Colorimetry is, put simply, the measurement of colour.
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While you can see with your eyes if water is cloudy or clear, or if it is coloured, it is not possible to get an accurate measurement of just how intense a colour is or how cloudy a solution may be.
A colorimeter is a device that passes light of a particular “wavelength” through a sample. Using a detector, the colorimeter can measure how much of the light has been absorbed by the sample. The amount of light absorbed by the sample is related to the concentration of the chemical of interest. This way it is possible to get numerical values for the amount of light and therefore the concentration of the substance in solution.