03 Feb

Beer’s Law

No it’s not a law about beer as in ale. Beer’s law, or the Beer-Lambert law describes the relationship between absorbance of light by a substance, the distance the light travels through the substance, and the concentration of the substance.


Where A* is absorbance; e is the molar absorptivity of the substance in L/mole cm; l is the length the light travels through the substance in cm, l is generally 1cm; c is the concentration of the substance in moles/L.  Colored solutions absorb in the visible region of the electromagnetic spectrum but proteins and DNA absorb in the ultraviolet region.

From Columbia University Science Teacher Program
From Columbia University Science Teacher Program

Absorbance is a unitless value so we refer to absorbance units.  If e is known, the concentration of a substance in solution can be determined. An unknown concentration may also be determined by comparing the absorbance of a known (standard) concentration of the substance with the absorbance of the unknown concentration.

For sample determinations a standard of known concentration of the analyte of interest is prepared and the absorbance is determined.  The absorbance of the unknown sample is then determined.

A Std      A Unk

Conc. Std   =  Conc. Unk

The response of absorbance vs. concentration is linear through 2 absorbance units but above 2 absorbance units linearity falls off.  Samples and standards must be diluted so that absorbance falls between 0 and 2 absorbance units.

Absorbance can be measured using an instrument as advanced as a double beam scanning UV/VIS absorption spectrophotometer or as simple as in colorimetry. The principle of absorbance is also utilized in other instruments such as HPLC and LC detectors.

*A=-log(I/Io) where Io is the initial intensity of the light beam and I is the intensity of the light beam after passing through the sample.

Robin Prymula