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Author: Subject: Need help with growing crystals of sodium cobalt oxalate complex
Vylletra Heart

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[*] posted on 12-6-2018 at 21:56
Need help with growing crystals of sodium cobalt oxalate complex

In my efforts as shown here to make the complex, it seems that the solid that forms is rather amorphous, and the tiny crystals that form on the surface of the solution do not maintain their structure when they grow larger. Apologies for the poor image quality, I have only a phone camera on end. I would like to ask if anyone who has dealt with this complex before could give me some advice on how to grow these crystals.

The process I used to make the solution is as follows: Mix cobalt chloride with sodium oxalate, then filter the cobalt oxalate precipitate. Mix the cobalt oxalate precipitate with a fresh solution of sodium oxalate. Heat the mixture until a dark reddish purple color forms, then filter. Evaporate at room temperature(30C where I live).

I suspect that the amorphous solid could be due to unreacted sodium oxalate as it looks rather white, however, this observation is not really reliable as when the solid starts to form a thick layer, it has a light pink coloration even when washed thoroughly with water.
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crystal grower
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[*] posted on 13-6-2018 at 09:31

I'd say the cobalt oxalate hydrolyzes in the solution. Maybe adding some oxalic acid would help.

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[*] posted on 13-6-2018 at 09:58

I haven't tried the cobalt complex, but the analogous copper complex crystallizes best with cooling. It's very soluble in hot water, and almost completely insoluble in cold. That might work better than evaporation.

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[*] posted on 14-6-2018 at 07:34

Another possibility is that some chloride has remained. I much prefer suction filtration above solely gravitational filtration for that reason. Also, rinse the residue with some extra water while still not yet entirely filtered to dryness. If I go for higher purity myself, I take a second step by mixing and thoroughly stirring the residue with water and then filtering again.

With crystal growth the presence of "foreign" ions may enhance as well as inhibit the formation of well-shaped crystals. Testing is required. E.g. I have a nickel complex with iodide as the free anion, and the addition of extra KI enhances the formation of large crystals. With ammonium-cobalt-sulphate, I found that extra ammonium sulphate gave the same effect as you describe. Also, my surfaces were not shiny, whereas others have lovely surfaces with that compound.
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[*] posted on 14-6-2018 at 08:53

Maybe the Na salt crystallize not as well as the K salt?
Just like Tutton's salts?

[Edited on 14/06/18 by fusso]

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