Detection of Cobalt

Salts of cobalt are usually of a rose-red colour when hydrated, and yield a beautiful blue colour in the borax bead test provided they are not present in excessive amount, otherwise the bead becomes opaque and appears black. When heated on charcoal with reduction mixture in the blowpipe test, metallic cobalt separates out in the form of small magnetic beads. When strongly heated with alumina a blue colour is produced, known as Thenard's blue.

In qualitative analysis by Wet Methods, it is usual to precipitate cobalt, along with any nickel, zinc, and manganese that may be present, by passage of hydrogen sulphide through an ammoniacal solution containing these metals in the presence of ammonium chloride. The sulphides are dissolved in aqua regia, and excess of acid boiled off. Sodium hydroxide or peroxide is now added, whereby the hydroxides of cobalt, nickel, and manganese are precipitated, the zinc remaining in solution. The washed precipitates are dissolved in hydrochloric acid, excess of ammonium acetate added, and the sulphides of cobalt and nickel precipitated with hydrogen sulphide, manganese remaining in solution. It now remains to separate the nickel from the cobalt. This may be effected by Liebig's Cyanide Process, which consists in dissolving the precipitates in aqua regia or hydrochloric acid with a little bromine, boiling off excess of acid or bromine, nearly neutralising with sodium carbonate, and adding potassium cyanide until the precipitated cyanides are just redissolved. After again boiling, which converts the cobalt into potassium cobalti-cyanide, K₃CoC₆N₆, sodium hypobromite is added, which precipitates black hydrated nickel oxide, leaving the cobalt in solution. The solution may now be evaporated to dryness with dilute sulphuric acid, and either tested for cobalt by the dry methods mentioned above, or the presence of cobalt confirmed by any of the reactions:

1. α-nitroso β-naphthol yields a red precipitate of cobalti-nitroso β-naphthol with acetic acid solutions of cobalt salts.
2. Potassium thiocyanate yields a blue colour with solutions of cobalt salts, a blue alcoholic layer being produced on addition of amyl alcohol or of alcohol and ether to the mixture and shaking.
3. Potassium nitrite yields a yellow precipitate of potassium cobalti-nitrite, K₃Co(NO₂)₆, when added to a solution of a cobalt salt containing free acetic acid.
4. Ammonium sulphide yields a black precipitate when added to an ammoniacal solution of a cobalt salt.