Next to sulphuric acid in the extent and importance of its applications in the arts and manufacturea, we may solely place nitric acid — the aqua fortis of the early chemists. This acid is manufactured on a commercial scale in enormous quantities, and of late years almost exclusively from the socalled Chilian saltpeter — a nitrate of soda — by decomposing this salt with sulphuric acid. By this treatment the nitric acid is set free, and is collected by condensation in vessels provided for the purpose. The vast expansion that the the chemical industries have attained, have mode the deposits of soda niter that occur abundantly in Chili and Peru, enormously valuable, as this material is the cheapest source of nitric acid that has yet been discovered, in consequeues of which the exportation of the product to Europe and to this country has become one of the leading industries of those countries.
We illustrate on the opposite page the method of manufacturing this important chemical product in one of the leading chemical manufactories of the United States (that of Martin Kalbfleisch's Sons, whose works are located in Brooklyn, N. Y.; Bayonne, N. J.; anti Buffalo, N. Y.)
For the manufacture of nitric mid, a series of cast-iron vessels are provided, suitably mounted over furnace, so that they may be heated. This construction is shown in Fig. 1. Them vessels are charged with a quantity of the before mentioned Chilian saltpeter, to which is then added an equal weight of strong sulphuric acid. The chambers are then closed and the fires started. The sulphuric acid speedily attacks the niter, dispincing the nitric acid, which is liberated in the form of vapor, and forming a bisulphate of soda as the product of the decomposition.
From the rear of each oven a clay-lined pipe projects, which conducts the vapors of nitric acid, all they are given off, into a set of stoneware or glass receivers, the latter being connected with each other by means of earthenware or glass tubes. The acid condensed in the receivers nearest to the ovens is highly concentrated. That which is condensed in the others is weaker, because, in order to condense all the acid given off, some water is introduced into the following ones; the product condensed in these is, therefore, more dilute than that obtained in the others.
Sometimes, instead of the iron vessels above named, the manufacturers employ a series of glass retorts, placed upon a sand-bath, heated by a fire below, as shown in Fig. 2 of our illustrations. The subsequent, operation of condensation in a series of connected vessels, however, remains the same as above described. The proportion of materials employed to produce the best results should be 17 parts of nitrate of soda to 14¼ parts of strong sulphuric acid.
The nitric acid obtained in the above operation, properly conducted, is a colorless, transpsrent liquid, having a specific gravity of 1.55 (water = 1), and boiling at 176° Fahrenheit. When diluted with water the boiling point is higher. An acid containing 100 parts dry acid and 10 parts water boils at 264.2° Fah., but if the dilution with water is continued further, the boiling point is again lowered, consequently when such acid is heated above 212° Fah., the first product of the distillation is water, containing only a trace of acid; and if the process be continued, the boiling-point gradually rises until it reaches about 264° F., at which point,what is known to the trade as double aqua fortis, passesover, which has a specific gravity of from 1.35 to 1.45, while ordinary, or single aqua fortis, has a gravity of 1.19 to 1.25. In contact with the air, nitric acid gives off fumes, owing absorption of moisture from the atmosphere.
The strongest acid made in the process here described is usually of a yellow color, owing to the presence of lower oxides of nitrogen (byponitric acid). To get rid of this, the acid is bleached by exposing it for a time, in suitable glass vessels, on the water-bath to a temperature from 176° to 194° Fah., where it is left until colored fumes cease to be given off, and the acid remaining in the vessels is colorless. If the acid is required to be very pure, the first products coming over are collected in a separate vessel. This will contain most of the hydrochloric acid which may be given off, if the nitrate employed contains chlorides, which it usually does in small quantities. In the bleaching process above described, most of the hydrochloric acid is removed from the acid product as chlorine. To still further purify the product, it is necessary to remove the small quantities of sulphuric acid carried over mechanically from the still. This is effected by distilling the nitric acid over pure nitrate of baryta, while the last traces of hydrochloric acid are removed by distillation over pure nitrate of silver. These last operations are only performed when it is desired to have the nitric acid chemically pure for analytical purposes, but are not carried out in commercial productions.
The residual product remaining in the chambers, or retorts, is a sulphate of soda, used be for the production of fuming sulphuric acid by subjecting it to a red heat, and condensing the acid fumes given off; or it is mixed with common salt and ignited, when muriatic (hydra chloric) acid is evolved, which is collected, and a neutral sulphate of soda remains behind, which can be utilised for the manufacture of soda by the common method, or for other purposes. The uses of nitric acid are extensive, and important both in the laboratory and in the arts and manufactures. Its usefulness is derived from the property which it possesses of yielding very freely a notable proportion of its oxygen to substances having an affinity for the same, a property which renders it one of the most energetic of oxidizing agents. On this account, as well at because of its cheapness, its use for oxidising purposes in the laboratory is very extensive.
We append below a list of the more important technical uses of nitric acid: Its property of energetically dissolving many of the common metals, renders it useful in etching steel, copper, bronze and the like. In the manufacture of sulphuric acid, it is introduced for the purpose of effecting the oxidation of the sulphurous acid given off in the burning of sulphur, or the roasting of pyrites, to sulphuric acid. It has the property of yielding, with certain organic substances, what are called nitro-compounds, which are of great value in the arts. So, for example, nitro-cellulose (gun-cotton), nitro-glycerine, nitro-benzole, nitro-mannite, and a number of analogous products are formed. Owing to its powerful oxidizing action, it acts powerfully upon coloring matters, and on this account has some important applications in dyeing. By prolonged treatment with nitric acid, starch, cellulose (wood fiber), and sugar are converted into oxalic acid; very dilute acid converts starch onto dextrine. The fact that it will not attack gold, while energetically dissolving nearly all the other metals, has long been taken advantage of in the erts, in assaying and metallurgy, to separate gold from silver and base metals.
Nitric acid is employed in the chemical industries in greet quantities in the manufacture of an immense number of chemical products, in addition to those we have already named. Of these, some of the more important are, the preparation of picric acid from carbolic acid, naphthaline yellow from naphthaline; the manufacture of nitro-benzole, nitro-talual, and phthalic acid; the preparation of nitrate of silver (lunar caustic), arsenic acid, fulminate of mercury, and, generally speaking, of the salts known as nitrates.
From the above, it will be seen that nitric acid is one one of the most important chemical agents employed in the arts and manufactures.
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