Preferred Label : isosbestic point;
IUPAC definition : wavelength, wavenumber or frequency at which the total absorbance of a sample does
not change during a chemical reaction or a physical change of the sample.;
Scope note : contrary to a widely accepted idea, the existence of an isosbestic point does not
prove that the reaction is a quantitative conversion of one species into a unique
other species or that an equilibrium exists between only two species. the observation
of isosbestic points only indicates that the stoichiometry of the reaction remains
unchanged during the chemical reaction or the physical change of the sample, and that
no secondary reactions occur during the considered time range, since
a¿ l -1 i 1 n ¿i ¿ ci
is invariant(a¿ is the absorbance at wavelength¿,l is the optical path,¿i is the molar
decadic absorption coefficient of the species i of concentration ci). for the reaction
a b ¿ c c d d e e, with c, d, and e the percentages of the products c, d, and
e, an isosbestic point will be observed at every wavelength where the condition
¿a ¿b c¿c d¿d e¿e , provided that the values
of the percentages c, d, and e remain constant during the chemical reaction or the
physical change. the use of the obsolete term isoabsorption point is not recommended.; contrary to a widely accepted idea, the existence of an isosbestic point does not
prove that the reaction is a quantitative conversion of one species into a unique
other species or that an equilibrium exists between only two species. the observation
of isosbestic points only indicates that the stoichiometry of the reaction remains
unchanged during the chemical reaction or the physical change of the sample, and that
no secondary reactions occur during the considered time range, since
a¿ l -1 i 1 n ¿i ¿ ci
is invariant(a¿ is the absorbance at wavelength¿,l is the optical path,¿i is the molar
decadic absorption coefficient of the species i of concentration ci). for the reaction
a b ¿ c c d d e e, with c, d, and e the percentages of the products c, d, and
e, an isosbestic point will be observed at every wavelength where the condition
¿a ¿b c¿c d¿d e¿e , provided that the values
of the percentages c, d, and e remain constant during the chemical reaction or the
physical change. the use of the obsolete term isoabsorption point is not recommended.; the name derives from the greek words: isos: equal, the same, and sbestos: extinguishable.; a simple example occurs when one molecular entity is converted into another that has
the same molar absorption coefficient at a given wavelength. as long as the sum of
the concentrations of the two molecular entities in the solution is held constant
there will be no change in absorbance at this wavelength as the ratio of the concentrations
of the two entities is varied.;
Origin ID : I03310;
See also
wavelength, wavenumber or frequency at which the total absorbance of a sample does
not change during a chemical reaction or a physical change of the sample.
