BEER’S LAW
P0
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
Absorbance
0% 100%
2.0 0
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 / PA = log10 1 / T A = log10 100 / %TA = 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 LAWIn 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)