Benzoic acid Molar Extinction Coefficient

DCalfine

Hi guys! First time post here.

I was wondering where on the internet you could find such a value? I have tried searching for sites that may have a list of molar extinction coefficient values for various compounds but have found them to be quite hard to access. Would any of you with perhaps considerably more experience in this direct me to the right place? Many thanks.

DDTea

What solvent did you use? I found a table in the following paper that had log ε values for benzoic acid in different solvents:

Herbert E. Ungnade, Robert W. Lamb
J. Am. Chem. Soc., 1952, 74 (15), pp 3789–3794

log ε for Benzoic Acid in:

Cyclohexane: 2.96
Chloroform: 2.95-2.96
0.01 N HCl: 2.96
Water: 2.96
Methanol: 2.81
Ethanol: 2.84
Isopropanol: 2.84
t-butanol: 2.88
Dioxane: 2.92

I don't know about particular websites that have this type of information, though. I'm sure there's a way to calculate it; I just don't know how.

Borek

These things are much easier determined experimentally than calculated.

DDTea - what wavelength is this information for?

DCalfine

0.1 M HCl was added to the decresing volumes of Benzoic acid each time before being made up to 50cm3 with distilled water. Then, each of these sultions of differing volumes of benzoic acid were analysed in a spectrophotometer for absorbance values.

DDTea

Whoops Borek, that was a pretty silly omission on my part wasn't it? I'd assume it was for the lambda-max value, corresponding to the pi-->pi* transition, but I don't know which wavelength they used...

Unfortunately, from my home computer, I cannot access that article! So this is from table 11-9 of Organic Structural Determination by Lambert et. al. *

Benzoic acid in:

Water: λ_max = 230nm, ε_max = 10 000
95% Ethanol: λ_max = 226nm, ε_max = 9 800

In less polar solvents, I would expect λ_maxand ε_max to decrease slightly due to solvatochromic effects.

In any case, for this particular experiment, you can calculate ε_max using the Beer-Lambert law, where:

A = ε_maxcL

where A = absorbance value, c = concentration (molarity), L = pathlength of the light beam (the width of your cuvette containing your sample).

The idea when using the Beer-Lambert law is to use concentrations that are within the "linear range" for the molecule and the spectrophotometer: i.e., to only analyze samples where a linear relationship can be made between absorbance and concentration. You're going to have to make a plot of A vs. c. The slope will be your experimentally-determined ε_max for the wavelength you used.

I thought you were wondering how ε_max could be calculated from quantum mechanics :P

*Joseph B. Lambert et. al. Organic Structural Determination. Prentice-Hall, Inc.: 1998. ISBN: 0-13-258690-8

DCalfine

Sorry, that misunderstanding may have been not providing enough detail on my part. Although I have already worked out the ε_max to be at 0.0011 from my gradient of y=mx+c. As I am working out the end concentration of an unknown product, this has led to calculating my concentration to 632.72 from the A=ε_maxcL equation. I assume I divide this concentration by 1000 twice to get a value that is closer to my other end concentration values that I have worked out, which are values that are x10^-4 M?

DCalfine

Sorry, ignore my previous question. I have realised that this whole confusion was mostly caused by one very stupid mistake that I should've seen.
But as for the values, thank you. They have been very useful.