Beer's Law Question

Homework Statement

If I measure an absorbance at 560nm of 0.80 absorbance units for a solution of the basic form of phenolphthalein, what is the concentration of phenolphthalein?

Homework Equations

Beer's Law -- A(sub lamda)=e*b*c

The Attempt at a Solution

The question is asking for concentration, so I'll need to solve for c. I think A is the absorbance units, but beyond that I believe I'm missing information.

b is supposed to be the path length, which I believe is the width of whatever cuvet is being used (the problem doesn't specify what that is). e is molar absorbtivity, but isn't that a constant?

Is this problem able to be solved as it is or do I need more information?

Thanks!

presbyope
Gold Member
b is supposed to be the path length, which I believe is the width of whatever cuvet is being used (the problem doesn't specify what that is). e is molar absorbtivity, but isn't that a constant?

Cuvet length is 1 cm by convention. Molar absorptivity is going to vary with temperature and pH as you can see here:

http://pubs.acs.org/doi/full/10.1021/ol016823q

Unfortunately they don't report a value for phenolphthalein but estimating from Fig. 6, e should be about 1770/(M.cm) at 20C and basic conditions.

Thank you for the reply. It seems there was no way to calculate an answer based on the question alone. I was worried I was missing something obvious.

Borek
Mentor
Cuvet length is 1 cm by convention.

I agree 1 cm is a common cuvette size, but calling it a "convention" is too far fetched.

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presbyope
Gold Member
I agree 1 cm is a common cuvette size, but calling it a "convention" is too far fetched.

Quite right, I stand corrected.