Synthetic Inorganic Chemistry

CAUSTIC ALKALI FROM ALKALI CARBONATE 183

PREPARATION 16

CAUSTIC ALKALI FROM ALKALI CARBONATE, NaOH

CAUSTICIZING REACTION

The industrial process by which large quantities of sodium

hydroxide are made consists in treating a 10 per cent solution of

sodium carbonate with an excess of calcium hydroxide (milk of

lime). After the reaction is complete the mixture is filtered to

remove the precipitated calcium carbonate and excess of calcium

hydroxide. This process involves a most important application

of the principle of solubility product: calcium hydroxide continues

to dissolve and calcium carbonate to precipitate according to the

reaction

Ca(OH) 2 ^ Ca++ + 2OH"

CO-T ~ + 2Na+

CaCO 31

until equilibrium between both solids and the solution is attained.

Solubility in pure water

moles per liter

Solubility product

Ca(OH) 2

CaCO 3

0 02

0 00013

0 02 X 0 04^2

0 00013 X 0 00013

At this point the concentration of both Ca++ and CO 3 ions
is so small (both close to the value 0.00013) that for practical pur-
poses it is disregarded and the filtered solution is said to be free
of either calcium or carbonate. However, with so small a Ca++
ion concentration the OH~ ion concentration can be quite large
without exceeding the solubility product of Ca(0H) 2. In fact
the filtered solution has a concentration of nearly 2-molal in NaOH.
If, however, the OH~ ion concentration gets above 2-molal, the
Ca++ ion concentration is depressed to such an extent that an
appreciable concentration of CO3" ions can remain without ex-
ceeding the solubility product of calcium carbonate. In other
words, if we started with a more concentrated solution of Na 2 CO 3
than 10 per cent, a complete conversion to the hydroxide would
not be possible.