count of its reducing properties. It
has also been used for the production
of sodium thiosulphate (for the pho-
tographic industry) and as a depila-
tory agent in leather preparation. It
is a strong skin irritant.
sodium sulphiteA white solid,
Na 2 SO 3 , existing in an anhydrous
form (r.d. 2.63) and as a heptahydrate
(r.d. 1.59). Sodium sulphite is soluble
in water and because it is readily oxi-
dized it is widely used as a conve-
nient reducing agent. It is prepared
by reacting sulphur dioxide with ei-
ther sodium carbonate or sodium hy-
droxide. Dilute mineral acids reverse
this process and release sulphur diox-
ide. Sodium sulphite is used as a
bleaching agent in textiles and in
paper manufacture. Its use as an an-
tioxidant in some canned foodstuffs
gives rise to a slightly sulphurous
smell immediately on opening, but
its use is prohibited in meats or foods
that contain vitamin B 1. Sodium sul-
phite solutions are occasionally used
as biological preservatives.
sodium–sulphur cell A type of
*secondary cell that has molten elec-
trodes of sodium and sulphur sepa-
rated by a solid electrolyte consisting
of beta alumina (a crystalline form of
aluminium oxide). When the cell is
producing current, sodium ionsÛow
through the alumina to the sulphur,
where they form sodium polysul-
phide. Electrons from the sodium
Ûow in the external circuit. The op-
posite process takes place during
charging of the cell. Sodium–sulphur
batteries have been considered for
use in electric vehicles because of
their high peak power levels and rel-
atively low weight. However, some of
the output has to be used to main-
tain the operating temperature
(about 370°C) and the cost of sodium
is high.
sodium superoxide (sodium diox-
ide)A whitish-yellow solid, NaO 2 ,
formed by the reaction of sodium
peroxide with excess oxygen at el-
evated temperatures and pressures. It
reacts with water to form hydrogen
peroxide and oxygen.
sodium thiosulphate (hypo)A
colourless efÛorescent solid, Na 2 S 2 O 3 ,
soluble in water but insoluble in
ethanol, commonly encountered as
the pentahydrate (monoclinic; r.d.
1.73; m.p. 42°C), which loses water at
100 °C to give the anhydrous form
(r.d. 1.66). It is prepared by the reac-
tion of sulphur dioxide with a sus-
pension of sulphur in boiling sodium
hydroxide solution. Aqueous solu-
tions of sodium thiosulphate are
readily oxidized in the presence of
air to sodium tetrathionate and
sodium sulphate. The reaction with
dilute acids gives sulphur and sul-
phur dioxide. It is used in the photo-
graphic industry and in analytical
chemistry.
soft acidSee hsab principle.soft baseSee hsab principle.
soft soapSee soap.soft waterSee hardness of water.solA *colloid in which small solid
particles are dispersed in a liquid
continuous phase.
solderAn alloy used to join metal
surfaces. A soft solder melts at a tem-
perature in the range 200–300°C and
consists of a tin–lead alloy. The tin
content varies between 80% for the
lower end of the melting range and
31% for the higher end. Hard solders
contain substantial quantities of sil-
ver in the alloy. Brazing solders are
usually alloys of copper and zinc,
which melt at over 800°C.solidA state of matter in which
there is a three-dimensional regular-
ity of structure, resulting from the493 solid
s