Isosbestic point

What is Isosbestic point. How to calculate this value?

In UV spectroscopy, an isosbestic point is a specific wavelength at which two chemical species have the same molar absorptivity (ε) or, more generally, are linearly related.

When an isosbestic plot is constructed by the superposition of the absorption spectra of two species (whether by using molar absorptivity for the representation, or by using absorbance and keeping the same molar concentration for both species), the isosbestic point corresponds to a wavelength at which these spectra cross each other.

When a 1-to-1 (one mole of reactant gives one mole of product) chemical reaction (including equilibria) involves a pair of substances with an isosbestic point, the absorbance of the reaction mixture at this wavelength remains invariant, regardless of the extent of reaction (or the position of the chemical equilibrium). This occurs because the two substances absorb light of that specific wavelength to the same extent, and the analytical concentration remains constant.

The above mentioned is theoretical concept drawn from wikipedia.

I came to know that, when we are doing the method development for two drugs in a formulation, we may decide the wavelength(for UV detector) of a detector at this point. Any one have an idea about this, please explain me in detail.

Thanks in advance

IMO, we can’t use this method for multi-APIs product (except, if the API contains more than one substance, such as some antibiotics). one example in USP that using this method is in dissolution procedure for aspirin. aspirin tends to be hydrolized in dissolution media so that isosbestic points between aspirin and salycylic acid is used to measure dissolved API.