Ultravioletvisible

Ultraviolet-visible spectroscopy (UV-Vis) measures the absorption of ultraviolet and visible light by chemical species. It detects electronic transitions, typically π→π* and n→π* in chromophores such as conjugated bonds, carbonyls, or aromatic rings. The range spans roughly 190–400 nm (UV) and 400–700 nm (visible).

Principle: Light absorbed by a sample reduces transmitted light. Beer-Lambert law relates absorbance A to concentration c and path length l via A = εcl, where ε is the molar absorptivity. The wavelength λmax is used for quantification; the full spectrum informs on the electronic structure.

Instrumentation: A UV-Vis spectrophotometer uses UV and visible light sources (deuterium for UV, tungsten for visible), a monochromator, a sample holder (cuvette), and a detector (photomultiplier or silicon diode). Measurements yield absorbance or transmittance as a function of wavelength.

Procedure and data: Samples are measured against a solvent blank. Calibration with standards yields concentrations. Spectra support qualitative identification, quantitative analysis, and kinetic studies; derivative spectra can resolve overlapping bands.

Applications: UV-Vis is used to quantify nucleic acids and proteins, monitor chemical reactions, assess dyes and pigments, and support pharmaceutical quality control, environmental analysis, and food chemistry.

Limitations: Only species absorbing in the UV-Vis range are detectable; solvents and samples must be transparent at measurement wavelengths. Turbidity, scattering, or strong color can interfere. Beer's law holds best at moderate concentrations and with well-behaved solvents.