P0
BEER’S LAW
USES OF BEER’S LAW


- Relates concentration to the optical
measurement of ‘absorbance’
- combined with spectrophotometry can be
used to distinguish and compare different
molecules in solution
THE RELATIONSHIP BETWEEN ABSORBANCE
AND TRANSMITTANCE IS ILLUSTRATED IN THE
FOLLOWING DIAGRAM:
Transmittance
0%
100%
2.0
0
Absorbance
THE AMOUNT OF RADIATION ABSORBED
MAY BE MEASURED IN A NUMBER OF WAYS:
Transmittance, T = P / P0
% Transmittance, %T = 100 T
Absorbance,
A = log10 P0 / P
A = log10 1 / T
A = log10 100 / %T
A = 2 - log10 %T
QUESTION : WHY DO WE PREFER TO
EXPRESS THE BEER-LAMBERT LAW USING
ABSORBANCE AS A MEASURE OF THE
ABSORPTION RATHER THAN

%T ?
Compare the two equations that we use:
A=abc
%T = 100 P/P0 = e -abc
COMPARE HOW EACH EQUATION GRAPHS
BEER’S LAW EQUATION
A=abc
Where,
a= molar absorptivity (is a measure of the
amount of light absorbed per unit concentration;
this value is a constant for a given solution)
b = cell path length (usually 1cm) (Cuvette)
c = concentration (M)
BEER’S LAW
A = abc
Where ab = constant,
then A = constant times c
So when we plot this y=mx+b
BEER’S LAW
In order to use A = abc, we need to define values for
a and b.
b = path length – “blank” – distilled water in a
cuvette
a = colorimeter(select a preset wavelength)
spectrophotometer (choose wavelength where
maximum absorption of photons occurs)