UV-visible spectroscopy — Chemistry, 14–17
Molecules can absorb particular wavelengths of ultraviolet or visible light. Measuring that absorption helps you investigate colour, concentration and electronic structure without using up much sample.
Light absorbed by molecules
White light contains many wavelengths. A molecule absorbs some of them when its electrons move to a higher energy state; the wavelengths left to reach your eyes can give a solution its colour.
Why measure absorption?
A solution may look pale even when a small amount of an important substance is present. A spectrometer turns invisible light absorption into data, allowing chemists to detect and measure substances more reliably than eyesight alone.
Using a calibration line
Standards give absorbances 0.20 at 2 mg/L and 0.50 at 5 mg/L, so the slope is 0.30 ÷ 3 = 0.10 per mg/L. An unknown has absorbance 0.40. Its concentration is 0.40 ÷ 0.10 = 4 mg/L.
The trap: darker always means more
It is reasonable to judge concentration by how dark a solution looks, because a stronger colour often means more absorption. But colour depends on the wavelength and the substance; very concentrated samples can also give unreliable readings.
Where it is used
UV-visible instruments measure dyes in drinks, pollutants in water and molecules in medicines. They are useful because measurements are quick, quantitative and often possible without destroying the sample.
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