Multicolored complexes with cobalt
name: Reactions of formation of colored complexes and colored sediments with cobalt
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
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.