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| [[File:bromocresol green spectrum.png|thumb|250 px|Isosbestic point in the [[bromocresol green]] spectrum. The spectra of basic, acid and intermediate pH solutions are shown. The [[analytical concentration]] of the dye is the same in all solutions.]]
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| In [[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. The word derives from two Greek words: "isos", meaning "equal", and "sbestos", meaning "extinguishable".<ref>[http://goldbook.iupac.org/I03310.html IUPAC Gold Book] ([[International Union of Pure and Applied Chemistry]])</ref>
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| == Isosbestic plot ==
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| When an isosbestic plot is constructed by the superposition of the [[spectroscopy|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.
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| A pair of substances can have several isosbestic points in their spectra.
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| When a 1-to-1 (one [[mole (unit)|mole]] of [[reactant]] gives one [[mole (unit)|mole]] of [[product (chemistry)|product]]) [[chemical reaction]] (including [[chemical equilibrium|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 [[absorbance|absorb]] light of that specific wavelength to the same extent, and the [[analytical concentration]] remains constant.
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| For the reaction:
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| :<math>X \rightarrow Y</math> | |
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| the analytical concentration is the same at any point in the reaction:
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| :<math> c_X + c_Y = c \,</math>.
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| The absorbance of the reaction mixture (assuming it depends only on X and Y) is:
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| :<math>A = l\cdot (\epsilon_{X} c_{X} + \epsilon_{Y} c_{Y} )</math>.
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| But at the isosbestic point both molar absorptivities are the same:
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| :<math>\epsilon_X = \epsilon_Y = \epsilon \,</math>.
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| Hence, the absorbance
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| :<math>A = l\cdot (\epsilon_{X} c_{X} + \epsilon_{Y} c_{Y} )=l\cdot\epsilon \cdot (c_{X} + c_{Y} )=l\cdot\epsilon\cdot c</math>
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| does not depend on the extent of reaction (i.e., in the particular concentrations of X and Y)
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| The requirement for an isosbestic point to occur is that the two species involved are related linearly by stoichiometry, such that the absorbance is invariant for one particular wavelength. Thus other ratios than one to one are possible. The presence of an isosbestic point typically does indicate that only '''''two''''' species that vary in concentration contribute to the absorption around the isosbestic point. If a third one is partaking in the process the spectra typically intersect at ''varying'' wavelengths as concentrations change, creating the impression that the isosbestic point is 'out of focus', or that it will shift as conditions change.<ref> page 49 of Kinetics and Mechanism
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| By John W. Moore, [[Ralph Pearson|Ralph G. Pearson]] and Arthur Atwater Frost (3rd
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| Edition, John Wiley and Sons, 1981)
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| ISBN 0-471-03558-0, ISBN 978-0-471-03558-9</ref> The reason for this is that it would be very unlikely for three compounds to have extinction coefficients linked in a linear relationship by chance for one particular wavelength.
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| ==Applications==
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| [[File:Isobestic graph.gif|thumb|250px|Isosbestic point as is used in oximetry.]]
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| In [[chemical kinetics]], isosbestic points are used as reference points in the study of [[rate of reaction|reaction rates]], as the absorbance at those wavelengths remains constant throughout the whole reaction.
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| Isosbestic points are used in medicine in a laboratory technique called [[oximetry]] to determine [[hemoglobin]] concentration, regardless of its saturation. [[Oxyhaemoglobin]] and [[deoxyhaemoglobin]] have (not exclusively) isosbestic points at 586 [[nanometer|nm]] and near 808 nm.
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| Isosbestic points are also used in [[clinical chemistry]], as a [[quality assurance]] method, to verify the [[accuracy]] in the [[wavelength]] of a [[spectrophotometer]]. This is done by measuring the [[spectroscopy|spectra]] of a [[standard substance]] at two different [[pH]] conditions (above and below the [[pKa]] of the substance). The standards used include [[potassium dichromate]] (isosbestic points at 339 and 445 nm), [[bromothymol blue]] (325 and 498 nm) and [[congo red]] (541 nm). The wavelength of the isosbestic point determined does not depend on the [[concentration]] of the substance used, and so it becomes a very reliable reference.
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| == References ==
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| {{reflist}}
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| [[Category:Spectroscopy]]
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