Beer–Lambert Law

The Beer–Lambert law says that the amount of light absorbed by a solution increases with both the number of absorbing particles in the light path and the distance the light travels through the sample. In other words, a more concentrated solution or a longer cuvette gives a larger absorbance because more photons are removed from the beam. The molar absorptivity, ε, is a constant for a given substance at a specific wavelength and measures how strongly that substance absorbs that light.

Use this law when determining the concentration of a coloured or UV-absorbing solution from spectrophotometer data, or when predicting how absorbance changes with concentration or path length. It is especially common in analytical chemistry, biochemistry, and environmental testing.

• Forgetting that ε is wavelength-dependent, so values from a different wavelength may give the wrong concentration
• Using the wrong path length units; l must be in cm when ε is in L mol^-1 cm^-1
• Applying the law to very concentrated or strongly scattering samples where the linear relationship breaks down