Application of graphical methods using absorbances

During thermal- and photoreaction periods a registration of spectra is performed. The obtained reaction spectra (see Section 4.2.6.1) can at one or some wavelength cross in just one point. These points are called isosbestic points. An example is given by the reaction spectrum of azobenzene (see Fig. 5.4). Such a point is found at a certain wavelength A if at all times 

If 5 1 and all reactants absorb then it is rather improbable that eq. (5.64) is fulfilled at any wavelength. For the consecutive reaction 

In general the existence of an isosbestic point or its absence can be used to draw the following conclusions  

Sometimes one can obtain further information from reaction spectra. For the reaction 

At this wavelength the concentration of the intermediate is directly proportional to the change in absorbance. One can recognise such a wavelength by the fact that the initial spectrum for r = 0 will cross with the final spectrum at r — oo, since at this wavelength the condition is given by 

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