B.Sc. Second year Undergraduate degree course (CBCS Pattern)
Semester Third
Physical Chemistry (CHE-312)
Chapter – Colorimetry
Limitations of Beer –Lambert’s law
Beer-Lambert’s law is strictly valid only in dilute solutions. For dilute solutions, a linear relationship is exhibited by a plot of absorbance (A) as a function of concentration of the absorbing substance (C), as shown in Figure
Fig.- Plot
of Absorbance versus Concentration
(i) Real deviations occur at higher concentration of the absorbing species. At higher concentrations (>10-3M), there is a change in the refractive index of the solution.
(ii) Chemical deviations occur when there is more than one absorbing species present in the solution. When the absorbing molecules associate or dissociate in the solution, there is a change in the number of absorbing species.
(iii)Instrumental deviation occurs due to
changes in absorptivity of the species as a function of instrumental bandwidth.
Deviation from Beers Law
From Beer's law it follows that if we plot
absorbance A against concentration, a straight line passing through the origin
should be obtained in figure 6.3. But there is usually a deviation from a linear
relationship between concentration and absorbance and an apparent failure of
Beer's law may ensure. Deviations from the law are reported as positive or
negative according to whether the resultant curve is concave upwards or concave
downwards. Even at constant temperature and in a proper solvent, absorptivity
deviate towards either greater or smaller values. Deviations from Beer's law
can arise due to following factors:
1. Real Limitations:
The law successfully describes the absorption
behavior of dilute solutions. At high concentrations (>0.01F) the average
distance between molecules of the absorbing solute diminishes to such an extent
that they affect the charge distribution of the neighboring molecules. As a consequence
the ability of the absorbing molecules to absorb a certain wavelength is also
affected. The magnitude of this interaction depends upon concentration and
therefore deviations are observed in the linear relationship between absorbance
and concentration.
Fig.-
Relationship between absorbance and concentration
The value of C depends upon the value of
refractive index of the medium and if this changes considerably with
concentration, the law also shows marked deviations. Ife is substituted by a
factor ε.n/(n2+2)2(n=refractive index of the solution), a
correction can be made in the deviations due to ε but this correction is
insignificant at concentrations below 0.01 F.
2. Chemical Deviation.
If the degree of dissociation, ionization,
hydrolysis, polymerisation and complex formation of the absorbing solute alters
on diluting or concentrating the solution, apparent deviations from Beer's law
are observed. For example, the absorbance of the weak acid would show
deviations from Beer's law on account of the change in its degree of
dissociation on dilution. Chemical deviations are also observed when a solution
of the dichromate ions is diluted with water resulting into colour change.
Cr2O7-2 +H2O
=2 HCrO4-2 = 2H+ + 2CrO4-2
Orange Yellow
3. Deviations may also occur due to the
presence of impurities that fluorescence or absorb at the absorption
wavelength. This interference introduces an error in the measurement of
absorption of radiation penetrating the sample.
4. Beer's law cannot be applied to
suspensions but the latter can be estimated colorimetrically after preparing a
reference curve with known concentrations
5. Beer's law is also not valid if the
absorbing material gets coagulated into a small number of large units. The
effect will be more pronounced if molar absorptivity is greater and an
absorption band may become distorted beyond recognition.
6. Deviations from Beer's law is observed if
concentration change causes significant alterations in the refractive index of
the solution.
7.
Deviations may occur if the width of slit is not proper and, therefore, it
allows
Undesirable radiations to fall on the
detector.
Reasons for Deviation from Beer's Law.
i. Beer's law may deviate because of instrumental errors like stability of radiation source, slit control, wave length selector, stray radiation and reliability of the optical parts.Colorimetry: Interaction of electromagnetic radiation with matter
Colorimetry:Lamberts Law,Beer's Law,Lambert-Beer's Law, Molar absorptivity
Colorimetry: Limitations of Beer –Lambert’s law,Deviation from Beers Law,Reasons for Deviation from Beer's Law.
Colorimeter: Principle, Construction and components, working, Applications
Colorimetry Multiple Choice questions
