Rainbow of cobalt

Multicolored complexes with cobalt

Scientific name: Reactions of formation of colored complexes and colored sediments with cobalt

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Chemistry experiment 9 - Cobalt complexes

by koen2all

Metal complexes 10. The chemistry of Cobalt Complexes.

by FranklyChemistry

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Reaction formula

CoSO4 + 6H2O ↔ (Co(H2O)6)SO4

CoSO4 + NaHCO3 → CoCO3 + NaHSO4

(Co(H2O)6)SO4 + 2NaOH → (Co(H2O)2(OH)2) + Na2SO4

CoSO4 + Na2SiO3 → CoSiO3 + Na2SO4

(Co(H2O)2(OH)2) + 6NH3 → (Co(NH3)6)(OH)2 + 4H2O

CoSO4 + Na2S → CoS + Na2SO4

CoSO4 + K4(Fe(CN)6) → K2Co(Fe(CN)6) + K2SO4

CoSO4 + K3(Fe(CN)6) → KCo(Fe(CN)6) + K2SO4

Step-by-step instruction

  1. Take 4 g of solid cobalt(II) sulfate-7-water and dissolve it in 40 ml of water in a beaker. You have a light pink solution. This solution will be added to other solutions.
  2. Take a test-tube, pour 3 ml of water into it, add a small amount of sodium bicarbonate and shake the mixture. Add 1-2 ml of the cobalt sulfate solution to the test tube and shake the mixture again. You have a pink-violet precipitate.
  3. Take a test-tube, pour 1 ml of water into it, add 2 ml of sodium hydroxide solution and shake the mixture. Add 1-2 ml of the cobalt sulfate solution to the test tube and shake the mixture again. You have an orange solution.
  4. Take a test-tube, pour 3 ml of water into it, add a small amount of sodium silicate and shake the mixture. Add 1-2 ml of the cobalt sulfate solution to the test tube and shake the mixture again. You have a bright blue precipitate.
  5. Take a test-tube, pour 1 ml of water into it, add 2 ml of ammonium hydroxide solution and shake the mixture. Add 1-2 ml of the cobalt sulfate solution to the test tube and shake the mixture again. You have a blue-green solution.
  6. Take a test-tube, pour 1 ml of water into it, add 2 ml of sodium sulfide solution and shake the mixture. Add 1-2 ml of the cobalt sulfate solution to the test tube and shake the mixture again. You have a black precipitate.
  7. Take a test-tube, pour 3 ml of water into it, add a small amount of potassium-ferricyanide and shake the mixture. Add 1-2 ml of the cobalt sulfate solution to the test tube and shake the mixture again. You have a dark-red precipitate.
  8. Take a test-tube, pour 3 ml of water into it, add a small amount of potassium-ferrocyanide and shake the mixture. Add 1-2 ml of the cobalt sulfate solution to the test tube and shake the mixture again. You have a green precipitate.

Scientific background

This experiment is based on a very important phenomenon - the ability of metals to form chemical structures called coordination compounds. In such structures the central metal atom is bounded with one or more ligands. A ligand is an atom or a molecule which contains one or more electron pairs that can be shared with a metal ion. Cobalt ions are well known to combine easily with different ligands to form coordination complexes. These compounds have very symmetrical, octahedral or tetrahedral geometry. Also cobalt forms different colored insoluble compounds with many substances.

An aqueous solution of cobalt sulfate has a light pink color. Cobalt sulfate is present as (Co(H2O)6)SO4 in aqueous solution. This compound is dissociated into (Co(H2O)6)2+ and SO42-.

When we added sodium bicarbonate to the solution to cobalt sulfate, a pink-violet precipitate was formed.

CoSO4 + NaHCO3 --> CoCO3 + NaHSO4.

Cobalt(II) carbonate is an inorganic pigment, and a precursor to catalysts.

Similarly, cobalt sulfate can react with sodium silicate to form an insoluble cobalt silicate CoSiO3. This compound is used in production of colored glass.

Also, cobalt sulfate reacts with sodium sulphide.

CoSO4 + Na2S → CoS + Na2SO4.

The reaction produces black cobalt sulphide.

Cobalt sulfate reacts with sodium hydroxide. The progress of this reaction depends on the amount of the alkali solution. Hydroxide ions gradually replace the water from the coordination sphere of the (Co(H2O)6)2+ ion. If the amount of hydroxide ions is sufficient then (Co(H2O)2(OH)2) is formed. Adding an aqueous solution of ammonia, which has an alkaline pH, results the formation of the (Co(NH3)6)(OH)2.

Cobalt sulfate reacts with K4(Fe(CN)6) and K3(Fe(CN)6) to form similar compounds. But the resulting compounds differ in their structure. Iron(II) is present in the K2Co(Fe(CN)6). Iron(III) is present in the KCo(Fe(CN)6). This explains the difference in the color of produced precipitations.

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Published on 13 April 2015

  • Fire
  • Heating with fire
  • Explosion
  • Poisoned gas
  • Organic
  • Electricity
  • Solution
  • Oxidation reduction
  • Color change
  • Precipitate
  • Gassing
  • Catalyst