Isosbestic points molar extinction coefficient

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. 

Present in the solution are [ H]MBTA and its hydrolysis product, 4- (A -maleamido) benzyltrimethylammonium iodide. The concentrations in moles per liter of [ HJMBTA (a) and its hydrolysis product (b) are determined from their molar extinction coefficients and absorbances at 224, 238.7 (isosbestic point), 260, and 290 nm. The absorbances at any... 

Even if A and B are interconverted in a reaction of the form A B or its reverse, then because their total concentration remains constant, so does A°. As a result, one or more isosbestic points, which are invariant points in the absorption spectrum, may be observed (Fig. 12.10). It is very unlikely that three or more species would have the same molar extinction coefficients at a single wavelength. Therefore, the observation of an isosbestic point, or at least not more than one such point, is compelling evidence that a solution consists of only two solutes in equilibrium with each other with no intermediates. 

If A and B are single chemical species, isosbestic points will appear at all wavelengths at which their molar absorption coefficients (formerly called extinction coefficients) are the same. (A more involved identity applies when A and B are mixtures of constant proportion.)... 

Fig. 1.4 Absorption spectra of two species that have the same extinction coefficient at 333 nm (A) and of mixtures of the same two components in molar ratios of 1 3, 1 1 and 3 1 (B). The total absorbance at the isosbestic point (333 nm) is constant...

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