SULPHUR 175
it must be heated before it is perceptibly dissociated. Hydrogen
iodide begins to show the violet color of free iodine as low as
180°; hydrogen sulphide does not deposit free sulphur below
310°; hydrogen bromide is first perceptibly decomposed at 800°;
hydrogen chloride at 1,800°; water at 2,000°; and hydrogen fluoride
is still undissociated at this temperature.
- Ionization of Hydrogen Sulphide. Pass hydrogen sul-
phide, which has been filtered through cotton wool, and
which has bubbled through one bottle containing water,
into distilled water in a flask until the solution is saturated.
Test the conductivity of the solution with electrodes B (see
page 84), and test it with litmus. The conductivity is almost
imperceptible; it does not cause the lamp to glow, but it is
shown by a slight evolution of bubbles from the'electrodes.
Litmus is turned towards the red, but not the full red color
produced by strong acids.
The effects observed in this experiment are caused by the
ionization H 2 S ^± H+ + HS ~ to the extent of 0.05 per cent
(page 100).
- Hydrogen Sulphide as a Precipitant. Pass hydrogen
sulphide into solutions of salts of the heavy metals acidified
with HC1; take for example a solution of copper chloride.
A heavy black precipitate is formed.
The precipitate is copper sulphide. The table gives the ioniz-
ation HS~ ;=± H+ + S~~ as 0.0002 per cent, but this applies only
to a 0.1 equivalent solution of Na+ HS~ in which all the H+ ions
come from the dissociation in question. In a solution of H 2 S con-
taining the H+ ions of the first dissociation, the concentration of
S~~ ions would be very much smaller. In a solution containing a
strong acid like HC1, the concentration of S~~ which could come
from H 2 S would be excessively small. But copper sulphide is
precipitated from such a solution. The solubility product of
copper sulphide must be extremely small.
The solubility of all the heavy metal sulphides is very small;
they are all insoluble in water, but some dissolve in hydrochloric
acid. The solubility product of these is not quite so small because
it is not reached when the ionization of H2S is driven back by the
strong acid.
