Sciencemadness Discussion Board

Cobalt carbonate decomposition

MeshPL - 27-8-2016 at 06:46

I wanted to made some cobalt carbonate from cobalt sulfate and sodium carbonate. I mixed solutions of both with an excess of carbonate, mixed well, decanted, washed few times with deionized water, gravity filtered and washed a bit more. So far it was good. But now I decided to heat it in oven to dry it up. And I chose 200C. Which apparently is too much. The bulk of my carbonate is good (as it didn't dried completly) and still nicely violet but tiny little pieces on the walls of my beaker turned black, indicating decomposition to Co3O4.

Do you guys have any experience with CoCO3? (Nevermind it actually is not plain CoCO3) Did you dry it and what temperature did you dry it at?

Edit: both violet and black stuff on the walls dissolves and bubbles in vinegar (I didn't bother taking actual acid to test this). But weirdly enough the black stuff makes solution yellow (Co 3+?), unlike violet which makes it red. I will dry the violet bulk of the carbonate in 150C. But I'm not sure if this is enough to make it lose hydratation water.

[Edited on 27-8-2016 by MeshPL]

[Edited on 27-8-2016 by MeshPL]

[Edited on 27-8-2016 by MeshPL]

NEMO-Chemistry - 27-8-2016 at 07:02

I think it decomposes to the oxide at around 140C, so dry it in a desiccator or pull a vacuum over a filter funnel until dry (like copper carbonate, or dry in the oven at around 110c?

MeshPL - 27-8-2016 at 08:11

Too late... the stuff I have doesn't look bad though, it's rather violet, not unlike the fresh one. I didn't dry it for long enough to start decomposing. It's weird because the lowest decomposition temperature I found was around 200C. Well... carbonates are weird.

And if I will be unsatissfied with the quality of this batch, I will try a different drying method. Thanks for pointing out the surprisingly low doecomposition temperature.

blogfast25 - 27-8-2016 at 08:16

Quote: Originally posted by MeshPL  

Do you guys have any experience with CoCO3? (Nevermind it actually is not plain CoCO3) Did you dry it and what temperature did you dry it at?




I believe there is a 'straight' Co(+2) carbonate, i.e. CoCO3. A product by that same formula I used some time ago to prepare CoCl2 hydrate. The stoichiometry worked out perfectly.

Wash the precipitated CoCO3 extensively with water, suck 'dry', then wash with several small aliquots of a water soluble low boiler like acetone, MeOH or EtOH. Then dry at low temperature or even RT.

[Edited on 27-8-2016 by blogfast25]

MeshPL - 27-8-2016 at 08:25

I wanted to wash my cobalt carbonate with some organic solvent-but turns out I ran out of most of them. I had acetone, but it was technical grade and yellow, so I didn't trust it.

NEMO-Chemistry - 27-8-2016 at 09:39

https://en.wikipedia.org/wiki/Cobalt(II)_carbonate

If i read it right, the decomp is around 140c, look at the melting point data on the right.

Nice colour compound

MeshPL - 27-8-2016 at 10:55

Yeah, I've seen that but assumed that hexahydrate dehydrates rather than completly decomposes at that temperature... oh well, I know what not to do next time.

And everybody knows what happens when one assumes sth...

Sure it has nice colour. And is slightly attracted by magnet, at least my impure sample.

NEMO-Chemistry - 27-8-2016 at 11:27

Quote: Originally posted by MeshPL  
Yeah, I've seen that but assumed that hexahydrate dehydrates rather than completly decomposes at that temperature... oh well, I know what not to do next time.

And everybody knows what happens when one assumes sth...

Sure it has nice colour. And is slightly attracted by magnet, at least my impure sample.


LOL i nearly mentioned that the wiki page says it is paramagnetic, so it does look like you have what you were after.

I dont know about the dehydration, i got the impression it decomposed at 140c, but like you say to ASSUME is erm... well.

Copper, manganese and Cobalt are all fascinating elements with some great chemistry. I am doing stuff with Copper and permanganate at the moment, but i will be getting some Cobalt salts soon.

Anyway hope it all goes well for you, sounds like you pretty much got the product you were after.

MeshPL - 29-8-2016 at 08:48

Thanks! Have fun with copper, manganates and coppervpermanganate too!

I'll use my cobalt carbonate to make nitrate and any other cobalt salts I will need in the future. I'll use it to coat titanium electrides with Co3O4 and maybe NiCo2O4. Fortunately I was abke to buy a decent quality nickel carbonate. We don't really have pottery stores here.