isosbestic point
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.
  1. A simple example occurs when one molecular entity is converted into another that has the same @[email protected] at a given @[email protected] 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 @[email protected] at this @[email protected] as the ratio of the concentrations of the two entities is varied.
  2. The name derives from the Greek words: isos: equal, the same, and sbestos: extinguishable.
  3. 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 @[email protected] 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 Aλ.l-1 = (n)∑(i=1) ɛi(λ)ci is invariant (Aλ is the @[email protected] at @[email protected] λ, l is the optical path, ɛi is the @[email protected] 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 @[email protected] where the condition ɛA + ɛB = cɛC + dɛD + eɛE , 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 @[email protected] is not recommended.
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 359 (