Isosbestic point

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

The (total) absorption of a system at a particular wavelength is the sum of the absorptions of the individual components. Does a photon, a particle it reacts with it, regardless of what's included in the system otherwise.

Dilute solutions can be according to the Lambert -Beer 's law, the absorption of the individual components of the concentration, a wavelength-dependent absorption coefficient, and the path length of light through the homogeneous system calculated. This leads to:

Now takes place, a reaction takes place, the composition of the components in the system changes. Is monitored using a spectrometer with the reaction course, it may occur that the absorption does not change at specific wavelengths, even though the reaction takes place. This is only possible if the amount of the substances formed, the absorption at the same wavelength as the amount which reacted substances. In one reaction, the concentrations of substances formed with the concentrations of the reacting substances over the stoichiometry associated.

For example, have in the reaction

Substance A at an isosbestic point, the substance B a. Because for each particle A that disappears two particles B arise and the absorption may not change, must be half the size.

If an absorption spectrum recorded at different points in time, then all curves intersect at the wavelength of the isosbestic point (see chart).

The existence of an isosbestic point says something about

• The system ( closed or open)
• The reaction ( subsequent reactions occur, the reaction proceeds stoichiometrically, all species involved known to occur intermediates in stationary concentrations [ no but entire area in which the absorption remains the same ] )