It is easy to see that no aqueous vapour, or as little as possible, must be incorporated with the ealt for sublimation. The loss sustained by the crystallized salt on dryiog is about 20 to 25 per oent. aud in subliming a further loss of 10 to 12 per cent. The chambers in which the salts of ammonia are sublimed being lined with iron or lead, various devices have beeu practibed with a view to prevent tho impregnation of the salt. Siliceous slabs, and a preparation known as lieat's alum plaster, are largely used for this purpose.
Messrs. Boggs and Braby proposed to treat the crude gas liquor with a eolution of chloride of iron to obtain the chloride of ammonium.
The double chloride of iron and ammonia, which ie formed iu the crude salt and sublimee with the chloride, is removed, as in Calvert's process, by adding 5 per cent. of biphosphate of lime, or 3 per cont. of the phosphate of ammonia.
Preparation of pure Chloride of Ammonium.—Profossor Stets has shown that to obtain ammonic chloride absolutely free from compound anamonias (amines), and other organic matters, requires very c,omplex manipulation. From the commercial sulphate he proceeds as follows. Two kilogrammee of sulphate is heated with kilogramme of concentrated sulphuric acid, the temperature being raised until the sulphate is decomposed with efferveacence. At this stage, nitric acid is gradually added, until tho liquid, which previously had a strong blackish colour, becomes quite colourless. The organic matters and compound ammonias are thus destroyed, with the liberation of carbonic acid gas. The acid sulphate, suitably cooled, is poured into about 10 times its volume of cold water, aud the excess of acid nearly saturated by lime water. The sulphate of calcium having subsided, the eupernatant liquor is mixed with an excess of slaked lime, contained in a very large globe and heated in a bath of a saturated solution of coinmon salt. The liberated ammonia, after washing, ie passed into pure water, and afterwards saturated with a current of pure hydrochloric acid gas. The solution is evaporated to dryness in a globe of hard glass, and the residue, eublimed in an atmosphere of ammonia obtained from a portion of the same chloride. The free ammonia is expelled by heating until a vapour appears. In obtaining the pure chloride from the ammonia contained in the commercial chloride, he proceeds by oxidizing the organic matter by the addition of nitric acid of sp. gr. 1.4, and boiling until all the chlorine is driven off. The solution
of the ammoniacal salt is treated with hydrate of lime as before, and the gas, well washed, passed into distilled water, through which a current of pure hydrochloric acid is passed to saturation. The liquid is evaporated, and the residuum sublimed in an atmosphere of ammonia. The first process would certainly recommend itself over the other, if it were necessary to produce this salt on a commercial scale with this high degree of purity.
Sulphate of Ammonium.—As it is easier to obtain this salt in a certain degree of purity than the chloride under the same circumstances, it is usual to convert the crude preparations into sulphate, even if required afterwards to be converted into chloride. One or two recrystallizations being generally sufficient, this is due not so much to any peculiarity of the different salts, but to the carbonizing action of the sulphuric acid on the tarry matters carried over mechanically in the first distillation.
The conversion of the ammonia into sulphate, or the formation of any other salts by a simple substitution of the acid may be carried out in the same way, is to convey the gas, as generated in a Coffey's still or retort; into a receiver containing the acid. Formerly the gas eutered a receiver packed with coke or charcoal, over which the acid was allowed to trickle gently, any ammonia which escapes the lower part of the apparatus will meet in its upward escape with sufficient acid to fix it. Sufficient vapour enters this receiver to keep the salt in solution, otherwise it would condense and clog the charcoal, and perhaps lead to an accident. If the acid be diluted, it will save the necessity of conveying the steam as the solvent, and will further help to keep the receiver cool. The solu tion of the sulphate condensed falls to the bottom of the receiver, and may be drawn off, as shown in Fig, 200.
The sulphate is converted into chloride by mixing a concentrated hot solution of the salt with a warm saturated solution of chloride of sodium, and removing the crystals of sulphate of sodium, which deposit on the sides of the pans.