This document discusses Beer's law and Lambert's law, which describe how the intensity of light passing through an absorbing medium decreases exponentially with increasing thickness and concentration of the medium. It states that Beer's law relates the decrease in intensity to both the thickness and concentration, while Lambert's law relates it only to thickness. The document also describes deviations from the linear relationship predicted by these laws that can occur, including positive deviations where concentration changes have a greater than expected effect, and negative deviations where changes have a smaller effect. Possible causes of deviations, both instrumental and physicochemical, are outlined.

2. INTRODUCTION
 BEERS LAW
 LAMBERTS LAW
 DEVIATIONS
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 When a beam of monochromatic radiation is passed through a
solution of an absorbing substance, the rate of decrease of
intensity of radiation with thickness of the absorbing solution is
proportional to the intensity of incident radiation as well as the
concentration of the solution.
 - di / dt  I c - di / dt = K’I c

 I = Intensity of incident radiation passing through a
thickness of “ t “ of the medium.
di = decrease in intensity of radiation
 - di / dt = rate of decrease of intensity of radiation with
thickness of the absorbing medium.


K’
c
= molar absorption coefficient.
= Concentration of the solution in moles/litre.

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 Lamberts law – when a beam of monochromatic radiation passes
through a homogenous absorbing medium, the rate of decrease of
intensity of radiation with the thickness of the absorbing medium
is proportional to the intensity of incident radiation.
- di / dt  I - di / dt = KI
I = Intensity of incident radiation passing through a thickness of “ t
“ of the medium.
di = decrease in intensity of radiation
- di / dt = rate of decrease of intensity of radiation with thickness
of the absorbing medium.
K = propionate constant or absorption coefficient.

5.  Let, Io be the intensity of the incidentradiation
 I be the intensity of the radiation after
passing through the medium.
 The intensity of the absorbed radiation can be
given as Iabs
 Iabs= Io -I
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-a’cx
 I = Io 10 , where a = extinction coefficient
of the absorbing medium.
 a,
= k’ / 2.303

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Negative deviation
Concentration
Absorbance

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 When a non linear curve is obtained, the
system is said to undergo deviation.
 The two types of deviations are positive and
negative deviations.
 Positive deviation results in when a small
change in concentration produces a greater
change in absorbance.
 Negative deviation results when a large
change in concentration produces smaller
change in absorbance.

8.  Instrumental deviations – stray
radiation, improper slit width,
fluctuations in single beam and
monochromatic light is not used.
 Physiochemical changes in solutions
factors like association, dissociation,
ionization (change in pH), faulty
development of colour (incompletion of
reaction).
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