Question

Why do we expect to see an isosbestic point in a UV-Vis experiment? What would it mean if the isosbestic point were not present?

Answer 1

An isosbestic point in a UV-Vis experiment occurs when there is a change from one state to another. If the process goes from one state to another state directly, and these two states have different absorption spectra that overlap, then there is an isosbestic point.

An example of a two state process would be the protonation of bromocresol green. The absorption spectra of the protonated form and deprotonated form overlap. As we overlay the spectra of bromocresol green at different values of pH, all the spectra will cross at one point, this point is isosbestic point.

If the isosbestic point were not present, this means that one of the species is being lost in the reaction.