Laboratory Methods of Inorganic Chemistry, 2nd English Ed. 1928

COLLOIDAL ANTIMONY SULPHIDE. 43

platinum passes in grayish brown clouds into the liquid. It is not

usually possible to preserve the arc for more than a short time, but

it can be repeatedly started again by bringing the electrodes

together, and then separating them; the process may be thus con-

tinued until the platinum solution is so dark as to be nearly opaque,

or until it has become too hot. The platinum electrodes should

never be held except by the insulated glass ends. If the experiment

does not succeed well at first, add a trace of dilute alkali to the

water. Finally, filter off the coarser platinum powder, and employ

the solution in the following experiments:

1. To a few cubic centimeters add a drop of a dilute salt solu-
   tion. After a short time the colloidal metal separates out in the
   form of a powder (precipitation by an electrolyte).


2. Add some of the platinum solution to dilute hydrogen
   peroxide. A reaction takes place with evolution of oxygen, while
   a comparison sample of the hydrogen peroxide itself remains clear.
   Concentrated 30 per cent hydrogen peroxide, when treated with
   the platinum solution, decomposes at first slowly, then more rap-
   idly, with rise of temperature, and finally with explosive violence.
   This is an instance of catalysis; i.e., the acceleration by means of
   a chemically indifferent substance (the platinum) of a reaction
   (the decomposition of hydrogen peroxide) which would of itself
   take place slowly.


3. To another portion of dilute hydrogen peroxide add a few
   drops of hydrogen sulphide water and then some platinum solu-

tion. This time no decomposition occurs; the catalyzer has

temporarily lost its accelerating action in consequence of the

presence of a third, likewise indifferent substance which acts as
a poison. The catalytic property of the platinum depends upon

the condition of the surface of the finely divided material; the

action of the poison is due perhaps to an alteration in the con-

dition of the surface.

21. Colloidal Antimony Sulphide.
    Allow a cold 1 per cent solution of tartar emetic to drop from a

dropping funnel into 200 c.c. of hydrogen sulphide water through

which a fairly strong stream of hydrogen sulphide is kept passing.

Antimony sulphide produced under these conditions does not form
a precipitate, but remains in colloidal suspension as a deep-orange-