SEMIPERMEABLE MEMBRANES. 49
reddish-purple precipitate is formed which can be filtered off; the
compound is characterized by its solubility in ammonia.
II. Zircon-gold-purple. Treat 130 c.c. of a boiling colloidal gold
solution with 28 c.c. of a boiling colloidal zircon solution (see foot-
note to No. 24). A precipitation of zircon-gold-purple takes place
even without the addition of an electrolyte. In the cold the pre-
cipitate forms slowly. If the deposition is incomplete, preliminary
tests must be made with small portions to find the right propor-
tions in which to mix the above solution.
III. Gold Solution and Protective Colloids. Treat 10 c.c. of col-
loidal gold solution with a few drops of dilute hydrochloric acid; a
blue coloration is first produced, later sedimentation of the metal.
Repeat the experiment after first adding one drop of a dilutegelatin solution (0.2 per cent) to the colloidal gold; no change
whatever in the color or the stability of the gold solution is observed.
- Hydrogels as Semipermeable Membranes.
The separation of colloids and electrolytes by dialysis depends on the
colloidal nature of the parchment wall, which is impervious to other colloids.
Certain colloids are impervious even to truly dissolved substances but still
pervious to water. By means of membranes of such semipermeable material
a solute can be separated from its solvent, and thus the osmotic pressure of
the dissolved substance can be both demonstrated and measured. Cupric
ferrocyanide has been found especially suitable as a semipermeable colloid.
(Pfeffer, 1877.) - Cupric Ferrocyanide Membrane. Let a drop of a cold, satu-
rated potassium ferrocyanide solution run from a fine, glass
capillary into a 0.5-molal copper sulphate solution, and detach it
by means of a slight motion, so that it sinks to the bottom of the
vessel. • The drop has at the moment of its entrance into the solu-
tion become surrounded with a thin film of cupric ferrocyanide,
which keeps growing at the cost of the dissolved components.
Since, however, the concentration of the solute within the mem-
brane is greater than that of the copper sulphate outside, the mem-
brane expands in consequence of the pressure caused by the water
entering through the walls. The membrane is at first trans-
parent and traversed by brown veins. A uniform growth can be
brought about by occasional, gentle stirring of the copper sulphate
solution. As the cell keeps expanding a point is reached when
the specific gravity of its contents having grown less through