which is soluble in water and in
ethanol but insoluble in diethyl
ether; cubic; r.d. 1.80; m.p. 840°C. It
is made industrially by reducing
potassium sulphate with carbon at
high temperatures in the absence of
air. In the laboratory it may be pre-
pared by the reaction of hydrogen
sulphide with potassium hydroxide.
The pentahydrate is obtained on crys-
tallization. Solutions are strongly al-
kaline due to hydrolysis. It is used as
an analytical reagent and as a depila-
tory. Potassium sulphide is generally
regarded as a hazardous chemical
with aÜre risk; dusts of K 2 S have
been known to explode.
potassium sulphiteA white crys-
talline solid, K 2 SO 3 , soluble in water
and very sparingly soluble in
ethanol; r.d. 1.51; decomposes on
heating. It is a reducing agent and is
used as such in photography and in
the food and brewing industries,
where it prevents oxidation.potassium superoxide (potassium
dioxide)A yellow paramagnetic
solid, KO 2 , produced by burning
potassium in an excess of oxygen; it
is very soluble (by reaction) in water,
soluble in ethanol, and slightly solu-
ble in diethyl ether; m.p. 380°C.
When treated with cold water or di-
lute mineral acids, hydrogen perox-
ide is obtained. The compound is a
powerful oxidizing agent and on
strong heating releases oxygen with
the formation of the monoxide, K 2 O.
potential barrierA region con-
taining a maximum of potential that
prevents a particle on one side of it
from passing to the other side. Ac-
cording to classical theory a particle
must possess energy in excess of the
height of the potential barrier to pass
it. However, in quantum theory there
is aÜnite probability that a particle
with less energy will pass through
the barrier (see tunnel effect). A po-tential barrier surrounds the atomic
nucleus and is important in nuclear
physics; a similar but much lower
barrier exists at the interface be-
tween semiconductors and metals
and between differently doped semi-
conductors. These barriers are impor-
tant in the design of electronic
devices.
potential energySee energy.potential-energy curveA graph
of the potential energy of electrons
in a diatomic molecule, in which the
potential energy of an electronic
state is plotted vertically and the in-
teratomic distance is plotted horizon-
tally, with the minimum of the curve
being the average internuclear dis-
tance. The *Morse potential gives a
good analytic description of a poten-
tial-energy curve. The dissociation
energy and certain quantities of in-
terest in vibrational spectroscopy can
be found from the potential-energy
curve.potential-energy surfaceA mul-
tidimensional surface used in the
theory of electronic states of poly-
atomic molecules and chemical reac-
tions. A *potential-energy curve is
the simplest type of potential-energy
surface. Each internuclear distance in
the molecule or reacting system is
represented by one dimension, as is
the potential energy. This means that
if a nonlinear molecule has N atoms
its potential-energy surface is a
(3N– 6)-dimensional surface in a
(3N– 5)-dimensional space. In the
case of a linear molecule the poten-
tial-energy surface is a (3N– 5)-dimen-
sional space. If the surface has a
minimum then that electronic state
is stable. It is possible for there to be
more than one minimum. In poten-
tial-energy surfaces for chemical re-
actions the reactants and products
are frequently ‘valleys’ connected by
a region of higher energy, which ispotassium sulphite 436p