In ion-exchange chromatography, when pouring an aqueous mixture of cobalt(II) an
ID: 1018337 • Letter: I
Question
In ion-exchange chromatography, when pouring an aqueous mixture of cobalt(II) and nickel(II) cations through a resin column that has been charged with 6M HCl, why do the cobalt(II) cations elute more slower. I did some research and found out that the nickel(II) cations forms a NiCl+ complex when passing through the Cl- charged resin, where as cobalt(II) cation forms a CoCl42- complex; what is the logic behind the formation of the two totally different complexes when they both have the same charge and nearly same atomic masses? Why does the CoCl42- complex elute with the addition of DI water, after the NiCl+ complex?
Explanation / Answer
Answer:
Principle behind elution process:
Ion exchange chromatography works on the charge present on central metal ion and technique helps in separation of two or more mixture of metal ions compounds. In solutions containing a large concentration of HCl, many metals are converted into complex anions like chlorides and are adsorbed in the resin. while other metal which is not converted into complex anions are eluted. Cobalt forms deep blue complex CoCl42- and is strongly adsorbed in resin. Ni is not adsorbed in resin, hence separation becomes easy. Co can be removed from resin by washing resin with less conc. HCl or DISTILLED WATER.
Formation of Complex: This is totally based on coordination and geometry of the metal complex. Co forms TETRAHEDRAL COMPLEX and Ni forms SQUARE PLANAR COMPLEX.
ION EXCHANGE EQUILIBRIA: It is treated by law of mass action.
K= [[resin+] [A-]] [Cl-] / [[res+][Cl-]] [A-]
The equilibrium constant (K) for Co complex is very large number at low pH and CoCl4 2- is retained on the resin.
But Ni complex elute out due to small equilibrium constant value.