102 NON-METALS AND THEIR COMBINATIONS. 



Sulphurous acid, Acidum sulphurosum, H 2 SO 3 = 82. One 

 volume of cold water absorbs about 40 volumes of sulphur dioxide, 

 equal to about 11 per cent, by weight. The official acid must contain 

 not less than 6.4 per cent, by weight, equal to about 2250 volumes of 

 gas dissolved in 100 of water. According to the U. S. P. the acid 

 is made by generating sulphur dioxide from charcoal and sulphuric 

 acid in a flask, and passing the gas through a wash-bottle containing 

 water, into distilled water for absorption. 



Experiment 10. Use an apparatus as shown in Fig. 11. Place in the flask 

 about 20 grammes of charcoal in small pieces, cover it with sulphuric acid, 

 apply heat, and pass the generated gas first through a small quantity of water 

 contained in the wash-bottle, and then into pure water, contained in the 

 cylinder. 



FIG. 11. 



Apparatus for making sulphurous acid. 



The solution, sulphurous acid, may be used for the tests mentioned below ; 

 when the neutral solution of a sulphite is required, make this by adding solu- 

 tion of sodium carbonate to a portion of the sulphurous acid until litmus-paper 

 shows neutral reaction. Examine also the contents of the wash-bottle by 

 means of the tests given below for sulphuric acid ; most likely some of the 

 latter will be found. How much carbon and how much H 2 S0 4 are required to 

 make 100 grammes of a 6.4 per cent, sulphurous acid ? 



Thus obtained, sulphurous acid is a colorless acid liquid, which has 

 the odor as well as the disinfecting and bleaching properties of sulphur 

 dioxide ; it is completely volatilized by heat. Sulphurous acid is a 

 dibasic acid, the salts of which are termed sulphites. 



