4.1: Background

Chemists can identify a substance by the color (wavelength) of light it emits. It is also possible to identify a substance by the color (wavelength) of light it absorbs. White light consists of all the wavelengths of electromagnetic radiation from about 400 nm to 700 nm. When white (colorless) light enters a sample and all of the colors of light are strongly absorbed the sample appears black. If a sample absorbs none of the colors of light, then it appears white or colorless.

A more interesting situation occurs when a sample absorbs some of the colors of light. The remaining colors pass through the solution. The color we observe for the solution is the sum of these remaining colors that strike your eyes.

The absorbance is directly proportional to the concentration, that is, the greater the concentration of a solution, the more light that is absorbed. Additionally, the path length, or the size of the sample, can affect the amount of light or “photons” absorbed. The greater the path length, the more molecules are present to absorb photons, thus, the greater the absorbance value. The relationship is known as Beer’s law or

A = kc

where c is the concentration (molarity) and k is a constant that is dependent on the wavelength. The absorbance is related to the intensity of the color of a solution. If we compare two solutions of the same substance and the color of one appears to be more intense than the color of the other, then we can reasonably assume that the darker solution is absorbing more light (has a higher absorbance) and is the more concentrated solution.

In determining the concentration of a solution, a calibration curve is normally used. Solutions of known concentration are prepared, and their absorbances are measured and plotted on a graph of absorbance versus concentration. The absorbance of the unknown solution is then measured and its concentration read from the calibration curve.

Calibration Curve:

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