74 HALOGEN COMPOUNDS.
dark. Place the evaporating dish, with its contents, in a desic-
cator over sulphuric acid, preferably in a vacuum; an almost
solid mass of crystals forms in a few days. Break up this mass
thoroughly once each day to accelerate the drying process. Cu-
pric bromide is of a deep-black color and glistens somewhat like
iodine.
Heat a portion of the cupric bromide cautiously in an evapo-
rating dish; it loses bromine and is changed into white cuprous
bromide. Cuprous bromide is practically insoluble in water.
The color of the cupric bromide solution varies with the dilution;
the most dilute solutions, which contain practically all the copper
in the state of the simple ions, are light blue; the most concentrated
ones have a dark-brown color from non-ionized cupric bromide and
complex ions; other concentrations have intermediate shades.
1- Cuprous Chloride.
Treat 50 g. crystallized cupric sulphate and 25 g. sodium chloride
(or 37 g. crystallized cupric chloride) in a flask with 150 g. concen-trated hydrochloric acid and 20 g. copper turnings. Heat upon
the water bath until, at the end of about an hour, the green color
has disappeared. Pour the clear solution into a liter of water
containing a little sulphurous acid, whereby white cuprous chlo-
ride, insoluble in dilute hydrochloric acid, is precipitated. Wash
the precipitate by decantation with water containing sulphurous
acid, finally drain it with suction and wash successively with
glacial acetic acid, alcohol, and ether. Dry it in the hot closet.
Yield, about 20 g.
- Potassium Iodide. Iodine from Iodide Residues.
By the action of iodine upon a caustic potash solution, a mixture of potas-
sium iodide and iodate is produced, and this can be reduced completely to the
iodide by heating with charcoal. By extracting the mass with water and
recrystallizing, the product can be purified.^2 Potassium iodide can also be
(^1) The light-blue color of the dilute solutions is the characteristic color of
all solutions of cupric ions. The changes in color with increasing concentra-
tion may be due, certainly in part, to an increasing proportion of undissociated
molecules, but it is also quite certain that changes in the state of hydration of
the dissolved salt have a large influence in altering the color.
(^2) Cf. Preparation of potassium bromate and bromide, No. 78.