Reason for Ferroin's color shift

In summary, Ferroin is a redox indicator that consists of a complex of phenanthroline and iron. The reduced state of the complex is red, while the oxidized state is blue. The difference in color can be attributed to the ligand's strong field strength, causing the reduced form to be low spin and have a transition in the blue region. The oxidized form is blue because the occupied d orbitals are lower in energy than the unoccupied d orbitals, allowing for an easier transition from the ground state. However, the main factor in the color change is the charge transfer from the metal to the ligand, rather than d-d* transitions.
  • #1
sekushi0range
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TL;DR Summary
The Fe -phen complex is a redox indicator. In the context of d-d orbital transitions why is the oxidized form blue?
Ferroin is a redox indicator. It is the complexation of phenanthroline and iron. In its reduced state the Fe (II) - phenanthroline complex is red and in its oxidized state the Fe(III)- phenanthroline complex is blue. I am trying to understand in the context of d-d orbital transitions why this is true. I know that phenanthroline is a strong field ligand so the Fe (II) would be low spin and the transition would be in the blue region explaining why the reduced form is red. But why is the oxidized form blue? My only thought is that an occupied orbital is lower in energy than an unoccupied orbital so the paired spin on the reduced form would need even more energy to make it to an unoccupied orbital than an electron from the ground state of the oxidized form, which would all be the same in energy. This thing about the difference in energy of occupied d orbitals vs unoccupied d orbital might me something I made up. I don't quite remember since I took inorganic 4 years ago.
 
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  • #2
The colour is mainly due not to d-d* transitions but due to charge transfer from the metal to the ligand.
 

Related to Reason for Ferroin's color shift

1. What is Ferroin and why does it change color?

Ferroin is a chemical compound that consists of iron ions and an organic molecule called 1,10-phenanthroline. It changes color due to a chemical reaction between the iron ions and the surrounding environment.

2. What causes the color shift in Ferroin?

The color shift in Ferroin is caused by a change in the oxidation state of the iron ions. When the iron ions are reduced, they appear colorless, but when they are oxidized, they appear red.

3. Can the color shift in Ferroin be reversed?

Yes, the color shift in Ferroin can be reversed by changing the oxidation state of the iron ions. This can be done by adding reducing or oxidizing agents to the solution.

4. How does the environment affect the color shift in Ferroin?

The environment plays a crucial role in the color shift of Ferroin. Changes in pH, temperature, and the presence of other chemicals can all affect the oxidation state of the iron ions and, therefore, the color of Ferroin.

5. What applications does the color shift in Ferroin have?

The color shift in Ferroin has various applications in analytical chemistry, such as detecting the presence of reducing or oxidizing agents in a solution. It is also used in chemical demonstrations and as a visual indicator in redox reactions.

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