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)