Extinction Coefficient from Time series data

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Discussion Overview

The discussion revolves around determining the extinction coefficient (ε) for bromine (Br2) from time series absorbance data obtained through UV-Vis spectroscopy. Participants explore the relationship between absorbance, concentration, and the reaction kinetics of Br2 formation and loss in a chemical reaction.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant describes the overall reaction and the need to find the extinction coefficient using the equation A=εcl, noting the availability of time series absorbance measurements and initial concentrations.
  • Another participant suggests that plotting absorbance versus time to find the slope is a valid approach, but recommends recording an additional point to ensure stoichiometric production of Br2 for calibration.
  • A different participant questions whether the extinction coefficient determined from the initial run is accurate, given that they are also measuring the loss of Br2 in a subsequent reaction.
  • Another participant argues that if the same cuvette, wavelength, and instrument are used, a calibration curve may be more appropriate than calculating the extinction coefficient directly, and questions the choice of time series data over a series of samples with different concentrations.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of determining the extinction coefficient versus creating a calibration curve, indicating that multiple competing approaches remain in the discussion.

Contextual Notes

There are unresolved questions regarding the validity of using time series data for determining the extinction coefficient and the implications of using different methods for measuring absorbance in relation to stoichiometry and reaction kinetics.

Who May Find This Useful

Researchers and students involved in spectroscopy, chemical kinetics, or those studying reaction mechanisms in chemistry may find this discussion relevant.

lee403
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I have some time series data of the absorbance of Br2 formation using UV Vis spectroscopy and I need to figure out the extinction coefficient/ absorptivity.
The overall reaction is
BrO3-+5Br- +6H+-->3Br2+3H2O
which is expcted to go to completion
I know that the equation relating absorbance to concentration is
A=εcl
and I have times series A measurements and can calculate the initial concentrations of BrO3, Br -and the expected concentration of Br2 from the solutions I made. I just need to find ε.

I first attempted to plot the absorbance v. time and find the slope where it was most linear but I don't know how valid this approach is.
 
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lee403 said:
I first attempted to plot the absorbance v. time and find the slope where it was most linear but I don't know how valid this approach is.

It is about as good as it can be.

I would record an additional point after waiting for some time to make sure the amount of Br2 produced is just stoichiometric. That would give a good calibration point.

Besides, if all you are after is a time series (for kinetic measurements), all you are interested in is the rate of changes - are you sure you need absolute values for that?
 
Maybe I'm thinking about this wrong. I need to know the extinction coefficient for Br2 because in another reaction I measure its loss over time. So I dd an initial run for the formation of Br2 in order to determine its extinction coefficient. Then in a second run I added a compound that reacts with it and measured the absorbance again. I am interested in the rate of loss so does that mean the extinction coefficient from the initial run is not an exact value?
 
If you are using the same cuvette, wavelength and the same instrument you don't need extinction coefficient but a calibration curve. Linear regression on the data is typically a way to go.

What I don't get about your setup is why you use time series instead of just making a series of samples of different concentrations?
 

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