Potassium ( ) is the
chemical element with the symbol
K ( , from
al-qalyah “plant ashes”, cf.
Alkali from the same root),
atomic number 19, and
atomic mass 39.0983. Potassium was first
isolated from
potash. 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
evolved hydrogen.
Potassium in nature occurs only as ionic salt. As such, it is found
dissolved in
seawater, and as part of many
minerals. 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 in
plant cells, and in a mixed diet, it is most highly concentrated in
fruits.
In many respects, potassium and
sodium are
chemically similar, although they have very
different functions in organisms in general, and in animal cells in
particular.
Occurrence
Elemental potassium does not occur in nature because it reacts
violently with water. 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.
History of the free element
Elemental 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.
The structure of potash was not then known, but is now understood
to be mostly potassium carbonate. By heating, the carbonate could
be freed of carbon dioxide, leaving "
caustic potash", so called because it caused
chemical burns in contact with human tissue.
Potassium metal was discovered in 1807 in England by Sir
Humphry Davy, who derived it from caustic
potash (K
OH), by the use of electrolysis
of the molten salt with the newly discovered
voltaic pile. Before the 18th century, no
distinction was made between potassium and sodium. Potassium was
the first metal that was isolated by electrolysis. Davy extracted
sodium by a similar technique, demonstrating the elements to be
different.
Production
Pure potassium metal may be isolated by
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 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.
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 1.250×10
9 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
tracer 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.
Properties
Physical
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The flame-test color for
potassium
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.
Chemical
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 CO
2
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
Potassium cations in the body
Biochemical function
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. 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. 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.
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.
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.
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. 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.
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 in counter flow
to sodium in response to an apparent (but not actual)
Donnan equilibrium, 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. 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,
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.
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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 only one kind of ion 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.
Potassium in the diet and by supplement
Adequate intake
A potassium intake sufficient to support life can generally be
guaranteed by eating a variety of foods, especially plant 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 eating a balanced diet. Foods with high sources of
potassium include
orange juice,
potatoes,
bananas,
avocados,
tomatoes,
broccoli,
soybeans,
brown rice,
garlic
and
apricots, although it is also common in
most
fruits,
vegetables and
meats.
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.
With these findings, the question of what is the intake of
potassium consistent with optimal health, is debated. 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.
Similarly, in the European Union, particularly in Germany
and Italy
,
insufficient potassium intake is somewhat common.
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. 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
About 93% of the world potassium production was consumed by the
fertilizer industry.
Biological applications
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Potassium and magnesium sulfate
fertilizer
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 ( ) or
nitrate ( ).
In
animal cell,
potassium ions are vital to keeping cells alive (see
Na-K pump).
In the form of
potassium
chloride, it is used to stop the heart, e.g. in
cardiac surgery and in a solution used in
executions by
lethal
injection.
Food applications
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 according to
potassium content per measure shown.
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.
Industrial applications
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 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
complex; 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
(K
2CrO
4) 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
(K
2SiF
6) is used in specialized glasses,
ceramics, and
enamel. Potassium sodium tartrate, or
Rochelle salt
(KNaC
4H
4O
6) is used in the
silvering of
mirrors.
The
superoxide KO
2 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 O
2 (g).
4 KO
2 + 2 CO
2 → 2 K
2CO
3
+ 3 O
2
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.
Precautions
Potassium reacts very violently with water producing
hydrogen gas which then usually catches fire.
Potassium is usually kept under a hydrocarbon oil such as mineral
oil or kerosene to stop the metal from 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).
Potassium also produces
potassium
hydroxide (KOH) in the reaction with water.
Potassium hydroxide is a strong
alkali and so is a caustic hazard, causing
burns.
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.
See also
References
- ; for a diagram of the potassium pores are viewed, see
- ; includes a detailed picture of atoms in the pump.
External links