This destroys any nitric acid that may be
present, and reduces selenic acid (H_{2}SeO_{4}) to selenious
(H_{2}SeO_{3}). The solution is diluted with water, and treated with a
solution of sulphite of soda. It is warmed, and at last boiled. The
selenium separates as a red precipitate, which (on boiling) becomes
denser and black. It is collected on a weighed filter, washed with hot
water, dried at 100° C., and weighed as pure selenium.
Selenium can be precipitated with sulphuretted hydrogen as a sulphide,
which is readily soluble in ammonium sulphide. This sulphide may be
oxidised with hydrochloric acid and chlorate of potash; and the selenium
separated in the manner described.
TELLURIUM.
Tellurium occurs in nature, native, and in combination with gold,
silver, bismuth and lead. It is sometimes met with in assaying gold
ores. It may be detected by the purple colour it imparts to strong
sulphuric acid when dissolved in the cold, and by the black precipitate
of metallic tellurium which its solutions yield on treatment with a
reducing agent. Telluric acid is reduced to tellurous (with evolution of
chlorine) on boiling with hydrochloric acid.
~Solution~ is effected by boiling with aqua regia, or by fusing with
nitre and sodium carbonate.
~Separation.~--Tellurium closely resembles selenium in its reactions.
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