The absorption of visible light is used in spectroscopy.

The idea

A UV-visible spectrometer passes light of each wavelength in turn through a sample and records how much is absorbed, producing an absorption spectrum of absorbance against wavelength. The peak of that curve, λmax, is the wavelength most strongly absorbed, and it corresponds to the d orbital gap through ΔE = hc/λ. Because every complex has its own λmax, the spectrum identifies the species present, and the same measurement tells you which colour filter to use in a colorimeter: choose the filter that transmits the light the solution absorbs most strongly, which is the complementary colour to the one you see. Absorbance is also proportional to concentration, and that is what makes the technique quantitative.

What earns the marks

• Describing an absorption spectrum as absorbance plotted against wavelength.
• Reading λmax from a spectrum as the wavelength of maximum absorbance.
• Using λmax in ΔE = hc/λ to find the d orbital splitting energy.
• Selecting a filter that transmits the light the solution absorbs most strongly.

Common mistakes and how to put them right

• Reading the wavelength at which absorbance is lowest as λmax.

• Choosing a colorimeter filter that matches the colour of the solution rather than its complement.

• Assuming λmax is the same for all complexes of the same metal.

• Quoting λmax in nanometres and then using it in ΔE = hc/λ without converting to metres.

Exam tips

• Label both axes if asked to sketch a spectrum: absorbance up the side, wavelength across the bottom.
• Say that the filter transmits the light the solution absorbs when justifying a filter choice.
• Use the spectrum for the quantity asked for: λmax gives energy, absorbance gives concentration.