bhattshivamm
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Synthesis of Potassium Bromate by KMnO4
In my textbook [CBSE text book of chemistry- India (part 1/2)] it is written that potassium bromide can be oxidized to potassium bromate(V) by
potassium permanganate in a alkeline medium. I have tried to do it many times, but failed. once i made a saturated solution of KMnO4 (almost black)
and added KOH to it. as expected, solution became very hot. meanwhile, i added solid powdered KBr to it.... on swirling, solution turned dark green
(Mn(VII) reduced to Mn(VI), right ?) but after crystallizing, and testing, i couldn't get KBrO3 !!! can anyone suggest what thing is wrong (other than
text book) ?? 
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woelen
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Keep in mind that KMnO4 is not stable in strong KOH-solution. Oxygen is produced and the KMnO4 is converted to dark green K2MnO4. In moderately strong
solutions of KOH the permanganate lasts a little longer and then the KBr might be able to reduce it.
Was the change of color after adding the KBr? If so, then the KMnO4 may indeed have been reduced by the KBr and then you'll have KBrO3 in solution.
But in this way you only can make very dilute solutions of KBrO3, because of the dilute nature of solutions of KMnO4.
So, if you have success with this method, then it most likely only is in the form of very dilute solutions and isolating the KBrO3 from the nearly
black KMnO4/K2MnO4/KOH solutions does not seem like an easy task to me.
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AJKOER
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OK, I have a different opinion. Bad text book. I would argue that the chemistry of Bromine largely parallels that of Chlorine. And guess what, ones
does not usually make, or recommend for the preparation of KClO3, the action of KMnO4 on KCl.
However, as there are old textbooks that note that the action of HOCl on KCl can lead to KClO3, the use of KMnO4 in place of HOCl is theoretically
possible, but still not a particular good path. So unless one is OK with the promulgation of questionable chemical examples and exercises, get another
book.
Now if I am wrong on this, than I am not much of a chemist or an educator, so do yourself a favor and don't buy or recommend my book either.
[Edited on 25-2-2013 by AJKOER]
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woelen
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I do not agree with you AJKOER. Indeed, making bromates by means of oxidation of KBr by KMnO4 does not look like the best way to go, but in principle
it should be possible.
Bromine chemistry is sufficiently different from chlorine chemistry to allow oxidation to bromate at high pH. I can imagine that KMnO4 is a
sufficiently strong oxidizer to oxidize bromide to bromate. Intermediates like hypobromide are not an issue with bromine. Hypobromite is very unstable
and very quickly (if formed at all!) disproportionates to bromide and bromate. This is the reason why making bromates electrolytically works so nicely
(see my website, I have a page on making bromate by means of electrolysis of a solution of KBr).
Whether the text book of bhattshivamm is bad or not I cannot judge from here. It probably mentions the reaction between KMnO4 and KBr at high pH,
without really recommending that as a viable route to bromates. Many textbooks mention what can be done theoretically, but this does not mean that
these reactions are good practice in real life. Sometimes, however, it may be an interesting experiment on its own to try such theoretically possible
reactions, regardless of their practical synthetic value.
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bhattshivamm
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the color didn't change on adding KOH, it changed after adding KBr, so i guess KBrO3 would certainly have formed. in this dark green solution, i added
some methanol, and left it for some time. the green Mn(VI) reduced to MnO2. i filtered MnO2 and chilled the solution, but no KBrO3 crystallized out
in another attempt, i added some oxalic acid to reduce Mn(VI), and I could reduce it. but again after filtering, no KBrO3
i really wanna make this work, but I can't. the only way to synthesize KBrO3 for me, is the oxidation by KMnO4. i dont have sufficient electrical
arrangements to perform electrochemistry. plz help me.... (and i have around 100 gm KMnO4 and 250 gm KBr, so dont worry about low yield.... i will
scale it up  )
[Edited on 26-2-2013 by bhattshivamm]
[Edited on 26-2-2013 by bhattshivamm]
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woelen
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Chilling of the solution is not enough for crystallizing KBrO3. The solution of KMnO4 already is quite dilute and the KBrO3 also will be quite dilute.
Try to boil down some of the solution.
Also, do not add too much methanol or oxalic acid. These may reduce bromate to bromide as well. Especially oxalic acid is quite a good reductor. I
would try adding just enough sulfite to the solution to just reduce the K2MnO4 to MnO2. Do not use large excesses of this reductor. Then, after
filtering, boil down the solution and see if you get crystals of KBrO3.
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bhattshivamm
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yup... you're right.... i will try. let me see
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