Beers Law plot

Question

I had to constructed a Beers Law plot for my graph (absorbance vs. concentration) I'm supposed to find the molar extinction coefficient from the slope. Well the slope of the line is y=0.2559x+0.0445 and r^2=0.98895. Where I'm confused is that I was told to multiply 100,000 to the slope of the line. Please advise.

Answer 1

The typical Beers Lambert plot for the equation:

 

A = ecl

 

where A = absorbance, e = extinction coefficient (M-1cm-1), c = concentration (M), and l = pathlength (cm)

has to be carried out with the units given in parentheses. If your manual is telling you to multiply by a factor of a million then you probably were working with solutions of micromolar concentration. In other words, the units of the extinction coefficient will be uM-1cm-1 instead of M-1cm-1. In order to convert them to the desired units, you essentially carry out the following conversion factor:

 

e = [1/(uMxcm)] x [(10^6 mM)/(1 M)]

 

I hope that helped out clarify the confusion.