Colorimetry and Visible Spectroscopy

Colourimetry

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 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.