Physical
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. 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.
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) → 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 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 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.
Biochemical
Potassium is important in nerve function and in influencing osmotic balance
between cells and the interstitiual fluid.
Potassium may be detected by taste because it triggers three of the five
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.
Occurence
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. It is never found free in nature, as it
reacts violently with water. 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 principle 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. 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 with 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
Agriculture and health
- It is primarily used in fertilizers as either the chloride, sulfate or
carbonate - not as the oxide.
- 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 a solution.
- Potassium bisulfite (KHSO3) is used as a food preservative (but
not in meats), bleaching of textiles and straw, wine and beer-making and in the
tanning of leathers.
- Potassium bromate (KBrO3) is used as a flour improver (E924).
- Potassium sodium tartrate, or Rochelle salt
(KNaC4H4O6) is used in baking powder and
medicine.
- Potassium pyrophosphate (K4P2O7) is used in
soaps and detergents.
- Potassium fluorosilicate (K2SiF6) is used as an
insecticide.
Industrial
- 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 manufacturing.
- Glass treated with liquid potassium is much stronger than regular glass.
- Potassium vapor is used in several types of magnetometers.
- NaK (usually 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.
- The superoxide KO2 is used as a portable source of oxygen and as
a carbon dioxide absorber. It is useful in portable respiration systems.
- Potassium bromide (KBr) is used in photographic film and in engraving.
- 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.
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 London England by Sir Humphry Davy. In 1807 he
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.
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%) 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 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.
Precaution
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 caesium 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 with care,
with full skin and eye protection.
Potassium fires are exacerbated by water, and only a few dry chemicals are
effective for them. Potassium has also been discovered to react violently with
iodine.
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