Rhodanide
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Mystery goop: Manganese Carbonate, Hydroxide, or something else? Help!
A few weeks ago I cleaned my battery-grade MnO2 from all soluble comtaminants (3+ washes) and reacted it with acceptably pure Sulfamic acid in water.
I then filtered the mixture to obtain a clear solution. I forgot to measure the PH afterwards. I then dissolved some anhydrous Na2CO3 in 100mL of
filtered water. This dissolved fine, leaving a reasonably clear solution, apart from some expected cloudiness and effervescence. I then added this by
dropper to the solution of my (hopefully) Manganese Sulfamate. The carbonate first reacted with a small amount of Sulfamic acid, however it was only a
few dropperfuls before CO2 evolution stopped. After this, I added it by beaker - not all at once though - and got a white-grey precipitate of unknown
composition.
My starting MnO2 shouldn't have had much Fe in it, the main contaminant was elemental C. What is this precipitate? It has almost the exact same
properties as Aluminium/Aluminum Hydroxide... wth is it? I'm at a loss. Hopefully you guys have a better idea than I do now or did when I first made
it as to what it is.
-T/Azide
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Sigmatropic
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Did you wash the crude MnO2 with neutral water? Could the percipitate be a zinc salt (hydroxide or carbonate)?
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Cryolite.
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Try heating some of the precipitate. If it darkens to brown or black, it contains manganese.
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Rhodanide
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Yes, I did wash it with clean water.
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Boffis
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Tetra, if I understand your description correctly you simply added sodium carbonate to a solution of manganese II salt and got a precipitate of
manganese II carbonate. What's the issue? What did you expect to happen? If your solutions are too concentrated then you tend to get a slimy,
difficult to filter, precipitate. In this case try diluting the resulting suspension and warm it for a few hours, this will often cause the initial
slimy ppt to crystallize, become granular and more easily filtered. This is a general comment for most precipitation reactions e.g. more dilute
solutions give more easily filtered precipitates. It is a common mistake for people to try using excessively concentrated solutions for precipitation
reaction and then wondering why they got an unfilterable gloop!
In the case of Mn2+ it is better to use warm (40 C) sodium bicarbonate solution because it is less alkaline than sodium carbonate and therefore
generates less basic Mn carbonate. The Mn2+ basic carbonates tend to oxidize and turn brown rapidly when dried. You should read an old inorganic
chemistry book or two, these are the sorts of things described under the preparation of individual compound such as manganese carbonate etc.
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Rhodanide
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Quote: Originally posted by Boffis  | Tetra, if I understand your description correctly you simply added sodium carbonate to a solution of manganese II salt and got a precipitate of
manganese II carbonate. What's the issue? What did you expect to happen? If your solutions are too concentrated then you tend to get a slimy,
difficult to filter, precipitate. In this case try diluting the resulting suspension and warm it for a few hours, this will often cause the initial
slimy ppt to crystallize, become granular and more easily filtered. This is a general comment for most precipitation reactions e.g. more dilute
solutions give more easily filtered precipitates. It is a common mistake for people to try using excessively concentrated solutions for precipitation
reaction and then wondering why they got an unfilterable gloop!
In the case of Mn2+ it is better to use warm (40 C) sodium bicarbonate solution because it is less alkaline than sodium carbonate and therefore
generates less basic Mn carbonate. The Mn2+ basic carbonates tend to oxidize and turn brown rapidly when dried. You should read an old inorganic
chemistry book or two, these are the sorts of things described under the preparation of individual compound such as manganese carbonate etc.
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The problem was that I'd never seen Manganese Carbonate in that sort of form before. It was like Aluminium Hydroxide, and not like the "normal" finely
powdered cream-colored precipitate I've encountered every other time.
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