Modern inorganic chemistry

248 GROUPV

The arsenate(III) ion can be reduced by systems which generate

hydrogen (for example metal/acid) to give arsine, for example

AsO^ + 3Zn + 9H+ -> AsH 3 T + 3Zn2+ + 3H 2 O

whilst other reducing agents give either arsenic or an arsenide.

Powerful oxidising agents, for example Cr 2 O7~ and MnO^ ions,

oxidise the arsenate(III) ion to arsenate(V). The reaction with iodine,

however, is reversible depending on the conditions:

AsOf ~ + I 2 + 2OH" ^ AsOr + 2I~ 4- H 2 O

THE + 5 ACIDS

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

obtained when arsenic is oxidised with concentrated nitric acid or

when arsenic(V) oxide is dissolved in water. It is a moderately strong

acid which, like phosphoric(V) acid, is tribasic; arsenates(V) in

general resemble phosphates(V) and are often isomorphous with

them.

Arsenates(V) are more powerful oxidising agents than phos-

phates(V) and will oxidise sulphite to sulphate, hydrogen sulphide

(slowly) to sulphur and, depending on the conditions, iodide to

iodine.

Antimony

No + 3 acid is known for antimony but antimonates(III) (anti-

monites) formed by dissolving antimony(III) oxide in alkalis are

known, for example sodium dioxoantimonate(III), NaSbO 2.

The + 5 acid is known in solution and antimonates(V) can be

obtained by dissolving antimony(V) oxide in alkalis. These salts

contain the hexahydroxoantimonate(V) ion, [Sb(OH) 6 ]~,

Bismuth

Bismuth(HI) oxide is basic. If, however, a suspension of bismuth(III)
hydroxide is oxidised with a strong oxidising agent such as the
peroxodisulphate ion (p. 304) the hexahydroxobismuthate(V) ion
[Biv(OH) 6 ]~ is formed. Evaporation of, for example, the sodium
salt, gives the trioxobismuthate(V), NaBiO 3. Bismuthates(V) are
extremely powerful oxidising agents and will oxidise, for example,
the manganese(II) ion to manganate(VII),