Potassium

Related Topics:
Potassium :: Potassium-Argon_Dating

Potassium is the chemical element in the periodic table. It has the symbol K (L. kalium) and atomic number 19. The name "potassium" comes from the word "potash", as potassium was first isolated from potash. Potassium is a soft silvery-white metallic alkali metal that occurs naturally bound to other elements in seawater and many minerals. It oxidizes rapidly in air and is very reactive, especially toward water. In many respects, potassium and sodium are chemically similar.

Notable characteristics

With a density less than that of water, potassium is the second least dense metal after lithium. It is a soft, low-melting solid that can easily be cut with a knife. Freshly cut potassium is silvery in appearance but it quickly tarnishes. It oxidizes in air rapidly and thus should be protected for storage. Often samples are maintained under an inert medium such as kerosene.

Similar to other alkali metals but more so than Li and Na, potassium reacts violently with water producing hydrogen. The reaction is sufficiently exothermic that the evolved hydrogen gas ignites. Potassium and its compounds emit a violet color in a flame. This fact is the basis of the flame test for the presence of potassium in a sample.

Applications

It is primarily used in fertilizer as either the chloride, sulfate or carbonate - not as the oxide.
Potassium hydroxide is an important industrial chemical used as a strong base.
Potassium nitrate is used in gunpowder (black powder). An older term for KNO₃ is saltpeter.
Potassium carbonate, known as potash, is used in glass manufacture.
Glass treated with liquid potassium is much stronger than regular glass.
NaK, an alloy of sodium and potassium, is used as a heat-transfer medium.
Potassium is an essential component needed in plant growth and is found in most soil types.
In animal cells potassium ions are vital to keeping cells alive (see Na-K pump)
Potassium chloride is used as a substitute for table salt and is also used to stop the heart, e.g. in cardiac surgery and in executions by lethal injection in solution.
The superoxide KO₂ is used as a portable source of oxygen and as a carbon dioxide absorber. It is useful in portable respiration systems.

Many potassium salts are very important, and include: potassium bromide, potassium carbonate, potassium chlorate, potassium chloride, potassium chromate, potassium cyanide, potassium dichromate, potassium iodide, potassium nitrate, potassium sulfate.

History

Potassium was discovered in 1807 by Sir Humphrey Davy, who derived it from caustic potash (KOH). Potassium was the first metal that was isolated by electrolysis.

Potassium was not known in Roman times, and its names are not Classical Latin but rather neo-Latin.

The name kalium was taken from the word "alkali", which came from Arabic al qalīy = "the calcined ashes".
The name potassium was made from the word "potash", which is English, and originally meant an alkali extracted in a pot from the ash of burnt wood or tree leaves.

Occurrence

Potassium makes up about 2.4% of the weight of the Earth's crust and is the seventh most abundant element in it. As it is very electropositive, potassium metal is difficult to obtain from its minerals.

Potassium salts such as carnallite, langbeinite, polyhalite, and sylvite are found in ancient lake and sea beds. These minerals form extensive deposits in these environments, making extracting potassium and its salts more economical. The principal source of potassium, potash, is mined in California, Germany, New Mexico, Utah, and in other places around the world. 3000 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 1960's. Saskatchewan pioneered the use of freezing of wet sands (the Blairmore formation) in order to drive mine shafts through them. See 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 to sodium.

Potassium can be isolated through electrolysis of its hydroxide in a process that has changed little since Davy. Thermal methods also are employed in potassium production, using potassium chloride. Potassium is never found as the free element in nature.

See also Potassium minerals.

Isotopes

There are seventeen known isotopes of potassium. Three isotopes occur naturally: ³⁹K (93.3%), ⁴⁰K (0.012%) and ⁴¹K (6.7%). Naturally occurring ⁴⁰K decays to stable ⁴⁰Ar (11.2%) by electron capture and by positron emission, and decays to stable ⁴⁰Ca (88.8%) 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 be used for nutrient cycling studies because potassium is a macro-nutrient 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 then ¹⁴C. In a human body of 70 kg mass, about 4,000 nuclei of ⁴⁰K decay per second. ^*

Precautions

Solid potassium reacts violently with water, and should therefore be kept under a mineral oil such as kerosene and handled with care. Unlike lithium and sodium however, potassium cannot 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 that can detonate upon opening. DO NOT store potassium, rubidium or cesium for longer than a year unless stored in an inert (argon) atmosphere or in a vacuum.

Potassium in diet

Potassium is an essential mineral micronutrient in human nutrition; it assists in muscle contraction and in maintaining fluid and electrolyte balance in body cells. Potassium is also important in sending nerve impulses as well as releasing energy from protein, fat, and carbohydrates during metabolism.

A shortage of potassium can 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 form respiratory paralysis and alkalosis and arrhythmia.

Eating a variety of foods that contain potassium is the best way to get an adequate amount. Healthy individuals who eat a balanced diet rarely need supplements. Foods with high sources of potassium include bananas, orange juice, avocados, celery, parsnips and turnips, although many other fruits, vegetables, and meats contain potassium. Research has indicated that diets high in potassium can reduce the risk of hypertension.

The 2004 guidelines of the Institute of Medicine specify an RDA of 4,700 mg of potassium. However, it is thought that most Americans consume only half that amount per day (Similarly, in the European Union, particularly in Germany and Italy, insufficient potassium intake is widespread ([http://content.karger.com/ProdukteDB/produkte.asp?Aktion=ShowPDF&ProduktNr=223977&Ausgabe=230671&ArtikelNr=83312&filename=83312.pdf).

Some people with kidney disease are advised to avoid large quantities of dietary potassium.