19
  K  
39.098300
Potassium

Name: Potassium
Symbol: K
Atomic Number: 19
Atomic Weight: 39.098300
Family: Alkali Metals
CAS RN: 7440-09-7
Description: Very soft silvery metal that quickly oxidizes to white in air and reacts violently in water.
State (25C): Solid
Oxidation states: +1

Molar Volume: 45.46 cm3/mole
Valence Electrons: 4s1

Boiling Point:  1032K, 759C, 1398F
Melting Point:
336.5K, 63.35C, 146.03F
Electrons Energy Level: 2, 8, 8, 1
Isotopes: 20 + 2 Stable
Heat of Vaporization: 79.87 kJ/mol
Heat of Fusion: 2.334 kJ/mol
Density: 0.862 g/cm3 @ 300K
Specific Heat: 0.75 J/gK
Atomic Radius: 2.77
Ionic Radius: 1.38
Electronegativity: 0.82 (Pauling); 0.91 (Allrod Rochow)
Vapor Pressure: 0.000106 Pa @ 63.35C
1
H
1.007
Potassium was discovered in 1807 by Sir Humphry Davy, who derived it from Caustic Potash (KOH).  Potassium was the first metal that was isolated by electrolysis and the seventh most abundant element.

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

  • The Latin name kalium was taken from the word "alkali", which came from Arabic al qalja = "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.

Potassium is a soft silvery-white metallic alkali metal that occurs naturally bound to other elements in seawater and many minerals.  It oxidizes so violently with water that it bursts into flame.  In many respects, Potassium and Sodium are chemically similar, although organisms in general, and animal cells in particular, treat them very differently.  Potassium is very soft also reacts rapidly  with Oxygen in air.

Potassium, like Sodium, melts below the boiling point of water (63oC) and is less dense than water also. Like most of the alkali metals, potassium compounds impart a characteristic color to flames.  In the case of the 19th element, the color is pale lavender.

Peroxides of Potassium exhibit a yellow coloration while Ozonides are red.

3
Li
6.941
11
Na
22.98
19
K
39.09
37
Rb
85.46
55
Cs
132.9
87
Fr
223.0
potassium.gif (443 bytes)

Alchemical Symbol, Potassium

1s2 2s2p6 3s2p6 4s1

Characteristics

Although potassium is the eighth most abundant element on earth and comprises about 2.1% of the earth's crust, it is a very reactive element and is never found free in nature. Metallic potassium was first isolated by Sir Humphry Davy in 1807 through the electrolysis of molten caustic potash (KOH). A few months after discovering potassium, Davy used the same method to isolate sodium.  Potassium can be obtained from the minerals sylvite (KCl), carnallite (KClMgCl26H2O), langbeinite (K2Mg2(SO4)3) and polyhalite (K2Ca2Mg(SO4)42H2O). These minerals are often found in ancient lake and sea beds. Caustic potash, another important source of potassium, is primarily mined in Germany, New Mexico, California and Utah.

Pure potassium is a soft, waxy metal that can be easily cut with a knife.  It reacts with oxygen to form potassium superoxide (KO2) and with water to form potassium hydroxide (KOH), hydrogen gas and heat. Enough heat is produced to ignite the hydrogen gas. To prevent it from reacting with the oxygen and water in the air, samples of metallic potassium are usually stored submerged in mineral oil.

Potassium forms an alloy with sodium (NaK) that is used as a heat transfer medium in some types of nuclear reactors.

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 a reducing medium such as kerosene.

1s2
2s2 2p6
3s2 3p6
4s1

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.

2K(s) + 2H2O(l) rarrow.gif (63 bytes) H2(g) + 2KOH(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 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.  The 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.

Potassium concentration in solution is commonly determined by flame photometry, atomic absorption spectrophotometry, inductively coupled plasma, or ion selective electrodes.   Methods of separating Potassium by precipitation, sometimes used for gravimetric analysis, include the use of Sodium Tetraphenyl Boron, Dihydrogen Hexachloroplatinate (IV) Hexahydrate, and Sodium Cobaltinitrite.

potassium1.jpg (1372 bytes) potassium2.jpg (1441 bytes) potassium3.jpg (1287 bytes)
Alchemical Symbols, Potassium

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.  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 Saskatchewan, 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 1960s.  Saskatchewan pioneered the use of freezing of wet sands (the Blairmore formation) in order to drive mine shafts through them. 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.

Applications

Potassium forms many important compounds. Potassium chloride (KCl) is the most common potassium compound. It is used in fertilizers, as a salt substitute and to produce other chemicals. Potassium hydroxide (KOH) is used to make soaps, detergents and drain cleaners. Potassium bicarbonate (KHCO3), also known as pearl ash, is used to make some types of glass and soaps and is obtained commercially as a byproduct of the production of ammonia.  It is used in respiratory equipment and is produced by burning potassium metal in dry air. Potassium nitrate (KNO3), also known as saltpeter or nitre, is used in fertilizers, match heads and pyrotechnics.

2KO2 + H2O + 2CO2 rarrow.gif (63 bytes) 2KHCO3 + O2

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, and Potassium Sulfate.

Compounds

Potassium Bisulfite, KHSO3 Potassium Chromate, K2CrO4
Potassium Fluorosilicate, K2SiF6 Potassium Pyrophosphate, K4P2O7
Potassium Nitrate, KNO3 Potassium Iodide, KI
Potassium Dichromate, K2Cr2O7 Potassium Sulfate, K2SO4
Potassium Chlorate, K2ClO3 Potassium Cyanide, KCN
Potassium Chloride or Sylvite, KCl Potassium Bromide, KBr
Carnallite, KClMgCl26H2O Langbeinite, K2Mg2(SO4)3
Polyhalite (K2Ca2Mg(SO4)42H2O)
Potassium Sodium Tartrate or Rochelle Salt, KNaC4H4O6
Potassium Hydroxide or Caustic Potash, KOH
Potassium Nitrate or Saltpeter or Niter, KNO3
Potassium Carbonate or Potash, K2CO3
Polyhalite, K2Ca2Mg(SO4)42H2O
Potassium Bicarbonate or Pearl Ash, KHCO3

Isotopes

There are 24 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 poitron emission, and decays to stable 40Ca (88.8%) by beta decay, 40K has a half-life of 1.250109 years.

The decay of 40K to 40Ar enables a commonly used method for dating rocks.  The conventiona 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.

atom.gif (700 bytes)

Isotope Atomic Mass Half-Life
K32 32.022  
K33 33.007 < 25 ns
K34 33.998 < 25 ns
K35 34.988 190 ms
K36 35.9813 342 ms
K37 36.9734 1.226 seconds
K38 37.9691 7.636 minutes
K39 38.9637 Stable
K40 39.964 1.277E 9 years
K41 40.9618 Stable
K42 41.9624 12.360 hours
K43 42.9607 22.3 hours
K44 43.9616 22.13 minutes
K45 44.9607 17.3 minutes
K46 45.962 105 seconds
K47 46.9617 17.5 seconds
K48 47.9655 6.8 seconds
K49 48.9675 1.26 seconds
K50 49.973 472 ms
K51 50.976 365 ms
K52 51.983 105 ms
K53 52.987 30 ms
K54 53.994 10 ms
K55 54.99  

Precautions

80px-Flammable.jpg (2186 bytes) 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.

It is recommended that Potassium, Rubidium or Cesium not be stored for longer than three months unless stored in an inert (Oxygen free) atmosphere, or under 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.

Biological Role

Nutrition & Medicine

Potassium is an essential mineral macronutrient 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.  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, 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, apricots, parsnips and turnips, although many other fruits, vegetable, 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 4,000mg 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.

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.

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.


atom.gif (700 bytes)

Potassium Data

 

Atomic Structure

Atomic Radius (): 2.77
Atomic Volume cm3/mol : 45.46cm3/mol
Covalent Radius: 2.03
Crystal Structure: Cubic body centered
Ionic Radius: 1.38

Chemical Properties

Electrochemical Equivalents: 1.4587 g/amp-hr
Electron Work Function: 2.3eV
Electronegativity: 0.82 (Pauling); 0.91 (Allrod Rochow)
Heat of Fusion: 2.334 kJ/mol
Incompatibilities: unknown
First Ionization Potential: 4.341
Second Ionization Potential: 31.625
Third Ionization Potential: 45.72
Valence Electron Potential(-eV): 10.4
Ionization Energy (eV): 4.341 eV

Physical Properties

Atomic Mass Average: 39.0983
Boiling Point: 1032K, 759C, 1398F
Melting Point: 336.5K, 63.35C, 146.03F
Heat of Vaporization: 79.87 kJ/mol
Coefficient of Lineal Thermal Expansion/K-1: 83E-6
Electrical Conductivity: 0.139 106/cm
Thermal Conductivity: 1.024 W/cmK
Density: 0.862 g/cm3 @ 300K
Elastic Modulus (Bulk): 3.1/GPa
Elastic Modulus (Rigidity): 1.3/GPa
Elastic Modulus Youngs: 3.53/GPa
Enthalpy of Atomization: 89.5 kJ/mole @ 25C
Enthalpy of Fusion: 2.32 kJ/mole
Enthalpy of Vaporization: 77.5 kJ/mole
Hardness Scale (Brinell): 0.363 MN m-2
Hardness Scale (Mohs): 0.4
Hardness Scale (Vickers): unknown
Flammability Class: unknown
Molar Volume: 45.46 cm3/mole
Optical Reflectivity: unknown
Optical Refractive Index: unknown
Relative Gas Density (Air=1): unknown
Specific Heat: 0.75 J/gK
Vapor Pressure: 0.000106 Pa @ 63.35C
Estimated Crustal Abundance: 2.09104 milligrams per kilogram
Estimated Oceanic Abundance:
3.99102 milligrams per liter


(English, potash - pot ashes; L.. kalium, Arab qali, alkali) Discovered in 1807 by Davy, who obtained it from caustic potash (KOH); this was the first metal isolated by electrolysis. The metal is the seventh most abundant and makes up about 2.4% by weight of the earth's crust. Most potassium minerals are insoluble and the metal is obtained from them only with great difficulty. Certain minerals, however, such as sylvite, carnallite, langbeinite, and polyhalite are found in ancient lake and sea beds and form rather extensive deposits from which potassium and its salts can readily be obtained. Potash is mined in Germany, New Mexico, California, Utah, and elsewhere. Large deposits of potash, found at a depth of some 3000 ft in Saskatchewan, promise to be important in coming years. Potassium is also found in the ocean, but is present only in relatively small amounts, compared to sodium. The greatest demand for potash has been in its use for fertilizers. Potassium is an essential constituent for plant growth and is found in most soils. Potassium is never found free in nature, but is obtained by electrolysis of the hydroxide, much in the same manner as prepared by Davy. Thermal methods also are commonly used to produce potassium (such as by reduction of potassium compounds with CaC2, C, Si, or Na). It is one of the most reactive and electropositive of metals. Except for iridium, it is the lightest known metal. It is soft, easily cut with a knife, and is silvery in appearance immediately after a fresh surface is exposed. It rapidly oxidizes in air and must be preserved in a mineral oil such as kerosene. As with other metals of the alkali group, it decomposes in water with the evolution of hydrogen. It catches fire spontaneously on water. Potassium and its salts impart a violet color to flames. Seventeen isotopes of potassium are known. Ordinary potassium is composed of three isotopes, one of which is 40K (0.0118%), a radioactive isotope with a half-life of 1.28 x 109 years. The radioactivity presents no appreciable hazard. An alloy of sodium and potassium (NaK) is used as a heat-transfer medium. Many potassium salts are of utmost importance, including the hydroxide, nitrate, carbonate, chloride, chlorate, bromide, iodide, cyanide, sulfate, chromate, and dichromate. Metallic potassium is available commercially for about $40/lb in small quantities.

Source: CRC Handbook of Chemistry and Physics, 1913-1995. David R. Lide, Editor in Chief. Author: C.R. Hammond