Sciencemadness Discussion Board - Potassium ferricyanide

Potassium ferricyanide

Arbacluce - 31-1-2016 at 14:52

Hello

I have problem with preparation of Potassium ferricyanide from Potassium ferrocyanide. I tried two methods, first to the solution of 26g K4Fe(CN)6 3(H2O) in 200ml of water I added 5g KMnO4 and 5ml 10% HCl, after 30 minutes solution turned black-green color and MnO2 precipitated, I took 1ml of solution and treated it with some FeCl3 and I saw blue color, it means that reaction is not done yet, I was dumping more and more KMnO4 and HCl and there was more and more MnO2, I poured 10g of KMnO4 and nothing changed, still dark green color and positive test with FeCl3

. Second method was passing chlorine gas through Potassium ferrocyanide solution (same as first solution) I used 50ml of 30% HCl and a lot of KMnO4 to produce chlorine and results were the same, after this large amount of chlorine gas FeCl3 test was still positive. What did I do wrong? How Can I prepare Potassium ferricyanide from Potassium ferrocyanide?

[Edited on 31-1-2016 by Arbacluce]

woelen - 1-2-2016 at 04:21

Are you sure your FeCl3 does not contain any iron(II)? Even a tiny amount of iron(II) in it will lead to the green/blue color you mention, even if you only have ferricyanide.

The chlorine method should give you a solution with K3Fe(CN)6 and KCl and normally this reaction runs to completion easily. Just bubble Cl2 through a warm (not hot) solution of K4Fe(CN)6. You see intensification of the color of the solution, from pale yellow to deep yellow. Keep on adding Cl2 until the liquid remains smelling of Cl2. Then heat somewhat stronger to drive off the excess Cl2.

blogfast25 - 1-2-2016 at 06:08

Quote: Originally posted by woelen

The chlorine method should give you a solution with K3Fe(CN)6 and KCl and normally this reaction runs to completion easily. Just bubble Cl2 through a warm (not hot) solution of K4Fe(CN)6.

Isn't this oxidation supposed to work with H₂O₂?

[Edited on 1-2-2016 by blogfast25]

Arbacluce - 2-2-2016 at 13:25

I treated my FeCl3 solution with 30% hydrogen peroxide, it turned more brown but after couple of minutes it turned yellow-green again, it did this 3 times. Finally I heated it and it turned brown again, when I added one drop of K3Fe(CN)6(?) nothing happend, but after 3 minutes it turned light-blue. I do not know what I am supposed to think about it.
[Edited on 2-2-2016 by Arbacluce]

[Edited on 3-2-2016 by Arbacluce]

woelen - 3-2-2016 at 00:04

If after the initial addition no blue color appears then it is OK. You get the brown complex FeFe(CN)6. This, however, is unstable, especially in the light, and slowly it decomposes to something which has some iron(II) in it and the color turns green/blue.

So, apparently your FeCl3 is the issue and not the K3Fe(CN)6.

The Volatile Chemist - 3-2-2016 at 17:35

Quote: Originally posted by blogfast25

Quote: Originally posted by woelen

The chlorine method should give you a solution with K3Fe(CN)6 and KCl and normally this reaction runs to completion easily. Just bubble Cl2 through a warm (not hot) solution of K4Fe(CN)6.

Isn't this oxidation supposed to work with H₂O₂?

[Edited on 1-2-2016 by blogfast25]

Yes.^{(Cite: https://www.researchgate.net/post/Is_there_any_way_and_react...)} It requires an acidic solution, though. And if the peroxide need be any more concentrated than 3%, it would be simple enough to do such by 'purification by freezing' (where you freeze almost all of the H₂O₂ and filter out the ice). Why mess with other oxidizing agents, then?

blogfast25 - 3-2-2016 at 17:47

Quote: Originally posted by The Volatile Chemist

Thanks, that's interesting.

The Volatile Chemist - 3-2-2016 at 18:15

Certainly. Sorry about the source, though, it isn't exactly quality. I grabbed the first one that showed what I remembered was true. Probably not the best way to cite...