The Emission Spectrum

  • This method is 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 combination of wavelengths produced depends on the ions charge and its electronic configuration.
  • This can be used to identify the ions since no two ions have the same charge and the same electronic configuration.
  • An apparatus inside the machine then converts the light into electrical signals.
  • The output is an emission spectrum in which different ions produce lines in different parts of the spectrum.

Emission-Spectrum-of-Mercury, IGCSE & GCSE Chemistry revision notes

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.

Calibration Curves

  • The concentration of an unknown sample can be determined by creating a calibration curve.
  • Solutions of different but known concentrations of the metal ion are prepared and analysed using flame emission spectroscopy.
  • The readings obtained are used to prepare a calibration curve which plots the instrument reading against the known concentrations.
  • This is then used to determine the concentration of the unknown solution using construction lines from the instrument reading of the unknown sample.
  • An example is shown below for a solution of calcium ions that produced a reading of 3.8 units and whose concentration is thus calculated at being 1.85 Ca
  • mol dm-3.

Calibration Curve Calcium graph, IGCSE & GCSE Chemistry revision notes

Calibration curve of calcium solutions for determining the concentration of the sample solution

Using Reference Data

  • 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:

Sodium-Spectrum, IGCSE & GCSE Chemistry revision notes

Diagram of an emission spectrum

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

Reference-Spectra, IGCSE & GCSE Chemistry revision notes

Diagram of a reference spectra including an emission spectrum for sodium

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.