very slightly soluble in water; r.d. 2.9;
m.p. 1000°C. It changes to a cubic
form at 580°C. The compound occurs
naturally as the mineral *cryolite but
a considerable amount is manufac-
tured by the reaction of aluminium
Ûuoride wth alumina and sodium hy-
droxide or directly with sodium alu-
minate. Its most important use is in
the manufacture of aluminium in
the *Hall–Heroult cell. It is also used
in the manufacture of enamels,
opaque glasses, and ceramic glazes.
sodium hydrideA white crys-
talline solid, NaH; cubic; r.d. 0.92; de-
composes above 300°C (slow);
completely decomposed at 800°C.
Sodium hydride is prepared by the
reaction of pure dry hydrogen with
sodium at 350°C. Electrolysis of
sodium hydride in molten LiCl/KCl
leads to the evolution of hydrogen;
this is taken as evidence for the ionic
nature of NaH and the presence of
the hydride ion (H–). It reacts vio-
lently with water to give sodium hy-
droxide and hydrogen, with halogens
to give the halide and appropriate
hydrogen halide, and ignites sponta-
neously with oxygen at 230°C. It is a
powerful reducing agent with several
laboratory applications.
sodium hydrogencarbonate
(bicarbonate of soda; sodium bicar-
bonate)A white crystalline solid,
NaHCO 3 , soluble in water and
slightly soluble in ethanol; mono-
clinic; r.d. 2.159; loses carbon dioxide
above 270°C. It is manufactured in
the *Solvay process and may be pre-
pared in the laboratory by passing
carbon dioxide through sodium car-
bonate or sodium hydroxide solution.
Sodium hydrogencarbonate reacts
with acids to give carbon dioxide
and, as it does not have strongly cor-
rosive or strongly basic properties it-
self, it is employed in bulk for the
treatment of acid spillage and inmedicinal applications as an antacid.
Sodium hydrogencarbonate is also
used in baking powders (and is
known as baking soda), dry-powder
Üre extinguishers, and in the textiles,
tanning, paper, and ceramics indus-
tries. The hydrogencarbonate ion has
an important biological role as an in-
termediate between atmospheric
CO 2 /H 2 CO 3 and the carbonate ion
CO 3 2–. For water-living organisms this
is the most important and in some
cases the only source of carbon.
sodium hydrogensulphate
(sodium bisulphate)A colourless
solid, NaHSO 4 , known in anhydrous
and monohydrate forms. The anhy-
drous solid is triclinic (r.d. 2.435; m.p.
> 315 °C). The monohydrate is mono-
clinic and deliquescent (r.d. 2.103;
m.p. 59°C). Both forms are soluble in
water and slightly soluble in alcohol.
Sodium hydrogensulphate was origi-
nally made by the reaction between
sodium nitrate and sulphuric acid,
hence its old name of nitre cake. It
may be manufactured by the reac-
tion of sodium hydroxide with sul-
phuric acid, or by heating equimolar
proportions of sodium chloride and
concentrated sulphuric acid. Solu-
tions of sodium hydrogensulphate
are acidic. On heating the compound
decomposes (via Na 2 S 2 O 7 ) to give sul-
phur trioxide. It is used in paper
making, glass making, and textile
Ünishing.sodium hydrogensulphite
(sodium bisulphite)A white solid,
NaHSO 3 , which is very soluble in
water (yellow in solution) and
slightly soluble in ethanol; mono-
clinic; r.d. 1.48. It decomposes on
heating to give sodium sulphate, sul-
phur dioxide, and sulphur. It is
formed by saturating a solution of
sodium carbonate with sulphur diox-
ide. The compound is used in the
brewing industry and in the steriliza-sodium hydride 490s