Title: molar absorption coefficient
Long Title: IUPAC Gold Book - molar absorption coefficient
DOI: 10.1351/goldbook.M03972
Status: current
Index: quantity

Definition
Absorbance divided by the absorption pathlength, \(l\), and the amount concentration, \(c\): \[\varepsilon(\lambda) = \frac{1}{c\, l}\, \rm{lg}\! \left ( \frac{P_{\lambda}^{0}}{P_{\lambda}}\right ) = \frac{A(\lambda)}{c\, l}\] where \(P_{\lambda}^{0}\) and \(P_{\lambda}\) are, respectively, the incident and transmitted spectral radiant power.

Notes
1) The term molar absorptivity for molar absorption coefficient should be avoided.
2) In common usage for \(l/\rm{cm}\) and \(c/\pu{mol dm-3}\) (\(\rm{M}\)), \(\varepsilon(\lambda)\) results in \(\pu{dm3 mol-1 cm-1}\) (\(\rm{M}^{-1}\ \rm{cm}^{-1}\), the most commonly used unit), which equals \(\pu{0.1 m2 mol-1}\) (coherent SI units).

Related Terms
- Absorbance: https://goldbook.iupac.org/terms/view/A00028
- absorption coefficient: https://goldbook.iupac.org/terms/view/A00037
- amount concentration: https://goldbook.iupac.org/terms/view/A00295
- molar absorptivity: https://goldbook.iupac.org/terms/view/M03973
- spectral radiant power: https://goldbook.iupac.org/terms/view/S05828

Source
- PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 371 (https://doi.org/10.1351/pac200779030293)

Other Outputs
- html: https://goldbook.iupac.org/terms/view/M03972/html
- json: https://goldbook.iupac.org/terms/view/M03972/json
- xml: https://goldbook.iupac.org/terms/view/M03972/xml

Citation: Citation: 'molar absorption coefficient' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.M03972
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Accessed: 2026-05-14T18:38:41+00:00