Molar Absorptivity Calculator

Calculate molar absorptivity epsilon from Beer-Lambert Law A = epsilon*c*l. Find the molar extinction coefficient from absorbance measurements at known concentration.

🔬 Spectroscopy📐 ε = A/(c·l)🧪 Chemistry
Absorbance A (measured)
Concentration c (mol/L)
Path length l (cm)
Please enter valid values.

Formula & Reference

VariableSymbolFormulaUnits
Molar Absorptivity Calculator—ε = A/(c·l)L/mol·cm

Step-by-Step Examples

Example 1
Find Epsilon

A=1.000 at c=0.00100 M, l=1.00 cm.

  • eps = A/(c x l) = 1.000/(0.00100 x 1.00)
  • eps = 1000 L/molcm
✓ eps = 1000 L/mol-cm
Example 2
Hemoglobin

Oxy-hemoglobin at 541 nm: A=0.850, c=1.50x10^-5 M, l=1.00 cm.

  • eps = 0.850/(1.50x10^-5 x 1.00)
  • eps = 5.67x10^4 L/molcm
✓ eps = 5.67x10^4 L/mol-cm (intense absorption)
Example 3
Food Dye

Tartrazine (FD&C Yellow 5) in water: A=1.20, c=2.00x10^-5 M, l=1.00 cm.

  • eps = 1.20/(2.00x10^-5 x 1.00) = 6.00x10^4 L/molcm
✓ eps = 6.0x10^4 L/mol-cm

Real-World Applications

🔬
Quantitative Analysis
High eps means very sensitive detection even at low concentrations.
🧬
Protein Biochemistry
Tryptophan eps at 280 nm ~ 5600, tyrosine ~ 1490 L/molcm. Used for protein quantification.
💊
Drug Analysis
Molar absorptivity allows calculation of drug concentration in plasma samples.
🏷️
Food Quality
Dye concentrations in food and beverages measured using known molar absorptivity values.

Common Mistakes to Avoid

⚠️
Average over multiple concentrations

Best practice: measure A at 3-5 concentrations, plot A vs c, use slope = eps*l. Single-point estimation less accurate.

⚠️
Must be at analytical wavelength

Molar absorptivity depends on wavelength. Measure A at the lambda-max (absorbance maximum) for highest sensitivity.

⚠️
Units check

eps = A/(c x l) = 1/(M x cm) = L/molcm. Dimensionally: eps (L/molcm) x c (mol/L) x l (cm) = dimensionless A.

Frequently Asked Questions

What is a chromophore? ▾
Functional group that absorbs light: C=C, C=O, aromatic rings, heme. High molar absorptivity (eps > 10^4) indicates efficient light absorption.
What are typical molar absorptivity values? ▾
Aliphatic C=O: ~15-20 L/molcm. Aromatic: ~10^3-10^4. Porphyrins (heme): ~10^5. Metal complexes vary widely.
What wavelength should I use? ▾
Use lambda-max: wavelength of maximum absorption. Provides highest sensitivity and best linearity for Beer-Lambert.
What is a molar extinction coefficient? ▾
Synonym for molar absorptivity (eps). Both describe how strongly a substance absorbs light per mole per centimeter path length.
How do I determine molar absorptivity experimentally? ▾
Prepare known concentrations, measure A, plot A vs c. The slope of the linear best-fit line = eps x l. If l=1 cm, slope = eps.

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Formula Explorer connections

Interpretation: This analytical relationship converts an instrument signal, separation measure or optical response into concentration, identity or performance. Assumption: Calibration, blank correction, linear range, path length, matrix effects and instrument settings must match the sample and method.

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