The Beer-Lambert law describes the relationship between absorbance and concentration, which combines the principles established by scientists Johann Heinrich Lambert and August Beer. Lambert's law states that when light passes through a medium, the loss in intensity is directly proportional to the original intensity and the path length of the light. Beer's law proposed that the transmittance of a solution remains constant if the product of concentration and path length is constant. The modern Beer-Lambert law combines these two laws and correlates the light's absorbance, concentration, and path length.
According to the Beer-Lambert law, absorbance is concentration multiplied by the molar absorptivity coefficient and the path length. At a single wavelength, absorbance can be given by the formula,
Where A is absorbance, ε is the molar absorptivity of the compound or molecule in solution (M−1cm−1), the path length of the cuvette is denoted by l, and c is the concentration of the solution (M). Absorbance of a solution, is the logarithmic function of the ratio of the intensity of the incident light, I0, to the intensity of the transmitted light, I.
The molar absorptivity, also known as the molar absorptivity coefficient, ε, is the absorbance of a compound when a 1.00 M solution is taken in a cell with a 1.00 cm path length. The absorbance of a solution is directly proportional to its concentration. The linear relationship between absorbance and concentration allows the determination of an unknown concentration by plotting the absorbance against known concentrations, creating a calibration curve.
The Beer–Lambert law gives the relationship between absorbance, concentration, and path length of light through the sample.
The absorbance, A, of a solution, is the logarithmic function of the ratio of the intensity of the incident light, I0, to the intensity of the transmitted light, I.
The absorbance also relies on molar absorptivity, ε, which is the absorbance of a 1 molar solution measured in a cell with a path length of 1 cm.
Absorptivity values above 104, are termed high-intensity absorptions, while those below 103 are classified as low-intensity absorptions.
Molar absorptivity is a constant for a compound, and it is a characteristic of that compound at a particular wavelength. For example, absorption bands in the benzoic acid spectrum are characteristic of its molar absorptivity at each wavelength.
The Beer–Lambert law can be used to determine the unknown concentration, if the absorbance and molar absorptivity at a particular wavelength are known.
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