6. Determination by UV-visible spectrophotometry

UV-visible spectrophotometry (or UV-visible molecular absorption spectroscopy) is the other major method for determining pK _a s, due to the large number of applications that have emerged in this field. Let's make it clear from the outset, however, that unlike potentiometry, its use implies the possession by the acidic or basic form of a particular molecular characteristic, namely the ability to absorb in the UV-visible range. Ideally, as we shall see, the two forms should absorb differently. Not all acid-base pairs are therefore suitable for this method.