UV–visible absorption: connect color, conjugation, and concentration

MCAT content library / Chemistry & physics / UV–visible absorption and color

Use an absorption spectrum and calibration to separate structural evidence from concentration measurement—and recognize when the linear model no longer applies.

About 16 minutes, including the calibration example and checks.

Absorbance reports light removed at each wavelength

UV–visible spectroscopy measures how strongly a sample attenuates light across ultraviolet or visible wavelengths. Absorbance is related to transmitted intensity by

A = −log₁₀(T), where T is the transmitted fraction.

For a species obeying the Beer–Lambert model under the tested conditions, absorbance at a chosen wavelength follows:

A = εbc

where ε is molar absorptivity, b is path length, and c is concentration. The relation is useful only when the measurement remains within the instrument and chemical system’s validated linear range.

Concept: conjugation can shift electronic absorption

Molecules absorb UV or visible light when photon energy matches an allowed electronic transition. Changing molecular structure can therefore change the wavelengths and intensities of absorption bands.

Extended conjugation often lowers the energy separation between relevant electronic states and can shift strong absorption toward longer wavelengths. That trend is useful, but it is not a universal rule that lets wavelength alone identify a structure. Solvent, substituents, geometry, protonation, and the specific transition also matter. Use the controlled comparison supplied by the experiment.

Worked application: use a calibration within its range

An original assay measures a compound at 500 nm with a 1.00-cm cuvette. Standards give the following blank-corrected values:

An unknown gives A = 0.50. The calibration slope is 0.020 absorbance units per µM, so the unknown concentration is 25 µM.

A second sample gives A = 1.30. Extrapolating the same line would produce 65 µM, but that lies outside the validated range. The defensible next step is to dilute the sample into the tested range, remeasure it, then apply the dilution factor.

Pattern: the blank defines the background

A spectroscopic blank contains the solvent and other non-analyte components needed to account for background absorption from the cuvette, solvent, or reagents. If the blank itself absorbs strongly at the measurement wavelength and is not handled correctly, the analyte signal can be biased.

A blank is not the same as a zero-concentration biological control for every experimental question. It corrects the optical baseline. A separate negative or matrix-matched control may still be needed to test whether other sample components alter the assay.

Rule: observed color and absorbed color are not the same

A solution appears colored because some visible wavelengths are transmitted or reflected more strongly than others. If a substance strongly absorbs one region of the visible spectrum, the observed color is generally dominated by the remaining transmitted light, often described approximately as the complementary color.

Therefore, a substance that appears blue is not necessarily absorbing blue light most strongly. It may be transmitting blue while absorbing strongly in an orange-red region. A spectrum is more reliable evidence than the everyday color label.

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Continue learning

• Electromagnetic spectrum and photons
• Infrared spectra and functional groups
• Experimental Controls
• Browse Chemistry & Physics MCAT lessons

Sources

1. AAMC. Content Category 4D.
2. OpenStax Chemistry 2e. Electromagnetic Energy.