3860. To Purify Oil of Vitriol

3860. To Purify Oil of Vitriol. Commercial sulphuric acid frequently contains nitrous acid, arsenic, and saline matter. These impurities must bo removed in order to obtain the acid in any high degree of purity.

Nitrous acid is removed by adding about l1/2 grains sugar to each fluid ounce of the sulphuric acid, heated to nearly its boiling point, and continuing the heat until the dark color at first produced disappears, when it should bo distilled. Another method is by adding 1/8 to 1/2 of 1 per cent, of sulphate of ammonia to the acid, and heating to ebullition for a few minutes. In this way the most impure acid may be rendered absolutely free from nitric acid and nitrous oxide.

Arsenic can be got rid of by adding a little sulphuret of barium, or of copper foil, to the acid, agitating the mixture well, and, after repose, decanting or distilling it.

Saline matter may be removed by simply redistilling {rectification.) The distillation is best conducted on the small scale, in a glass retort containing a few platinum chips, heated by a sand-bath or gas flame, rejecting the first 1/2 fluid ounce that comes over.

3861. Test for Nitric Acid in Sulphuric Acid

3861.    Test for Nitric Acid in Sulphuric Acid. Place in a watch glass a small portion pure and concentrated sulphuric acid at a density of 1.84; then pour, drop by drop, half the quantity of a solution of sulphate-of aniline, prepared by mixing commercial aniline with diluted sulphuric acid. A glass rod is dipped in the liquid to bo tested, and then stirred in the contents of the watch glass; from time to time the experimenter should blow slowly on the agitated liquid; if the liquid thus stirred contains traces of nitric acid, circular lines of a deep red are soon visible, coloring the whole liquid to a pink. On adding a very small quantity of nitric acid to the mixture, the liquid becomes of a carmine color; the addition of a single drop of very dilute nitric acid renders the liquid a deep red, and afterwards a dead red.

3862. To Remove Nitric Acid from Sulphuric Acid

3862.    To Remove Nitric Acid from Sulphuric Acid. Diluted sulphuric acid may be deprived of any small quantity of nitric acid it may contain, by shaking it up for a few minutes with a little powdered (freshly burned) charcoal, and afterwards filtering it. This will not answer for concentrated sulphuric acid; nitric acid is separated from it with great difficulty, and only by very protracted methods.

3863. To Decolorize Sulphuric Acid

3863.    To Decolorize Sulphuric Acid. Acid which has become brown by exposure may be decolorized by heating it gently; the carbon of the organic substances is thus converted into carbonic acid.

3864. Sulphurous Acid

3864.    Sulphurous Acid. This acid is used to bleach silks, woolens, etc., (see No. 171G), and to remove vegetable stains and iron-moulds from linen. For these purposes it is prepared from sawdust, or any other refuse carbonaceous matter. The salts formed by the combination of sulphurous acid with a base arc called sulphites. (See Nos. 1717 (Sulphuration) and 1718.)