Potassium

Potassium is a chemical element. 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 towards water. In many respects, potassium and sodium are chemically similar, although organisms in general, and animal cells in particular, treat them very differently.

Notable characteristics

The flame test

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 in air it begins to tarnish toward gray visibly and immediately. Potassium must be protected from air for storage to prevent disintegration of the metal from oxide and hydroxide corrosion. 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.

K + H₂O → ½ H₂ + KOH

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 is an essential element for all living organisms.

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.

Potassium compounds generally have excellent water solubility, due to the high hydration energy of the K⁺ ion. Potassium ion is colorless in water.

Potassium may be detected by taste because it triggers all the types of tastebuds, 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.

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, pronounced "nack", an alloy of sodium and potassium which 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 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 in feldspar

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. It is never found free in nature.

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.

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 than ¹⁴C. In a human body of 70 kg mass, about 4,400 nuclei of ⁴⁰K decay per second. [1]

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, which can detonate upon opening. DO NOT store potassium, rubidium or caesium for longer than a year unless stored in an inert (argon) atmosphere or in a vacuum.

The extremely alkaline potassium hydroxide (KOH) residue on the surface of potassium which has been exposed to moisture, is a caustic hazard. As with sodium metal, the "soapy" feel of potassium metal on skin is due to caustic breakdown of the fats in skin into crude soft potassium soap, and represents the beginning of an alkali burn. Potassium should obviously be handled only with careful skin and eye protection.

Potassium fires are exacerbated by water, and only a few dry chemicals are effective for them. For a fire discussion which applies to alkali metals in general, see the precaution section for sodium.

Potassium in nutrition and medicine

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.

Potassium is also important in allowing muscle contraction and the sending of all nerve impulses in animals. See action potential for an explanation of the interplay of sodium and potassium in all excitable animal cells. Because of the interaction of the charge on a potassium ion and its surrounding water molecules, K⁺ ions are actually a little smaller than Na⁺ ions, and ion channels and pumps in cell membranes can easily 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 (see article for detail), 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 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 orange juice, potatoes, bananas, avocados, 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 ([2]). Similarly, in the European Union, particularly in Germany and Italy, insufficient potassium intake is widespread ([3]).

Supplements of potassium in medicine are most widely used in conjunction with the most powerful 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.

Some people with kidney disease are advised to avoid 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 heart dysrhythmias.

References

Los Alamos National Laboratory – Potassium

MXO

External links

WebElements.com – Potassium