# Explaining the result of experiment---Aluminum foil heated in boiling KMnO4 solution

I've recently been thinking back to an impromptu "experiment" I performed almost exactly 3 years ago. The setup was as follows:

A concentrated solution of KMnO$$_4$$ was produced by dissolving 2 grams of Condy's crystals in 250 ml of water. This was subsequently poured into an Erlenmyer flask containing a strip of aluminum foil, and the entire system was then heated at high on a stove until the water came to a boil. As the water evaporated, the solution lost the intensity of its violet hue, turning a light pink-violet instead. The stove was then turned off, and the aluminum strip was removed, having turned a golden color.

My questions are two:

• What, precisely, was the chemical reaction(s) that occurred?

• How much of a risk did the steam from the boiling solution pose in terms of potential acute manganese inhalation toxicity---and what are the potential long-term effects? Unfortunately, this experiment was performed on an ordinary kitchen stove and not in a fume hood (I'm kicking myself to this day).

My guess as to the former is that MnO$$_2$$ was produced, hence the observed color changes of the solution and foil. I am not sure whether this occurred via heat decomposition of the MNO$$_4^-$$ (which seems to occur at temperatures of >240$$^\circ$$C, and thus should not have been able to take place in boiling water) or oxidization of the aluminum (having somehow penetrated the surface layer of aluminum oxide). As for the latter, welp.

Thanks in advance!

• Maybe solid KMnO4 decomposes at $>240^\circ\rm C$. A solution, however, is quite a different story. – Ivan Neretin Sep 19 '19 at 9:32

## 1 Answer

Potassium permanganate solution tends to decompose with time, especially those which are not acidified. Just let an aqueous solution sit in a glass bottle and it will leave a brown film of tenacious manganese dioxide on the surface at room temperature. If water has any organic material, it will become brown and tap water is not pure water. It decomposes on the skin as well leaving a brown stain. You Al surface is no different, with a thin deposit of manganese dioxide, it may appear yellowish-golden.

Long time ago potassium permanganate was added to water wells to disinfect water. Of course it would decompose with all the organics. If you are well after three years, what is the problem?