potash alumSee aluminium potas-
sium sulphate; alums.potash micaSee muscovite.
potassamideSee potassium
monoxide.
potassiumSymbol K. A soft silvery
metallic element belonging to group
1 (formerly IA) of the periodic table
(see alkali metals); a.n. 19; r.a.m.
39.098; r.d. 0.86; m.p. 63.7°C; b.p.
774 °C. The element occurs in seawa-
ter and in a number of minerals,
such as sylvite (KCl), carnallite
(KCl.MgCl 2 .6H 2 O), and kainite
(MgSO 4 .KCl.3H 2 O). It is obtained by
electrolysis. The metal has few uses
but potassium salts are used for a
wide range of applications. Potas-
sium is an *essential element for liv-
ing organisms. The potassium ion,
K+, is the most abundant cation in
plant tissues, being absorbed through
the roots and being used in such
processes as protein synthesis. In ani-
mals the passage of potassium and
sodium ions across the nerve-cell
membrane is responsible for the
changes of electrical potential that
accompany the transmission of im-
pulses. Chemically, it is highly reac-
tive, resembling sodium in its
behaviour and compounds. It also
forms an orange-coloured superox-
ide, KO 2 , which contains the O 2 – ion.
Potassium was discovered by Sir
Humphry *Davy in 1807.
A- Information from the WebElements site
potassium–argon datingA *dat-
ing technique for certain rocks that
depends on the decay of the radioiso-
tope potassium–40 to argon–40, a
process with a half-life of about 1.27
× 1010 years. It assumes that all the
argon–40 formed in the potassium-
bearing mineral accumulates within
it and that all the argon present is
formed by the decay of potas-sium–40. The mass of argon–40 and
potassium–40 in the sample is esti-
mated and the sample is then dated
from the equation:(^40) Ar = 0.1102 (^40) K(eλt– 1),
where λis the decay constant and t is
the time in years since the mineral
cooled to about 300°C, when the^40 Ar
became trapped in the crystal lattice.
The method is effective for micas,
feldspar, and some other minerals.
potassium bicarbonateSee
potassium hydrogencarbonate.
potassium bichromate See potas-
sium dichromate.
potassium bromide A white or
colourless crystalline solid, KBr,
slightly hygroscopic and soluble in
water and very slightly soluble in
ethanol; cubic; r.d. 2.75; m.p. 734°C;
b.p. 1435°C. Potassium bromide may
be prepared by the action of bromine
on hot potassium hydroxide solution
or by the action of iron(III) bromide
or hydrogen bromide on potassium
carbonate solution. It is used widely
in the photographic industry and is
also used as a sedative. Because of its
range of transparency to infrared ra-
diation, KBr is used both as a matrix
for solid samples and as a prism ma-
terial in infrared spectroscopy.
potassium carbonate (pearl ash;
potash)A translucent (granular) or
white (powder) deliquescent solid
known in the anhydrous and hy-
drated forms. K 2 CO 3 (monoclinic; r.d.
2.4; m.p. 891°C) decomposes without
boiling. 2K 2 CO 3 .3H 2 O (monoclinic;
r.d. 2.04) dehydrates to K 2 CO 3 .H 2 O
above 100°C and to K 2 CO 3 above
130 °C. It is prepared by the Engel–
Precht process in which potas-
sium chloride and magnesium
oxide react with carbon dioxide to
give the compound Engel’s salt,
MgCO 3 .KHCO 3 .4H 2 O. This is decom-
posed in solution to give the hydro-
potash alum 432
p