Modern inorganic chemistry

GROUP V 245
Both the acid and its salts are powerful reducing agents. They
reduce, for example, halogens to halides, and heavy metal cations
to the metal. Copper(II) ion is reduced further to give copper(I)
hydride, a red-brown precipitate:

3H 3 PO 2 + 3H 2 O + 2Cu2+ -> 2CuHi + 3H 3 PO 3 + 7H +

Phosphonic acid, H 3 PO 3 , often called just 'phosphorous acid', is

prepared by the hydrolysis of phosphorus trichloride; a stream of

air containing phosphorus trichloride vapour is passed into ice-cold

water, and crystals of the solid acid separate:

PC1 3 + 3H 2 O -> H 3 PO 3 + 3HC1

The acid is dibasic (see structure p. 244). Like phosphinic acid it

disproportionates when heated :

4H 3 PO 3 -» PH 3 + 3H 3 PO 4

and is a strong reducing agent. Also like phosphinic acid it reduces

heavy metal ions to the metal, but copper(II) ions are not reduced

to CuH.

THE + 5 ACIDS

The important phosphoric acids and their relation to the anhydride

P 4 O 10 are:

hot

P 4 O10H20 HPO 3 , H 4 P 2 O 7 J!!^ H 3 PO 4

^(P 4 O 10 ,2H 2 O)^^r(P 4 O 10 ,4H 2 O)^h^r(P 4 O 10 ,6H 2 O)

(poly)trioxophosphoric heptaoxodiphosphoric tetraoxophosphoric

(meta) (pyro) (ortho)

(The formulae P 4 O 10 ,xH 2 O are merely to illustrate the inter-

relationship and have no structural meaning.)

Tetraoxophosphoric acid, H 3 PO 4 :—This is prepared in the labora-

tory either by dissolving phosphorus(V) oxide in water (giving

trioxophosphoric acid) and then heating to give the tetraoxo-acid;

or by heating violet phosphorus with 33% nitric acid, which

oxidises it thus:

4P 4- 10HNO 3 + H 2 O -> 4H 3 PO 4 + 5NO? + 5NO 2 T

Caution is required in both methods. In the second case, in
particular, gentle heating only is essential once the reaction starts.