- Isosbestic point
In
spectroscopy , an isosbestic point is a specificwavelength at which two (or more) chemical species have the samemolar absorptivity (ε).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 usingabsorbance and keeping the same molar concentration for both species), the isosbestic point corresponds to a wavelength at which these spectra cross each other.A pair of substances can have several isosbestic points in their spectra.
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 theextent 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 theanalytical concentration remains constant.For the reaction:
:X ightarrow Y
the analytical concentration is the same at any point in the reaction:
:c_X + c_Y = c ,.
The absorbance of the reaction mixture (assuming it depends only on X and Y) is:
:A = lcdot (epsilon_{X} c_{X} + epsilon_{Y} c_{Y} ).
But at the isosbestic point both molar absorptivities are the same:
:epsilon_X = epsilon_Y = epsilon ,.
Hence, the absorbance
:A = lcdot (epsilon_{X} c_{X} + epsilon_{Y} c_{Y} )=lcdotepsilon cdot (c_{X} + c_{Y} )=lcdotepsiloncdot c
does not depend on the extent of reaction (i.e. in the particular concentrations of X and Y)
Applications
In
chemical kinetics , isosbestic points are used as reference points in the study of reaction rates, as the absorbance at those wavelengths remains constant throughout the whole reaction.Isosbestic points are used in medicine in a laboratory technique called
oximetry to determinehemoglobin concentration, regardless of its saturation.Oxyhaemoglobin anddeoxyhaemoglobin have isosbestic points at 590 nm and near 800 nm.Isosbestic points are also used in
clinical chemistry , as aquality assurance method, to verify theaccuracy in thewavelength of aspectrophotometer . This is done by measuring the spectra of astandard substance at two differentpH conditions (above and below thepKa of the substance). The standards used includepotassium dichromate (isosbestic points at 339 and 445 nm),bromothymol blue (325 and 498 nm) andcongo red (541 nm). The wavelength of the isosbestic point determined does not depend on theconcentration of the substance used, and so, it becomes a very reliable reference.
Wikimedia Foundation. 2010.