This section is from the book "Materia Medica And Therapeutics Inorganic Substances", by Charles D. F. Phillips. Also available from Amazon: Materia medica and therapeutics.
The pharmacopoeial sulphurous acid is a solution of the gas in water, containing 9.2 per cent. by weight = twenty times its volume.
By deoxidizing sulphuric acid by distilling it with coarsely-powdered wood charcoal. The carbon combines with part of the oxygen of the acid to form carbonic acid and probably carbonic oxide, while sulphurous acid gas (anhydrous) passes over into a receiver containing distilled water, being previously washed from sulphuric acid and other impurities.
2C + H1SO4 = CO2 + 2H1O + 2SO2
A nearly saturated solution of sulphurous anhydride, SO2, colorless, of strong suffocating odor and pungent acid taste, which, however, is not unpleasant in moderate dilution. It bleaches vegetable colors, and is an energetic oxidizing agent: it is said to absorb radiant heat in a high degree (R. Bird). The hydrated acid can be obtained in crystals but is very unstable. A solution of the officinal strength and upward oxidizes on exposure to light and air (e.g., when kept in partially-filled transparent bottles), with formation of sulphuric acid - an important change, since the special properties of the drug are thereby impaired or lost; contact with chlorine at once induces this change. It should therefore be carefully kept; or better still, be freshly prepared when wanted. Water would absorb forty or fifty times its bulk, alcohol still more; but Professor Redwood recommends a 5 per cent. solution, as more easily made and more stable than that of the British Pharmacopoeia.