Transition metals can be identified by their colour. Their colour arises from partly filled d-orbitals, which allow electrons to move from one d-orbital to another. All transition metals by itself have d-orbitals of the same energy, but in the presence of lignands, d-orbitals splitting will occur, such that the d-orbitals start getting different energies.
When an electron moves from its ground state (its lowest energy level) to an excited state (to a higher d-orbital), it needs to absorb photons of a certain wavelength. The rest of the wavelengths belonging to the visible light spectrum combine to give the colour we see.
The equation linking frequency to the difference in energy levels of the d-orbitals is given:
where delta E is the difference in energy levels;
h is Planck's constant 6.63 x 10-34 Js;
f is the frequency of the wavelength;
c is the speed of light in vacuum 3 x 108 ms-1;
and lambda is the wavelength.
This means that the colour can be affected by:
- the lignad itself
- the coordination number, which affects the geometry of the complex
- the oxidation state and therefore, charge on the transtion metal ion
We can compare the concentration of transition metal ions relative to each other by comparing how deep their colours are, using a colorimeter:
- place sample in a small cylindrical container called a cuvette
- a filter is chosen
- a light shines through this filter and the %absorbance is measured through how much of the light makes it through the mixture and detected in the photocell
- a graph calibration graph is plotted
Drafted by Eunice (Chemistry)