260 OXY-ACIDS AND SALTS OF THE NON-METALSThe iodine is then reduced by more sulphurous acid to hydriodic
acid, which is colorless,I 2 + H 2 O + H 2 SO 3 -» H 2 SO 4 + 2HI- Sulphur Dioxide. Burn some sulphur in a deflagrating
spoon in a bottle. Note the characteristic odor of the gaseous
product. Lower into the bottle a piece of filter paper soaked
in potassium iodate-starch solution and note the same effects
as in Experiment 6.
Sulphur burns forming sulphur dioxide SO 2. The effect with
the potassium iodate-starch paper constitutes a convenient test
for sulphur dioxide.
- Sulphurous Acid, (a) Bubble sulphur dioxide into a
bottle of water and note that it dissolves freely. The solu-
tion colors litmus red and it conducts electricity fairly well.
(b) Test for Sulphurous Acid. To 5 cc. of sulphurous acid
add 1 cc. of 6N HC1 and 2 cc. of BaCl 2 solution. A small
precipitate or a slight clouding will probably occur at this
point, for the sulphurous acid solution invariably contains
some sulphuric acid if it has stood exposed to the air. Pour
the liquid through a filter; the filtrate may come through
cloudy, in which case pour it repeatedly through the same
filter until it is clear. To this filtrate add bromine and note
that the red color disappears and a white precipitate is formed.
Sulphurous acid is a rather weak acid, and presence of the strong
acid HC1 prevents the formation of SO 3 ~~ ions in sufficient con-
centration to precipitate with Ba++ ions. The first precipitate
removes all the sulphuric acid. Then the addition of bromine
oxidizes the sulphurous acid to sulphuric acid and another pre-
cipitate of barium sulphate is produced.H 2 SO 3 + Br 2 + H 2 O -»• H 2 SO 4 + 2HBr- Reducing Action of Sulphurous Acid. To 5 cc. of
potassium permanganate solution add sulphurous acid. The
deep purple solution becomes colorless.
5H 2 SO 3 + 2KMnO 4 -»• K 2 SO 4 + 2MnSO 4 + 2H 2 SO 4 + 3H 2 O