Colorimetry and Visible Spectroscopy

Colorimetry and Visible Spectroscopy


  • Colourimetry is a technique used to determine the concentration of coloured compounds in a solution.
  • It works by measuring the amount of light that is absorbed by a substance at a particular wavelength.
  • The concentration of the solution is proportional to the absorbance of light.
  • The relationship between concentration and absorption is described by the Beer-Lambert law: A=εcl, where A is the absorbance, ε is the molar absorptivity, c the concentration, and l the path length.
  • Calibration curves are used in colourimetry. They are plots of absorption against the concentration of a series of known solutions.
  • The absorbance of an unknown sample is then measured, and its concentration determined from the calibration curve.
  • Colourimetery can be used to determine concentrations of a wide range of chemicals, from transition metal complexes to organic compounds.

Visible Spectroscopy

  • Visible spectroscopy involves studying how a sample absorbs light in the visible part of the electromagnetic spectrum (around 400-700nm).
  • As with colourimetry, the Beer-Lambert law is applicable and we can use a calibration curve to determine the concentration of an unknown.
  • Spectrophotometers are used in visible spectroscopy to measure the absorption of light at every wavelength across the visible spectrum, producing an absorption spectrum.
  • The absorption spectra can provide information about the type of chemical bonds, the molecular structure, and the concentration of a substance in solution.
  • Visible spectroscopy can also indicate the presence of particular functional groups in organic molecules.
  • In transition metal complexes, the different colours result from d-orbital splitting, which is clearly illustrated by the absorption spectrum.
  • Visible spectroscopy is more comprehensive than colourimetry as it covers a wider range of wavelengths, allowing for the identification of more compounds and more detailed information.


  • Colorimetry and visible spectroscopy are valuable tools in fields such as chemistry, biology, physics, and environmental science.
  • They assist in quality control in industrial processes, determining, for instance, metal ion concentrations in water samples or drug concentrations in blood samples.
  • They are used in medical labs for diagnosing and monitoring patient’s health.
  • In environmental science, they help monitor pollution levels in air and water samples.