Sulphur dioxide can be used as :

..Sulfur dioxide gas turns moist blue litmus red but bleaches the colour of litmus.  Sulfur dioxide gas dissolves in water to form sulphurous acid.
SO₂​+H₂​O→H₂​SO_3​
When blue litmus changes its color to red, it indicates acidic solution.
Sulfur dioxide gas acts as a bleaching agent because in presence of water, it is oxidised to sulphuric acid with liberation of nascent hydrogen which reduces colouring matter to colourless.