How does permanganate cause stains- and also remove them?

I did a redox titration experiment with acidified potassium permanganate and iron(II) sulphate. I wonder why potassium permanganate stains our skin on drying...? Phoebe 10/07/99

	Vocabulary
	insoluble oxidation oxidizing agent permanganate reduction

Potassium permanganate (KMnO₄) stains fabrics, fingers, and paper a dark brown color. The coffee-colored stains often can't be scrubbed off, and they may persist until the outer layer of stained skin wears off (in a day or two). It's more than just a nuisance for students and laboratory workers. Municipal water supplies are sometimes overtreated with permanganate ion to kill microbes and parasites, and when that water is chlorinated, swimming pools and porcelain can be badly stained.

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( ell ) + 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( ell )

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 [1]. The same trick can be used to remove indelible inks from concrete [2]. Photographers use permanganate to bleach stains caused by certain developing chemicals.

References

Bookbinding and the Conservation of Books, D. Etherington and M. T. Roberts. URL: http://palimpsest.stanford.edu/don/dt/dt2556.html, viewed 10/06/99
CBD-153. Removal of Stains from Concrete Surfaces,V.S. Ramachandran, J.J. Beaudoin, National Research Council Canada (1972). URL:http://www.nrc.ca/irc/cbd/cbd153e.html, viewed 10/06/99

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! How does permanganate cause stains- and also remove them?