Unformatted text preview: ation, C, of absorbing substance is increased or the path length, ℓ, of the sample
is increased, since both increase the number of absorbing species in the path of the light. 48 A related quantity is the absorbance, A, which is given by
A - log T - log 10 %T
100 2 log 10 %T %T is read directly from the spectrometer. However, the absorbance is particularly
important since it, and not the transmittance, is directly proportional to the concentration
of the absorbing substance and the path length. This proportionality constitutes Beer’s
Law, and is commonly written as
A = ℓ C.
The concentration, C, is expressed in moles per liter (M) and the path length, ℓ, is
generally expressed in cm. The quantity is called the molar absorptivity and has units
of M-1 cm-1. Note that the quantity ℓC is dimensionless, as are both the absorbance, A,
and the transmittance, T. The molar absorptivity is characteristic of the substance. It
tells us how much light the substance absorbs at a particular wavelength. A graphical
plot of either the absorbance at constant path length or the molar ab...
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This note was uploaded on 04/09/2013 for the course CHE CHE 2C taught by Professor Nasiri during the Spring '07 term at UC Davis.
- Spring '07