Action of Nitric Acid on Metals

Nitric acid has no action on gold, platinum, iridium, tantalum, rhodium, and titanium. All other metals are attacked by the acid, often with the formation of nitrates, and reduction products of the nitric acid, which vary with the temperature and concentration of the acid.

Pelouze and Fremy described the reactions of copper and silver with nitric acid, and showed that tin differed from these in the production of, ammonia.

The mechanism of the reaction between nitric acid and various metals has been the subject of considerable controversy for many years. Armstrong and Ackworth put forward the theory that nascent hydrogen was the initial substance which brought about reduction of nitric acid:

M + HNO₃ = MNO₃ + H.

This nascent hydrogen did not escape from the solution owing to the powerful oxidising nature of nitric acid, but produced as secondary products nitrous acid, hyponitrous acid, hydroxylamine, and ammonia. Tertiary reactions occurred from the decomposition of these secondary products, with the formation of nitric oxide, nitrogen trioxide, and nitrous oxide; while double decomposition between the secondary products resulted in the formation of nitrogen and nitrous oxide.

Thus the action of dilute nitric acid on copper, silver, mercury, and bismuth was similar, as shown by the equations

3Cu + 6HNO₃ = 3Cu(NO₃)₂ + 6H,
3HNO₃ + 6H = 3HNO₂ + 3H₂O,
3HNO₂ = HNO₃ + 2NO + H₂O,

which gives, on summing up,

3Cu + 8HNO₃ = 3Cu(NO₃)₂ + 2NO + 4H₂O.

Divers considered that metals may be divided into two classes with reference to their action on nitric acid. In the first class are placed the four metals copper, silver, mercury, and bismuth, and the primary products are nitrite, nitrate, and water:

1) 2Ag + HO.NO₂ = AgOH + AgNO₂;
2) AgOH + HNO₃ = AgNO₃ + H₂O.

Further action of the nitric acid on the nitrite produces nitrous acid and nitrate:
3) AgNO₂ + HNO₃ = AgNO₃ + HNO₂;

while the nitrous acid reacts to produce nitrogen peroxide or nitric oxide according as the nitric acid is concentrated or dilute, as shown in the following equations: -
4) HNO₂ + HNO₃ = 2NO₂ + H₂O;
5) 3HNO₂ = HNO₃ + 2NO + H₂O.

Divers assumed that a very small quantity of nitrous acid is necessary to initiate the reaction and functions in a catalytic manner. There is no formation of ammonia or hydroxylamine at any stage of the reaction.

The second class of metals includes zinc, magnesium, aluminium, cadmium, tin, lead, iron, and the alkali metals, and no nitrous acid is required to start their reaction with nitric acid. According to Divers, nitrous acid is not produced in appreciable amounts, because further reduction occurs which is due to the action of nascent hydrogen:

HNO₃ + 2H = HNO₂ + H₂O; (Nitrous acid.)
2HNO₃ + 8H = H₂N₂O₂ + 4H₂O; (Hyponitrous acid.)
HNO₃ + 6H = NH₂OH + 2H₂O; (Hydroxylamine.)
HNO₃ + 8H = NH₃ + 3H₂O. (Ammonia.)

The production of hydroxylamine and ammonia occurs chiefly with tin and zinc, although under suitable conditions other metals may give traces. The nature and quantity of both primary and secondary products depend upon the concentration of the acid. Secondary products result from the following reactions: -

3HNO₂ = HNO₃ + 2NO + H₂O;
H₂N₂O₂ = N₂O + H₂O;
HNO₂ + NH₂OH = N₂O + 2H₂O;
HNO₂ + NH₃ = N₂ + 2H₂O.

Thus the reduction products of nitric acid produced by zinc under various conditions may be shown in the following reactions: -

3Zn + 7HNO₃ = 3Zn(NO₃)₂ + NH₂OH + 2H₂O;
4Zn + 9HNO₃ = 4Zn(NO₃)₂ + NH₃ + 3H₂O;
4Zn + 10HNO₃ = 4Zn(NO₃)₂ + N₂O + 5H₂O;
5Zn + 12HNO₃ = 5Zn(NO₃)₂ + N₂ + 6H₂O.

Veley suggests that nitrous acid is necessary to initiate the reaction between all metals and nitric acid, and his experiments go to prove that really pure nitric acid has no action on pure metals. There is considerable difficulty in obtaining complete purity of the reacting substances, and traces of nitrous acid are produced probably by electrolytic action set up by the most minute quantities of impurities in the metals. The very slight action of pure nitric acid of 30 per cent, concentration on pure metals was almost entirely prevented if the metals were agitated in the solution, because it was not possible for nitrous acid to concentrate round the metal. No action occurred at all if the formation of nitrous acid was inhibited by adding such substances as urea, hydrogen peroxide, potassium chlorate, etc. While Veley agrees that the metals copper, silver, mercury, and bismuth differ in their action from the rest of the metals, yet he maintains that in all cases nitrous acid is necessary to start the reaction, and that nitrous acid is the primary product in all cases. Thus, in the case of copper the reaction can be represented by the equations

Cu + 4HNO₂ = Cu(NO₂)₂ + 2NO + 2H₂O,
Cu(NO₂)₂ + 2HNO₃ = Cu(NO₃)₂ + 2HNO₂.

The decomposition or formation of nitrous acid is shown by the reversible reaction

3HNO₂ ⇔ HNO₃ + 2NO + H₂O.

In the case of both classes of metals Veley found that rapid solution occurred in nitrous acid, less rapid in a mixture of nitrous and nitric acids, and much slower action still in the case of pure nitric acid solution.

Higley has investigated the action of nitric acid on iron, and also the electrolytic reduction of nitric acid. The products of the reaction in each case are closely comparable. Thus, in the electrolytic reduction of nitric acid, ammonia and nitric oxide are the chief products of dilute acids, while hydrogen is also produced. Increasing concentration of the acid yields correspondingly larger amounts of nitrogen peroxide.

In the case of the reduction by iron, the products are ammonia, nitric oxide, and nitrogen peroxide, and while there is no free hydrogen, yet there are considerable amounts of nitrogen and nitrous oxide. Very weak acid yields large quantities of ammonia, but nitrogen peroxide is practically the only product with concentrated acid.

The only metal which produces free hydrogen is magnesium, and the nitric acid must be very dilute. As magnesium is next to the alkali metals with regard to its high solution tension, Webb suggests that hydrogen is liberated first by all metals with a higher solution tension than hydrogen, while those with a lower solution tension (copper, mercury, silver) are polarised in a solution of nitric acid. The function of nitrous acid in starting the reaction between these metals and nitric acid is that of a depolariser.

Very few metals can be used as containers, etc., for nitric acid, but aluminium seems to have possibilities in this respect, and the action of nitric acid on this metal has received much attention.3 Temperature and concentration are the chief factors, and it would seem that aluminium can be used at low temperatures for either very weak or very concentrated acids. Extensive use is made of aluminium air-elevator pipes at Notodden in Norway in connection with the absorption system for nitric acid produced by the arc process.

Generally speaking, the action of nitric acid upon alloys is of a very varied and complex nature. One of the commonest substances used for handling and storing nitric acid is iron containing varying amounts of silicon (12 to 20 per cent.). Narki, ironac, and tantiron are examples of silicon-iron alloys which are used alternatively to stoneware for towers, pumps, fans, etc., in various absorption systems.