Beer-Lambert law

Beer-Lambert Law

The Beer-Lambert Law, also known as Beer's Law, the Lambert-Beer Law, or the Beer-Lambert-Bouguer Law, is a fundamental principle in the field of Spectroscopy and Analytical Chemistry. It describes the linear relationship between the absorbance of a solution and the concentration of an absorbing species present in the solution, as well as the path length through which the light passes. This law is essential for understanding how light interacts with matter and is widely applied in quantitative analysis to determine the concentration of solute in a solution.

Overview

The Beer-Lambert Law can be mathematically expressed as:

A = εlc

where:

• A is the absorbance (no units, as it is a logarithmic ratio),
• ε (epsilon) is the molar absorptivity or extinction coefficient of the substance (L mol^-1 cm^-1),
• l is the path length of the sample cell (cm),
• c is the concentration of the compound in solution (mol L^-1).

The law assumes that the system follows monochromatic light absorption, and the medium is homogeneous and does not scatter the light. It is also presupposed that the absorbing species acts independently of each other.

Applications

The Beer-Lambert Law is utilized in various scientific fields, including Biochemistry, Molecular Biology, and Environmental Science, for the quantitative determination of substances. Applications range from measuring blood alcohol content, monitoring pollution levels, to determining the concentration of DNA in a sample. It is also fundamental in the operation of instruments like UV-Vis Spectrophotometers, which are used to measure the absorbance of light by a solution.

Limitations

While the Beer-Lambert Law provides a basis for the analysis of solution concentration, it has limitations. Deviations can occur due to high concentrations leading to molecular interactions, the presence of particulates causing scattering, or when the sample absorbs significantly outside the measured wavelength, known as the stray light effect. Additionally, the law does not apply to non-linear optical phenomena such as two-photon absorption.

Historical Background

The law is named after August Beer and Johann Heinrich Lambert, who independently formulated their respective laws in the 18th and 19th centuries. Beer extended Lambert's work on light absorption by incorporating concentration variables into the equation, leading to the combined form known today.

See Also

• Spectrophotometry
• Absorption Spectroscopy
• Molecular Absorption Spectroscopy
• Analytical Chemistry