The
measured absorbance, A, can be used to
determine the concentration of a coloured
material in solution. Absorbance is a
function of three factors:
- the pathlength, b, of the light passing through
the solution
- the absorptivity, a, which is characteristic
of the material
- the concentration, C, of the material
The
relationship of these three parameters
is expressed in Beer's Law:
A
= abC
If
the concentration of the material measured
is expressed in units of molarity, the
absorptivity, a, is referred to as the molar
absorptivity and given the symbol
?. In the older literature, ? is called
the extinction coefficient.
The units of the molar absorptivity are
?, with the pathlength in cm.
Absorbance,
A, is the logarithm of the ratio of the
radiant power of the incident light falling
on the solution, Po, relative to the
power of the light emerging from that
solution P.
A
= log (Po / P)
Transmittance,
T, of a solution is defined as the fraction
of the original light that passes through
the sample:
T
= P / Po |