Nitric Acid may be made in various ways, but that most commonly employed is to decompose saltpetre, or nitrate of potash, by concentrated sulphuric acid, in an apparatus similar to that used in the preparation of muriatic acid, except that four cylinders are usually heated by one fire. The cylinders communicate by tubes with three or four rows of earthen vessels, the two first of which are plunged in water. The tubes which proceed from the cylinders must be of glass, that the colour of the gas which passes may be seen, as it shows the progress of the operation; the other tubes may be of earthenware.
Each cylinder is charged with 170lbs. of nitrate of potash, and l00lbs. of sulphuric acid, of 1.845 spec. grav. The heat must be equally applied, and the fire conducted slowly. As the operation advances, the vapours become red; and it is finished when these vapours are no longer visible: a brisk fire is made towards the close to disengage all the gas. The acid condensed in the first row of bottles is always the least pure: that contained in the second range and in part of the third contains only nitrous acid; this is disengaged by carrying it to ebullition in glass retorts; the ebullition is gradually stopped when it becomes white, and it is in this state sold in commerce. All the weak acid in the last vessels is again put into the first or second range instead of pure water, and water is always put in the last row to complete the condensation. The acid thus obtained is not sufficiently pure for all purposes for which it is wanted, but requires to be distilled in glass retorts, taking care to separate the products.
The first portions which are volatilized are chlorine and nitrous acid; these are separated when the liquor in the retorts becomes white, after which the pure nitric acid comes over.
The distillation should be stopped when nine-tenths of the acid in the retort is volatilized. Nitric acid is employed in the manufacture of sulphuric, oxalic, and other acids, in the composition of aqua regia, in making the red precipitate, in dyeing, gilding, assaying money, in parting gold, and numerous other processes.