January 1, 2010
I actully wrote up this piece quite some time ago, but thought I'd include it here.
Sodium permanganate! Yes, it exists, but why doesn't anybody talk of it? I've always wondered this and I finally have myself a legitimate answer. And with this new knowledge, I may just be able to synthesize in my backyard.
Generally speaking, sodium salts are cheaper than their potassium analogs, but often they are not as useful because either they 1) are hygroscopic, 2) have less favorable solubility paramaters, or 3) don't crystallize easily. In the case of sodium permanganate, all three of these cases apply.
Still, it is odd that there is so little information available about sodium permanganate. A quick google search of "potassium permanganate" gives 1,080,000 results, while "sodium permanganate" only yields 41,100 results.
Potassium permanganate is most commonly produced by fusing molten potassium hydroxide and manganese dioxide together. An optional oxidizer such as potassium nitrate or potassium chlorate may be added to speed this process up. The fusion results in the production of the green solid, potassium manganate. Potassium manganate is then further oxidized to potassium permanganate either through a disproportionation reaction with a weak acid that is resistant to oxidation (usually carbonic acid is employed here) or via electrolysis.
Not a terribly complex affair! In fact, several preparative inorganic chemistry books describe the entire process in great length.
Here's where things get interesting. I had always assumed that sodium permanganate would be produced in an analogous manner; namely by reacting manganese dioxide with molten sodium hydroxide. And in this, some literature, albeit sparse, seemingly backed me up.
"Sodium manganate (Na2MnO4), prepared by fusion of a mixture of natural manganese dioxide and sodium hydroxide; green crystals, soluble in cold water, decomposed by hot water."
"Sodium permanganate, NaMnO4, is obtained in a similar way to the potassium salt, and is distinguished from it by being deliquescent, and therefore, crystallizing with difficulty."
So off I went, performing nearly a dozen of experiments that all had the same underlying principle: fusing solid sodium hydroxide with manganese dioxide. I encountered problems right away. At first, I thought it was because I wasn't reaching the required temperatures, then because of carbon impurities and later zinc impurities in my manganese dioxide extracted from old batteries, and then because I discovered there was fumaric acid in my potassium chloride salt substitute! After purifying both my manganese dioxide and potassium chloride, I was still met with failure. I once did somehow manage to obtain the characteristic purple color of the permanganate ion by adding bleach to my sludge, but as you can see from the photograph of it to your right, it is extremely dilute. I had no chance to crystallize anything out and as soon as I began to boil the solution down it completely turned into the sad manganese dioxide.
This last experiment gave me hope at least, and I thought that maybe I just needed to increase my yields by heating this sucker at still higher temperatures and for longer periods of time. Out of frustration, I took an extended hiatus from the project and during this interim, I discovered what may be the key to this mystery!
"The price of sodium permanganate is about 5 to 8 times that of KMnO4. This is mainly due to the fact that NaMnO4 cannot be made in the same way as KMnO4, because the oxidation of MnO2 in a NaOH melt does not lead to the required Na2MnO4 (with hexavalent Mn) but only to Na3MnO4 with pentavalent Mn. The latter is very unstable in dilute NaOH solution (and therefore cannot be converted electrolytically to the desired NaMnO4). Even if electrolytic oxidation were possible, there would still be the difficult problem of isolating the extremely soluble NaMnO4 from the alkaline mother liquor."
Aha! This excerpt from Ullmann's Encyclopedia seems to answer all of my questions! You cannot produce sodium permanganate in the same way that you can produce potassium permanganate! Chemically, I still don't understand why. If anyone has an inkling as to why molten potassium hydroxide is a more potent oxidizer than molten sodium hydroxide in this case, please leave me a comment on the guestbook or e-mail me at admin at backyardchem dot com (at and dot are spelled out to avoid spam)!
So basically, during all of my experiments, I was essentially performing the following reaction:
4MnO2 + 12NaOH + O2 --> 4Na3MnO4 + 6H2O
And then when I extracted the mass with water, I got:
2Na3MnO4 + 2H2O --> Na2MnO4 + MnO2 + 4NaOH
On one lucky occasion this occurred:
3Na2MnO4 + 2H2O --> 2NaMnO4 + MnO2 + 4NaOH
These equations seem to accurately describe what I witnessed in my experiments. It is probably true that in all cases, my yields were extremely low.
At this point, there was still one part of the puzzle that was missing. I had discovered these passages describing the industrial production of sodium permanganate:
"Sodium manganate, Na2MnO4, is formed when a mixture of equal parts of manganese dioxide and soda-saltpetre is heated for sixteen hours; the mass is then lixiviate with a small quantity of water and the solution cooled down, when the salt separates out in small crystals isomorphous with Glauber's salt, and having the composition Na2MnO4-10H2O. These dissolve in water with partial decomposition, yielding a green solution."
"For disinfecting purposes it is not necessary to employ the pure, well-crystallized salt [potassium permanangate], which is used in the laboratory, but a commercial article consisting of a mixture, more or less pure, of manganate and permanganate of sodium is used. The substance is obtained by mixing the caustic soda obtained from 1,500 kilos of soda-ash with 350 kilos of finely-divided manganese dioxide in a flat vessel, and heating this mixture for forty-eight hours to dull redness. The product is then lixiviated with water, and the solution either boiled to the requisite degree of strength or evaporated to dryness. If the manganate is to be completely converted into permanganate it is neutralized with sulfuric acid, the solution concentrated until Glauber's salt separates out, and these crystals are then removed and the liquid further evaporated."
What? These sources point to the direct production of sodium permanganate! I thought Ullmann's Encylopedia said that was impossible!
Well, it is impossible with sodium hydroxide, but not with good old soda, sodium carbonate at elevated temperatures (probably somewhere around 600C) and with prolonged heating. The set of reactions is probably:
4MnO2 --> 2Mn2O3 + O2
2Mn2O3 + 4Na2CO3 + 3O2 --> 4Na2MnO4 + 4CO2
This proposal has instilled hope in me once again for the backyard production of sodium permanganate (and from there potassium permanganate)! Once I finish building my furnace capable of withstanding such high temperatures, I am giving this one a try!