MadSci Network: Chemistry

Re: How does peroxide bleach remove color?

Date: Sat Jan 22 21:54:28 2000
Posted By: Jonathan Feldman, Undergraduate, Applied Chemistry, University of Western Sydney
Area of science: Chemistry
ID: 942268095.Ch

Hi Larry,

Hydrogen peroxide bleaching works by oxidizing the colour pigments.

The colours in the pigments are due to alternating single and double carbon bonds, which absorb light in the visible spectrum. When bleaches attack these pigments, the some of the double bonds are split into single bonds, which breaks up the colour-absorbing system.

In the bleach process, Hydrogen peroxide (H202) is reduced to water (H20) in the prescence of acid (H+).

  H202 + 2H+ + 2e-  ---->  2 H20
In terms of whether a higher or lower pH is more effective, all the textbooks I consulted agree that bleaching is generally more effective in an acidic (low pH) solution (hence, the equation above).

I hope this answer has been of some assistance,

Jonathan Feldman
University of Technology, Sydney

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