文档介绍：Introduction
pounds absorb ultraviolet (UV) or visible (Vis.) light. The diagram below shows a beam
of monochromatic radiation of radiant power P0, directed at a sample solution. Absorption takes
place and the beam of radiation leaving the sample has radiant power P.
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
The last equation, A = 2 - log10 %T , is worth remembering because it allows you to easily calculate
absorbance from percentage transmittance data.
The relationship between absorbance and transmittance is illustrated in the following diagram:
So, if all the light passes through a solution without any absorption, then absorbance is zero, and
percent transmittance is 100%. If all the light is absorbed, then percent transmittance is zero, and
absorption is infinite.
The Beer-Lambert Law
Now let us look at the Beer-Lambert law and explore it's significance. This is important because
people who use the law often don't understand it - even though the equation representing the law is
so straightforward:
A=abc
Where A is absorbance (no units, since A = log10 P0 / P )
a is the molar absorbtivity with units of L mol-1 cm-1
b is the path length of the sample - that is, the path length of the cuvette in which the sample is contained. We will express this measurement in
centimetres.
c is the concentration of pound in solution, expressed in mol L-1
The reason why we prefer to express the law with this equation is because absorbance is directly
proportional to the other parameters, as long as the law is obeyed. We are not going to deal with
deviations from the law.
Let's have a look at a few questions...
Question : Why do we prefer to express the Beer-Lambert law using absorbance as a measure of
the absorption rather than %T ?
Answ