I'm trying to synthesize phases M2EF6:Mn4+ (M - Na, K, E - Si, Ti, Ge etc.) in nanostate using hydrothermal process. As a source of manganese I used KMnO4. Other materials in the typical synthesis include, for example, SiO2, HF, KF and, naturally, water (deionized). Temperature of synthesis is around 200C.
At first I was trying to introduce a reducing agent to transform Mn7+ into Mn4+, but after couple of attempts I found out that not only it is not needed at all, but on the contrary, even without ANY additives the problem is to avoid reduction up to Mn2+.
Therefore, I have two questions: 1) how to avoid reduction to Mn2+ in conditions as such? 2) what is the reducing agent in my process? As it puzzles me a lot too.
In acidic medium Mn(VII) is reduced to Mn(II).
While in neutral or basic (except strongly basic) madia Mn(VII) is reduced to Mn(IV). For example permanganate is reduced to the insoluble mangnanese dioxide MnO2.
In strongly basic medium Mn(VII) is reduced to Mn(VI) for example permanganate is reduced to the green manganate,
So to avoid formation of Mn(II) the reaction medium should not be acidic.
If the medium is quite acidic due to use of HF, then reduction of Mn(VII) will not stop at Mn(IV).
It is well known that reaction of an aqueous solution of a Mn(II) salt (for example manganous sulfate) with potassium permangante results in the formation of MnO2 (where permangnate is reduced to Mn(IV) level). The reducing agent is Mn(II).
Dear Bikshandarkoil R Srinivasan,
thank you for your answer, but the main question is WHAT reduces Mn7+ in this system. There's no Mn2+/4+ initially, so comproportionation is not the case. The only thing I can imaging is reducing F- with release of F2. How realistic is that I don't know. Such reaction is possible with HCl, but about HF - I don't know. After I understand this point, it will be easier to find out, how to stop this process (if possible).
If KMnO4 is strongly heated in a pure state, it decomposes itself to K2MnO4 and MnO2. This could be a complex cascade of reactions which eventually lead to lower oxidation states when other elements are present. My understanding was that KMnO4 cannot oxidize F(-) to elemental F2, since this is the most powerful oxidizing agent known.
Dear M. Farooq Wahab, thank you! It's an interesting idea, but, well, I can imagine formation of MnO2 in the process like you've mentioned. But that it decomposes itself up to Mn2+. How this is possible? It means, that MnO2 is stronger oxidizer than oxygen, which would be released.
I think there is some other component which is further reducing MnO2 under the action of heat. Do you have access to Mellor's Treatise on Inorganic Chemistry? He wrote multivolume descriptive chemistry of elements. It is an old treatise, but best in my opinion in terms of descriptive chemistry. Please check the volume on Mn. There you may find decomposition reactions of MnO2. When you detect Mn2+, what counter-ions do you have in solution?
It's F- mostly, as there is huge excess of KF and HF in this process. Also SiF62- and maybe MnF62-.
The microorganisms (organic in nature) present in water can be a possible source of reducing agent to reduce Mn+7. To verify if this is so one can try using sterile water or preferably water distilled over KMnO4. Vogel's textbook of quantitative inorganic analysis recommends an elaborate procedure for preparation of KMnO4 solution starting from using glassware cleaned with chromic acid; Please note that commercial KMnO4 contains a very trace amount of MnO2 due to which KMnO4 is not a primary standard.
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