BaMnO4 in alkaline solution is a blue color of precipitant. After drying, the color of BaMnO4 precipitant changes its color from blue to brown. Does anyone have a good idea to keep out this change?
Not much can be said, since the actual synthesizing/precipitating protocol was not mentioned in the query enunciate. You may possibly have to further purify the raw barium manganate precipitate. Also note that barium manganate can oxidize many substances, including common organic solvents.
Hi Carlos,
Thanks for your responce. The precipitation method is as following
1) KMnO4(purple) mixed with KOH and then heat to make a K2MnO4(green)
2) K2MnO4 add to BaCl2 to get a precipitant of BaMnO4 (blue)
3) wash the precipitant with Ba(OH)2 for three times
4) pour the precipitant into the petri dish for drying overnight
Finally, the color of BaMnO4 changes from blue to brown though.
Dear Samuel:
I. As noticed from the sequence disclosed on above post, Ba(OH)2 was used to wash the precipitate (§3) ― it thus seems that the mother solution is somehow expected to be acidic. That might perhaps be intended to neutralize the mother solution after precipitation of barium manganate: Ba(OH)2 + 2 HCl → BaCl2 + 2 H2O. Generated BaCl2 might then possibly induce some post-precipitation, thus improving yield, as long as some excess K2MnO4 remains in the mother solution: BaCl2 + K2MnO4 → 2 KCl + BaMnO4↓.
II. That being indeed the case, it seems that the mother solution should have been somehow acidified prior precipitation of barium manganate ― Such step was however not mentioned at above post.
III. Precipitation from acidic mother solution may possibly induce some degree of manganate disproportionation: 3 MnO42–+ 4 HCl ⇌ MnO2 + 2 MnO4– + 2 H2O + 4 Cl-.
Dear Carlos,
The reason ,by using Ba(OH)2 solution in wash step, is following the note from Wikipedia to maitain the stability of K2MnO4 and BaMnO4. Also there is not HCl exit in reaction. Second , I can see the BaMnO4(blue) precipitated in the solution. My question is after pouring the precipitated BaMnO4 into petri-dish that the color of BaMnO4 changes from blue (wet) to brown (dry) though.
Dear Carlos,
Because of brown color of final product, I guess it which may be a "Disproportionation" during dehydration of BaMnO4. BaMn(VI)O4 -> Ba(Mn(VII)O4)2 + Mn(OH)x -- etc. But the exactly reaction is still unknown though. I may try to prevent this " Disproportionation " during dehydration of BaMnO4 with no clue.
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