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Cobalt(III)-ammonium sulfate

From Crystal growing


   Cobalt(III)-ammonium sulfate   
Names: ammonium-cobalt(III) sulfate
cobalt-ammonium alum
cobaltic alum
Formula: NH4Co(SO4)2 (anhydrous)
NH4Co(SO4)2 · 12H2O (dodecahydrate)
Molar mass: 269.094 g/mol (anhydrous)
485.272 g/mol (dodecahydrate)
Crystal system: cubic (dodecahydrate)
α=90°, β=90°, γ=90°
Shapes:
https://en.crystalls.info/w/uploads/media/3/37/Octahedron.json

Color: dark-blue
Stability: decomposes (dodecahydrate)
Hardness: strong
Toxicity: moderately toxic

Description

Inorganic compound, double salt of transitional metal cobalt, ammonium ion and inorganic sulfuric acid. Unlike complexed ions, aqueous cobalt(III) ion is extremely oxidizing and can oxidize water quickly, so cold sulfuric acid of at least 5mol/L is needed to handle it. From water solutions crystallizes as dodecahydrate.

Precursors

Reaction between sodium tris(carbonato)cobaltate(III), sulfuric acid and ammonium sulfate

Chemical equation:

2Na3[Co(CO3)3] + (NH4)2SO4 + 6H2SO4 = 3Na2SO4 + 2NH4Co(SO4)2 + 6CO2↑ + 6H2O


For preparation of 100.00g of cobalt(III)-ammonium sulfate dodecahydrate а 63.45g of sodium tris(carbonato)cobalt(iii) and 13.61g of ammonium sulfate is required.



Add the carbonato complex carefully into cold sulfuric acid, then add cold sulfuric acid solution of ammonium sulfate and cool this solution down to crystallize the alum.

Electrolysis of solution of cobalt(II) sulfate in sulfuric acid then add ammonium sulfate

Chemical equation:

(NH4)2SO4 + 2CoSO4 + H2SO4 = 2NH4Co(SO4)2 + H2


For preparation of 100.00g of cobalt(III)-ammonium sulfate dodecahydrate а 31.94g of cobalt(ii) sulfate and 13.61g of ammonium sulfate is required.



Electrolyze cold solution with a Pt or PbO2 anode and a cathode in separated cell, until solution becomes blue, then add cold sulfuric acid solution of ammonium sulfate and cool this solution down to crystallize the alum. If you want mixed crystals with ammonium alum, you can simply electrolyze a solution saturated with ammonium alum containing cobalt(ii) sulfate at room temperature without partitioning, until solution becomes brown, and freeze it to crystallize.

Notes

Pure compound is very unstable at room temperature, but mixed crystals with ammonium alum are quite stable.

Storage conditions

For crystal storing use hermetic vessel with small amount of saturated solution at the bottom or place wadding moistened with such solution. You can use such alternatives as vessel with vaseline or vegetable oil, organic non-hygroscopic solvent (kerosene, benzine or paraffin). Also you can use acrylate polymer or another kinds of solidifying plastic.

Solubility

Insoluble in ethanol and acetone.


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