Colourimetry is a technique used to find the concentration of a solution using the absorbance of light.

The Colourimeter

The colourimeter measures the absorbance of specific wavelengths of light through a solution in order to find out its concentration.

The colourimeter works by:
1. Emitting white light from a light source.
2. The white light then gets filtered through a filter so only specific wavelengths get through.
3. The incident light then passes through the solution and gets partially absorbed.
4. The transmitted light is recorded and displayed.
Technique
-Step one is to choose a filter of the colour most strongly absorbed by the solution.
-Then solutions are made up of varying concentrations above and below that of the unknown concentration.
-The colorimeter is then calibrated with a pure solution in the cuvette to get a base reading of minimal absorbance.
-Solutions of known concentration are then put into the colourimeter and the absorbance is measured.
-The absorbance is plotted against concentration on a calibration curve.
-Absorbance is then measured of the unknown concentration solution in the colourimeter.
-Finally the absorbance is plotted on the calibration curve to find the concentration of the unknown solution.