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8.8 Instrumental methods and flame emission spectroscopy

The advantages of instrumental methods of analysis, how flame emission spectroscopy identifies metal ions from their line spectra, and how a calibration curve is used to find the concentration of an ion in a solution.

Separate only AQA Edexcel OCR 2 min read

Instrumental methods

Key terms

  • Instrumental method: a method of chemical analysis that uses a machine, such as flame emission spectroscopy.
  • Calibration curve: a graph made using solutions of known concentration, used to find the concentration of an unknown sample.

Compared with chemical tests, instrumental methods are:

  • accurate: they give reliable, precise results
  • sensitive: they detect very small amounts of a substance
  • rapid: results are produced quickly, and many samples can be tested automatically

Flame emission spectroscopy

  1. The sample is put into a flame.
  2. The light given out is passed through a spectroscope, which splits it into its wavelengths.
  3. The output is a line spectrum. Each metal ion produces its own unique pattern of lines, like a fingerprint.
Line spectra of lithium, sodium and potassium compared with sample X, whose lines match lithium and sodium, and a calibration graph of light intensity against concentration used to find an unknown concentration of 25 mg/dm3
Line spectra of lithium, sodium and potassium compared with sample X, whose lines match lithium and sodium, and a calibration graph of light intensity against concentration used to find an unknown concentration of 25 mg/dm3

Identifying ions

Compare the spectrum of the sample with reference spectra of known metal ions. If all the lines of a known ion appear in the sample, that ion is present. Unlike a flame test, this works for mixtures, because each ion's lines can be seen separately.

Finding the concentration

The intensity (brightness) of the lines depends on the concentration of the metal ion.

  1. Measure the intensity for several solutions of known concentration.
  2. Plot a calibration curve of intensity against concentration.
  3. Measure the intensity for the unknown solution, then read across to the line and down to find its concentration.
Flame test or flame emission spectroscopy?
FeatureFlame testFlame emission spectroscopy
Identifies ions in mixturesno, colours can be hiddenyes
Measures concentrationnoyes, with a calibration curve
Sensitivityneeds a fairly large amountdetects tiny amounts

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