The Use of Isosbestic Points to Differentiate between Single-Step

3 The Use of Isosbestic Points to Differentiate between Single-Step and Complex Reaction Mechanisms 

A simple example of the occurrence of an isosbestic point is when two absorbing substances, one of which can be converted into the other (a reactant and a product), have the same molar extinction coefficient at a given wavelength. 

Discussions of isosbestic points began as early as 1937. The first complete isosbestic point theory was offered by Cohen and Fischer in 1962. Their conclusion is that the appearance of an isosbestic point in a closed system requires that the changes in the concentrations of the various components be linearly related. Although criticized for lack of considerations of temperature and solvent composition, Cohen and Fisher s theory is generally accepted in practical kinetic studies with respect to the developments in both theoretical and methodological aspects. 

In order to avoid comphcations due to overlapping of absorbance, kinetic measurements using spectrophotometry are most conveniendy carried out at wavelengths where only product or reactant absorbs. However, analyses near isosbestic points can also be carried out quahtatively and quantitatively. For example, isosbestic points have been widely used as reference points for the cafibration of spectrophotometers and for quahtative analysis of the composition of a binary mixture.  

According to the theory developed by Cohen and Fisher, the existence of well-defined isosbestic points (true isosbestic points) has often been considered to be proof of a single-step conversion or a parallel mechanism. On the other hand, the absence of a tme isosbestic point, but rather a series of steadily shifting intersection points, has been attributed to complications in the reaction mechanism. The shift of isosbestic wavelength has been utilized to estimate the conversion fraction of a precursor to the product. Recently, more and more chemical physicists have paid attention to the occurrence of isosbestic points, and compared these to the fingerprints of electronic correlations. 

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