2
$\begingroup$

I have built a 12-band spectrophotometer, and i'm currently keen on proving Beer-Lamberts law through a small practical example.

The solution that I'm going to test with my spectrophotometer, is orange juice that I thin with water. I naturally know I0 and I1, I.e. the absorption, and the path length that the light has to travel. Meaning that I have 2 unknowns, respectively the concentration and the molar absorptivity.

I am aware that I can figure out the molar absorptivity by doing multiple assays at different concentrations, plot them into a coordinate system and find the slope between the coordinates, which will be equal to epsilon*pathlength.

Unfortunately, I don't know how I find the molarity concentration of the orange juice that I intend investigate. I do however have the weight and the amount. Will that be sufficient to calculate the concentration molarity?

$\endgroup$
3
$\begingroup$

The molar absorptivity is a proportionality constant. You do not know what is the compound responsible by the color. So, this constant will not be in L/mol.cm but will be in terms of the concentration units. For example, if you use the mass to express the concentration (g/L) the constant will be express in this unit L/g.cm. With this system, you can estimate concentrations and so on. To known the molar absorptivity you must know the concentration in mol/L and it is not possible with this experiment.

$\endgroup$
  • $\begingroup$ Thank you very much for your answer. What would you suggest that I do instead? I only have access to "home-chemicals", and my spectrophotometer measures intensities from 450 nm to 860nm with roughly 50nm intervals between each of the 12 bands. $\endgroup$ – Jeppe Christensen Nov 16 '18 at 0:45
  • $\begingroup$ I know that carotenoid is responsible for the yellow color, but i don't know the molecule structure and exactly what kind of carotenoid that is in oranges. $\endgroup$ – Jeppe Christensen Nov 16 '18 at 0:47
  • $\begingroup$ Why you need the value in mol? $\endgroup$ – Koba Nov 16 '18 at 21:02
  • $\begingroup$ To know the molar absorptivity you need the pure compound to produce a solution with a known concentration. Why are you using orange juice? What is your objective? $\endgroup$ – Koba Nov 16 '18 at 21:10
  • 1
    $\begingroup$ Exactly, if you plot the absorbance versus concentration (g/ml) you prove the linearity of your equipment. In this case, your epsilon will be in L.g-1.cm-1 instead L.mol-1.cm-1 but it doesn't matter. If you want you can buy some copper sulphate used in flower shops, purify by crystallization and obtain a constant in molar units for this case... $\endgroup$ – Koba Nov 17 '18 at 0:04

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.