solvents in the presence of crown ethers and cryptand. These organic ligands envelop K+ ions, giving lipophilic coordination complexes. …
Potassium is formed in supernovae by nucleosynthesis from lighter atoms. Potassium is believed to be created in Type II supernovae via an oxygen-consuming nuclear reaction, probably during the explosive stage of the nova. 40K is also formed in s-process nucleosynthesis and the neon burning process.
Potassium is the 20th most abundant element in the Solar System and the 17th most abundant element by weight in the Earth. It makes up about 2.6% of the weight of the Earth's crust and is the seventh most abundant element in the crust. The potassium concentration in seawater is 0.39 g/L (0.039 wt/v%), about one twenty-seventh the concentration of sodium.
Elemental potassium does not occur in nature because of its high reactivity with water and oxygen.
Orthoclase (potassium feldspar) is a common rock-forming mineral. Granite for example contains 5% potassium, which is well above the average in the Earth's crust. Sylvite (KCl), carnallite (KCl·MgCl2·6H2O), kainite (MgSO4·KCl·3H2O), and langbeinite (MgSO4·K2SO4) are the minerals found in large evaporite deposits worldwide. The deposits often show layers starting with the least soluble at the bottom and the most soluble on top. Deposits of niter (potassium nitrate) are formed by decomposition of organic material in contact with atmosphere, mostly in caves; because of the good water solubility of niter the formation of larger deposits requires special environmental conditions.
Potassium ions are an essential component of plant nutrition and are found in most soil types. They are used as a fertilizer in agriculture, horticulture, and hydroponic culture in the form of chloride (KCl), sulfate (K2SO4), or nitrate (KNO3), representing the 'K' in 'NPK'. Agricultural fertilizers consume 95% of global potassium chemical production, and about 90% of this potassium is supplied as KCl. The potassium content of most plants ranges from 0.5% to 2% of the harvested weight of crops, conventionally expressed as amount of K2O. Modern high-yield agriculture depends upon fertilizers to replace the potassium lost at harvest. Most agricultural fertilizers contain potassium chloride, while potassium sulfate is used for chloride-sensitive crops or crops needing higher sulfur content. The sulfate is produced mostly by decomposition of the complex minerals kainite (MgSO4·KCl·3H2O) and langbeinite (MgSO4·K2SO4). Only a very few fertilizers contain potassium nitrate. In 2005, about 93% of world potassium production was consumed by the fertilizer industry. Furthermore, potassium can play a key role in nutrient cycling by controlling litter composition.
Potassium citrate is used to treat a kidney stone condition called renal tubular acidosis.
Potassium, in the form of potassium chloride is used as a medication to treat and prevent low blood potassium. Low blood potassium may occur due to vomiting, diarrhea, or certain medications. …
There are 25 known isotopes of potassium, three of which occur naturally: 39K (93.3%), 40K (0.0117%), and 41K (6.7%) (by mole fraction). Naturally occurring 40K has a half-life of 1.250×109 years. It decays to stable 40Ar by electron capture or positron emission (11.2%) or to stable 40Ca by beta decay (88.8%). This decay results in a relatively higher concentration of Argon in the atmosphere. The decay of 40K to 40Ar is the basis of a common method for dating rocks. The conventional potassium–argon dating method depends on the assumption that the rocks contained no argon at the time of formation and that all the subsequent radiogenic argon (40Ar) was quantitatively retained. Minerals are dated by measurement of the concentration of potassium and the amount of radiogenic 40Ar that has accumulated. The minerals best suited for dating include biotite, muscovite, metamorphic hornblende, and volcanic feldspar; whole rock samples from volcanic flows and shallow instrusives can also be dated if they are unaltered. Apart from dating, potassium isotopes have been used as tracers in studies of weathering and for nutrient cycling studies because potassium is a macronutrient required for life on Earth.
40K occurs in natural potassium (and thus in some commercial salt substitutes) in sufficient quantity that large bags of those substitutes can be used as a radioactive source for classroom demonstrations. 40K is the radioisotope with the largest abundance in the human body. …
Potassium is crucial for diverse plant species. It has a vital role in maintaining cell sap and internal root pressure required for plant growth. Potassium increases plant metabolism and uptake of carbon dioxide.
Potassium is the eighth or ninth most common element by mass (0.2%) in the human body, so that a 60 kg adult contains a total of about 120 g of potassium. The body has about as much potassium as sulfur and chlorine, and only calcium and phosphorus are more abundant (with the exception of the ubiquitous CHON elements). Potassium ions are present in a wide variety of proteins and enzymes. 98% of the potassium in a human body is inside of individual cells.
Potassium levels influence multiple physiological processes, including
resting cellular-membrane potential and the propagation of action potentials in neuronal, muscular, and cardiac tissue. Due to the electrostatic and chemical properties, K+ ions are larger than Na+ ions, and ion channels and pumps in cell membranes can differentiate between the two ions, actively pumping or passively passing one of the two ions while blocking the other.
hormone secretion and action
vascular tone
systemic blood pressure control
gastrointestinal motility
acid–base homeostasis
glucose and insulin metabolism
mineralocorticoid action
renal concentrating ability
fluid and electrolyte balance
local cortical monoaminergic norepinephrine, serotonin, and dopamine levels, and through them, sleep/wake balance, and spontaneous activity. …