229. Sodium Sulphate.—What acid and what base are represented by Na2SO4? Which is the stronger acid, HCl or H2SO4? Would the latter be apt to act on NaCl? Why?
230. Manufacture.—This comprises two stages shown by the following reactions, in which the first needs moderate heat only; the last, much greater.
(1) 2 NaCl + H2SO4 = HNaSO4 + NaCl + HCl: (2) NaCl + HNaSO4 = Na2S4 + HCl.
The operation is carried on in large furnaces. The gaseous HCl is passed into towers containing falling water in a fine spray, for which it has great affinity. The solution is drawn off at the base of the tower. Thus all commercial HCl is made as a by- product in manufacturing Na2SO4.
When crystalline, sodium sulphate has ten molecules of water of crystallization (Na2SO4, 10 H2O); it is then known as Glauber's salt. This salt readily effloresces; i.e. loses its water of crystallization, and is reduced to a powder. Compute the percentage of water.
231. Uses.—The leading use of Na2SO4 is to make Na2CO3; it is also used to some extent in medicine, and in glass manufacture. 232. Sodium Carbonate.—Note the base and the acid which this salt represents. Test a solution of the salt with red and blue litmus, and notice the alkaline reaction. Do you see any reason for this reaction in the strong base and the weak acid represented by the salt?
233. Manufacture.—Na2CO3 is not made by the union of an acid and a base, nor is H2CO3 strong enough to act on many salts. The process must be indirect. This consists in reducing Na2SO, to Na2S, by taking away the O with C, charcoal, and then changing Na2S to Na2O3 by CaCO3, limestone. The three substances, Na2SO4, C, CaCO3, are mixed together and strongly heated. The reactions should be carefully studied, as the process is one of much importance.
(1) Na2SO4 + 4 C = Na2S + 4 CO. (2) Na2S + CaCO3 = CaS + Na2CO3.
Observe that C is the reducing agent. The gas CO escapes. The solid products Na2CO3 and CaS form black ash, the former being very soluble, the latter only sparingly soluble in water. Na2CO3 is dissolved out by water, and the water is evaporated. This gives commercial soda. CaS, the waste compound in the process, contains the S originally in the H2SO4 used. This can be partially separated and again made into acid. Describe the manufacture of NaCO3 in full, starting with NaCl. This is called the Le Blanc process, but is not the only one now employed to produce this important article.
234. Occurrence.-Sodium carbonate is found native in small quantities. It forms the chief surface deposit of the "alkali belt" in western United States, where it often forms incrustations from an inch to a foot in thickness. It was formerly obtained from sea-weeds, by leaching their ashes, as, by a like process, K2CO3 was obtained from land plants.