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Author: Subject: Cobalt Oxalate complexes
Vylletra Heart

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[*] posted on 25-6-2017 at 00:56
Cobalt Oxalate complexes

Hello, I am currently trying to synthesize a sodium cobalt oxalate complex. However, I am unable to find any extensive research on such a compound. My current procedure was this:
--Add black cobalt oxide to excess oxalic acid(interestingly enough, I heard distinct crackling during this, like ice melting rapidly but softer)
--Add sodium bicarbonate to the resultant mixture, but still leaving the acid in excess
--The solution was green at first, like that of ferrioxalate(i'm worried that there may be iron oxide impurities)
--Gradually the solution began to turn to a light pink, like that of the hexaaquo cobalt ion. The pink intensified after heating to a boil.

The issues I'm currently having is that I noticed a lot of websites citing the complex with cobalt in the +3 oxidation state. However, this oxidation state is only achievable with ammine ligands and the anhydrous fluoride compound. I am not so sure if the oxalate ion is bound to the cobalt ion strongly enough to permit its oxidation from +2.

Has anyone done any experiments similar to this? I would like to know if the final color of my solution(intense pink) is evident of the [Co(C2O4)2] 2- ion. Does pH of the solution affect the solubility of this complex? Should I have an excess of oxalic acid or sodium bicarbonate during the crystallization of the cobalt complex?

Additionally, is the Co(III) related complex possible? If it is possible, is it stable in air? What would be its color?

Any additional information on cobalt oxalate complexes is greatly appreciated! Thanks!

[Edited on 25-6-2017 by Vylletra Heart]
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International Hazard

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[*] posted on 25-6-2017 at 08:45

I don't have any experience with the cobalt(III) oxalate complexes, but the tris(carbonato) complex is an intense dark green, and is perfectly stable. I beleive that carbonate and oxalate have similar field-splitting characterisitics, so I would not be surprised if the tris(oxalato) complex is also green.

That being said, the tris(oxalato)ferrate(III) complex is light-sensitive because the iron(III) can oxidize the oxalate. Cobalt(III) is a stronger oxidizing agent, so I'd expect the complex to be even less stable.

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International Hazard

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[*] posted on 25-6-2017 at 10:22

In my experience, transition metal oxalates are pretty insoluble, including cobalt oxalate. Merck lists cobalt (II) oxalate as "almost insoluble in oxalic acid". So, I'm not sure how well a direct reaction from the oxide will work, although it does sound like you've coaxed some cobalt ions into solution.

Merck also lists cobalt (II) oxalate as "decayed on heating with aqueous KOH or Na2CO3 solution", if that helps.

What I might try, especially if the cobalt source is of lower purity, is extract and refine the cobalt as a salt like the sulfate, and then precipitate cobalt oxalate powder using oxalic acid. You can recrystallize this from hot HCl if you want. Then, maybe try reacting it with sodium oxalate somehow? Solubility might still be a barrier.

Cobal (III) is a pretty high oxidation state that I often hear in the context of things like elemental fluorine, and it appears to oxidize mineral acids when dissolved in them. I'd be pretty surprised if it appeared spontaneously in such mild aqueous conditions.

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