Beer–Lambert absorption model
Absorbance can be proportional to path length and absorbing-species concentration.
Key knowledge
Decadic absorbance is A = log10(Pref/Psample), where Pref and Psample are the transmitted radiant powers through a matched reference cell and sample cell at the selected wavelength. Reflection, scattering, luminescence and cell/solvent backgrounds need appropriate treatment; a raw loss of transmitted light is not automatically molecular absorption. In the Beer–Lambert regime, A = εℓc for the concentration of the fixed absorbing species. Chemical equilibria, interactions at higher concentrations, spectral bandwidth and instrumental stray light can produce departures from linearity.
Values and equations
A = log10(Pref/Psample) = −log10(τ); τ = Psample/Pref; A = εℓc
- A
- absorbance 1
- ε
- molar absorptivity at the selected wavelength L mol⁻¹ cm⁻¹
- ℓ
- optical path length cm
- c
- concentration of the absorbing species mol L⁻¹
- Pref
- Transmitted radiant power through the matched reference cell at the selected wavelength W
- Psample
- Transmitted radiant power through the sample cell at the selected wavelength W
- τ
- Sample-to-reference transmitted radiant-power ratio after the applicable optical corrections 1
Conditions and scope
- Use a homogeneous, sufficiently dilute, non-scattering sample and a suitable narrow spectral band with a fixed absorbing form.
- Absorptivity depends on wavelength and conditions; extinction from scattering is not automatically molecular absorption.
- Use the decadic coefficient with log10; changing to a Napierian logarithm requires the corresponding coefficient. The measured ratio needs a matched blank/cell and the stated optical corrections.
- For a finite absorbance, Pref and Psample must be strictly positive. A zero or negative background-corrected signal is a measurement or detection-limit issue and is not evaluated with this logarithm.
Only the selected optical pages and algebraic finite-log domain are adopted. The fourth abridged edition is2023, officially released2025; its front-matter constants refer toCODATA2018 and are not used as current constants evidence. No whole-book, all optical losses, current certified standard or human laboratory review is claimed.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
- 13.2: Beer's Law — LimitationsDavid Harvey, DePauw University; adaptation in Chemistry LibreTexts Instrumental AnalysisTextbook chapter
- Quantities, Units and Symbols in Physical Chemistry, fourth edition, abridged version (2023): §§4.7–4.7.1IUPAC Physical and Biophysical Chemistry Division; international Green Book author teamReference guide · PDFFourth abridged edition, 2023; official PDF release March 2025. Only the cited optical sections are adopted here.
Evidence and review
Harvey's absorption expression and fundamental, chemical and instrumental limitations.
Add the decadic matched-reference/sample transmitted-radiant-power definition, Greekτ ratio, positive finite-log domain and corresponding coefficient/optical-correction scope. IUPAC fourth abridged edition §§4.7–4.7.1, printed37–39/PDF48–50, optical notes11,24–26 were actually inspected. Existing Harvey Beer–Lambert linear-regime evidence and the original two conditions are retained.
2026-10-10 · Checked concentration/path units and the validated linear absorption regime.
2026-10-11 · Continuation author/editor source-check scope: Add the decadic matched-reference/sample transmitted-radiant-power definition, Greekτ ratio, positive finite-log domain and corresponding coefficient/optical-correction scope. IUPAC fourth abridged edition §§4.7–4.7.1, printed37–39/PDF48–50, optical notes11,24–26 were actually inspected. Existing Harvey Beer–Lambert linear-regime evidence and the original two conditions are retained. Only the selected optical pages and algebraic finite-log domain are adopted. The fourth abridged edition is2023, officially released2025; its front-matter constants refer toCODATA2018 and are not used as current constants evidence. No whole-book, all optical losses, current certified standard or human laboratory review is claimed. Exact independent adoption review is pending; historical reviews retain their original date and scope.
Entry details
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- chem-beer-lambert
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- university-reference.1.continuation-20261011.1
- Structured reference record
- University reference