Acidum Sulphurosum, Sulphurous Acid, Hso, Official -. This aqueous solution of sulphur dioxide, SO2, 6.4 p. c, by weight, and water about 94 p. c, is obtained by heating in a flask sulphuric acid 60 Ml. (Cc.) and charcoal 20 Gm., passing the evolved gas through a wash-bottle containing water 50 Ml. (Cc), thence into well-cooled distilled water 500 Ml. (Cc), contained in a bottle of 1000 Ml. (Cc.) capacity, adding water q. s. to give 6.4 p. c, by weight, of sulphur dioxide. The charcoal acts as a reducing agent upon the sulphuric acid - C + 2H2SO4 = 2SO2 + 2H2O; sulphurous acid does not exist in the free state, but undoubtedly is present in solution when sulphur dioxide is passed into water. It is a colorless liquid, characteristic odor of burning sulphur, acid sulphurous taste, volatile, sp. gr. 1.028; exposed to air oxidizes to sulphuric acid of same strength (6.4 p. c). Antiseptic, germicide, parasiticide, deoxidizant, disinfectant, destiuctive to plant-life, deodorizer (due to great affinity for oxygen, abstracting it from organic bodies), irritant (SO2), arrests fermentation, putrefaction by destroying the germs - 1 p. c solution by volume, killing most micro-organisms; throat affections locally (spray, mop), diphtheria, chronic bronchitis, tonsillar ulcers, aphthae, stomatitis, laryngitis, wounds, scabies, thrush, whooping-cough; internally - urticaria, fermentative dyspepsia, flatulence, vomiting (frothy, yeasty). Should be kept cool, dark, in completely-filled, glass-stoppered, amber-colored bottles, and not dispensed unless of full strength (strong odor of burning matches - blue-head). Dose, 3ss-2 (2 8 Ml. (Cc.)), diluted with water; externally, diluted with water, glycerin, or fat.