Prep. By distilling sulphuric acid with wood charcoal in coarse powder.

Prop. & Comp. Water takes up 33 times its bulk of the gas: when fully saturated the solution is colourless, with the suffocating odour of burning sulphur; sulphurous acid is a powerful deoxidizing agent, liberating iodine from iodic acid, decomposing sulphuretted hydrogen, etc. Composition of sulphurous acid (So2).

The solution gives no precipitate, or a very slight one, with chloride of barium, indicating that no sulphuric acid is present, but a copious one if solution of chlorine be also added (sulphate of baryta). Sp. gr. 1.04. [Sp.gr. 1.035. U. S.] One fluid drachm mixed with a little mucilage of starch does not acquire a permanent blue colour with the volumetric solution of iodine, until 164 measures of the latter have been added, representing about 5 1/4 grains of sulphurous acid.

The formula representing the decomposition which ensues in the use of this volumetric test, is as follows: So2 + I + HO = So3 + HI; therefore, no blue colour appears until more than an equivalent of iodine has been added to each equivalent of sulphurous acid, or 3.2 grains of sulphurous acid require 12.7 grains of iodine, or 100 measures of its volumetrical solution; which is in the same ratio as 5.25 to 164 measures.

A solution of sulphurous acid when evaporated leaves no residue.

Therapeutics. Sulphurous acid has a destructive influence on vegetable life, and upon this its therapeutic value, probably, for the most part depends. Externally applied, it causes irritation and redness, and has been used for the treatment of skin affections, especially when connected with vegetation, as porrigo. It may be used in solution, more or less diluted, or applied as vapour from burning sulphur.

Internally it is not often employed in the free state, the vapour being suffocating in character.

See Sodae Sulphis, and Sodas Hyposulphis.

Dose. A strong solution of the acid may be diluted with about an equal bulk of glycerine or some other liquid, and painted on the affected skin.