potash alum See aluminium potassium sulphate; alums.
potash mica See muscovite.
potassamide See potassium
monoxide.
potassium Symbol 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 seawater 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. Potassium is an *essential element for living 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 animals 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 impulses. Chemically, it is highly reactive, resembling sodium in its
behaviour and compounds. It also
forms an orange-coloured superoxide, 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 dating A *dating technique for certain rocks that
depends on the decay of the radioisotope potassium–40 to argon–40, a
process with a half-life of about 1.27
× 10
10 years. It assumes that all the
argon–40 formed in the potassiumbearing mineral accumulates within
it and that all the argon present is
formed by the decay of potassium–40. The mass of argon–40 and
potassium–40 in the sample is estimated and the sample is then dated
from the equation:
40 Ar = 0.1102
40 K(e
λt – 1),
where λ is 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 bicarbonate See
potassium hydrogencarbonate.
potassium bichromate See potassium 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 radiation, KBr is used both as a matrix
for solid samples and as a prism material in infrared spectroscopy.
potassium carbonate (pearl ash;
potash) A translucent (granular) or
white (powder) deliquescent solid
known in the anhydrous and hydrated 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 potassium chloride and magnesium
oxide react with carbon dioxide to
give the compound Engel’s salt,
MgCO 3 .KHCO 3 .4H 2 O. This is decomposed in solution to give the hydropotash alum
432
p
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potash mica See muscovite.
potassamide See potassium
monoxide.
potassium Symbol 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 seawater 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. Potassium is an *essential element for living 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 animals 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 impulses. Chemically, it is highly reactive, resembling sodium in its
behaviour and compounds. It also
forms an orange-coloured superoxide, 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 dating A *dating technique for certain rocks that
depends on the decay of the radioisotope potassium–40 to argon–40, a
process with a half-life of about 1.27
× 10
10 years. It assumes that all the
argon–40 formed in the potassiumbearing mineral accumulates within
it and that all the argon present is
formed by the decay of potassium–40. The mass of argon–40 and
potassium–40 in the sample is estimated and the sample is then dated
from the equation:
40 Ar = 0.1102
40 K(e
λt – 1),
where λ is 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 bicarbonate See
potassium hydrogencarbonate.
potassium bichromate See potassium 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 radiation, KBr is used both as a matrix
for solid samples and as a prism material in infrared spectroscopy.
potassium carbonate (pearl ash;
potash) A translucent (granular) or
white (powder) deliquescent solid
known in the anhydrous and hydrated 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 potassium chloride and magnesium
oxide react with carbon dioxide to
give the compound Engel’s salt,
MgCO 3 .KHCO 3 .4H 2 O. This is decomposed in solution to give the hydropotash alum
432
p
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