mrjeffy321
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Manganese Salt Problems
My goal is to make a soluble Manganese+2 salt (either Chloride, Nitrate, Sulfate).
I have [two types of] Manganese Dioixde at my disposal.
The obvious procedure would be to dissolve the MnO2 in Hydrochloric acid to produce Manganese(II) Chloride (giving off Cl2 in the process). I have
done this reaction before several times in the past using pottery grade MnO2 and it has worked fine.
But when I tried it this time, specifically for the purpose of creating MnCl2 instead of Cl2, I see no signs of a reaction occurring. It is very
strange, but the MnO2 does not seem to be reacting with the HCl at all.
I thought the problem might be with my MnO2, perhaps it is too impure, or the off chance that it is not MnO2 at all and it was labeled wrong by the
pottery supply company who sold it to me. So I went and bought some MnO2 from a laboratory supply company thinking that this would surely work.
But the MnO2 I received does not visibly react either and, to make maters stranger, it does not even look like the MnO2 I know.
All the MnO2 I have ever seen is a dark, dull, black powder. But the store-bought chemical that came in the bottle is shinny (maybe because the
particle size is larger?). Also, in addition to the “Manganese Dioxide, MnO2” on the label, it also says in parenthesis, “Manganese
Peroxide”. But the “Peroxide” ion is O2-2, so if the chemical formula written on the bottle is to be trusted (MnO2), then that would mean it is
Manganese(II) Oxide instead of Manganese(IV) Oxide, which would explain the different appearance. But I looked it up and I have seen that “Manganese
Peroxide” is supposed to be synonymous with Manganese Dioxide.
I have tested the MnO2 by mixing it with Hydrogen Peroxide to see if it would decompose the H2O2 into water and Oxygen gas. Both MnO2 powders pass
the test, a substantial amount of bubbles are observed when H2O2 is poured onto the powders.
So this shifts my suspicion onto the Hydrochloric acid I am using. But as near as I can tell, my HCl is fine….it is EXTREMELY old, but otherwise
fine.
Solutions of the HCl have a very low pH, will clean the oxide coating off Copper, and will react (slowly) with CuO to give a green colored solution.
So what is going on here? Why isn’t my MnO2 reacting with the acid?
Maybe you guys can think of something that I have overlooked.
Of course, one solution to this problem would be to simply order whatever soluble Mn+2 salt I wish and forget about making it myself, but this would
cost me a lot of time (it will probably take 2 weeks to get here, and I don’t have that long to wait, time is of the essence).
Another option would be the break open some batteries and take the MnO2 out from there, but the problem with that is the impurities/electrolytes
present in that source of MnO2….it is also important to me to have the Mn+2 (aq) be as pure as possible, without any KOH or other soluble compounds
mixed in.
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not_important
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If the HCl rapidly attacks aluminium foil, then it's strong enough. The reactivity of MnO2 depends on it history, how it was made and how hot it has
gotten. The chem supply MnO2 is likely made by thermal decomposition of Mn(NO3)2, which gives the purest grade; it is more stable to heat and has a
larger crustal size than many other forms of the dioxide.
You did heat the acid-dioxide mix ? :-)
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mrjeffy321
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Oh yes, that was a test I meant to perform but forgot to.
Using the old HCl and a few pieces of Aluminum foil (and even powder), the reaction begins VERY slowly. Eventually, the Aluminum will bubble
vigorously, as it should, but it has to warm up for several minutes first.
Using some HCl which I know to be MUCH newer than the HCl I was originally using, the HCl + Al reaction begins in only a few seconds and becomes quite
vigorous.
So it seems that the old HCl I was using is somewhat lacking in comparison to the new HCl….but it still should be strong enough to react with the
MnO2.
Using some of the newer HCl I added some of the chem. Store bought MnO2. Initially nothing at all appeared to happen. After several minutes had
elapsed it became apparent that the color of the solution was darkening but that it was not due to MnO2 in suspension. So a chemical reaction
apparently occurred, but I watched very carefully and I did not see (or smell) any Chlorine produced.
I did heat the HCl + MnO2 mixture, but it did not seem to help the reaction occur.
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not_important
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Yes, I'd expect the chem grade MnO2 to be less reactive, the ceramics grade is both fines particle size and has a bit of Mn(III) in it, I assume as
Mn3O4.
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12AX7
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Well obviously, you've performed the reaction before, so...just make more Cl2! Does it know what you're trying to produce here or something?
One thing I've done to try to reduce the Cl2 output is to reduce MnO2 with charcoal. I get a brown powder typically;
MnO2 + 2C = MnO + 2CO(g)
This is the ideal reaction, but light-colored MnO requires high temperatures and may be endothermic. As I recall, it occurs above orange heat and MnO
is prone to oxidation so needs to be protected from atmosphere when complete.
MnO2 + C = Mn2O3 + CO(g)
This is most likely the process, as I'm left with a brown, rather than black, powder. It is exothermic when heated to the reaction temperature, so is
convienient to produce.
Changing the form will probably make it a bit easier to react.
Do mind the gas release from these powdered reactions! The "boiling" mixture tends to "evaporate" itself, leaving you with a nasty mess.
Tim
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mrjeffy321
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| Quote: | Originally posted by 12AX7
Well obviously, you've performed the reaction before, so...just make more Cl2! Does it know what you're trying to produce here or something?
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It just might.
When I performed this reaction before (on several occasions), specifically for the goal of Cl2 production, I was using my own chemicals from my
private stash.
Right now, I am borrowing someone else’s chemicals and it all is very old (the HCl is 17 years old!) and from varied sources so I am not sure what
is still good and what is not.
I might try getting some brand new HCl and seeing if that makes a difference, that and seeing if I can reduce the MnO2 to MnO on a hot plate.
I am extraordinarily close to just giving up and buying it premade to save myself the aggravation.
How about Nitric acid?
Will the Nitric acid oxidize the Mn to some higher oxidization state in the process, or might this be a way to produce Mn(NO3)2?
I did quickly try this reaction, but to no avail, I did not see anything happening (but then again, the HNO3 is also 17 years old and I am unsure of
the concentration).
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12AX7
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Oh, and did you try HCl + H2O2? Or H2SO4. H2O2 ordinarily decomposes with MnO2, but if you catch it in the act with some acid, it reduces it quite
nicely. Do mind that excess H2O2 will slowly oxidize chloride.
Tim
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woelen
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mrjeffy321, the problem is not the HCl. If the HCl is concentrated, then it is OK, regardless of age. I also still have a glass bottle with 30% HCl,
20 years old, and it is as good as brend new HCl.
The problem is with your MnO2. As is the case with many metal oxides, when they are calcined/heated for prolonged time at high temperature, then the
oxides become VERY inert. A crystalline structure is formed, which makes them inert for dissolving in acids and in bases.
I have similar problems with MnO2. I have reagent grade MnO2, which was very expensive (EUR 13 for 100 grams), but to my disappointment, it cannot be
dissolved in HCl. It looks like a nice crystalline glittering powder. Nice to see, but very hard to dissolve.
The pottery grade MnO2 is a dark grey/black very fine powder, and that does work.
A similar problem I also have with Fe2O3, which looks like a red/brown crystalline powder. I also did not manage to dissolve that in acid. It only
dissolves VERY slowly, giving a pale yellow color to HCl. Almost every transition metal oxide suffers from this behavior when strongly calcined. The
only type of reactions, which can make the metal accessible again, is either melting with NaOH/KOH, or using a thermite-like reaction.
[Edited on 28-6-07 by woelen]
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not_important
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Woelen got it regarding the structure of the solid. Overheating can cause a recrystallization into a less reactive form.
| Quote: | Originally posted by mrjeffy321
I might try getting some brand new HCl and seeing if that makes a difference, that and seeing if I can reduce the MnO2 to MnO on a hot plate.
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Mn3O4 is the most stable of the oxides in air, heating most manganese compounds in air to 1000 C will give this oxide.
Mn2O3 is the next most stable. Heating MnO2 in air for 20 hours at 700 C gives Mn2O3.
You might try heating it with H2SO4, slowly raising the temparature into the acid starts to fume off.
Or try adding something to oxidise, like alcohol or sugar, to the dioxide and acid - but only a little at a time.
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mrjeffy321
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| Quote: | Originally posted by woelen
The problem is with your MnO2. As is the case with many metal oxides, when they are calcined/heated for prolonged time at high temperature, then the
oxides become VERY inert. A crystalline structure is formed, which makes them inert for dissolving in acids and in bases.
I have similar problems with MnO2. I have reagent grade MnO2, which was very expensive (EUR 13 for 100 grams), but to my disappointment, it cannot be
dissolved in HCl. It looks like a nice crystalline glittering powder. Nice to see, but very hard to dissolve.
The pottery grade MnO2 is a dark grey/black very fine powder, and that does work. |
Ok, it does sound like the MnO2 has become calcinated in the production process and that is what is causing me these problems. I had never heard of
this being a problem before.
The two types of MnO2 you have, Woolen, also sound just like mine.
I will try heating the MnO2 powder to a high temperature and see if that does any good.
Looking at various MSDSs for MnO2, it has a melting point listed as being only 535 °C, seems a little low, but if this is truly the case then melting it and then
re-crystallizing it back into a [more reactive] solid should be no problem.
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not_important
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The "melting point" is a decomposition point, where it starts to lose oxygen. Even if it were a melting point, melting will only take you further in
the direction that calcining does - larger, lower surface area, less reactive crystals. The reative stuff is very small crystals, the somewhat
hydrated form is more reactive, and the freshly precipitate stuff the most; heating first drives off the water then causes restructuring into denses
more crystalline forms.
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YT2095
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I have the Calcined MnO2 reagent grade here too, I can`t make give of Cl2 either, same problem as Woelen had (in fact we probably bought from the same
supplier).
the only was I can get it to react at all with HCl is with the addition of H2O2.
\"In a world full of wonders mankind has managed to invent boredom\" - Death
Twinkies don\'t have a shelf life. They have a half-life! -Caine (a friend of mine)
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DerAlte
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I'm a manganese nut, so I'll throw in my 2 cent's worth. Dilute acid will not react much with any lower Mn oxide except MnO, which is what you've got
isn't. It's a dull green, and slowly absorbs O2 from the air to turn brown.
Peroxide is an old technical name for dioxide. In sufficently concentrated HCl all the lower oxides react. I am not sure of the limit, maybe around 1N
say 4%. You will have to neutralise this at the end of the Cl2 reaction. If you want to crystallize the MnCl2, it is best to attempt it from acid
solution. It, too, turns brown due to oxidation in time like all Mn Salts.
I suspect the reason for no or slow reaction is either low HCl content, as has been pointed out, or large particle size which is often true of tech
grade pyrolusite. I also understand that there are several different crystalline forms of MnO2 with different reactivity. Purity is no problem,
Mn3O4, Mn2O3 and MnO2, and the large number of hydrated forms, all react to give chlorine. MnO gives the Mn(II) salts directly.
To make any desired salt, I use and make the carbonale (see my Permangante thread)
Regards,
DerAlte
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mrjeffy321
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I placed 20.0 grams of the chem. grade MnO2 on a hot plate at a temperature of ~> 600 degrees C for a length of time of around 10 hours today.
The layer of MnO2 on the bottom (contacting the bottom of the container and closest to the heat source) exhibited a slight color change (got a little
darker), but mostly nothing much was observed.
Tomorrow I will repeak the same process and then retest the MnO2.
I will also try the method suggested by "YT2095" of using a combination of H2O2 and HCl.
But I do now have a bottle of premade MnCl2 on the way now so I can continue on with my other experiments while I keep playing with the MnO2 and try
to get it to react.
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guy
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What is there to do with MnCl2 anyway?
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mrjeffy321
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I really don’t care a whole lot whether I get MnCl2 or another Manganous salt, I just need some water-soluble Mn+2 salt.
What I hope to do with it is to dope Zinc Oxide nanorods I am growing with Manganese and, in doing so, make the nanorods exhibit magnetic properties.
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