COMMERCIAL HYDROCHLORIC ACID. 
107 
As to the sulphurous acid, which now and then exists in 
this acid, and arises from the circumstance, that towards the 
end of the operation a part of the sulphuric acid in excess 
contained in the residue has been decomposed by the eleva- 
tion of temperature, it is easy to recognize its presence by 
saturating the acid diluted with water, by a solution of baryta, 
collecting the precipitate upon a filter and drying it at a mo- 
derate heat. This precipitate of sulphite of baryta, heated 
in a tube, is decomposed, and furnishes sulphur, which sub- 
limes, and is converted into sulphate of baryta: treated with 
hydrated sulphuric acid, it immediately exhales a lively and 
penetrating odor of sulphurous acid. 
M. Girardin, Professor of Chemistry at Rouen, proposes, 
in order to discover the existence of sulphurous acid in the 
commercial hydrochloric acid, a process founded upon the de- 
composition of sulphurous acid by protochloride of tin. 
This is the method of operating — - 
. Put into a test glass about sixteen grammes (half ounce) of 
the acid to be tested, add to it eight or ten grammes of crys- 
tallised protochloride of tin, which dissolve by stirring with 
a glass tube, and pour upon the whole two or three times its 
volume of distilled water. 
When the acid is pure no change of color takes place, but 
however small a quantity of sulphurous acid it may contain, 
it will be seen, as soon as the protochloride of tin is added, 
that the acid loses its transparency, becomes yellow, and 
from the time the water is added, the odor of hydrosulphuric 
acid is perceived, and the liquid turns brbwn, depositing a 
powder of the same color: this precipitate is a mixture of 
the protosulphuret of tin and bioxide of the same metal. 
In this reaction the tin of a part of the protochloride be- 
comes free, and decomposes the sulphurous acid, so as to pro- 
duce all at once the two compounds above indicated; as to 
the disengagement of the hydrosulphuric acid, it is owing to 
the solution of a little protosulphuret of tin formed by the 
hydrochloric acid which is present. 
This simple and ready process offers the means of testing, 
