Personne prefers to heat with caustic soda or potash, and then pass chlorine gas into the mixture; the excess of chlorine is expelled by boiling, mercuric chloride in presence of an alkaline chloride not being volatilised at 100°. The standard solutions required for this process are:—
(1) 33·2 grms. of potassic iodide in 1 litre of water, 1 c.c. = 0·01 grm. Hg, or 0·01355 grm. HgCl2.
(2) A solution of mercuric chloride containing 13·55 grms. to the litre, 1 c.c. = 0·1 grm. Hg.
The process is founded on the fact that, if a solution of mercuric chloride be added to one of potassic iodide, in the proportion of one of the former to four of the latter, mercuric iodide is formed, and immediately dissolved, until the balance is overstepped, when the red colour is developed; the final reaction is very sharp, and with solutions properly made is very accurate. The mercuric solution must always be added to the alkaline iodide; a reversal of the process does not answer. It therefore follows that the solution to be tested must be made up to a definite bulk, and added to a known quantity of the potassic iodide until the red colour appears.
Mercurous Salts may be titrated with great accuracy by a decinormal solution of sodic chloride. This is added to the cold solution in very slight excess, the calomel filtered off, the filtrate neutralised by pure carbonate of soda, and the amount of sodic chloride still unused found by titration with nitrate of silver, the end reaction being indicated by chromate of potash. Several other volumetric processes are fully described in works treating upon this branch of analysis.
III.—PRECIPITATED BY HYDRIC SULPHIDE FROM A NEUTRAL SOLUTION.
Zinc—Nickel—Cobalt.
1. ZINC.
§ 856. Zinc—At. wt., 65; specific gravity, 6·8 to 7·1; fusing-point, 412° (773° F.)—is a hard, bluish-white, brittle metal, with a crystalline fracture. Between 100° and 150° it becomes ductile, and may be easily wrought, but at a little higher temperature it again becomes brittle, and at a bright red heat it fuses, and then volatilises, the fumes taking fire when exposed to the air. In analysis, zinc occurs either as a metallic deposit on a platinum foil or dish, or as a brittle bead, obtained by reducing a zinc compound with soda on charcoal.
The salts of zinc to be briefly described here are the carbonate, the oxide, and the sulphide,—all of which are likely to occur in the separation and estimation of zinc, and the sulphate and chloride,—salts more especially found in commerce, and causing accidents from time to time.