What are sulphur dioxide bleaches in the presence of?

1. (b) Sulphur dioxide is a good reductant. In the presence of water, SO2 is able to decolorise substances. Specifically it is a useful reducing bleach for papers and delicate materials such as clothes. This bleaching effect normally does not last very long. Oxygen in the atmosphere reoxidises the reduceded dyes, restoring the colour. read more

Sulphur dioxide access is a bleaching agent and bleaches vegetable colouring matter in presence of moisture. You can observe here how moist flower looses colour. The bleaching action sulphur dioxide is due to its reducing action. read more