Potassium Permanganate (KMnO4) is an extremely powerful oxidising agent and is generally found as interesting purple crystals which look reasonably similar to iodine crystals. Its oxidising properties come from the permanganate ion which contains manganese in the formal +VII oxidation state. The permanganate ion reacts readily to give various manganese oxides, the simplest being manganese oxide (MnO) with manganes being in the formal +II oxidation state, a potential oxidation of 5 electrons per molecule.

Permanganate is readily available for general uses that usually focus on its bactericidal properties (which are a result of its oxidising power, similar to hypochlorite bleach) such as disinfecting aquariums where bacteria/fungi would prove harmful to fish (obviously, the fish are removed prior to cleaning the tank).

It is also used in organic chemistry for converting alkenes to diols, an occasionally useful reaction in synthesis due to the low cost of permanganate and many recognised methods of producing alkenes.

It was originally marketed in solution by Henry Condy in the mid 1800s, who later crystallised the active permanganate and sold it as 'Condy's Crystals'

Permanganate has relatively few dangers for such a reactive chemical. It is relatively non-toxic and is classified as harmful (meaning unless you eat/snort/otherwise ingest it you should be fine but gloves would be a good idea when handling the stuff and washing hands afterwards is always sensible). It stains lots of things brown (apparently from the formation of MnO2) and I have had stains on my hands for about a week in the past. Wikipedia informs me that acetic acid (vinegar) can work in some cases for removing stains from clothes. Obviously, as a strong oxidising agent it is a good idea to keep away from heat (it's surprising how many things can be oxidised if heated enough), strong acids (but unless you're in a chem lab or a hobby chemist that's unlikely, it's dangerous because it forms explosive and unstable manganese compounds) and not to flush down the drain (oxidisers play merry havoc with marine life, see the bit about fish tanks above). Finally, and this may seem obvious, do not mix with easily oxidised compounds. This is anything that burns easily. Because KMnO4 is solid it will take heat or liquid to make it react and it will be delayed but it is still dangerous. The classic example is addition of KMnO4 to glycerin (glycerol is just pure glycerin, often found in cough medicines). After a short delay the mixture bursts into flame with enough heat to ignite thermite.

It should be noted that because of its oxidising properties, any mixture of permangantes and a fuel will be flammable and potentially explosive. In effect, mixing the right ratio of permangante to fuel will provide a gunpowder substitute with a substantially faster release of energy, generally as LOTS of heat and light. This is because gunpowder is also a mixture of fuel and oxidiser, however, the oxidiser used in black powder is a nitrate (traditionally potassium but sodium nitrate is used as well) which are less powerful than permanganates. It's the metaphorical difference between lighting a barbeque with a match and a flamethrower, both will get it to light eventually but the flamethrower will have a much faster effect. Increasing the reactivity of the other reactant will also increase the burn rate and amount of energy released, to carry on the metaphor it's now lighting a stick of dynamite rather than a barbeque.

I should stress at this point that I have played with permanganate in making small amounts of firework mixtures and then lighting them. I do not recommend you do this, it is frankly suicidal. This is not the obligatory, 'I've posted instructions that I know people will follow, I'd better put up a warning about informational purposes in case I get sued' warning, it's a 'I have done this, been blown up/set on fire, spent weeks in hospitals and been left permanently scarred on various bits of my body' warning. I will create a node at some point detailing my adventures with pyrotechnics, for now please just accept that this node is details on permanganate and it's dangers, not a 'How-to' guide on loss of skin/fingers/life. I have intentionally left out fuels and ratios, any half decent chemist should be able to work out the ideal fuel and stochiometrics.

Potassium Permanganate is also good for treating athlete's foot, that is if you don't mind walking around with brown feet for a few weeks.

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