Potassium
Potassium (
/pɵˈtæsiəm/ po-TAS-ee-əm) is the chemical element with the symbol K (Neo-Latin kalium), atomic number 19, and atomic mass 39.098. Elemental potassium is a soft silvery-white metallic alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.
Potassium and sodium are alkali metals and are chemically very similar. For this reason, historically their salts were not differentiated. They were finally realized to be different elements when the metals were isolated by electrolysis in the early 19th century. Potassium in nature occurs only as ionic salt. As such, it is found dissolved in seawater (which is 0.04 % potassium by weight), and as part of many minerals. Industrial chemical applications of potassium tend to employ potassium ion's extreme water-solubility as part of chemicals which depend for activity on their non-potassium components. Potassium metal has only a few specialty applications, being replaced in most chemical reactions with sodium metal.
Potassium ion is necessary for the function of all living cells, and is thus present in all plant and animal tissues. It is found in especially high concentrations within plant cells, and in a mixed diet, it is most highly concentrated in fruits. (Dietary potassium, discussed below, should not be confused with Vitamin K).
The high concentration of potassium in plants, associated with comparatively very low amounts of sodium there, historically resulted in potassium first being isolated from the ashes of plants (potash), which in turn gave the element its modern name. Heavy crop production rapidly depletes soils of potassium, and agricultural fertilizers consume 93% of the potassium chemical production of the modern world economy.
The functions of potassium and sodium in living organisms are quite different. Animals, in particular, employ sodium and potassium differentially to generate electrical potentials in animal cells, especially in nervous tissue. Potassium depletion in animals, including humans, results in various neurological dysfunctions.
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Occurrence
- See also Category: Potassium minerals
Elemental potassium does not occur in nature because it reacts violently with water (see section Precautions below). As various compounds, potassium makes up about 1.5% of the weight of the Earth's crust and is the seventh most abundant element. As it is very electropositive and highly reactive potassium metal is difficult to obtain from its minerals.[1]
History
Neither elemental potassium nor potassium salts (as separate entities from other salts) were known in Roman times, and the Latin name of the element is not Classical Latin but rather neo-Latin. The Latin name kalium was taken from the word "alkali", which in turn came from Arabic: القَÙ„ْÙŠَÙ‡ al-qalyah "plant ashes." The similar-sounding English term alkali is from this same root (more commonly known in Modern Standard Arabic as بوتاسيوم ‹bootasyoom›). Potassium was the secret ingredient that was mixed with ash, indigo dye and hot water to produce a deep blue textile by Hausa merchants in Kano.[2]
The English name for the element potassium comes from the word "potash",[3] referring to the method by which potash was obtained (leaching the ash of burnt wood or tree leaves and evaporating the solution in a pot). Potash is primarily a potassium salt, because plants have little or no sodium content, and the rest of a plants' major mineral content consists of calcium salts of low water solubility. While potash has been used since ancient times, it was not understood to be a fundamentally different substance from sodium mineral salts. Its chemical composition, and its status as a compound of a element distinct from sodium, was not actually known until after the application of electrolysis to sodium and potassium hydroxide salts in the early 19th century, by Davy.
Potassium metal was first isolated in 1807 in England by Sir Humphry Davy, who derived it from caustic potash (KOH), by the use of electrolysis of the molten salt with the newly discovered voltaic pile. Potassium was the first metal that was isolated by electrolysis.[4] Later in the same year, Davy reported extraction of the metal sodium from a mineral derivative (caustic soda, NaOH, or lye), not a plant salt, by a similar technique, demonstrating the elements, and thus the salts, to be different.[5]
Creation
 | This section requires expansion. |
The stable form of potassium is created in supernovas via the r-process.[6]
Commercial production
Pure potassium metal may be isolated by electrolysis of its hydroxide in a process that has changed little since Davy.[1] Thermal methods also are employed in potassium production, using potassium chloride.
Potassium salts such as carnallite, langbeinite, polyhalite, and sylvite form extensive deposits in ancient lake and seabeds,[citation needed] making extraction of potassium salts in these environments commercially viable. The principal source of potassium, potash, is mined in Canada, Russia, Belarus, Germany, Israel, United States, Jordan, and other places around the world.[7] Three thousand feet below the surface of the Canadian province of Saskatchewan are the largest discovered deposits of potash on Earth. Saskatchewan, where several large mines were in operation since the 1960s, pioneered the use of freezing of wet sands (the Blairmore formation) in order to drive mine shafts through them. The main potash mining company in Saskatchewan is the Potash Corporation of Saskatchewan.
The oceans are another source of potassium, but the quantity present in a given volume of seawater is much lower than that of sodium.[8][9]
Potassium metal in reagent-grade sells for about $10.00/pound ($22/kg) in 2010 when purchased in tonnage quantities. Lower purity metal sells for considerably less. The market in this metal is volatile due to the difficulty in its long term storage. It must be stored under a dry inert gas atmosphere or anhydrous mineral oil to prevent the formation of a surface layer of potassium superoxide. This superoxide is a pressure sensitive explosive which will detonate when scratched. The resulting explosion will usually start a fire which is difficult to extinguish.[10]
Kilogram quantities of potassium cost far more, in the range of $700/kg. This is partially due to the cost of hazardous material shipping requirements. [11]
Isotopes
There are 24 known isotopes of potassium. Three isotopes occur naturally: 39K (93.3%), 40K (0.0117%) and 41K (6.7%). Naturally occurring 40K decays to stable 40Ar (11.2% of decays) by electron capture or positron emission, or decays to stable 40Ca (88.8% of decays) by beta decay; 40K has a half-life of 1.250×109 years. The decay of 40K to 40Ar enables a commonly used method for dating rocks. The conventional K-Ar dating method depends on the assumption that the rocks contained no argon at the time of formation and that all the subsequent radiogenic argon (i.e., 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 that are best suited for dating include biotite, muscovite, plutonic/high grade metamorphic hornblende, and volcanic feldspar; whole rock samples from volcanic flows and shallow instrusives can also be dated if they are unaltered.
Outside of dating, potassium isotopes have been used extensively as tracers in studies of weathering. They have also been used for nutrient cycling studies because potassium is a macronutrient required for life.
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. In healthy animals and people, 40K represents the largest source of radioactivity, greater even than 14C. In a human body of 70 kg mass, about 4,400 nuclei of 40K decay per second.[12] The activity of natural potassium is 31 Bq/g.
Physical properties
Potassium is the second least dense metal; only lithium is less dense. It is a soft, low-melting solid that can easily be cut with a knife. Freshly cut potassium is silvery in appearance, but in air it begins to tarnish toward grey immediately.[1]
In a flame test, potassium and its compounds emit a lilac color, which may be masked by the strong yellow emission of sodium if it is also present. Cobalt glass can be used to filter out the yellow sodium color.[13] Potassium concentration in solution is commonly determined by flame photometry, atomic absorption spectrophotometry, inductively coupled plasma, or ion selective electrodes.
Chemical properties
Potassium must be protected from air for storage to prevent disintegration of the metal from oxide and hydroxide corrosion. Often samples are maintained under a hydrocarbon medium which does not react with alkali metals, such as mineral oil or kerosene.
Like the other alkali metals, potassium reacts violently with water, producing hydrogen. The reaction is notably more violent than that of lithium or sodium with water, and is sufficiently exothermic that the evolved hydrogen gas ignites.
- 2 K (s) + 2 H2O (l) → H2 (g) + 2 KOH (aq)
Because potassium reacts quickly with even traces of water, and its reaction products are nonvolatile, it is sometimes used alone, or as NaK (an alloy with sodium which is liquid at room temperature) to dry solvents prior to distillation. In this role, it serves as a potent desiccant.
Potassium hydroxide reacts strongly with carbon dioxide to produce potassium carbonate, and is used to remove traces of CO2 from air. Potassium compounds generally have excellent water solubility, due to the high hydration energy of the K+ ion. The potassium ion is colorless in water.
Methods of separating potassium by precipitation, sometimes used for gravimetric analysis, include the use of sodium tetraphenylborate, hexachloroplatinic acid, and sodium cobaltinitrite
Biological role
Biochemical function
Potassium is the eight 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 grams of potassium. The body has about as much potassium as sulfur. Only the major minerals calcium and phosphorus are more abundant, while sodium and chloride are each only about 2/3 of the potassium content.
Potassium cations are important in neuron (brain and nerve) function, and in influencing osmotic balance between cells and the interstitial fluid, with their distribution mediated in all animals (but not in all plants) by the so-called Na+/K+-ATPase pump.[14] This ion pump uses ATP to pump 3 sodium ions out of the cell and 2 potassium ions into the cell, thus creating an electrochemical gradient over the cell membrane. In addition, the highly selective potassium ion channels (which are tetramers) are crucial for the hyperpolarisation, in for example neurons, after an action potential is fired. The most recently resolved potassium ion channel is KirBac3.1, which gives a total of five potassium ion channels (KcsA, KirBac1.1, KirBac3.1, KvAP, MthK) with a determined structure.[15] All five are from prokaryotic species.
Potassium may be detected by taste because it triggers three of the five types of taste sensations, according to concentration. Dilute solutions of potassium ion taste sweet (allowing moderate concentrations in milk and juices), while higher concentrations become increasingly bitter/alkaline, and finally also salty to the taste. The combined bitterness and saltiness of high potassium content solutions makes high-dose potassium supplementation by liquid drinks a palatability challenge.[16]
Membrane polarization
Potassium is also important in preventing muscle contraction and the sending of all nerve impulses in animals through action potentials. By nature of their electrostatic and chemical properties, K+ ions are larger than Na+ ions, and ion channels and pumps in cell membranes can distinguish between the two types of ions, actively pumping or passively allowing one of the two ions to pass, while blocking the other.[17]
A shortage of potassium in body fluids may cause a potentially fatal condition known as hypokalemia, typically resulting from vomiting, diarrhea, and/or increased diuresis. Deficiency symptoms include muscle weakness, paralytic ileus, ECG abnormalities, decreased reflex response and in severe cases respiratory paralysis, alkalosis and cardiac arrhythmia.
Filtration and excretion
Potassium is an essential mineral micronutrient in human nutrition; it is the major cation (positive ion) inside animal cells, and it is thus important in maintaining fluid and electrolyte balance in the body. Sodium makes up most of the cations of blood plasma at a reference range of about 145 milliequivalents per liter (3.345 grams) and potassium makes up most of the cell fluid cations at about 150 milliequivalents per liter (4.8 grams). Plasma is filtered through the glomerulus of the kidneys in enormous amounts, about 180 liters per day.[18] Thus 602 grams of sodium and 33 grams of potassium are filtered each day. All but the 1–10 grams of sodium and the 1–4 grams of potassium likely to be in the diet must be reabsorbed. Sodium must be reabsorbed in such a way as to keep the blood volume exactly right and the osmotic pressure correct; potassium must be reabsorbed in such a way as to keep serum concentration as close as possible to 4.8 milliequivalents (about 0.190 grams) per liter.[19] Sodium pumps in the kidneys must always operate to conserve sodium. Potassium must sometimes be conserved also, but as the amount of potassium in the blood plasma is very small and the pool of potassium in the cells is about thirty times as large, the situation is not so critical for potassium. Since potassium is moved passively[20][21] in counter flow to sodium in response to an apparent (but not actual) Donnan equilibrium,[22] the urine can never sink below the concentration of potassium in serum except sometimes by actively excreting water at the end of the processing. Potassium is secreted twice and reabsorbed three times before the urine reaches the collecting tubules.[23] At that point, it usually has about the same potassium concentration as plasma. If potassium were removed from the diet, there would remain a minimum obligatory kidney excretion of about 200 mg per day when the serum declines to 3.0–3.5 milliequivalents per liter in about one week,[24] and can never be cut off completely. Because it cannot be cut off completely, death will result when the whole body potassium declines to the vicinity of one-half full capacity. At the end of the processing, potassium is secreted one more time if the serum levels are too high.
The potassium moves passively through pores in the cell membrane. When ions move through pumps there is a gate in the pumps on either side of the cell membrane and only one gate can be open at once. As a result, 100 ions are forced through per second. Pores have only one gate, and there only one kind of ion can stream through, at 10 million to 100 million ions per second.[25] The pores require calcium in order to open[26] although it is thought that the calcium works in reverse by blocking at least one of the pores.[27] Carbonyl groups inside the pore on the amino acids mimics the water hydration that takes place in water solution[28] by the nature of the electrostatic charges on four carbonyl groups inside the pore.[29]
In diet
Adequate intake
A potassium intake sufficient to support life can generally be guaranteed by eating a variety of foods. Clear cases of potassium deficiency (as defined by symptoms, signs and a below-normal blood level of the element) are rare in healthy individuals. Foods rich in potassium include parsley, dried apricots, dried milk, chocolate, various nuts (especially almonds and pistachios), potatoes, bamboo shoots, bananas, avocados, soybeans, bran, although it is present in sufficient quantities in most fruits, vegetables, meat and fish. [30]
Optimal intake
Epidemiological studies and studies in animals subject to hypertension indicate that diets high in potassium can reduce the risk of hypertension and possibly stroke (by a mechanism independent of blood pressure), and a potassium deficiency combined with an inadequate thiamine intake has produced heart disease in rats.[31] There is some debate regarding the optimal amount of dietary potassium. For example, the 2004 guidelines of the Institute of Medicine specify a DRI of 4,000 mg of potassium (100 mEq), though most Americans consume only half that amount per day, which would make them formally deficient as regards this particular recommendation.[32] Similarly, in the European Union, particularly in Germany and Italy, insufficient potassium intake is somewhat common.[33] Italian researchers reported in a 2011 meta-analysis that a 1.64 - gram higher daily intake of potassium was associated with a 21 percent lower risk of stroke.[34]
Medical supplementation and disease
Supplements of potassium in medicine are most widely used in conjunction with loop diuretics and thiazides, classes of diuretics which rid the body of sodium and water, but have the side effect of also causing potassium loss in urine. A variety of medical and non-medical supplements are available. Potassium salts such as potassium chloride may be dissolved in water, but the salty/bitter taste of high concentrations of potassium ion make palatable high concentration liquid supplements difficult to formulate.[16] Typical medical supplemental doses range from 10 milliequivalents (400 mg, about equal to a cup of milk or 6 oz. of orange juice) to 20 milliequivalents (800 mg) per dose. Potassium salts are also available in tablets or capsules, which for therapeutic purposes are formulated to allow potassium to leach slowly out of a matrix, as very high concentrations of potassium ion (which might occur next to a solid tablet of potassium chloride) can kill tissue, and cause injury to the gastric or intestinal mucosa. For this reason, non-prescription supplement potassium pills are limited by law in the U.S. to only 99 mg of potassium.
Individuals suffering from kidney diseases may suffer adverse health effects from consuming large quantities of dietary potassium. End stage renal failure patients undergoing therapy by renal dialysis must observe strict dietary limits on potassium intake, as the kidneys control potassium excretion, and buildup of blood concentrations of potassium (hyperkalemia) may trigger fatal cardiac arrhythmia.
Applications
In 2005, about 93% of the world potassium production was consumed by the fertilizer industry.[9]
Biological
Potassium ions are an essential component of plant nutrition and are found in most soil types. Its primary use in agriculture, horticulture and hydroponic culture is as a fertilizer as the chloride (KCl), sulfate (K2SO4) or nitrate (KNO3). Potassium content of most plants typically ranges from 1/2 to 2 percent of the harvested weight of crops, expressed as (K2O), which is the conventional way fertilizer analysis is shown, in the order N, P, K. Modern high yield agriculture removes potassium from soils at a much faster rate than it can be replenished from weathering soil K containing minerals, which may not present in sufficient quantity.
In animal cells, potassium ions are vital to cell function. They participate in the Na-K pump.
In the form of potassium chloride, it is used to stop the heart, e.g. in cardiac surgery and execution by lethal injection.
Food
Potassium ion is a nutrient necessary for human life and health. Potassium chloride is used as a substitute for table salt by those seeking to reduce sodium intake so as to control hypertension. The USDA lists tomato paste, orange juice, beet greens, white beans, potatoes, bananas and many other good dietary sources of potassium, ranked in descending order according to potassium content per measure shown.[35]
Potassium sodium tartrate, or Rochelle salt (KNaC4H4O6) is the main constituent of baking powder. Potassium bromate (KBrO3) is a strong oxidiser, used as a flour improver (E924) to improve dough strength and rise height.
The sulfite compound, potassium bisulfite (KHSO3) is used as a food preservative, for example in wine and beer-making (but not in meats). It is also used to bleach textiles and straw, and in the tanning of leathers.
Industrial
Potassium vapor is used in several types of magnetometers.
An alloy of sodium and potassium, NaK (usually pronounced "nack"), that is liquid at room temperature, is used as a heat-transfer medium. It can also be used as a desiccant for producing dry and air-free solvents.
Potassium metal reacts vigorously with all of the halogens to form the corresponding potassium halides, which are white, water-soluble salts with cubic crystal morphology. Potassium bromide (KBr), potassium iodide (KI) and potassium chloride (KCl) are used in photographic emulsion to make the corresponding photosensitive silver halides.
Potassium hydroxide KOH is a strong base, used in industry to neutralize strong and weak acids and thereby finding uses in pH control and in the manufacture of potassium salts. Potassium hydroxide is also used to saponify fats and oils and in hydrolysis reactions, for example of esters and in industrial cleaners.
Potassium nitrate KNO3 or saltpeter is obtained from natural sources such as guano and evaporites or manufactured by the Haber process and is the oxidant in gunpowder (black powder) and an important agricultural fertilizer. Potassium cyanide KCN is used industrially to dissolve copper and precious metals particularly silver and gold by forming complexes; applications include gold mining, electroplating and electroforming of these metals. It is also used in organic synthesis to make nitriles. Potassium carbonate K2CO3, also known as potash, is used in the manufacture of glass and soap and as a mild desiccant.
Potassium chromate (K2CrO4) is used in inks, dyes, and stains (bright yellowish-red colour), in explosives and fireworks, in safety matches, in the tanning of leather and in fly paper. Potassium fluorosilicate (K2SiF6) is used in specialized glasses, ceramics, and enamels. Potassium sodium tartrate, or Rochelle salt (KNaC4H4O6) is used in the silvering of mirrors.
The superoxide KO2 is an orange-colored solid used as a portable source of oxygen and as a carbon dioxide absorber. It is useful in portable respiration systems. It is widely used in submarines and spacecraft as it takes less volume than O2 (g).
4 KO2 + 2 CO2 → 2 K2CO3 + 3 O2
Potassium chlorate KClO3 is a strong oxidant, used in percussion caps and safety matches and in agriculture as a weedkiller. Glass may be treated with molten potassium nitrate KNO3 to make toughened glass, which is much stronger than regular glass.
Potassium cobaltinitrite K3[Co(NO2)6] is used as artist's pigment under the name of Aureolin or Cobalt yellow.
Precautions
Potassium reacts very violently with water producing potassium hydroxide (KOH) and hydrogen gas.
- 2 K (s) + 2 H2O (l) → 2 KOH (aq) + H2 (g)
This reaction is exothermic and temperature produced is sufficient to ignite the resulting hydrogen. It in turn may explode in the presence of oxygen. Potassium hydroxide is a strong alkali which causes skin burns.
Finely divided potassium will ignite in air at room temperature. The bulk metal will ignite in air if heated. Water makes a potassium fire worse. Because its density is 0.89 g/cm3, burning potassium floats in water, which exposes it to more atmospheric oxygen. The water also produces potentially explosive hydrogen gas. Many common fire extinguishing agents are either ineffective or make a potassium fire worse. Sodium chloride (table salt), sodium carbonate (soda ash), and silicon dioxide (sand) are effective if they are dry. Some Class D dry powder extinguishers designed for metal fires are also effective. These powders deprive the fire of oxygen and cool the potassium metal. Nitrogen or argon are also effective.
Potassium reacts violently in the presence of halogens and will detonate in the presence of bromine. It also reacts explosively with sulfuric acid. During combustion potassium forms peroxides and superoxides. These peroxides may react violently with organics present such as oils. Both peroxides and superoxides may react explosively with metallic potassium.[36]
Since potassium reacts with water vapor present in the air, it is usually stored under anhydrous mineral oil or kerosene. Unlike lithium and sodium, however, potassium should not be stored under oil indefinitely. If stored longer than 6 months to a year, dangerous shock-sensitive peroxides can form on the metal and under the lid of the container, which can detonate upon opening. It is recommended that potassium not be stored for longer than three months unless stored in an inert (oxygen free) atmosphere, or under vacuum.[37]
Due to the highly reactive nature of potassium metal, it must be handled with great care, with full skin and eye protection being used and preferably an explosive resistant barrier between the user and the potassium.
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