3865. To Obtain Sulphurous Acid

3865.    To Obtain Sulphurous Acid. In the gaseous form this acid is freely evolved by burning sulphur in air or in dry oxygen. It is also given off during the digestion of metals in hot sulphuric acid. "When charcoal, wood, or cork chips, or sawdust are digested in hot sulphuric acid, a mixture of sulphurous and carbonic acids is obtained, which is used for bleaching and cleansing purposes.

3866. Pure Gaseous Sulphurous Acid

3866.      Pure Gaseous Sulphurous Acid. This is evolved during the action of sulphuric acid on mercury or clippings of copper. It is also obtained pure by heating in a glass retort, a mixture of 100 parts black oxide of manganese, and 12 or 14 parts sulphur. The gas evolved should be collected in a receiver over mercury.

3867. Sulphurous Acid Solution

3867. Sulphurous Acid Solution. The gas obtained according to the last method is to be passed through water, which is capable of dissolving or absorbing 30 times its bulk of the gas. To avoid waste in preparing the solution, the unabsorbed gas -which escapes from the water is usually again passed through water, and the same arrangement repeated through a series of vessels of water so long as any gas escapes undissolved.

3868. Pure Sulphurous Acid

3868.     Pure Sulphurous Acid. In order to prepare sulphurous acid from sulphuric acid and charcoal, it is better to employ an acid of .74 per cent., or 1.825 specific gravity. If we take a stronger acid, a part of it is entirely deoxidized to sulphur, and if weaker acid be employed, sulphuretted hydrogen is evolved. To obtain absolutely pure sulphurous acid, it is well to put sulphite of lead and coarse charcoal in the wash bottle. "With these precautions, it is possible to obtain pure sulphurous acid from sulphuric acid and charcoal.

3869. Pure Liquid Sulphurous Acid

3869.    Pure Liquid Sulphurous Acid. This can only be obtained by passing the pure dry gas through a glass tube surrounded by a powerful freezing mixture. The specific gravity of the pure liquid gas is 1.45; its boiling point is 14° Fahr., and causes intense cold by its evaporation.

3870. Hydrosulphuric Acid, also Called Sulphuretted Hydrogen

3870. Hydrosulphuric Acid, also Called Sulphuretted Hydrogen. When sulphur acts upon paraffine at a temperature a little above the melting point of sulphur, hydrosulphuric acid gas is evolved in large quantities, and this method may be advantageously used for its generation in the laboratory. A flask, holding about a pound of the material, is fitted with a tube bent at right angles, about 1/2 inch bore and 12 to 18 inches long, containing cotton wool, and to this is attached the small tube for precipitation. The production of gas may be stopped by removing the heat. Heavy paraffine oil, stearic acid, or suet, may be used as a substitute for paraffine.