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The Element
Potassium
Potassium is a chemical element in the periodic
table that has the symbol K (Latin, Kalium)
and atomic number 19. This 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, is very reactive,
especially in water, and resembles sodium chemically.
Notable characteristics
Potassium is the second lightest metal. It is a soft solid that
easily is cut with a knife and is silvery in color on fresh surfaces.
It oxidizes in air rapidly and must be stored in mineral oil for
preservation.
Similar to other alkali metals, potassium reacts violently with
water producing hydrogen. When in water it may catch fire spontaneously.
Its salts emit a violet color when exposed to a flame.
Applications
- Potassium oxide, best known as potash, is primarily used in
fertilizer.
- Potassium nitrate is used in gunpowder.
- Potassium carbonate is used in glass manufacture.
- NaK an alloy of sodium and potassium is used as a heat-transfer
medium.
- This element 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.
Many potassium salts are very important, and include, potassium;
bromide, carbonate, chlorate, chloride, chromate, cyanide, dichromate,
hydroxide, iodide, nitrate, sulfate.
History
Potassium (English, potash L. kalium) was discovered in 1807 by
Sir Humphry Davy who derived it from caustic potash (KOH. This alkali
metal was the first metal that was isolated by electrolysis.
Occurrence
This element makes up about 2.4% of the weight of the Earth's crust
and is the seventh most abundant element in it. Due to its insolubility,
it is very difficult to obtain potassium from its minerals.
However, other minerals, 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 California, Germany, New Mexico, Utah, and in other
places around the world. At 3000 ft below the surface of Saskatchewan
are large deposits of potash which may become important sources of
this element and its salts in the future.
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. Potassium is almost
never found unbound in nature. However, in living organisms K+
ions are important in the physiology of excitable cells.
Isotopes
There are seventeen isotopes of potassium known to exist. The non-synthetic
form of potassium are composed of three isotopes: K-39 (93.3%),
K-40 (0.01%) and K-41 (6.7%). Naturally occurring K-40 decays to
stable Ar-40 (11.2%) by electron capture and by positron emission,
and decays to stable Ca-40 (88.8%) by negatron emission; K-40 has
a half-life of 1.250 × 109 years.
The decay of K-40 to Ar-40 is commonly used as a 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., Ar-40) was quantitatively
retained, i.e., closed system. Minerals are dated by measurement
of the concentration of potassium, and the amount of radiogenic
Ar-40 that has accumulated. The minerals that are best suited for
dating include biotite, muscovite, and 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.
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