Colorimetry

colorimetry

Measurement of the intensity of electromagnetic radiation in the visible spectrum transmitted through a solution or transparent solid. It is used to identify and determine the concentrations of substances that absorb light of a specific wavelength or colour according to Lambert's law, which relates the amount of light absorbed to the distance traveled through the absorbing medium, and Beer's law, relating it to the concentration of absorbing substance in the coloured solution. A photocell is often used to measure the amount of light transmitted through a glass tube containing the solution to be analyzed; the result is compared with results from a similar tube containing solvent alone. Most elements and many compounds, in appropriately treated samples, may be identified by colorimetry or spectrophotometry, a closely related technique.

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colorimetry

The science of measuring color. The International Commission on Illumination (Commission Internationale de l'Eclairage) governs this subject along with all aspects of lighting and illumination. See CIE Lab and colorimeter.

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colorimetry [‚kəl·ə′rim·ə·trē]

(optics)

Any technique by which an unknown color is evaluated in terms of standard colors; the technique may be visual, photoelectric, or indirect by means of spectrophotometry; used in chemistry and physics.

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Colorimetry 

(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 = I_o · e^-kcl

where I is the intensity of light after passing through the absorbing medium, I_o 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 (I_o/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.