How to Make a Pure Sample of Cobalt
Cobalt is a chemical element used to prepare high-strength alloys, smalt (cobalt glass) and gamma rays. In nature, cobalt only occurs as part of a compound, but when manufacturing materials that incorporate pure cobalt, it is essential to rid cobalt of all impurities. Impurities potentially weaken cobalt materials, making them porous. Certain impurities, such as cadmium, can be toxic to lab workers handling cobalt and to the environment.
Things You'll Need
- Cobalt-containing solids
- Beakers
- Concentrated hydrochloric acid
- Filters
- Activated carbon
- Ammoniacal ammonium halide solution
- Sodium hydroxide
- Cation exchange resin
Instructions
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1
Digest cobalt-containing solids in hydrochloric acid (concentrated between 1 molar and 12 molar, preferably closer to 12 molar) until the solids are dissolved. Digestion results in a hydrated cobalt (II) chloride, CoCl 2 .6H 2 O.
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2
Contact the hydrated cobalt (II) chloride with an ammoniacal ammonium halide solution and a catalyst (ie, activated carbon), causing oxidation to form a solution of hexammine cobalt (III) halide.
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3
Precipitate crystals of the hexammine cobalt (III) halide from the solution by adding hydrochloric acid (!2 molar) to increase the chloride ion concentration of the solution. These crystals contain the activated carbon catalyst.
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4
Remove the catalyst. Dissolve the crystals in warm water and adjust the pH of the aqueous solution to about 9.0 by adding a strong base, such as concentrated (19 Molar) sodium hydroxide. Filter the solution to remove the insoluble activated carbon.
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5
Pass the hexammine cobalt (III) halide solution through a cation exchange resin in order to remove divalent cationic impurities. The resin will take up divalent cations from the solution, and release two monovalent ions in turn. A cation exchange resin is typically loaded with sodium ions, but other suitable forms include the proton (hydrogen) form and the ammonium form, which has the greatest ion exchange efficiency.
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6
Monitor, if you wish, the efficiency of the resin, by determining the concentration of divalent cations therein (ie, using atomic absorption spectroscopy, inductively-coupled plasma spectroscopy, or optical emission spectroscopy).
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7
Decompose the purified hexammine cobalt (III) halide solution to an insoluble cobalt oxide-containing material. Between 80 and 100 degrees Celsius, add a strong base, such as concentrated (19 Molar) sodium hydroxide, to the solution, increasing the pH to between 12.5 and 13; you will obtain an amorphous solid containing both cobalt oxides and cobalt hydroxides. Use a reducing agent (ie, hydrogen gas) to reduce this insoluble material to form pure cobalt metal powder.
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