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(1) In analytical chemistry, colorimetry is a group of photometric methods of quantitative analysis based on the determination of the concentration of substances in a colored solution by measuring the amount of light absorbed by the solution. The amount of light absorbed, the thickness of the layer of solution, and the concentration of the solution are related by a formula that obeys the Bouguer-Lambert-Beer law:
(1) I = Io · e-kcl
where I is the intensity of light after passing through the absorbing medium, Io is the intensity of the incident light, l is the thickness of the layer of solution (in cm), c is the concentration of the absorbing substance (in moles per liter), and K is a constant for light of a particular wavelength. After the logarithm is taken, equation (1) takes the form
(2) In (Io/I) = Kcl = D
where D is the optical density of the solution. It follows from equation (2) that D is directly proportional to the concentration of the substance in the solution.
A distinction is made between subjective (visual) and objective (photocolorimetric) methods of colorimetry. In visual methods, the optical density is determined by comparing the color of the solution being studied with the colors of a series of standard solutions, as well as by using visual colorimeters. In objective methods, photoelectric colorimeters are used.
(2) In physics, colorimetry is the methodology of measuring and expressing color in quantitative terms, and also the set of such methods.