BeerLambertregel

BeerLambertregel, more commonly known as the Beer-Lambert law or Beer-Lambert-Bouguer law, describes how the intensity of light is attenuated as it passes through a chemical sample. In its standard form, absorbance A equals the product of molar absorptivity ε, path length l, and concentration c: A = ε·l·c. Absorbance is related to the transmitted light I through I0, the incident light, by A = log10(I0/I). Equivalently, the transmitted fraction is I/I0 = 10^(−εlc). In some contexts a natural-log form is used with a corresponding coefficient change.

Key variables include ε (the molar absorptivity, typically in L·mol−1·cm−1), l (the optical path length in cm), and c (the solute concentration in mol/L). The law assumes monochromatic light and a homogeneous, non-scattering solution, with no chemical or physical changes during measurement. It is most accurate for dilute solutions where solute–solvent interactions do not alter ε appreciably.

Limitations arise at higher concentrations, where deviations occur due to solute–solute interactions, changes in refractive index, inner-filter effects, or scattering. Stray light, instrumental noise, and deviations from monochromatic light also reduce accuracy. The molar absorptivity ε can vary with wavelength, so measurements are typically performed at a single wavelength or across a spectrum with calibration.

Applications are widespread in quantitative chemistry and bioscience, including spectrophotometric determination of concentrations, enzyme kinetics, and quality control. The law is credited to Lambert and Beer, with Bouguer contributions often cited in the combined name Beer-Lambert-Bouguer law.