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

88 SULPHIDES.

air. The voltmeter rises slowly until it registers the temperature

of the hot vapor, after which it remains constant. It is neces-

sary, however, that the liquid should boil vigorously. The

boiling-point determined in this way in the author's laboratory

with the substance prepared as above, was 507° to 508° at 710 mm.,

which value agrees with that obtained by Bodenstein (508° at

atmospheric pressure).

55. Black Mercuric Sulphide; Transformation into Cinnabar.
    The black sulphide of mercury is always formed by the direct combination
    of the elements, as well as by the precipitation of a mercuric salt with hydrogen
    sulphide, but this modification changes slowly, of itself, into the more stable,
    red form (cinnabar). The transformation may be accelerated greatly by
    allowing the black sulphide to stand in a warm place in contact with a solution
    containing caustic alkali and alkaline sulphide; little by little the relatively
    more soluble black sulphide dissolves, and the more insoluble red form
    separates in a crystalline condition.

Triturate 50 g. mercury, 20 g. flowers of sulphur, and a little

ammonium sulphide solution, in a large porcelain mortar. Mix

the resulting black paste of mercuric sulphide, sulphur, and

globules of mercury with 60 g. of a 20% caustic potash solution,

and allow the mixture to stand at a temperature of about 50°

(e.g., on top of the hot closet). Replace the evaporated water

daily, mixing up the mass each time with the pestle. When, at

the end of a week at most, the mass has become of a pure red
color, wash it by decantation with water, whereby the greater

part of the excess of sulphur is removed, and then decant the

cinnabar itself with water into an evaporating dish, allowing any

lumps of the black sulphide to remain behind. Boil the material
with a sodium sulphite solution to remove the remainder of the

free sulphur, and wash the final product by decantation with

boiling water. Drain the product on the suction-filter and dry it

in the hot closet. Yield, 80 to 90% of the theoretical.

56. Sulphides of Tin.
    Stannous Sulphide. Dissolve 12 g. of tin in 50 g. of concen-

trated hydrochloric acid, warming gently.
1
Dilute the filtered

(^1) The addition of 2.5 c.c. of nitric acid, of sp. gr. 1.2, together with the
hydrochloric acid will cause the metal to dissolve very much more rapidly.
The nitric acid is reduced to ammonium salt. (Translators.)