absorbance
Summary
Absorbance (A) measures how much light a sample absorbs at a given wavelength: A = log₁₀(I₀/I) = εbc. It is unitless and, for dilute solutions, directly proportional to concentration and path length. Higher A means less transmitted light.
Detail
Meaning: Absorbance is a logarithmic measure of how much light a sample absorbs at one wavelength. A molecule absorbs when the photon energy matches a gap between its energy levels. In UV-Vis spectroscopy, the wavelength of maximum absorbance (λmax) moves to longer wavelength as conjugation increases.
Equation: A = log₁₀(I₀/I) = εbc, where I₀ is incident intensity (W/m²), I is transmitted intensity (W/m²), ε is molar absorptivity (L·mol⁻¹·cm⁻¹), b is path length (cm), and c is concentration (mol/L). A has no units.
On the MCAT: Passages give a spectrum or a calibration line and ask for an unknown concentration. They may also ask what happens to A when concentration or path length doubles (A doubles). Biochemistry uses the same idea: A280 for protein, A260 for nucleic acids, and a colored product tracked over time in an enzyme assay.
Common confusion: Absorbance is not the percent of light absorbed, and it is not transmittance. It is logarithmic: A = 1 means 10% of the light is transmitted, and A = 2 means 1%. So A = 2 − log₁₀(%T). Linearity with concentration generally holds only for dilute solutions.
Sources
- Organic Chemistry (Clayden)
- Organic Chemistry (Wade)
- Quantitative Chemical Analysis (Harris)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.