Pilgaard Solutions

Cobalt


Main page



Other names/abbreviations: Co



Element no: 27
Element group: 9 (IUPAC)
VIII B (American labelling)
VIII A (European labelling)
Element type: Transition metals
CAS-Number: 7440-48-4
EINECS-Number: 231-158-0



Chemical properties

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 + 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)


Solubility
Water: Insoluble (20 °C) [5]


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


Safety

Symbol: Xn
R-phrases: 42/43
S-phrases: 22-24-37


Physical properties

Appearance

Physical state @ 20°C: Solid [5]
Color: Dark gray [5]
Odor: Odourless [5]


Bulk properties

Molecular weight (g/mol): 58.933200(9) [1]
Melting point (°C): 1495 [1]
Boiling point (°C): 3100 [1]
Density (g/cm3): 8.90 [1]
Molar volume (cm3): 6.60 (20 °C) [3]
Velocity of sound (m/s): 4720
Elastic properties:
Young's modulus (GPa):
Rigidity modulus (GPa):
Bulk modulus (GPa):
Poisson ratio:
211-213 [3]
76.7 [3]
182 [3]
0.31 [3]
Hardness:
Mineral hardness:
Brinell hardness (MN/m2):
Vicker hardness (MN/m2):
5.0
700
1043


Optical properties

Reflectivity (%): 67


Thermodynamic properties

ΔH°atomization (KJ/mol): 426
ΔfH°gas (KJ/mol): 425±17 (Monoatomic) [1]
ΔH°fusion (KJ/mol): 16.3 [1]
ΔH°vaporization (KJ/mol): 382 [1]


Cp (solid) (J/K·mol): 25.653 (20 °C) [3]
26.640 (100 °C) [3]
28.120 (200 °C) [3]
29.600 (300 °C) [3]
31.080 (400 °C) [3]
33.793 (600 °C) [3]
42.180 (800 °C) [3]
47.113 (1000 °C) [3]
52.046 (1200 °C) [3]


Coeff. of linear thermal expansion (106 K-1): 12.3 (100 °C) [3]
13.1 (200 °C) [3]
13.6 (300 °C) [3]
14.0 (400 °C) [3]


Crystal structure

Crystal type: HCP [3] Hexagonal


Electronic properties

Electron configuration: 1s2-2s2-2p6-3s2-3p6-3d7-4s1
Ionic radius (Å): Co(IV): 0.53 (6-coordinate) [1]
Co(III): 0.545 (6-coordinate, low-spin) [1]
Co(III): 0.61 (6-coordinate, high-spin) [1]
Co(II): 0.65 (6-coordinate, low-spin) [1]
Co(II): 0.745 (6-coordinate, high-spin) [1]


Conductivity

e°(V): -0.277
Electrical resisitvity (μΩ·cm): 5.68 (20 °C) [3]
9.30 (100 °C) [3]
13.88 (200 °C) [3]
19.78 (300 °C) [3]
26.56 (400 °C) [3]
40.20 (600 °C) [3]
58.60 (800 °C) [3]
77.40 (1000 °C) [3]
91.90 (1200 °C) [3]
Thermal (W/m·K): 87 (27 °C) [3]
79 (127 °C) [3]
70 (227 °C) [3]
63 (327 °C) [3]
56 (427 °C) [3]


References

1: Greenwood,N.N., Earnshaw,A. Chemistry of the elements
2nd edition (1997) Edited by Greenwood,N.N., Earnshaw,A. pp. 1-1340, Butterworth-Heinemann. Oxford. Great Britain

2: Standard Methods for the Analysis of Water and Wastewater, APHA, 1992, 18th edition

3: Buch,A. Pure Metals Properties. A Scientific-Technical Handbook
1st edition (1999) Edited by Buch,A. pp. 1-306, ASM International and Freund Publishing House Ltd. Ohio. USA

4: CRC Handbook of Chemistry and Physics
75th edition (1994) Edited by Lide,D.R. , CRC Press Inc. Boca Raton, USA

5: Merck. ChemDAT The Merck Chemical Database Ver. 1.1.5







© Michael Pilgaard
Created: October 3, 2006
Last update: October 8, 2008