Concentrated sulphuric acid is usually a brownish colored liquid, which chars or corrodes wood or other organic matter brought into contact with it, and when mixed with water gives out heat. When diluted, its presence may be thus detected:
1. The liquid is known to be acid by its action on litmus paper.
2. Add to a portion of the suspected liquid a few drops of nitric acid, and then a solution of nitrate of barium; a white precipitate (sulphate of barium) will fall if sulphuric acid be present. This test is extremely delicate; for although other acids yield a precipitate on the addition of nitrate of barium, yet as such deposits are all soluble in nitric acid the previous addition of this acid will prevent their formation.
3. The precipitate should next be collected, dried, and reduced with charcoal by the blow-pipe flame to the condition of barium sulphide. This, when treated with a drop of hydrochloric acid, gives off sulphuretted hydrogen, known by blackening paper dipped in acetate of lead solution.
To examine a piece of cloth stained with this poison it is only necessary to boil it in distilled water and then apply to the liquid the barium test as before.
Nitric Acid (Aqua fortis, Red Spirit of Nitre).—This substance has been employed as a poison for upwards of four centuries. Like the oil of vitriol it is found in commerce in a concentrated and in a diluted state. Cases of poisoning by it are rare. It produces a yellow stain on the skin. Two drachms is the smallest quantity which has destroyed life; but less than this would probably prove fatal, if it produced much corrosion about the wind-pipe. Death has occurred from it, in one hour and three quarters; the average would be within twenty-four hours.
Several cases ending fatally have followed the inhalation of the fumes of this acid, probably by inducing very extensive inflammation of the lung.
Tests.—The concentrated acid may be known by its orange-colored irritating fumes, and by its action on copper, tin, or mercury.
1. When poured on copper-filings, effervescence takes place, a red acid vapor is given off, and a green liquid remains (solution of nitrate of copper).