GROUP V 239converter (Figure 9.6). This contains a gauze of platinum, or
platinum-rhodium, heated at first electrically, then maintained at
red heat by the exothermic reaction which takes place on it. The
air-ammonia mixture must only remain in contact with the catalyst
for a fraction of a second, otherwise the nitrogen oxide decomposes
to give nitrogen and oxygen. From this converter, the nitrogen oxide
is mixed with more air, to convert it to nitrogen dioxide. This reaction
is also exothermic and the heat from it may be used to pre-heat the
air stream entering the converter.
AirPt gauzeFigure 9.6. Manufacture oj nitric acidThe nitrogen dioxide is then passed up a water-cooled steel tower,
fitted with baffles down which water flows. Here the nitrogen
dioxide dissolves to give nitric acid and nitrogen oxide ; air is also
passed up the tower to oxidise the latter to give more nitrogen
dioxide, which is absorbed in turn, so that ultimately almost
complete conversion of the nitrogen oxides to nitric acid is complete ;
the acid is collected, at a strength of 50-65 %, at the base of the tower.
Properties. Pure nitric acid is a colourless liquid, density 1 .52 g cm~^3 ,
dissociating slightly above its melting point into dinitrogen pent-
oxide and water, as already mentioned : on boiling, more oxides of
nitrogen are formed and the liquid obtained is then the constant
boiling-point acid, density 1.41gcm~^3 ; hence this latter acid
('concentrated nitric acid*) is usually yellow in colour due to
dissolved oxides formed during distillation. The colour deepens on
exposure to daylight because nitrogen dioxide is formed in solution
by the photochemical reaction :
- g-^ 4NO + 2HO -h O