Using Beers
law (Beer-Lambert law) for light travelling
through a curvette thickness d cm
containing n molecules/cm3
ln (Io/I) = snd
where Io and I are
the light entering and leaving and s is
the molecular property called
the absorption cross section
Now we can state that
ln (Io/I) = and where C is the concentration and a is the absorption coefficient
which reflects the capacity of the absorbing substance to absorb light
If there are n (molecules/cm3 ; d in cm, s must be in cm2 so if a is in cm2/mol, C must be in mol/cm3 do C=a/103
giving
log10 (Io/I)
= ead = A
where A is the absorbance or optical
density
and e is the decadic molar exctinction coeficient in dm3mol-1cm-1