|
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
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.
|
