Detection of Sulphurous Acid, fyc. 225 
The mode of operating is as follows: 
About half an ounce of the hydrochloric acid to be tested, 
is to be poured into a glass, and two or three drachms of white 
and unaltered salt of tin added to it, the mixture is to be stirred 
with a glass tube, and two or three times as much distilled 
water added. 
When the hydrochloric acid contains no sulphurous acid, 
no. remarkable phenomenon takes place on the addition of the 
hydrochloride of tin or the water ; the first dissolves, and the 
solution only becomes turbid after standing some time, owing 
to the action of the air on the salt. 
But if it contains sulphurous acid, immediately after the 
addition of the salt of tin, the acid will become turbid, yel- 
low, and when the water is added, an odour of sulphuretted 
hydrogen is perceptible, and the liquor assumes a brown col- 
our in depositing a powder of the same colour. These phe- 
nomena are so apparent, that there can be no hesitation in 
deciding on the presence or absence of sulphurous acid. 
Sometimes the brown colour is not developed for some 
minutes, and it is dark in proportion as the quantity of sul- 
phurous acid is greater. The disengagement of sulphuretted 
hydrogen takes place only on the addition of the water. By 
leaving the coloured liquid to stand, it deposites a powder of 
a yellow brown colour, this is a mixture of sulphuret of tin, 
and peroxide of the same metal. 
This curious reaction is readily explainable. A portion of 
the salt of tin is transformed into perchloride at the expense 
of another portion, whilst the tin thus set free reacts on the 
sulphurous acid, so as to produce both the peroxide and pro- 
tosulphuret. As to the small quantity of sulphuretted hydro- 
gen which escapes on the addition of the water, it arises from 
the decomposition of a little of the sulphuret formed in the 
hydrochloric acid. 
It is essential to obtain these phenomena, that the salt of 
tin should be added to the hydrochloric acid before the water, 
for if the acid be first diluted, the addition of the salt will in- 
duce no change of colour. 
This method is recommended from its simplicity and easy 
Vol. L— No. 3. 29 
