How does ultraviolet-visible spectroscopy differentiate molecules?

Ultraviolet-visible spectroscopy differentiates molecules based on their unique absorption of light in the ultraviolet and visible range.

Ultraviolet-visible (UV-Vis) spectroscopy is a technique that examines the interactions of radiation from the ultraviolet and visible light spectrum with molecules. Every molecule absorbs light differently, and this absorption is dependent on the structure of the molecule. This means that each molecule will have a unique absorption spectrum, much like a fingerprint, which can be used to identify it.

The principle behind UV-Vis spectroscopy is that when light is shone onto a molecule, it can absorb the energy from the light. This energy can cause electrons within the molecule to be excited from a lower energy level to a higher one. The wavelength of light that a molecule absorbs is directly related to the amount of energy required to excite an electron to a higher energy level. Therefore, different molecules, with different electron arrangements, will absorb light of different wavelengths.

In UV-Vis spectroscopy, a beam of light is split into two equal halves. One half is passed through a sample solution, while the other half is passed through a reference solution. The intensity of light in both beams is then measured. The difference in intensity between the two beams gives a measure of the amount of light absorbed by the sample. This is then plotted against the wavelength of the light to give an absorption spectrum.

The absorption spectrum of a molecule is unique and can be used to identify the molecule. For example, if you have a sample and you don't know what molecule it contains, you can use UV-Vis spectroscopy to obtain the absorption spectrum of the sample. You can then compare this with the known absorption spectra of different molecules to identify the molecule in the sample.

In addition, UV-Vis spectroscopy can also be used to determine the concentration of a molecule in a solution. This is because the amount of light absorbed by a solution is proportional to the concentration of the absorbing species in the solution. Therefore, by measuring the amount of light absorbed by a solution, you can determine the concentration of the molecule in the solution.