the slow release of equally toxic hy-
drogen cyanide gas.potassium dichromate (potas-
sium bichromate) An orange-red
crystalline solid, K 2 Cr 2 O 7 , soluble in
water and insoluble in alcohol; mon-
oclinic or triclinic; r.d. 2.68; mono-
clinic changes to triclinic at 241.6°C;
m.p. 396°C; decomposes above
500 °C. It is prepared by acidiÜcation
of crude potassium chromate solu-
tion (the addition of a base to solu-
tions of potassium dichromate
reverses this process). The compound
is used industrially as an oxidizing
agent in the chemical industry and in
dyestuffs manufacture, in electroplat-
ing, pyrotechnics, glass manufacture,
glues, tanning, photography and lith-
ography, and in ceramic products.
Laboratory uses include application
as an analytical reagent and as an ox-
idizng agent. Potassium dichromate
is toxic and considered aÜre risk on
account of its oxidizing properties.potassium dioxideSee potassium
superoxide.potassium hydrideA white or
greyish white crystalline solid, KH;
r.d. 1.43–1.47. It is prepared by pass-
ing hydrogen over heated potassium
and marketed as a light grey powder
dispersed in oil. The solid decom-
poses on heating and in contact with
moisture and is an excellent reduc-
ing agent. Potassium hydride is aÜre
hazard because it produces hydrogen
on reaction with water.potassium hydrogencarbonate
(potassium bicarbonate)A white
crystalline solid, KHCO 3 , soluble in
water and insoluble in ethanol; r.d.
2.17; decomposes about 120°C. It oc-
curs naturally as calciniteand is pre-
pared by passing carbon dioxide into
saturated potassium carbonate solu-
tion. It is used in baking, soft-drinks
manufacture, and in CO 2 Üre extin-guishers. Because of its buffering ca-
pacity, it is added to some detergents
and also used as a laboratory reagent.potassium hydrogentartrate
(cream of tartar) A white crystalline
acid salt, HOOC(CHOH) 2 COOK. It is
obtained from deposits on wine vats
(argol) and used in baking powders.potassium hydroxide (caustic
potash; lye)A white deliquescent
solid, KOH, often sold as pellets,
Ûakes, or sticks, soluble in water and
in ethanol and very slightly soluble
in ether; rhombic; r.d. 2.044; m.p.
360.4°C; b.p. 1320°C. It is prepared
industrially by the electrolysis of con-
centrated potassium chloride solu-
tion but it can also be made by
heating potassium carbonate or sul-
phate with slaked lime, Ca(OH) 2. It
closely resembles sodium hydroxide
but is more soluble and is therefore
preferred as an absorber for carbon
dioxide and sulphur dioxide. It is also
used in the manufacture of soft soap,
other potassium salts, and in Ni–Fe
and alkaline storage cells. Potassium
hydroxide is extremely corrosive to
body tissues and especially damaging
to the eyes.
potassium iodateA white crys-
talline solid, KIO 3 , soluble in water
and insoluble in ethanol; monoclinic;
r.d. 3.9; m.p. 560°C. It may be pre-
pared by the reaction of iodine with
hot concentrated potassium hydrox-
ide or by careful electrolysis of
potassium iodide solution. It is an
oxidizing agent and is used as an
analytical reagent. Some potassium
iodate is used as a food additive.potassium iodideA white crys-
talline solid, KI, with a strong bitter
taste, soluble in water, ethanol, and
acetone; cubic; r.d. 3.13; m.p. 681°C;
b.p. 1330°C. It may be prepared by
the reaction of iodine with hot potas-
sium hydroxide solution followed bypotassium dichromate 434p