Synthetic Inorganic Chemistry

180 ALKALI AND ALKALINE EARTH METALS

Much ingenuity has been exercised by chemists in attempts to
effect this change through a series of reactions. In the older
Le Blanc soda process sulphuric acid is used in the first step and
coal (carbon) in another, and neither of these auxiliary substances
is recovered. The soda process which is employed exclusively
today is known as the Solvay or ammonia process and uses am-
monia as an auxiliary substance. The ammonia is almost 100
per cent recoverable, however, and can be used over and over
indefinitely. The successive steps in the process are as follows:

1. Heating limestone in kilns to obtain quicklime and carbon
   dioxide:
   CaCO 3 -» CaO + CO 2


2. Passing carbon dioxide and ammonia into saturated brine
   in an absorption tower:
   NH 3 + CO 2 + H 2 O + NaCl^ NH4CI + NaHCO 3 j


3. Collecting the precipitated NaHCO 3.


4. Recovery of ammonia by treating the ammonium chloride
   filtrate with quicklime, expelling the ammonia gas to be used
   again in step 2, and working up the calcium chloride left behind:
   2NH 4 C1 + Ca(OH) 2 -» CaCl 2 + 2NH 3 + 2H 2 O


5. Heating the sodium bicarbonate to obtain sodium carbonate,
   the carbon dioxide expelled supplying one-half of the amount
   needed in step 2:
   2NaHCO 3 -» Na 2 CO 3 + CO 2 + H 2 O
   This preparation stresses particularly the principle involved
   in step 2. Carbon dioxide produces no precipitate in a neutral
   sodium chloride solution, the ionization of H 2 CO 3 producing too
   CO 2 + H 2 O ^ H 2 CO 3 ^ H+ HC(V
   Na+ Cl~
   NH 3 + H 2 O ^ NEUOH ^OH" I NH4+
   1 1
   H 2 O NaHCO 31

low a concentration of HCO 3 ~ to saturate the solution with

NaHCO 3. The presence of the base prevents the accumulation

of H+ and in consequence the carbonic acid may continue to ionize

until a sufficient HCO 3 " concentration to precipitate NaHCO 3 has

accumulated.