148 COMPLEX HALOGEN ACIDS AND SALTS.
close attention, since both are deep blue. Extract the aqueous
layer once or twice more with 10 to 15 c.c. of amyl acetate. To
obtain a good product it is necessary, before evaporating the amyl
acetate solution, to free it mechanically from drops of aqueous
solution by pouring it back and forth into clean, dry beakers. In
order to crystallize the salt, evaporate off one-tenth of the amyl
acetate under the hood, and to the cooled solution add slowly
50 to 60 c.c. of low-boiling ligroin. Drain off the crystals, wash
them with ligroin, and dry them over sulphuric acid. The product
consists of deep-blue needles; when dissolved in water, it disso-
ciates and a red solution is formed; upon the addition of potassium
thiocyanate the dissociation is driven back and the solution be-
comes blue. Instead of amyl acetate, ethyl acetate can be used
in the above procedure.
By allowing a solution of this substance in amyl acetate to
evaporate at room temperature, fairly large, dark blue, flat
crystals can be obtained.
- Cadmium Iodide, Cd[CdIJ (Autocomplex Compound).
Cadmium iodide is, according to Hittorf, the cadmium salt of the com-
plex iodocadmic acid, H 2 [CdIJ. A compound of this nature, in which one and
the same metal exercises different functions, is known as auto-complex. (W.
Biltz, 1902.) In harmony with the complex nature of cadmium iodide, the
simple ions Cd++ and I~ show a strong tendency to combine with each other,
as is illustrated, for example, by the fact that when a solution of potassium iodide
is treated with cadmium hydroxide, the liquid becomes strongly alkaline, due
to the formation of undissociated cadmium iodide and potassium hydroxide.
Allow several rods of pure zinc to stand for about 24 hours in con-
tact with a solution of 26 g. crystallized cadmium sulphate in 100 c.c.
of water, until all the cadmium is precipitated as spongy metal. To
test for complete precipitation treat a few drops of the solution with
hydrogen sulphide; no cadmium sulphide should form. Purify
the finely divided cadmium by boiling it repeatedly with water.
Boil the cadmium with 24 g. of iodine and 50 c.c. of water, in a
flask with return condenser, until all the metal is dissolved (1 or
2 hours). Then continue the boiling in an open flask to remove
any excess of iodine, filter, and concentrate the filtrate to crys-
tallization. Work up the mother-liquor. Yield, 30 to 35 g. of
nearly colorless, lustrous plates.
To show that the amount of cadmium ions in a concentrated