isosbestic point
This term is usually employed with reference to a set of absorption spectra,
plotted on the same chart for a set of solutions in which the sum of the
concentrations of two principal absorbing components, A and B, is constant.
The curves of absorbance against wavelength (or frequency) for such
a set of mixtures often all intersect at one or more points, called isobestic
points.
Isosbestic points are commonly met when electronic spectra are taken (a)
on a solution in which a chemical reaction is in progress (in which case
the two absorbing components concerned are a reactant and a product, A
+ B), or (b) on a solution in which the two absorbing components are in
equilibrium and their relative proportions are controlled by the concentration
of some other component, typically the concentration of hydrogen
ions, e.g. an acid–base indicator equilibrium.
A  B + H+
aq
The effect may also appear (c) in the spectra of a set of solutions of two
unrelated non-interacting components having the same total concentration.
In all these examples, A (and/or B) may be either a single chemical species
or a mixture of chemical species present in invariant proportion.
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.)
If absorption spectra of the types considered above intersect not at one or
more isosbestic points but over progressively changing wavelength, this is
prima facie evidence in case (a) for the formation of a reaction intermediate
in substantial concentration (A ® C
® B), in case (b) for the involvement of a third absorbing species in the
equilibrium, e.g.
A  B + H+
aq  C + 2H+
aq
or in case (c) for some interaction of A and B, e.g.
A + B  C
1994, 66, 1129，

见这几个资料
http://en.wikipedia.org/wiki/Molar_absorptivity

非常感谢！