During the first semester of General Chemistry, you learn about electromagnetic radiation. In the spectrum, light that ranges between 440 nm to 700 nm is the visible radiation. We also learn that the radiation results from electrons moving from one energy level to another. When an electron absorbs radiation (light), it is excited to a higher energy level. The absorption of light results in color in solutions of transition metal complexes.

A spectrophotometer is an instrument that measures the intensity of the light entering a sample and the light exiting a sample and compares the two intensities. Information about the two intensities can be expressed as transmittance (the ratio of the intensity of the exiting light to the entering light) or percent transmittance (%T). Different materials absorb different wavelengths of light. Therefore, the wavelength of maximum absorption by a material is one of the characteristic properties of that material. The %T can be related to the absorbance (A) by the equation below.

A = 2.00 -[log (%T)]

If T = 85%, then

Absorbance = 2 - log[85] = 0.071

Beer's law states that the absorbance is directly proportional to the concentration of a solution. If you plot absorbance versus concentration, the resulting graph yields a straight line. The equation for the straight line (termed regression line) can be used to determine the concentration of an unknown solution once the %T has measured.

In general chemistry laboratory, we use a Spec-20 to measure the %T. To view a demonstration on the use of a Spec-20, please visit the video webpage.