Q. Bleaching action of moist sulphur dioxide is because of its
- oxidising property
- reducing property
- basic property
- acidic property
Answer: Reducing Property
The bleaching action sulphur dioxide is due to its reducing action. The colour of pigment on exposure to SO2 gets reduced to a colourless compound. On standing in an air the colourless reduced product is reoxidised and gain the original colour. Bleaching with sulphur dioxide is therefore of temporary nature.
Bleaches work by reacting with many colored organic compounds, such as natural pigments, and turning them into colorless ones. While most bleaches are oxidizing agents (chemicals that can remove electrons from other molecules), some are reducing agents (that donate electrons).
Sulfur dioxide is also a good reductant. In the presence of water, sulfur dioxide is able to decolorize 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 reoxidizes the reduced dyes, restoring the color. In municipal wastewater treatment, sulfur dioxide is used to treat chlorinated wastewater prior to release. Sulfur dioxide reduces free and combined chlorine to chloride.
Sulfur dioxide is fairly soluble in water, and by both IR and Raman spectroscopy; the hypothetical sulfurous acid, H2SO3, is not present to any extent. However, such solutions do show spectra of the hydrogen sulfite ion, HSO3−, by reaction with water, and it is in fact the actual reducing agent present:
- SO2 + H2O ⇌ HSO3− + H+
Sulfur dioxide is one of the few common acidic yet reducing gases. It turns moist litmus pink (being acidic), then white (due to its bleaching effect).
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