This method involves measuring the intensity of a color in a solution and relating it to the concentration of the analyte. While some materials of interest are already colored, most of these analyses require the analyst to add some chemical reagents (reacting chemicals) to a sample to produce a characteristic color.
The simplest type of measurement is visual comparison of the intensity of the color to a set of color standards which represent various concentrations of the analyte. While this is method does not require any expensive equipment, color perception is rather subjective-- and many people have some degree of color-blindness.
A more precise measurement can be made using a colorimeter. A colorimeter is a device consisting of 1) a light source, which can be as simple as tungsten-filament light bulb; 2) some optics for focusing the light 3) a colored filter, which passes light of the color which is absorbed by the treated sample; 4) a sample compartment to hold a transparent tube or cell containing the sam
ple, 5) a light-sensitive detector, like the light meter on a camera, which converts the light intensity into an electric current, and 6) electronics for measuring and displaying the output of the detector. Some colorimeters may be designed to read out directly in concentration units, while others may show the results in units of light absorbance which need to be compared to a calibration curve. (An interesting point is that the filter is not the same color as the solution being tested, but rather the complementary color. We want to use a filter which transmits light of the color which the solution absorbs. A yellow solution looks yellow because it absorbs blue light, so a blue filter would be used.)
ple, 5) a light-sensitive detector, like the light meter on a camera, which converts the light intensity into an electric current, and 6) electronics for measuring and displaying the output of the detector. Some colorimeters may be designed to read out directly in concentration units, while others may show the results in units of light absorbance which need to be compared to a calibration curve. (An interesting point is that the filter is not the same color as the solution being tested, but rather the complementary color. We want to use a filter which transmits light of the color which the solution absorbs. A yellow solution looks yellow because it absorbs blue light, so a blue filter would be used.)
