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Author: Subject: Gold Coloured Mg?
National Hazard

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[*] posted on 7-5-2011 at 16:11
Gold Coloured Mg?

In attempt to try out some of the transition metal chemistry we've been learning at school, I put Mg turnings in an acidified (H2SO4) solution of Mn7+ ions to try to reduce them. A lot of frothing ensued once it was heated and the Mg turnings I was left with appear a dull gold colour. I have both washed them and left them for a period of time and the colour remains. Any ideas why?

Thanks for the help!
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[*] posted on 8-5-2011 at 04:46

I assume you had an excess magnesium in there and not all of it reacted away. Most likely cause in that case is a thin oxide/hydroxide film was formed, only a few molecules thin. This can cause thin layer diffraction and cause the metal to take on ‘unnatural’ colours. Very slight surface oxidation (usually electrolytically) is often used to colour the surface of various metals.
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[*] posted on 8-5-2011 at 05:09

Another explanation in this particular case may be that a very thin layer of (hydrous) MnO2 is deposited on the Mg. Hydrous MnO2 has a strong tendency to stain everything with a brown color, including glass and metal items. It can easily be removed by putting the item in a very dilute solution of H2O2 and some acid.

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