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Author: Subject: Quick Question about Manganese(II) Sulphate
Hydragyrum
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[*] posted on 4-6-2009 at 04:17
Quick Question about Manganese(II) Sulphate


Hi All,

I have managed to grow crystals of what I think is manganese(II) sulphate, but these crystals are colourless, not pink. Let me describe my method:

I reacted small chips of manganese metal with aqueous copper sulphate, whereby the manganese became coated with copper and the colour of the solution faded from blue to a pale yellow. The solution was filtered, with a lot of the yellow coming out as colloidal particles (or appeared to be anyway) - these I presumed might have been higher oxides of Mn, probably originating from an oxide layer on the Mn metal used.

The filtrate was allowed to evaporate under ambient conditions (because I am not set up for distillations, I have learned patience; also, I found evaporating by heating increased the yellow colouration - aerial oxidation I suppose, so I kept things at room temp to preserve my product), and became yellow again over time but also produced hard chunky colourless crystals. Now, I keep reading everywhere that MnSO4 (both hydrated and anhydrous) are pale pink in colour (and I've seen pink Mn(II) salts before, it is an obvious colour despite being pale), but these crystals show no pink colouration at all. I have analysed them as follows:

1. A solution of the crystalline material in water gives a white ppt with NaOH soln, which rapidly darkens to a yellow/brown colour --> Mn(II) oxidised by O2 under alkaline conditions.

2. More of the same solution (fresh, not from the NaOH reaction) gave a white ppt with a solution of Ba(NO3)2 --> crystals contain sulphate ion.

3. A crystal was heated in a clean, dry test tube, giving off water --> crystal is hydrated.

So, my conclusion is that the salt must be MnSO4.xH2O, x > 0.

Can anyone explain to me why this salt is not pink? (has anyone come across colourless Mn(II) salts before? - I would prefer first hand accounts)




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Jor
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[*] posted on 4-6-2009 at 04:59


Manganese sulfate monohydrate is almost colorless-white, it has a slight pinkish hue.

Member woelen has gallery of compounds, and it includes
Mn(II)-sulfate and Mn(II)-chloride. The first is almost colorless and the second is more pink.

Woelen's gallery of compounds


[Edited on 4-6-2009 by Jor]
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Hydragyrum
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[*] posted on 4-6-2009 at 05:13


Thank you Jor, I visited Woelen's MnSO4.H2O page and I see that this is really pale, just about white.

I guess the next question is: why is it so weakly hydrated when crystallising out from an aqueous solution? I guess this is a separate question altogether though.




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Sedit
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[*] posted on 4-6-2009 at 07:12


My issue with MnSO4 is the complete opposite. My crystals look almost orange fractal crystal clumps when completely dehydrated. I think there maybe some Iron contamination.




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Hydragyrum
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[*] posted on 4-6-2009 at 14:44


Quote: Originally posted by Sedit  
My issue with MnSO4 is the complete opposite. My crystals look almost orange fractal crystal clumps when completely dehydrated. I think there maybe some Iron contamination.

How did you make your MnSO4?




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Mossydie
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[*] posted on 4-6-2009 at 14:46


Mn+H2SO4
MnO2+SO2

[Edited on 4-6-2009 by Mossydie]
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[*] posted on 4-6-2009 at 15:52


I've prepared hydrated MnSO4 from the black electrolyte paste from D-cell carbon zinc batteries. The paste was calcined in order to burn off the carbon and vaporize some of the ammonium and zinc chloride contaminants. The slightly more pure MnOx was reacted with strong HCl, the Cl2 captured for other use. I recovered the Mn from solution by adding it to a solution of NaHCO3 with bicarbonate in excess. MnCO3 was thrown down immediately, the light brown precipitate filtered and washed before dissolving in diluted H2SO4.

On concentrating, the solution became very pale pink. As more H2O evaporated, nice delicate pink and rather deliquescent crystals were deposited.

I have seen the orange before as Sedit describes. If I remember correctly it appeared after reacting the MnOx w/ HCl and persisted when the solution was alkalinized. I'd have to dig up my notes, but seem to recollect the orange being completely discharged by H2O2 at neutral to alkaline pH. What sort of oxychlorides, if any, does manganese form in the aqueous melieu?




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[*] posted on 4-6-2009 at 22:46


The orange color I also know. It is manganese in a higher than +2 oxidation state, probably +3 and/or +4. You can also observe this color in solution, when your liquid with Mn(2+) is just somewhat basic (pH 9 or something like that). Slowly the liquid turns opalescent brown/orange. This is ultrafine hydrous Mn2O3 or MnO2. Once formed, the orange/brown opalescence persists when acid is added, but when a drop of H2O2 is added to the acidic solution, then it disappears at once.

If you have this contamination in your crystals, you can try dissolving all of it, adding just a tiny drop of dilute H2SO4 and adding a tiny drop of H2O2. Then let the liquid evaporate such that most of the solid crystallizes again. Do not evaporate to dryness, because that would add H2SO4 to the crystalline solid and makes it hygroscopic.




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Hydragyrum
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[*] posted on 4-6-2009 at 23:12


Thank you all for your input so far - I have a yellow colouration in my mixture too - it is mostly staying in the aqueous phase, but a couple of the crystals have a touch of yellow on them (not throughout the entire crystal though - there are still clear/colourless regions, and most of the crystals I've recovered have no yellow colour at all). This yellow colouration is quite a deep colour, and must be the same as the orange colouration several of you mentioned. I have some H2SO4 (acquired since I began this experiment) and dilute H2O2 so I might try to clean the product up at some stage.

Incidentally, since getting the H2SO4 I have tried dissolving the Mn metal in the acid, and that gave a dirty brown/black mixture (I couldn't refer to it as a solution) with a funny smell given off (possibly SO2?), but simple filtration cleaned it up nicely and I now have a pink solution from this experiment (no crystals as yet).

Manganese chemistry is certainly not straightforward!




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[*] posted on 24-1-2015 at 22:51
mso4 industrial route


i am trying to figure out an economical industrial route of processing mnso4 from natural ore. essentially mno2. the first process is to burn sulphur and inject the so2 formed into the crushed ore and water solution. the slurry thus obtained is kept at ph4-4.5 by adding lime. after the reaction the slurry is filtered and the liquid which is pink in colour is obtained. now the problem is how to seperate the mnso4 present in the solution. can anyone guide me, as when i try to heat the solution its ph goes down and it becomes acidic with evolution of so2 gas and probably the mnso4 is converted baqck to mno2.
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