UVVisabsorpties

UV-Vis absorpties refer to the ability of chemical species to absorb ultraviolet and visible light. In UV-Vis spectroscopy, the strength of absorption at a given wavelength is described by the molar absorptivity ε (also called the extinction coefficient). The absorbance spectrum A(λ) or ε(λ) provides a fingerprint of electronic transitions within a molecule.

The relationship between absorbance and concentration is given by Beer-Lambert law: A = ε c l, where A is absorbance (dimensionless), c is concentration (mol/L), and l is the optical pathlength (cm). ε has units of L mol−1 cm−1. The law implies a linear dependence of absorbance on concentration for a fixed pathlength and wavelength within a valid range.

Measurement is performed with a UV-Vis spectrophotometer, which compares transmitted light through a sample with a blank reference. Typical pathlengths are 1 cm. The resulting data can be presented as an absorbance spectrum A(λ) or a molar absorptivity spectrum ε(λ). The position and shape of peaks reflect electronic transitions such as π–π*, n–π*, and charge-transfer transitions. Transitions in the UV generally occur below 400 nm, while visible absorption (400–800 nm) imparts observed colors.

Practical considerations include the validity range of Beer's law, which is best for dilute solutions; deviations can arise from scattering, aggregation, solvent effects, or high absorbance (inner-filter effects). Applications span quantitative analysis, reaction monitoring, kinetics, and determination of extinction coefficients, making UV-Vis absorpties a fundamental tool in chemistry, biochemistry, and materials science.

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