at which the total absorbance
of a sample does not change during a chemical reaction or a physical change of the
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.
The name derives from the Greek words: isos: equal, the same, and sbestos: extinguishable.
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,
is the absorbance at wavelength,
is the optical path,
is the molar decadic absorption coefficient of the species
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
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.
PAC, 2007, 79, 293
(Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006))
on page 359
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by
A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997).
XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic,
J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. https://doi.org/10.1351/goldbook