Potassium is a soft, silver-white metal. Physically and chemically it resembles the other alkali metals in Group 1 of the periodic table. It is extremely reactive, more so than sodium. It combines so readily with oxygen that it is usually stored submerged in kerosene or some other hydrocarbon, out of contact with air. It reacts violently with water to form potassium hydroxide, KOH, releasing hydrogen, which usually ignites. It combines directly with the halogens, sulfur, and other nonmetallic elements (except nitrogen). It reacts with many organic compounds.
The metal has limited use since it so closely resembles sodium, which is readily available at lower cost. Nonetheless, potassium compounds are widely used in industry, although they are usually more expensive than the similar sodium compound. Potassium carbonate, or potash, K2CO3, is used principally in soap and glass manufacture. The chloride, KCl, is used in fertilizers and in the production of other potassium compounds. The chlorate, KClO3, and perchlorate, KClO4, are used in explosives and fireworks. The hydroxide, or caustic potash, KOH, is used in soaps. The nitrate, saltpeter (or niter), KNO3, is used in matches and explosives. Other commercially useful compounds include the bromide, KBr, the cyanide, KCN, the chromate, K2CrO4, the dichromate, K2Cr2O7, and the iodide, KI.
Javelle water contains potassium hypochlorite, KClO, a compound found only in solution. The metasilicate, K2SiO3, is used in water glass. Potassium has several useful tartaric acid salts, e.g., Rochelle salt (sodium potassium tartrate), tartar (argol) and cream of tartar (potassium hydrogen tartrate), and tartar emetic (potassium antimony tartrate). Potassium aluminum sulfate, KAl(SO4)2·12H2O, is a compound used in tanning, in water purification, and in baking powder; usually called alum, it is also called potash alum to distinguish it from other alkali aluminum sulfates. Potassium permanganate, KMnO4, a purple-black, crystalline compound that forms deep purple, aqueous solutions, is used in the chemical laboratory as a powerful oxidizing agent and in medicine as an antiseptic and disinfectant.
With sodium the metal forms alloys that are liquid at room temperature; these alloys are sometimes used in chemical reactions. Substances containing potassium impart a purple color to a flame. Potassium does not occur uncombined in nature but is found widely distributed in sylvite (KCl), carnallite (MgCl2·KCl), feldspar, mica, and other minerals. It is the seventh most abundant element in the earth's crust and the sixth most abundant of the elements in solution in the oceans. It is found in mineral waters, brines, and salt deposits. Potassium is an essential nutrient for plants and animals.
Potassium metal is produced commercially by a thermochemical process in which molten potassium chloride is reacted with sodium vapor; this method is also used to produce liquid sodium-potassium alloys. The metal may be produced electrolytically from fused potassium hydroxide, but, unlike sodium and lithium, it reacts with carbon electrodes and may form explosive compounds. Potassium was discovered in 1807 by Humphry Davy, who decomposed potash with an electric current. Potassium was the first metal so discovered; Davy discovered sodium a few days later by a similar experiment.
Potassium metal is never found free, as it reacts violently with the abundant water in nature. 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, potassium metal is difficult to obtain from its minerals. Potassium salts such as carnallite, langbeinite, polyhalite, and sylvite form extensive deposits in ancient lake and seabeds, making extraction of potassium salts in these environments commercially viable. The principal source of potassium, potash, is mined in Saskatchewan, California, Germany, New Mexico, Utah, and in other places around the world. It is also found abundantly in the Dead Sea. Three thousand feet below the surface of Saskatchewan are large deposits of potash which are important sources of this element and its salts, with several large mines in operation since the 1960s. Saskatchewan pioneered the use of freezing of wet sands (the Blairmore formation) in order to drive mine shafts through them. The main mining company is the Potash Corporation of Saskatchewan. The oceans are another source of potassium, but the quantity present in a given volume of seawater is relatively low compared with sodium.
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.
The activity of natural potassium is 31 Bq/g.
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.
In a flame test, potassium and its compounds emit a pale violet 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. Potassium concentration in solution is commonly determined by flame photometry, atomic absorption spectrophotometry, inductively coupled plasma, or ion selective electrodes.
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.
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.
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.
Potassium is also important in allowing 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.
A shortage of potassium in body fluids may cause a potentially fatal condition known as hypokalemia, typically resulting from diarrhea, increased diuresis and vomiting. Deficiency symptoms include muscle weakness, paralytic ileus, ECG abnormalities, decreased reflex response and in severe cases respiratory paralysis, alkalosis and cardiac arrhythmia.
The potassium moves passively through pores in the cell wall. When ions move through pumps there is a gate in the pumps on either side of the cell wall 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 one kind of ion only can stream through at 10 million to 100 million ions per second. The pores require calcium in order to open although it is thought that the calcium works in reverse by blocking at least one of the pores. Carbonyl groups inside the pore on the amino acids mimics the water hydration that takes place in water solution by the nature of the electrostatic charges on four carbonyl groups inside the pore.
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 supplements are available. If potassium supplements are used, such as sodium free baking powder and sodium free table salt, inadequate thiamine can cause beriberi.
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, since the kidneys control potassium excretion, and buildup of blood concentrations of potassium may trigger fatal cardiac arrhythmia. Acute hyperkalemia can be reduced through eating baking soda, or glucose, hyperventilation and perspiration.
Potassium sodium tartrate, or Rochelle salt is the main constituent of baking powder. Potassium bromate is a strong oxidiser, used as a flour improver (E924) to improve dough strength and rise height.
The sulfite compound, potassium bisulfite 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.
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 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 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 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 , also known as potash, is used in the manufacture of glass and soap and as a mild desiccant.
Potassium chromate (K2CrO4) is used in 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).
4KO2 + 2CO2 --- 2K2CO3 + O2
Potassium chlorate 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 to make toughened glass, which is much stronger than regular glass.
Potassium reacts very violently with water producing hydrogen gas which then usually catches fire. Potassium is usually kept under a mineral oil such as kerosene to stop the metal reacting with water vapour present in the air. 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, rubidium or caesium not be stored for longer than three months unless stored in an inert (oxygen free) atmosphere, or under vacuum.
As potassium reacts with water to produce highly flammable hydrogen gas, a potassium fire is only exacerbated by the addition of water, and only a few dry chemicals are effective for putting out such a fire (see the precaution section in sodium).
Due to the highly reactive nature of potassium, it should be handled with great care, with full skin and eye protection being used and preferably a explosive resistant barrier between the user and the source of the potassium.