In ion-exchange chromatography, when pouring an aqueous mixture of cobalt(II) an
ID: 1018336 • 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
Ni atomic number 28; electronic configuration [Ar] 4s23d8
Coatomic number 28; electronic configuration [Ar] 3d74s2
Here in aqueous solution of Cobalt(II), cobalt is present as [Co(H2O)6] 2+ and Nickel(II) is present as [Ni(H2O)6]2+.
Now when HCl solution is charged, Cl- ions are stronger ligand than water. So, Cl- ions replace the H2O ligand. Cl- ions in solution can cause lower energy exitation of d orbital electrons.
Cobalt has tendancy to from CoCl4- easilywhich are tetrahedral in shape.Here in this case oxidation state of Co is 4+. Which makes d orbital half filled (d5) that means it is stable configuration.
Where as Nickel can not form such complex very easily with Cl- ion as it is a weak ligand for Ni2+ . Because to approch Ni2+ ionis difficult as orbitals surrouing Ni are more than half filled and Cl- ion is also large in size with high electronic cloud which makes ligand like Cl- to approch Ni difficult. Ni4+ (d6) is also not as stable as Co4+ (d5).
Now,
Ion-exchange chromatography is a form of liquid chromatography in which the stationary phase is an inert polymer that is derivatized with charged groups that can bind (exchange) ions of opposite charge. The most common cation and anion exchange resins consist of a styrene-divinylbenzene crosslinked copolymer.
Replacing neutral water molecules with chloride anions also changes the charge on the complex. After 3 H2O have been replaced by 3 Cl- , the cobalt complex in solution is anionic. At this point it will have some affinity for an anion-exchange resin. So, it takes more time for eluting out than NiCl+ complex.
For Co ions Dv increases as concentration of HCl increases. Dv is binding affinity to the column. thus, Co complex elutes after Ni complex.