Beer lambert law calculator
This Beer lambert law calculator solves A = ε·l·c for absorbance, concentration or molar absorptivity. Example: ε = 6220 L/(mol·cm), 1 cm and 50 µmol/L give A = 0.311.
Example: What should be calculated: Absorbance A (and transmittance) · Molar absorptivity ε 6,220 L/(mol·cm) · Path length l (cuvette) 1 cm · Concentration c 50 µmol/L → Result: A = 0.311 · transmittance T = 48.8652 %. Source: IUPAC Gold Book: Beer–Lambert law (A = ε·c·l). Updated: .
How it is calculated
The Beer–Lambert law describes how strongly a solution attenuates light of a given wavelength. Absorbance A is proportional to the concentration of the substance and to the path length of the light – the basis of photometry.
Formula
A = ε · l · c with the molar absorptivity ε in L/(mol·cm), the path length l in cm and the concentration c in mol/L. Transmittance is T = I / I₀ = 10^(−A), and conversely A = −log₁₀(T). Rearranged: c = A / (ε·l) or ε = A / (l·c).
Worked example
NADH absorbs at 340 nm with ε = 6,220 L/(mol·cm). In a 1 cm cuvette at c = 50 µmol/L, A = 6,220 · 1 · 50 · 10⁻⁶ = 0.311 and the transmittance is T = 10^(−0.311) = 48.8652 %. Conversely, A = 0.311 with the same ε and l gives c = 50 µmol/L, and from A, l and c you get ε = 6,220 L/(mol·cm). At A = 1, 10 % of the light gets through, at A = 2 only 1 %.
- Absorbance is logarithmic: an increase of 1 means ten times less transmitted light.
- Measure preferably between A = 0.1 and 1; dilute above that.
- The measurement needs a blank (cuvette with solvent) as the zero point.
Frequently asked questions
What is the difference between absorbance and transmittance?
Transmittance T is the percentage of light passing through; absorbance is A = −log₁₀(T/100). A = 0.311 corresponds to T = 48.8652 %.
How do I calculate concentration from absorbance?
Use c = A / (ε·l). For A = 0.311, ε = 6,220 L/(mol·cm) and l = 1 cm that is 50 µmol/L.
Up to which absorbance does the law hold?
Depending on the instrument up to about A = 1 to 1.5. Above that stray light matters and the sample should be diluted.
Sources and legal basis
- IUPAC Gold Book: Beer–Lambert law (A = ε·c·l)
- Harris, Quantitative Chemical Analysis, Kap. „Spectrophotometry“
As of: · Editorial policy
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