isosbestic point
@W06659@, @W06664@ or @F02525@ at which the total @A00028@ 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 @M03972-1@ at a given @W06659@. 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 @A00028@ at this @W06659@ 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 @S06026@ 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 \(A_{\lambda }l^{-1} = \sum_{i=1}^{n}\varepsilon _{i}(\lambda)\, c_{i}\) is invariant (\(A_{\lambda}\) is the @A00028@ at @W06659@ \(\lambda\), \(I\) is the optical path, \(\varepsilon_{i}\) is the @M03972-2@ of the species \(i\) of concentration \(c_{i}\)). 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 @W06659@ where the condition \(\varepsilon_{A} + \varepsilon_{B} = c\, \varepsilon_{C} + d\, \varepsilon_{D} + e\, \varepsilon_{E}\), 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 @I03259@ is not recommended.
PAC, 2007, 79, 293. (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 359 [Terms] [Paper]