Question

What is the difference between IR and UV/vis spectroscopies?

Answer 1

IR spectroscopy is largely qualitative, whereas UV-vis spectroscopy can be (highly) quantitative.

Explanation:

IR spectroscopy gives a good indication of the functional groups present in a molecule. Absorptions from carbonyl groups `C=O` stick out like things on a dog, and if there is a carbonyl group present, the `IR` spectrum will identify it straightforwardly. `C-H` groups are of course present in the IR spectrum, but because all organic species have `C-H` bonds (at least usually), this is not very definitive. Where the compound has an active hydrogen (i.e. an alcohol, or a carboxylic acid) sometimes the `H` can be replaced by a deuterium label, which can be observed at at a predictable lower frequency (this sounds much easier than in fact it is; I have seen some fabulous stretches of imagination to identify a `O-D` stretch).

UV-vis spectroscopy is a much more quantitative technique. The Beer-Lambert law (sometimes!) allows quantitative determination of some analyte. This form of spectroscopy also allows the use of aqueous solutions (water absorbs in the IR spectrum far too strongly to be used as a solvent!), at very low concentrations.