Flame Emission Spectroscopy (AQA GCSE Chemistry)

• Flame emission spectroscopy is a technique used to analyse metal ions in solution
• When substances are heated they often emit energy in the form of light
• This is due to electrons falling back to their original energy levels after becoming excited which causes them to jump up one or more energy levels
• Flame emission spectroscopy works by exposing the sample to a very hot flame and then measuring the intensity and wavelength of the light emitted
• The output is an emission spectrum in which different elements produce lines in different parts of the spectrum

Diagram of an emission spectrum for mercury obtained from flame photometry

• The emission spectrum consists of brightly coloured thin lines on a dark background and each element ion produces a unique spectrum
• Flame emission spectroscopy also works for mixtures of ions
• This is a major advantage over flame testing which can only analyze one ion at a time
• The intensity of the light produced is proportional to the number of ions vaporised, so the technique can be used to determine the concentration of metal ions in a solution by reference to a standard solution of known concentration

• Ions in unknown samples can be identified by comparing the sample spectrum to reference spectra
• This is particularly useful if the sample contains a number of different ions
• The following flame spectrum for example was obtained for a solution containing an unknown metal:

Flame spectrum for an unknown metal 

• When compared to the reference spectra below we can see that the solution must contain sodium ions:

Diagram of a reference spectra including an emission spectrum for sodium