529 PRINCIPLES OF CHEMISTRY 



and boiled in a cast-iron, wrought-iron, or silver boiler (sodium hydrox- 

 ide does not act on these metals), and lime is added, little by little, 

 during the boiling. This latter is soluble in water, although but very 

 slightly. The clear solution becomes turbid on the addition of the lime 

 because a precipitate is formed ; this precipitate consists of calcium 

 carbonate, almost insoluble in water, whilst caustic soda is formed and 

 remains in solution. The decomposition is effected according to the 

 equation Na 2 CO 3 + Ca(HO) 2 = CaC0 3 + 2NaHO. On cooling the 

 solution the calcium carbonate easily settles as a precipitate, and the 

 clear solution or alkali above it contains the easily soluble sodium 

 hydroxide formed in the reaction. 25 After the necessary quantity of 

 lime has been added, the solution is allowed to stand, and is then 

 decanted off and evaporated in cast or wrought iron boilers, or in silver 

 pans if a perfectly pure product is required. 20 The evaporation cannot 



fluence of the mass of water is evident. According to Gerberts, however, strong 

 solutions of sodium carbonate are decomposed by lime, which is very interesting if con- 

 firmed by further investigation. 



* 5 As long as any undecomposed sodium carbonate remains in solution, excess 

 of acid added to the solution disengages carbonic anhydride, and the solution after dilu- 

 tion gives a white precipitate with a barium salt soluble in acids, showing the presence of 

 a carbonate in solution (if there be .sulphate present, it also forms a white precipitate, 

 but this is insoluble in acids). For the decomposition of sodium carbonate, milk of lime 

 that is, slaked slime suspended in water is employed. Formerly pure sodium hydrox- 

 ide was prepared (according to Berthollet's process) by dissolving the impure substance in 

 alcohol (sodium carbonate and sulphate are not soluble), but now that metallic sodium has- 

 become cheap and is purified by distillation, pure caustic soda is prepared by acting on 

 a small quantity of water with sodium. Perfectly pure sodium hydroxide may also be 

 obtained by allowing strong solutions to crystallise (in the cold) (Note 27). 



In alkali works where the Leblanc process is used, caustic soda is prepared directly from 

 the alkali remaining in the mother liquors after the separation of the sodium carbonate 

 by evaporation (Note 14). If excess of lime and charcoal have been used, much sodium 

 hydroxide may-be obtained. After the removal as much as possible of the sodium carbon- 

 ate, a red liquid (from iron oxide) is left, containing aodium hydroxide mixed with com- 

 pounds of sulphur and of cyanogen (see Chapter IX.) and also containing iron. This red 

 alkali is evaporated and air is blown through it, which oxidises the impurities (for this 

 purpose sometimes sodium nitrate is added, or bleaching powder, &c.) and leaves fused 

 caustic soda. The fused mass is allowed to settle in order to separate the ferruginous 

 precipitate, and poured into iron drums, where the sodium hydroxide solidifies. Such 

 caustic soda contains about 10 p.c. of water in excess and some saline impurities, but 

 when properly manufactured is almost free from carbonate nd from iron. The greater 

 part of the caustic soda, which forms so important an article of commerce, is manufactured 

 in this manner. 



26 Lowig gave a method of preparing sodium hydroxide from sodium carbonate by 

 heating it to a dull red heat with an excess of ferric oxide. Carbonic anhydride is given 

 off, and warm water extracts the caustic soda from the remaining mass. This reaction, 

 as experiment shows, proceeds very easily, and is an example of contact action, similar 

 to that of ferric oxide on the decomposition of potassium chlorate. The reason 

 of this may be that a small quantity of the sodium carbonate enters into double decom- 

 position with the ferrie oxide, and the ferric carbonate produced is decomposed into 

 carbonic anhydride and ferric oxide, the action of which is renewed. Similar explaua- 



