One useful application of Beer’s Law is to determine the equilibrium constant of a reaction. Reactants (iron (III) and thiocyanate) are mixed in a ratio such that one reactant is presumed to be nearly 100% converted into a colored product. The product is assumed to follow Beer’s Law, and the relationship between relative absorbance and concentration is determined from the first experiment. After that, the reactant originally in excess is reduced in concentration. From the colorimetric determination of product concentration, the concentrations of reactant remaining at equilibrium is determined and the quantitative relationships among these concentrations are studied. One of these relationships, the mass action expression, is found to be nearly constant.

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