Preparation of Nitric Acid

Nitric acid can be prepared by distilling a mixture of a nitrate salt with concentrated sulphuric acid. As nitric acid is a volatile acid it can be expelled by a stronger but less volatile acid such as concentrated sulphuric acid. But concentrated hydrochloric cannot be used for this process, as this acid is more volatile and so escapes out without reacting with the nitrate salt.

However in the laboratory, nitric acid is prepared by distilling a mixture of concentrated sulphuric acid and potassium nitrate or sodium nitrate salt (cheap and easily available). The apparatus is set up as shown in figure 6.13.

Potassium nitrate or sodium nitrate is taken in the glass retort and concentrated sulphuric acid is added to it. The retort is heated carefully to about 200^oC. The vapour coming out of the retort is collected and condensed inside a flask, which is kept under running water from a tap.

Nitric acid being strongly corrosive, attacks rubber, cork etc. Hence the complete apparatus should be made of glass only.

Although pure nitric acid is colourless, the acid obtained by this method is slightly yellowish brown. This is due to the formation of reddish brown fumes of nitrogen dioxide gas when the acid is exposed to hot conditions.

This gas dissolves in the freshly formed vapours of the acid and gives it the yellowish brown colour. The colour of nitric acid can be removed by bubbling air or carbon dioxide through the warm acid. Also the yellowish brown colour almost disappears, when the acid is diluted with water.

Remember:-

In the above method of preparation the working temperature should be maintained only at about or slightly below 200^oC for the following reasons:

The glass apparatus may break.

A good quantity of the nitric acid gets decomposed at higher temperatures.

At higher temperatures, potassium or sodium sulphates are formed, which cannot be easily removed from the apparatus.

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The reactions in the above method of preparation at the lower and higher temperatures are as follows:

First stage - lower temperature

Second stage - higher temperature

Combining both equations, we get

The potassium sulphate or sodium sulphate so formed are actually in aqueous solution. But at higher temperature the solution loses water and the salt is left behind as a solid, which is difficult to remove from the apparatus.

It should be noted that the nitric acid obtained by this procedure contains water, nitrogen dioxide etc. as impurities.