The French Academy offered a prize for the discovery of a profitable method of preparing it from common salt. Leblanc then proposed the above-mentioned process, which is remarkable for its great simplicity.16
Of all other industrial processes for manufacturing sodium carbonate, the ammonia process is the most worthy of mention.17
In this the vapours of ammonia, and then an excess of carbonic anhydride, are directly introduced into a concentrated solution of sodium chloride in order to form the acid ammonium carbonate, NH4HCO3.
Then, by means of the double saline decomposition of this salt, sodium chloride is decomposed, and in virtue of its slight solubility acid sodium carbonate, NaHCO3, is precipitated and ammonium chloride, NH4Cl, is obtained in solution (with a portion of the sodium chloride and acid sodium carbonate).
The reaction proceeds in the solution owing to the sparing solubility of the NaHCO3 according to the equation NaCl + NH4HCO3 = NH4Cl + NaHCO3.
The ammonia is recovered from the solution by heating with lime or magnesia,18 and the precipitated acid sodium carbonate is converted into the normal salt by heating. It is thus obtained in a very pure state.19
Sodium carbonate, like sodium sulphate, loses all its water on being heated, and when anhydrous fuses at a bright-red heat (1098°).
A small quantity of sodium carbonate placed in the loop of a platinum wire volatilises in the heat of a gas flame, and therefore in the furnaces of glass works part of the soda is always transformed into the condition of vapour.
Sodium carbonate resembles sodium sulphate in its relation to water.20
Here also the greatest solubility is at the temperature of 37°; both salts, on crystallising at the ordinary temperature, combine with ten molecules of water, and such crystals of soda, like crystals of Glauber's salt, fuse at 34°. Sodium carbonate also forms a supersaturated solution, and, according to the conditions, gives various combinations with water of crystallisation (mentioned on page 108), &c.
At a red heat superheated steam liberates carbonic anhydride from sodium carbonate and forms caustic soda, Na 2 CO 3 + H 2 O = 2NaHO + CO 2 . Here the carbonic anhydride is replaced by water; this depends on the feebly acid character of carbonic anhydride. By direct heating, sodium carbonate is only slightly decomposed into sodium oxide and carbonic anhydride; thus, when sodium carbonate is fused, about 1 per cent. of carbonic anhydride is disengaged. The carbonates of many other metals—for instance, of calcium, copper, magnesium, iron, &c.—on being heated lose all their carbonic anhydride. This shows the considerable basic energy which sodium possesses. With the soluble salts of most metals, sodium carbonate gives precipitates either of insoluble carbonates of the metals, or else of the hydroxides (in this latter case carbonic anhydride is disengaged); for instance, with barium salts it precipitates an insoluble barium carbonate (BaCl 2 + Na 2 CO 3 = 2NaCl + BaCO 3 ) and with the aluminium salts it precipitates aluminium hydroxide, carbonic anhydride being disengaged: 3Na 2 CO 3 + Al 2 (SO 4 ) 3 + 3H 2 O = 3Na 2 SO 4 + 2Al(OH) 3 + 3CO 2 . Sodium carbonate, like all the salts of carbonic acid, evolves carbonic anhydride on treatment with all acids which are to any extent energetic. But if an acid diluted with water be gradually added to a solution of sodium carbonate, at first such an evolution does not take place, because the excess of the carbonic anhydride forms acid sodium