Modern inorganic chemistry

244 GROUP V

decomposition. They are usually prepared by heating the alkali

metal nitrate, alone or with lead as a reducing agent—the latter

method being the one used in the manufacture of sodium nitrite for

use in the dye industry. Lead will also reduce nitrate to nitrite if

present as lead sponge':

2NaNO 3 -> 2NaNO 2 + O 2 T

or KNO 3 + Pb -* KNO 2 + PbO

The addition of even a weak acid (such as ethanoic acid) to a

nitrite produces nitrous acid which readily decomposes as already

indicated. Hence a nitrite is distinguished from a nitrate by the

evolution of nitrous fumes when ethanoic acid is added.

Phosphorus

Phosphorus forms a large number of oxoacids, many of which

cannot be isolated but do form stable salts. In general, ionisable

hydrogen is bonded to the phosphorus through an oxygen atom;

hydrogen atoms attached directly to phosphorus are not ionisable.

THE + 3 ACIDS

Two of these are important:

HPH 2 O 2 phosphinic (hypophosphorous) acid

and H 2 PHO 3 phosphonic (orthophosphorous) acid

X-Ray diffraction studies of the oxoacid anions indicate the following
probable arrangements for the acids:

O O

HO

and

H HO OH

H H

phosphinic acid phosphonic acid

In each case the P—O bonds have some multiple character.
Phosphinic acid is a moderately strong monobasic acid. On heating
the acid and its salts they disproportionate evolving phosphine:

4H 2 PO 2 ~ -> 2PH 3 -f 2