Determination of ionization constants by spectrophotometry

Abstract

The determination of ionization constants by ultraviolet or visible spectrophotometry is more time-consuming than by potentiometry. However, spectrometry is an ideal method when a substance is too insoluble for potentiometry or when its pK _a value is particularly low or high (e.g. less than 2 or more than 11). The method depends upon the direct determination of the ratio of molecular species (neutral molecule) to ionized species in a series of non-absorbing buffer solutions (whose pH values are either known or measured). For this purpose, the spectrum of the molecular species must first be obtained in a buffer solution whose pH is so chosen that the compound to be measured is present wholly as this species. This spectrum is compared with that of the pure ionized species similarly isolated at another suitable pH. A wavelength is chosen at which the greatest difference between the absorbances of the two species is observed. This is termed the ‘analytical wavelength’. Fig. 4.1, in which the base 2-amino- pyridine is used as an example, shows how these two pH values can be found.