Isosbestic point

Example of an isosbestic point

The isosbestic point (from the Greek iso "equal" and sbesis "extinction") describes a wavelength at which in a system in which a reaction takes place, the light absorption does not change in the course of the reaction. The term thus comes from the field of absorption spectroscopy .

The (total) absorption of a system at a certain wavelength is the sum of the absorptions of the individual components . If a photon hits a particle, it reacts with it, regardless of what else is in the system. ${\displaystyle A(\lambda _{n})}$ ${\displaystyle A_{k}(\lambda )}$

${\displaystyle A(\lambda )=\sum A_{k}(\lambda )}$

For dilute solutions, the absorptions of the individual components can be calculated according to the Lambert-Beer law from their concentration , a wavelength-dependent absorption coefficient and the path length of the light through the homogeneous system . That leads to: ${\displaystyle A_{k}(\lambda )}$ ${\displaystyle c_{k}}$ ${\displaystyle \epsilon (\lambda )}$ ${\displaystyle l}$

${\displaystyle A(\lambda )=\sum \epsilon _{k}(\lambda )\cdot l\cdot c_{k}}$

If a reaction now takes place, the composition of the components in the system changes. If you follow the course of the reaction with a spectrometer, it can happen that the absorption does not change at certain wavelengths, although the reaction takes place. This is only possible if the amount of substances formed has the same absorption at the wavelength as the amount of substances reacted. In a reaction, the concentrations of the formed substances are linked to the concentrations of the reacted substances via the stoichiometry .

${\displaystyle \sum \nu _{k}\cdot \epsilon _{k}(\lambda _{iso})=0}$

For example, got in response

${\displaystyle A\rightarrow 2B}$

substance A at the isosbestic point , substance B at . Because for every particle A that disappears, two particles B arise and the absorption must not change, must be half as large as . ${\displaystyle \epsilon _{A}}$ ${\displaystyle \epsilon _{B}}$ ${\displaystyle \epsilon _{B}}$ ${\displaystyle \epsilon _{A}}$

If an absorption spectrum is recorded at different times, then all curves intersect at the wavelength of the isosbestic point (see graphic).

The existence of an isosbestic point says something about

• the system (closed or open)
• the reaction ( if subsequent reactions occur, does the reaction proceed stoichiometrically, are all the species involved known, intermediate products occur in steady-state concentrations [no but entire range in which the absorption remains the same]) ${\displaystyle \lambda _{iso}}$

literature

• E. Meister: Basic practical course in physical chemistry , vdf & UTB, 2006

Web links

• Entry on Isosbestic Point . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.I03310 Version: 2.3.1.
• Comprehension exercises on the isosbestic point (PDF file; 341 kB)