Optics
Isosbestic Point is relevant, when you are performing......
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study of chemical process in a system.
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study of variations in absorption spectrum.
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study of the mechanism.
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study of the nature of the products.
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study of the reaction rate as a function of concentration.
The concept of Isosbestic Point is discussed in two contexts:
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chemical reaction
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inter-conversion
Isosbestic Point in context of chemical reaction is discussed in two themes:
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involving two species.
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involving more than two species.
In case of chemical reaction from reactant (R) to product (P), Isosbestic Point can be defined as the characteristic wavelength at which extinction coefficient (absorbance) of two chemical substances remain equal, throughout the chemical reaction.
What is the spectroscopic basis for such phenomena? According to Beer-Lambert Law, determining factors for absorbance intensities are extinctions coefficients (ε) and concentration (c).
A = ε. l. c
Asum = AR + AP
csum = cR + cP
Asum is constant at those specific wavelengths, as long as csum is constant, even though the ratio of cR and cP is being varied.
At the Isosbestic Point,
Molar Absorptivity of R = Molar Absorptivity of P
What does Isosbestic Point indicate?
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equilibrium is established between two species.
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formation of 1:1 complex.
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stoichiometry remains unchanged.
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no secondary reactions (intermediate) occur.
Isosbestic Point does not depend on:
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wavelength
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extent of reaction
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analytical concentration
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position of the equilibrium
Why ?
Because, at the specific wavelength λi ,
A1 = A2 ,
A1 = absorbance of substance 1
A2 = absorbance of substance 2
as the analytical concentration remains constant.
Experimental steps to obtain an Isosbestic Point:
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Find two principal absorbing components R and P ,
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Prepare 10 sample solutions with varying ratios of R / P or varying pH,
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Plot scans A vs. λ for these 10 solutions in one frame.
a combination of
Hypochromic Effect
+
Bathochromic Shift
3 Isosbestic Points
inter-conversion
typical case:
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acid-base indicator equilibrium
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oxidation (loss of electron)
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reduction (gain of electron)
for example, oxidation reaction of Myoglobin.
requirements for
an Isosbestic Point to occur:
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2 molecular entities are involved.
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They are linearly related by stoichiometry.
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They have identical absorption at specific wavelengths.
