# Extinction Coefficient from Time series data

## Main Question or Discussion Point

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.

Borek
Mentor
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?

Borek
Mentor
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?