Technique Primer

Spectrophotometry

What we see as "color" is the result of the absorption According to theory, the amount of light of a and/or reflection of light of specific wavelengths. White specific wavelength absorbed by a sample depends on 1) light consists of electromagnetic radiation having the concentration of the absorbing substance in the wavelengths ranging from 360 nm to 700 nm. When sample, 2) the thickness of the sample, and 3) the white light strikes a colored object, radiation of certain chemical characteristics of the absorbing (colored) wavelengths is absorbed. The radiation that is reflected species. When the sample is in solution, the relationship from or transmitted through the object will not contain among these factors is expressed by Beer's Law: the absorbed wavelengths, and the object will appear colored. Table I below lists the colors of various − log T = lc (1) (3) wavelengths of visible light:

Wavelength (nm) Color where T is transmittance, the fraction of incident 750 − 610 Red light that is transmitted

610 − 590 Orange  is the molar absorptivity, which is a constant 590 − 570 Yellow characteristic of the absorbing species 570 − 500 Green l is the sample thickness

500 − 450 Blue c is the concentration of the absorbing 450 − 370 Violet species in solution.

Table 1. Wavelengths and corresponding colors of light. The absorbance, A, of a sample is defined as follows:

The color wheel in Figure 1 shows the complementary A = - log T = - log (%T/100) (2) (4) relationship between colors absorbed by a solution and those transmitted. When a sample absorbs light of a The percent transmittance, %T, is 100 times the particular color, we perceive the object as the transmittance. Most instruments report %T or complementary color, i.e., the color opposite the absorbance, A, rather than T. Substituting the definition absorbed color on the color wheel. For example, if a of absorbance into Beer's Law yields the useful form: sample absorbs red light, the sample will appear blue- green to our eyes. A = lc (3) (5)

Figure 1: The color wheel.