Intermediate Topic 914 of 1102 Analytical, Lab & Spectroscopy
Beer-Lambert Law
Absorbance equals molar absorptivity times path length times concentration.
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In-depth explanation of Beer-Lambert Law
The Beer-Lambert law states that absorbance A equals the molar absorptivity epsilon multiplied by the path length l and the concentration c, written A = epsilon l c. Absorbance is defined as the base-10 logarithm of the ratio of incident intensity I0 to transmitted intensity I, so it is a logarithmic measure of how much light is absorbed. Because absorbance is proportional to concentration, a plot of absorbance against concentration for standards is a straight line that can be used for quantification. The molar absorptivity is a property of the compound at a specific wavelength and is high for strongly absorbing species. The law holds best for dilute solutions, with absorbance below about 1, and for monochromatic light.
Going deeper: Beer-Lambert Law
Key ideas: Absorbance A = log10(I0/I) = minus log10(T), where T is transmittance. A of 1 means 10 percent transmitted, 2 means 1 percent. To find an unknown, use c = A/(epsilon l), or compare with a calibration curve. Subtract a reagent blank first. Keep readings between 0.1 and 1.0 for best accuracy. Units of epsilon are L mol-1 cm-1.
Common mistakes and connections: Students confuse absorbance with percent absorbed, assuming A of 0.5 means 50 percent absorbed (it is about 68 percent). Using percent transmittance in the linear equation is another error. Cell fingerprints and bubbles scatter light. Measuring at a wavelength on a steep slope magnifies error, so use lambda max. Connected to colorimetry, calibration and kinetics by absorbance.
Visualization of Beer-Lambert Law
With epsilon 5000 L/mol/cm and a 1 cm cell, absorbance rises linearly with concentration; A = 0.50 corresponds to 1.0 x 10^-4 mol/L. The interactive lesson shows this as a live animation or chart.
Worked example and use case
With epsilon 5000 L mol-1 cm-1, a 1.00 cm cell and A = 0.50, the concentration is 1.0 x 10^-4 mol/L.
Frequently asked questions about Beer-Lambert Law
What is Beer-Lambert Law?
Absorbance equals molar absorptivity times path length times concentration.
Is Beer-Lambert Law a beginner, intermediate or advanced topic?
Beer-Lambert Law is a intermediate topic in Analytical, Lab & Spectroscopy. It builds on the vocabulary of beginner topics in this area.
What should I learn before Beer-Lambert Law?
Study these earlier topics in Analytical, Lab & Spectroscopy first: Molecular Ion Peak, Fragmentation Patterns, UV-Visible Spectroscopy.
What comes after Beer-Lambert Law?
The next topics in Analytical, Lab & Spectroscopy are Colorimetry and Calibration Curve.
Can I practise Beer-Lambert Law with quizzes?
Yes. The interactive lesson includes a self-check question on Beer-Lambert Law, and the Analytical, Lab & Spectroscopy quiz draws on every topic in this area.
Where Beer-Lambert Law fits in Analytical, Lab & Spectroscopy
Beer-Lambert Law is topic 38 of 99 in Analytical, Lab & Spectroscopy.