separation from the iodate (which is
also formed) by fractional crystalliza-
tion. In solution it has the interesting
property of dissolving iodine to form
the triiodide ion I 3 – , which is brown.
Potassium iodide is widely used as an
analytical reagent, in photography,
and also as an additive to table salt to
prevent goitre and other disorders
due to iodine deÜciency.
potassium manganate(VII)
(potassium permanganate)A com-
pound, KMnO 4 , forming purple crys-
tals with a metallic sheen, soluble in
water (intense purple solution), ace-
tone, and methanol, but decomposed
by ethanol; r.d. 2.70; decomposition
begins slightly above 100°C and is
complete at 240°C. The compound is
prepared by fusing manganese(IV)
oxide with potassium hydroxide to
form the manganate and elec-
trolysing the manganate solution
using iron electrodes at about 60°C.
An alternative route employs produc-
tion of sodium manganate by a simi-
lar fusion process, oxidation with
chlorine and sulphuric acid, then
treatment with potassium chloride to
crystallize the required product.
Potassium manganate(VII) is widely
used as an oxidizing agent and as a
disinfectant in a variety of applica-
tions, and as an analytical reagent.
potassium monoxideA grey crys-
talline solid, K 2 O; cubic; r.d. 2.32; de-
composition occurs at 350°C. It may
be prepared by the oxidation of
potassium metal with potassium ni-
trate. It reacts with ethanol to form
potassium ethoxide (KOC 2 H 5 ), and
with liquid ammonia to form potas-
sium hydroxide and potassamide
(KNH 2 ).
potassium nitrate (saltpetre) A
colourless rhombohedral or trigonal
solid, KNO 3 , soluble in water, insolu-
ble in alcohol; r.d. 2.109; transition to
trigonal form at 129°C; m.p. 334°C;
decomposes at 400°C. It occurs natu-
rally as nitre and may be prepared by
the reaction of sodium nitrate with
potassium chloride followed by frac-
tional crystallization. It is a powerful
oxidizing agent (releases oxygen on
heating) and is used in gunpowder
and fertilizers.potassium nitriteA white or
slightly yellow deliquescent solid,
KNO 2 , soluble in water and insoluble
in ethanol; r.d. 1.91; m.p. 440°C; may
explode at 600°C. Potassium nitrite is
prepared by the reduction of potas-
sium nitrate. It reacts with cold di-
lute mineral acids to give nitrous
acid and is also able to behave as a
reducing agent (if oxidized to the ni-
trate) or as an oxidizing agent (if re-
duced to nitrogen). It is used in
organic synthesis because of its part
in diazotization, and in detecting the
presence of the amino groups in or-
ganic compounds.
potassium permanganateSee
potassium manganate(vii).
potassium sulphateA white crys-
talline powder, K 2 SO 4 , soluble in
water and insoluble in ethanol;
rhombic or hexagonal; r.d. 2.66; m.p.
1069 °C. It occurs naturally as
schönite(Strassfurt deposits) and in
lake brines, from which it is sepa-
rated by fractional crystallization. It
has also been produced by the Har-
greaves process, which involves the
oxidation of potassium chloride with
sulphuric acid. In the laboratory it
may be obtained by the reaction of
either potassium hydroxide or potas-
sium carbonate with sulphuric acid.
Potassium sulphate is used in ce-
ments, in glass manufacture, as a
food additive, and as a fertilizer
(source of K+) for chloride-sensitive
plants, such as tobacco and citrus.
potassium sulphideA yellow-red
or brown-red deliquescent solid, K 2 S,435 potassium sulphide
p