GROUPS I AND II 133
Some carbonates are important industrial chemicals. Calcium
carbonate occurs naturally in several forms, including limestone,
and is used in the production of quicklime, calcium oxide CaO,
slaked (or hydrated) lime, calcium hydroxide Ca(OH) 2 and cement.
Several million tons of sodium carbonate are used every year,
almost one third of this being used in glass making and the rest
being used for a variety of purposes including paper manufacture,
chemicals, and as a water softener in soap powder. Sodium sesqui-
carbonate, Na 2 CO 3. NaHCO 3. 2H 2 O, occurs naturally in the US
and approximately 1 000 000 tons of sodium carbonate are pro-
duced from this annually. Until recently almost all the sodium
carbonate required commercially in the UK (5 000 000 tons annu-
ally) was manufactured by the soda-ammonia process but some is
now produced by carbonation of sodium hydroxide, surplus to
requirements, made during the electrolysis of brine :2NaOH + CO 2 -» Na 2 CO 3 + H 2 OThe soda-ammonia process occurs in two main stages. First,
brine is saturated with ammonia gas and this "ammoniacal brine'
is then treated with carbon dioxide. The equilibrium
CO 2 4- 2H 2 O ^HCO 3 ~ + H 3 O +is moved to the right by the competition of the ammonia for protons.
i.e. NH 3 + H 3 O + ?± NH + 4- H 2 O. The ions then present are
NH^. HCOa, Cl~ and Na+ and the least soluble salt sodium
hydrogen carbonate, is precipitated when ionic concentrations
increase, and is removed by vacuum filtration.
When heated, sodium hydrogencarbonate readily decomposes
evolving carbon dioxide, a reaction which leads to its use as baking
powder when the carbon dioxide evolved 'aerates' the dough. In the
soda-ammonia process the carbon dioxide evolved is used to
supplement the main carbon dioxide supply obtained by heating
calcium carbonate :
CaCO 3 -* CaO 4- CO 2
The calcium oxide so produced is slaked to give a suspension of
calcium hydroxide and this is heated with the filtrate from the
carbonator which contains ammonium chloride:
2NH 4 C1 + Ca(OH) 2 -> CaCl 2 + 2NH 3 f + 2H 2 OThe ammonia gas is used again and the only by-product, calcium
chloride, is used to melt snow, prevent freezing of coal in transit and
as an antidust treatment since it is hygroscopic and forms a solution
of low freezing point.