Instrumentation

Specification Point 9.8C:
  • Describe that instrumental methods of analysis are available and that these may improve sensitivity, accuracy and speed of tests
  • Advancements in technology and computing have allowed for the development of instruments designed to analyse chemical substances.
  • Methods of analysis include X-ray, Infra-Red and Mass Spectroscopy, Gas Chromatography and Flame Photometry.
  • These analytical techniques  require modern day instruments which are a vital part of busy chemistry laboratories.
  • The advantage of using these instruments over more traditional methods include:
    • They provide greater accuracy.
    • They are faster and easier to use.
    • Modern instruments are very sensitive and can work with very small sample sizes.

Flame Photometry

Specification Point 9.9C:
  • Evaluate data from a flame photometer:
    a) to determine the concentration of ions in dilute solution using a calibration curve
    b) to identify metal ions by comparing the data with reference data (no knowledge of the instrument or how it works is required)

Calibration Curve

  • When substances are heated they often emit energy in the form of light.
  • Flame photometry works by exposing the sample to a very hot flame and then measuring the intensity and wavelength of the light emitted.
  • 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 Mercury, Edexcel GCSE Chemistry

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 photometry also works for mixtures of ions.
  • This is a major advantage over flame testing which can only analyze one ion at a time.
  • The concentration of an unknown sample can be determined by creating a calibration curve.
  • Solutions of different but known and concentrations of the metal ion are prepared and analysed using the flame photometer.
  • 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, Edexcel GCSE Chemistry

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

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 solution containing an unknown metal:

Sodium Spectrum, Edexcel GCSE Chemistry

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

Reference Spectra, Edexcel GCSE Chemistry

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Edexcel GCSE Chemistry Notes

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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.