Modern inorganic chemistry

GROUP V 243

The nitrates of other metals give nitrogen dioxide, oxygen and

the metal oxide, unless the latter is unstable to heat, in which case

the metal and oxygen are formed (for example from nitrates of silver

and mercury):

2Cu(NO 3 ) 2 -> 2CuO + 4NO 2 + O 2

2AgNO 3 -> 2Ag -h 2NO 2 + O 2

Nitrates are detected by:

1. The action of heat on the solid (above).


2. By the brown ring test with iron(II) sulphate and cold con
   centrated sulphuric acid.


3. By their oxidising action; heating with copper and concen
   trated sulphuric acid yields brown fumes of nitrogen dioxide.


4. By the evolution of ammonia with Devarda's alloy in alkaline
   solution in absence of ammonium ions; this is used quantitatively,
   the ammonia being absorbed in excess standard acid and the excess
   acid back-titrated.

NITROUS ACID

Nitrous acid, HNO 2 , is known as a gas, but otherwise exists only in
solution, in which it is a weak acid. Hence addition of a strong acid
to a solution of a nitrite produces the free nitrous acid in solution.
Nitrous acid is unstable, decomposing to give nitric acid and
evolving nitrogen oxide :

3HNO -* NO + HO+ 4- 2NO
It is an effective oxidising agent and can oxidise iodide to iodine,
and the ammonium ion to nitrogen. The reduction products of
nitrous acid vary greatly with conditions. For example, nitrogen
oxide or ammonia may be formed when hydrogen sulphide is
oxidised to sulphur, according to the acidity of the solution.
Hydrazine is oxidised by nitrous acid to hydrogen azide. Nitrous
acid can itself be oxidised to nitric acid, but only by strong oxidising
agents such as manganate(VII). Nitrous acid is important in
organic chemistry for its ability to diazotise primary aromatic
amines — an important step in the manufacture of dyestuffs.

NITRITES

These all contain the ion NO^. They are much more stable than
nitrous acid, and those of the alkali metals can be fused without