Modern inorganic chemistry

GROUP V 237

ARSENIC(V) OXIDE, As 4 O 10

Unlike phosphorus pentoxide, this oxide cannot be made directly.

Arsenic(V) acid, H 3 AsO 4 (strictly, tetraoxoarsenic acid), is first

prepared by oxidising arsenic(III) oxide with concentrated nitric

acid or some other strong oxidising agent:

2H 3 AsO 3 + 2HNO 3 -> 2H 3 AsO 4 + NOT + NO 2 T + H 2 O

On concentrating the solution, a solid of formula As 4 O 10 .8H 2 O

(which may be composed by hydrated arsenic(V) acid) is obtained,

and this, on fairly prolonged heating to 800 K, loses water and leaves

arsenic(V) oxide. No compounds corresponding to the other acids

of phosphorus are formed, but salts are known.

Arsenic(V) oxide is a white deliquescent solid, which liberates

oxygen only on very strong heating, leaving the (III) oxide:

As 4 O 10 -> As 4 O 6 + 2O 2

It dissolves in water to give arsenic(V) acid, and in alkalis to form

arsenates( V}.

Oxides of antimony

Antimony forms both a + 3 and a + 5 oxide. The -f 3 oxide can be

prepared by the direct combination of the elements or by the action

of moderately concentrated nitric acid on antimony. It is an ampho-

teric oxide dissolving in alkalis to give antimonates(III) (for example

sodium 'antimonite', NaSbO 2 ), and in some acids to form salts, for

example with concentrated hydrochloric acid the trichloride, SbCl 3 ,

is formed.

Antimony(V) oxide can be prepared by treating antimony with

concentrated nitric acid. It is an oxidising agent and when gently

heated loses oxygen to form the trioxide. (The change in oxidation

state stability shown by antimony should be noted since it corres-

ponds to increasing metallic character.)

Unlike the amphoteric +3 oxide, the +5 oxide is acidic and

dissolves only in alkalis to give hydroxoantimonates which contain

the ion [Sb(OH) 6 J~. A third oxide, Sb 2 O 4 , is known but contains

both antimony(Ili) and antimony(V), Sbm(SbvO 4 ), cf. Pb 3 O 4.

Oxides of bismuth

Bismuth forms both + 3 and + 5 oxides. The + 3 oxide, unlike the
corresponding oxides of the other Group V elements, is insoluble