Chemical SciencesMolecular Absorption Spectroscopy Lab →List Of Experiments

Effects of sample concentration Dependence of Absorbance (Beer Law)

Have you noted that the color intensity of a colored substance (say, an indicator dye) decreases with the increase in dilution? This is because the magnitude of light absorbed depends on the quantity of light absorbing substance present in a sample, in addition to the sample path length. For a light absorbing substance present in solution, the light attenuation depends on the concentration of the substance. We know that many organic molecules do not absorb electromagnetic radiation in the ultraviolet-visible (UV-VIS) regions (~wavelengths range 190 nm - 800 nm). Absorption spectroscopy is, therefore, of limited utility in this range of wavelengths. However, a number of chemical analysis can be done in the case of light absorbing organic molecules. The quantity of a colored substance present in a solution can be determined if the relation between the sample concentration and the light absorption is known. German scientist August Beer formulated a relationship in this regard in 1852, which is known as Beer Law. This law is the basis for quantitative UV-visible measurements.