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How is ultraviolet-visible spectrophotometry used to analyze atoms and molecules?

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Ultraviolet-visible (UV-Vis) spectrophotometry is a powerful analytical technique used to identify and quantify atoms and molecules by measuring the absorption of radiant energy as a function of wavelength.12

Principles of Analysis

The method relies on electronic transitions: when a substance absorbs radiation in the UV-Vis region, electrons in the outermost occupied orbital are promoted to a higher, unoccupied orbital. The energy required for this transition, $Δ E = h ν$ , corresponds to specific wavelengths.34

Quantitative analysis is governed by the Beer-Lambert law, which states that the amount of radiation absorbed is directly proportional to the concentration of the absorbing substance and the length of the path the light travels through the medium.12

Applications

Quantitative Measurement: Because many organic and inorganic compounds exhibit strong absorption peaks in this region, the technique is ideal for measuring small analyte concentrations.3
Indirect Analysis: For substances that do not absorb strongly, chemists often add a reagent to form a product that does, allowing for indirect measurement through spectrochemical analysis.3
Structural Characterization: By observing shifts in absorption caused by specific chromophores or molecular substitutions, researchers can characterize electron distributions within chemical bonds.5