Modern inorganic chemistry

236 GROUP V

value. However, in the presence of water vapour the line powder

soon becomes covered with a layer of glassy trioxophosphoric acid,

and this reduces the rate at which drying can occur. For this reason,

gases are better dried by passing them through loosely-packed

kpentoxide\ rather than merely over the surface.

Oxides of arsenic

Arsenic forms two important oxides, As 4 O 6 and As 4 O 10.

ARSENIC(lIl) OXIDE, As 4 O 6

This is formed when arsenic burns in air (cf. phosphorus which

gives P 4 O 10 ). It can exist in two crystalline modifications ; the stable

one at room temperature, which also occurs naturally as arsenolite,

has an octahedral form. Solid arsenic(III) oxide is easily reduced,

for example by heating with charcoal, when arsenic deposits as a

black shiny solid on the cooler parts of the tube.

Arsenic(III) oxide is slightly soluble in water, giving a solution

with a sweetish taste — but as little as 0.1 g can be a fatal dose! (The

antidote is freshly-precipitated iron(III) hydroxide.) The solution

has an acid reaction to litmus, due to the formation of arsenic(III)

acid:

As 4 O 6 + 6H 2 O ^ 4H 3 AsO 3

Arsenic(III) acid is an extremely weak acid; in fact, the oxide is

amphoteric, since the following equilibria occur :

Hence arsenic(III) oxide dissolves readily in alkalis to give arsen-

ates(III), for example

As 4 O 6 + 6CO^~ -> 4AsO^~ + 6CO 2 T

but in strong acid solution tripositive arsenic ions may be formed.
This reaction indicates very clearly the increased electropositive
character of arsenic.
In aqueous solution arsenic(III) oxide is a reducing agent being
oxidised to arsenate(V) by halogens, chlorate(I), nitric acid and even
iron(III) chloride.