A method that has been proposed for the production of the red, fuming, nitric acid is the following :-100 parts of nitrate of potash are roughly ground with 4 part-4 of starch, the mixture introduced into a retort, and 100 parts of oil of vitriol of 1.85 sp. gr. added. The mouth of the retort leads into a piece of glass tubing three or four feet long, and from thence the products of distillation pass into an ordinary cooled receiver. A very slight degree of heat is sufficient to complete the operation, the proportions named yielding about 60 parts of a deep red, fuming, acid.
The thief impurities in nitric acid are sulphuric and hydrochloric acids, and chloride of iron. Occasionally, too, with a view to increase the strength artificially, nitrato of potash is dissolved in it. The last-named substance can be readily detected by evaporating the acid, when the nitrate, if present, will be left behind. To discover the presence of sulphuric acid, a small quantity of the sample may be evaporated in a platinum dish to about one-eighth its bulk, diluted with water, and a solution of nitrate of barium added. If sulphuric acid be present a white precipitate of sulphate of barium is produced, insoluble in water, acids, and ammonia. Dilution with water is necessary to dissolve any precipitate of nitrate of barium and nitrate of silver which might form and pass for sulphate of barium.
The presence of chlorine, hydrochloric acid, or the chlorides, is detected by diluting the sample with three or four times its bulk of water, and adding a solution of nitrate of silver. The formation of a white curdy precipitate, soluble in excess of ammonia, but reappearing again upon the addition of an excess of acid, shows the presence of one or more of the impurities in question. For laboratory or special purposes a pure acid may be obtained by adding a sufficient amount of nitrate of silver, decanting or filtering the liquid from whatever precipitate may be formed, and distilling it in a glass retort with a glass receiver.
The following constitute tests for nitric acid :—(1) Copper wire or turnings reduce the acid to nitric oxide, which forms bright yellowish-red fumes in the vessel. (2) Sulphuric acid decomposes
all nitrates, freeing nitric acid, which may be recognized by a purple discoloration of starch paper moistened with iodide of potassium. (3) Strong sulphuric acid is added to a solution of a nitrate, and the mixture allowed to cool upon the addition of a solution of ferrous sulphate, or chloride, the iron liquid turns a deep brown from the formation of a compound of nitric oxide and the ferrous salt. (4) A minute quantity of nitric acid added to water coloured by solution of sul phate of indigo, upon boiling bleaches the liquid, by the oxidation of the indigo. (5) Hydrochloric acid added to nitric acid confers upon it the power of dissolving gold leaf.
The ordinary rough estimation of nitric acid is made by the hydrometer. A more exact estima tion may he made by careful neutralization with carbonate of barium, filtering, evaporating to dryness, and weighing the dry nitrate of barium produced. The equivalent proportions readily give the original amount of acid. Many other methods are employed, for a detailed description of which the reader is referred to any good work upon chemical analysis. Among the best may be named neutralization and volumetric determination ; the oxidation of a ferrous into a ferric salt ; the reducing action of mercury or copper at a red heat ; conversion into ammonia, &c., &c.
It has already been said that nitric acid forms a series of well-defined salts termed nitrates. These are for the most part crystalline, and soluble in water. They melt readily, and decompose at a high temperature. Heated with combustible substances, a more or less violent deflagration ensues. The nitrates of lime, soda, potash, and ammonia occur largely in nature, and are formed whenever nitrogenized organic matters are acted upon by the air in contact with a base. The more important members of this series of salts are the nitrates just named, together with those of alumina, barium, cobalt, copper, iron, lead, magnesium, mercury, nickel, silver, strontium, tin, and zinc. A detailed description of the nitrates will be given under the heads of their respective bases.
J. L.