Modern inorganic chemistry

254 GROUP V

ammonium molybdate and nitric acid. Specific tests for various

oxophosphates are known.

Arsenic

Because of its toxicity, it is often necessary to be able to detect

arsenic when present only in small amounts in other substances.

Arsenic present only in traces (in any form) can be detected by

reducing it to arsine and then applying tests for the latter. In Marsh's

test, dilute sulphuric acid is added dropwise through a thistle

funnel to some arsenic-free zinc in a flask; hydrogen is evolved

and led out of the flask by a horizontal delivery tube. The arsenic-

containing compound is then added to the zinc-acid solution, and

the delivery tube heated in the middle. If arsenic is present, it is reduced

to arsine by the zinc-acid reaction, for example :

AsOl~ + 4Zn + 11H+ -» AsH 3 + 4Zn^2 + + 4H 2 O

The evolved arsine is decomposed to arsenic and hydrogen at the

heated zone of the delivery tube; hence arsenic deposits as a shiny

black mirror beyond the heated zone.

Antimony and bismuth

As can be expected, antimony compounds resemble those of
arsenic. In the Marsh test, antimony compounds again give a
black deposit which, unlike that formed by arsenic compounds, is
insoluble in sodium chlorate(I) solution.
Solutions of many antimony and bismuth salts hydrolyse when
diluted; the cationic species then present will usually form a pre-
cipitate with any anion present. Addition of the appropriate acid
suppresses the hydrolysis, reverses the reaction and the precipitate
dissolves. This reaction indicates the presence of a bismuth or an
antimony salt.
When hydrogen sulphide is bubbled into an acidic solution of an
antimony or a bismuth salt an orange precipitate, Sb 2 S 3 , or a brown
precipitate, Bi 2 S 3 , is obtained. Bismuth(III) sulphide, unlike
antimony(III) sulphide, is insoluble in lithium hydroxide.

QUESTIONS

1. Give an account of the oxides and the chlorides of arsenic,
   antimony and bismuth, including an explanation of any major