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guy
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Weird Mn compound
I prepared Mn2+ compound by oxidizing an acidified (sulfamic acid) aqueous solution of glycerol with KMnO4. This resulted in a clear solution which
was expected. Then I wanted to precipitate the Mn2+ with NaOH. It formed a gelatinous precipitate. Then 3% was added to it. First it formed
orange-brown ppt which was MnO2, but after adding more H2O2, it formed a clear red-brown solution with no ppt. This solution will turn clear when the
pH is lowered. What is this compound? I know this doesn't happen with regular Mn(OH)2 and peroxide, but this could have formed a complex with the
oxidation products of glycerol.
[Edited on 7/17/2006 by guy]
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woelen
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What you obtained could be a manganese (III) solution. Under highly acidic conditions, this ion forms a nice rose/red/brown aqueous ion, and at
raising the pH it hydrolyses.
This can be made by careful reduction of permanganate or MnO2 in acidic solution. I made it with malonic acid as reductor, but it also works with some
more difficulty with citric acid, and according to your post, also with glycerol.
Could you compare your solution with the one I obtained and which is manganese (III)? Here follows the page with pictures. Look at the section for
oxidation state +3:
http://woelen.scheikunde.net/science/chem/solutions/mn.html
[Edited on 17-7-06 by woelen]
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guy
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Mine is an orange-red solution that only forms in BASIC solution, when concentrated, it looks like a nice cherry red color. It acid with a reducing
agent(H2O2 in my case), it quickly turns to Mn2+.
I wonder what are the ligands that stabilize this stage. Oxidation of glycerol with KMnO4 can form many things since it has 3 hydroxyl groups. So
either I formed mesoxalic acid (COOH)2CO, or glyceric acid, or (COOH)2CHOH (what would that be called?). I think in basic solution, these will form
the conjugate bases which would allow complexing.
I will take pictures tommorow.
[Edited on 7/17/2006 by guy]
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woelen
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Sorry that I missed that point of formation in basic solution. I was mislead by the phrase "This solution turns clear when the pH is lowered". I
should have read "turns colorless". With "clear" I mean "no solid particles", but it still can have color. Native language-speaking people, am I
correct with this?
But guy, that experiment looks really interesting. I definitely will try this experiment this evening. What is remarkable is that on addition of a
small amount of H2O2 to the basic solution with gelatinous Mn(OH)2 precipitate, you get a brown precipitate (which can be a manganese (III) compound)
and that on addition of more H2O2 you get the red/brown clear solution. This is an indication of formation of a complex of higher than +3 oxidation
state, but if the initial brown precipitate already is hydrous MnO2, then this even is an indication of a higher than +4 oxidation state! That would
be really interesting.
Time to take the test tubes out of their container again  
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YT2095
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yes something can be clear but have a color, like copper sulphate soln for example.
\"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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woelen
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As promised, I have taken the test tubes from the cabinet, despite the VERY warm weather (35 C in my lab  , 30+ C outside) I did the following experiment:
Dissolve some MnSO4.H2O in water.
Add a small amount of glycerol and dissolve that as well.
In a separate test tube dissolve quite some NaOH
Add both solutions together.
Result is a slimy precipitate, light brown. This is impure Mn(OH)2 with traces of brown oxidized material (there always is some oxygen dissolved in
the solutions).
Add a small amount of H2O2 (3%). This results in formation of a dark brown precipitate, and marginal bubbling.
Add much more H2O2 (3%). The solution becomes clear again, but it has a very intense red color, much more intense than the rose/red/brown color of
manganese (III) in acidic solution. This deep red color most likely is due to some peroxo complex of manganese (IV), together with glycerol. It is
remarkable how intense the color of the complex is.
On dilution with water, the complex decomposes and brown MnO2 is precipitated.
On dilution, however, with a moderately concentrated solution of NaOH, the complex does not decompose. The resulting solution then has a beautiful red
wine like color. Very neat. On addition of a small amount of HCl, the complex decomposes again, forming brown MnO2.
So, one can conclude that this is a complex of glycerol, possibly together with peroxo, and this complex only is stable at very high pH. When things
have cooled down a little in my lab (< 30 C), then I'll put some more effort in researching this interesting phenomenon.
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guy
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Yes that is exactly what my results were. So now I now that glycerol is the ligand. That is interesting, I wonder what the formula could be and why
it only forms in basic conditions.
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guy
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A paper I found on Metal-gylcerol complexes.Some Complexes of Glycerol and Their Applications
Does anyone know what the pH of glycerol is or how easy it is to deprotonate? Its should be more acidic than ordinary alcohols.
[Edited on 7/17/2006 by guy]
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12AX7
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Hmm interesting, that's the same color I would describe Mn(III) acid solutions. I have some sulfuric solution downstairs, obtained by reacting
pottery grade MnO2 with pretty strong H2SO4.
Seems suprising to me that another ligand would have such a similar color at such a different pH!
Tim
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chemoleo
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I'd like to see what'd happen if glycerol was substituted with other organic solvents, alcohols etc.
How about ethylene glycol, or methanol, or ethanol? Acetone, Isopropanol? What if glycerol is left out altogether?
How about erythritol, or higher sugars? mannitol?
I am not entirely convinced that this is a complex with an organic ligand ... what is the evidence for this so far?
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guy
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Well I've seen a picture of Mn(III) without ligands and the color of mine is much more red and darker. And this will only work in the presence of
glycerol. And this will only work in a basic solution. If glycerol is let out, only MnO2 will be formed.
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mericad193724
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Woelen,
AWSOME site. I love it! So much good information. I really like and admire it.
Is there anyone else with a site like that besides you and Bromic acid (awesome site too).
Mericad
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12AX7
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I have some stuff, mostly how to make some basic metal salts...
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guy
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Sodium manganese triglycerolate?
Concentrated vs. Dilute
2Mn(OH)<sub>2</sub> + H<sub>2</sub>O<sub>2</sub> + 6OH<sup>-</sup> +
6C<sub>3</sub>H<sub>8</sub>O<sub>3</sub> ----->
2[Mn(C<sub>3</sub>H<sub>6</sub>O<sub>3</sub><sub>3</sub>]<sup>3-</sup> + 12H<sub>2</sub>O
[Edited on 7/18/2006 by guy]
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woelen
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| Quote: | Originally posted by chemoleo
I'd like to see what'd happen if glycerol was substituted with other organic solvents, alcohols etc.
How about ethylene glycol, or methanol, or ethanol? Acetone, Isopropanol? What if glycerol is left out altogether?
How about erythritol, or higher sugars? mannitol?
I am not entirely convinced that this is a complex with an organic ligand ... what is the evidence for this so far? |
Best evidence is that without glycerol, addition of H2O2 to a suspension of Mn(OH)2 in a solution of NaOH only results in formation of a brown
flocculent precipitate and a lot of bubbles of oxygen. With the glycerol present in the alkaline solution, a deep wine-red solution is formed, and no
precipitate is formed. That is enough evidence for me that there is a complex with an organic ligand.
| Quote: | | Sodium manganese triglycerolate? |
Now the second step. What is this complex? I'm absolutely not sure about this being a glycerolato manganate (III) complex, as suggested by guy. It
might as well be a mixed ligand complex, e.g. with glycerol (or a deprotonated version of that) and with peroxo ligands. This is something which also
can be tested easily. Take another oxidizer than H2O2. I'll try tonight with Na2S2O8 as oxidizer. One could also try with oxygen from air as oxidizer
by prolonged shaking of the liquid with contact with air.
It might well be true that also complexes can be formed with other organic compounds. This evening I can try the same experiment with glucose, with
pentaerythritol and with plain sugar. All of them have plenty of -OH groups attached. I'll come back on this later.
I once did the experiment with ethanol, and that does not give such a complex. Most likely because this can be at most a mono-dentate ligand. I do
think that ligands with multiple bites are needed for this type of complex.
| Quote: | | Is there anyone else with a site like that besides you and Bromic acid (awesome site too). |
Thanks for the compliment . There is an index of websites of many members over
here on sciencemadness. It is a sticky topic in the forum matters subsection. Many websites are mentioned there, and some are quite interesting. So,
yes, there are more good websites around, fortunately!
[Edited on 18-7-06 by woelen]
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guy
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Well this is kind of Off-Topic, but I tried this with Cu2+ and glycerol in NaOH forms a dark blue solution, much like ammonia does. (See the link I
provided before for reference). This leads me to think that glycerol will only form complexes in basic conditions because, it will deprotonate with
the aid of a metal ion. Glycerol is more acidic than normal alcohols because there are less electron donating groups. The metal ligand will form a
bond between the ion and the OH, which will cause the hydrogen to become more positive and more easily deprotonated.
Also, I will try the Mn experiment with a different oxidizer and see the result too.
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guy
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I have just conformed that this is compound is at least Mn4+ or higher. I started this time with pure MnO2. I added a basified solution of glycerol
to it. Then H2O2 was added. The same red solution was formed.
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woelen
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I also just did some experiments.
I did the original experiment, starting off with manganese (II) hydroxide, suspended in a solution of NaOH.
I did the experiments with Na2S2O8 as oxidizer.
The procedure was as follows:
Step 1: Prepare a solution of MnSO4
Step 2: Add some ligand** to the solution of MnSO4 and dissolve that as well.
Step 3: In a separate test tube, prepare a solution of NaOH, fairly concentrated.
Step 4: Add the solution of MnSO4, ligand** and NaOH to each other.
Step 5: Add some solid Na2S2O8 as oxidizer to the result of step 4.
**: I repeated the experiment for the following four ligands:
A) Glycerol
B) Pentaerythritol
C) Glucose
d) Saccharose (plain sugar)
With glycerol the result is the same as with H2O2 as oxidizer. From this I conclude that the complex is not a peroxo complex, it is sufficient to have
glucose as ligand.
With pentaerythritol as ligand, no complex is formed. On addition of Na2S2O8 the suspension of Mn(OH)2 turns dark brown and more flocculent.
With glucose also no complex is formed. Same brown precipitate.
Finally, with plain sugar a really interesting effect is created. The solution becomes VERY eager for oxygen. No suspension of Mn(OH)2 is formed when
the solution of NaOH is added to the MnSO4/sugar solution. At the surface of the liquid a red/brown complex is formed, which slowly moves into the
liquid. A little later, the liquid becomes covered by a yellow/brown crust and the walls of the test tube, which are exposed to air, also first become
brown, and lateron scales of a yellow/brown compound slowly move downwards along the glass. When the liquid is shaken, then all brown scales
redissolve again, and the liquid becomes clear and red/brown. On standing, all places, which are in contact with air soon become covered by the
yellow/brown solid again. After some time, the entire liquid becomes dark brown and turbid and I think, finally MnO2 is formed.
It looks as with sugar, that a complex is formed with manganese in the +3 oxidation state and that on further oxidation this is destroyed again.
===================================================
Your experiment with H2O2, added to MnO2 also makes me think more and more that it is a complex with manganese in the +3 oxidation state. You start
off with a suspension of MnO2 in NaOH/glycerol solution and on addition of H2O2 it becomes red and clear? If that is the case, then I would say that
the H2O2 in this case acts as reductor and reduces the MnO2 to Mn(III), itself being converted to water and oxygen. It is well known that H2O2 can act
as reductor. That even is applied in cleaning glassware from MnO2 stains, by rinsing it with a dilute H2O2/acid mix. So, why could H2O2 not act as
reductor in the alkaline medium with the ligand as stabilizer for the +3 species of manganese?
This is very interesting stuff and I'll perform a few more experiments, trying to see whether this is a +4 (or higher) oxidation state complex, or +3.
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guy
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Well I tried the experiment this time with MnO2, NaOH, glycerol, and NaOCl(household bleach). The results were almost the same. It formed a yellow
solution first, but on heating, the same red solution was formed.
What does this mean? Can hypochlorite reduce Mn4+ to Mn3+?
Can you try all your experiments this time starting only with MnO2? Then use substances that can ONLY oxidize, such as persulfate or hypochlorite.
This will be more conclusive.
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woelen
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Your experiment with NaClO indeed is a strong argument against my +3 oxidation state hypothesis. NaClO indeed cannot reduce as far as I know.
Did you use commercially obtained MnO2, or a hydrous suspension, made from Mn(OH)2. I have MnO2 from a pharmacy, but this is a dry crystalline very
fine and glittering powder. It seems to me that such MnO2 would be very slow in its reactions.
Tomorrow evening I again have the time to experiment further. If you could give some info on your MnO2 before that, that would be very helpful. Right
now, I'm determined to solve this "riddle", at least the oxidation state part of the riddle.
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guy
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It is formed from the decomposition of the red compound. It is the hydrous form. I cleaned and washed it of course, so it would be fairly pure.
[Edited on 7/18/2006 by guy]
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woelen
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I did some further experimenting on this and now I come to the conclusion that the complex either is a manganese (V) or higher compound with glycerol
ligands, or it is a manganese (IV) compound with an oxidized species of glycerol as ligand.
I did the following:
Prepare some hydrous MnO2 from MnSO4, NaOH and Na2S2O8. Filter and rinse the precipitate well with water.
The still wet, but cleaned, precipitate is added to a solution of NaOH, to which also some glycerol is added. If this is done, then there is no
visible reaction. The solid does not dissolve, the liquid becomes turbid and very dark brown.
When some Na2S2O8 is added, then the solid MnO2 dissolves and a deep red/brown complex is formed. So, indeed the oxidizer is needed. Hydrous MnO2,
NaOH and glycerol alone do not form the complex.
I also tried the reaction with some commercial MnO2 from a pharmacy. This does not react at all under these conditions. This is something I expected.
The MnO2 I have consists of many small very hard refractive glittering crystals, which only can be dissolved with difficulty in conc. HCl.
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guy
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Is there anyway to isolate this compound? It is soluble in alcohol and it wont decompose if you attempt to evaporate it. Maybe it will be easier to
analyze once it has been isolated, such as how much MnO2 will be formed with X grams of it.
OR
If you have the time, you can try to see how many grams of a certain reactant is needed to fully react, assuming that it goes to completion. You
could find out the number of glycerol ligands there are at least.
This is a way to determine oxidation state: How much of a certain oxidizing agent is required to form the compound.
I would do this but I don't have any measuring tools.
[Edited on 7/22/2006 by guy]
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guy
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What is the reason why MnO4(2-) has to be in basic solutions? My guess is that it is unstable enough to react with H+ formed by water. What do you
think?
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woelen
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The ion MnO4(2-) is resonance stabilized, while the molecule H2MnO4 is not.
H2MnO4 has structure MnO2(OH)2, with the O-atoms without hydrogen double bonded to the Mn, and the O-atoms with the H-atom single bonded to Mn. Hence,
the O-atoms are not equivalent in this compound. When the two H-atoms are removed, then all O-atoms become equivalent, each having "on average" 1.5
bonds. This structure is much more stable than the structure of HMnO4(-) and the structure of H2MnO4.
Many ions are much more stable than their corresponding acids. E.g. NO3(-) is very stable in water, HNO3 is a strong oxidizer. The same is true for
ClO4(-), which is very inert in water, while molecular HClO4 is an extremely dangerous oxidizer (which only exists in 72+% solutions, at lower
concentration all is ionized).
Resonance stabilization is present in MANY compounds, and also in MANY ions. The classical example is benzene, but lateron it became clear that it is
not special at all.
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