The gas is a reducing agent and reduces a Fe(III) salt to Fe(II) salt.
`({:(SO_(2)+2H_(2)O to SO_(4)^(2-)+4H^(+)+2e^(-)),(2Fe^(3+)+2e^(-) to 2Fe^(2+)):})/(2Fe^(3+)+SO_(2)+2H_(2)O to2Fe^(2+)+SO_(4)^(2-)+4H^(+))`
For example, ferric sulphate is reduced to ferrous sulphate.
`2Fe_(2)(SO_(4))_(3)+SO_(2)+2H_(2)O to 2FeSO_(4) +2H_(2)SO_(4)`.
