Chemical reactions

Reaction of cobalt with acids

Cobalt metal dissolves slowly in dilute sulphuric acid to form solutions containing the aquated Co(II) ion together with hydrogen gas, H2. The actual occurrence of Co(II) in aqueous solution is as the complex ion [Co(OH2)6]2+.

Co(s) + H2SO4(aq) Co2+(aq) + SO42−(aq) + H2(g)

Co(II) reacts with HCl(aq) under strongly acidic conditions, forming tetrahedral chloro complexes

[Co(H2O)6]2+(aq) + 4 Cl(aq) [CoCl4]2−(aq) [deep blue] + 6 H2O(l)

Reaction of cobalt with air

Cobalt does not react readily with air. Upon heating the oxide Co3O4 is formed, and if the reaction is carried out above 900°C, the result is cobalt(II) oxide, CoO.

3 Co(s) + 4 O2(g) 2 Co3O4(s)
2 Co(s) + O2(g) 2 CoO(s)

Cobalt does not react directly with nitrogen, N2.

Reaction of cobalt with ammonia

Cobalt(II)ions are precipitated by ammonia:

[Co(H2O)6]2+(aq) [pink, octahedral] + 2 NH3(aq) [Co(OH)2(H2O)4](s) [blue/pink] + 2 NH4+(aq)

The precipitate dissolves in excess amonia.

[Co(OH)2(H2O)4](s) + 6 NH3(aq) [Co(NH3)6]2+(aq) + 4 H2O(l) + 2 OH(aq)

The Co(II)ammonia complex is unstable and will oxidize to Co(III) e.g. with oxygen

[Co(NH3)6]2+(aq) [yellow/brown octahedral] [Co(NH3)6]3+(aq) [red/brown octahedral] + e

Reaction of cobalt with halogens

Metallic cobalt reacts with bromine, forming cobalt(II) bromide, CoBr2

Co(s) + Br2(l) CoBr2(s) [green]

The corresponding reaction with chloride and iodide does occur, but as a synthetic pathway other reactions are used.

Co(s) + Cl2(g) CoCl2(s) [blue]
Co(s) + I2(s) CoI2(s) [blue-black]

Co(II) forms complexes with Cl:

[Co(H2O)6]2+(aq) + 4 Cl(aq) [CoCl4]2−(aq) + 6 H2O(l)

Reaction of cobalt with hydroxide

Co(II) is precipitated by hydroxide ions:

[Co(H2O)6]2+(aq) [pink, octahedral] + 2 OH(aq) [Co(OH)2(H2O)4](s) [blue/pink depending on temperature] + 2 H2O(l)

The precipitate dissolves partially in excess hydroxide, and is oxidated:

[Co(OH)2(H2O)4](s) + O2(g) + H2O(l) Co(OH)3(s) [dark brown]

Reaction of cobalt with sulfide

Co(II) is precipitated by sulfide ions, either H2S under weakly acidic conditions with HAc or Na2S under neutral to slightly alkaline conditions:

Co2+(aq) + S2−(aq) CoS(s)

The precipitate is quickly changed into a more stable version which can only be dissolved in nitric acid or aqua regis:

CoS(s) + 2 NO3(aq) + 8 H+(aq) 3 Co2+(aq) + 3 S(s) + 2 NO(g) + 4 H2O(l)

Reaction of cobalt with thiocyanate

Co(II) forms complexes with SCN under acidic and neutral conditions

[Co(H2O)6]2+(aq) + SCN(aq) [Co(H2O)5(SCN)]+(aq) [red] + H2O(l)
[Co(H2O)6]2+(aq) + 4 SCN(aq) [Co(SCN)4]2−(aq) [blue] + 5 H2O(l)

In the blue [Co(SCN)4]2− complex there is also formed some H2Co(SCN)4(aq) which can be dissolved in solvents like isopentanol and ether.

Reaction of cobalt with water

Water has little effect upon cobalt metal. The reaction between red hot cobalt metal and steam produces cobalt(II) oxide, CoO.

2 Co(s) + O2(g) 2 CoO(s)

Quantitative analysis

Method 3500-Co C Inductively Coupled Plasma Method [2]. A portion of the sample is digested in a combination of acids. The digest is aspirated into an 8,000 K argon plasma where resulting light emission is quantified for 30 elements simultaneously.

Method limit of detection in water = 0.005 mg/L
Method limit of detection in soil = 2.00 mg/kg