Permanganate itself is a deep purple color (almost black). The brown stain develops when permanganate decomposes into crystalline manganese dioxide on the skin or fabric surface:
4 MnO4-(aq) + 4 H+(aq) = 3 O2(g) + 2 H2O() + 4 MnO2(s)The tiny crystals of MnO2 become trapped in the pores and complex surfaces of the skin. The crystals are insoluble, and washing with water will only dislodge a few loose crystals. If the stain is on a nonliving surface, try treating it with a weak hydrochloric acid solution. Hydrochloric acid can reduce the MnO2 to water-soluble Mn2+:
MnO2(s) + 4 HCl(aq) = MnCl2(aq) + Cl2(aq) + 2 H2O()
Curiously, permanganate's oxidizing power makes it very useful for removing stains. Permanganate oxidation breaks the molecules of large, highly colored substances into smaller pieces. The excess permanganate is then removed chemically or by quickly rinsing.
For example, book collectors remove stains from old paper by washing the paper with a weak solution of permanganate; a rinse of sulfurous acid is used to remove the permanganate before it locks into the paper fibers and turns it brown . The same trick can be used to remove indelible inks from concrete . Photographers use permanganate to bleach stains caused by certain developing chemicals.
Author: Fred Senese email@example.com
Copyright © 1997-2010 by Fred Senese
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/redox/faq/print-permanganate-stains.shtml