© 2002 J.Paul Robinson, Purdue University
Exctinction
•Using Beer’s 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
3rd ed. Shapiro p 86