November 13, 2019, 04:20:39 PM
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Topic: Oxidation of organic liquid scintillators  (Read 218 times)

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Offline pmaxd

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Oxidation of organic liquid scintillators
« on: July 04, 2019, 09:50:21 AM »
Hello,
I am currently working with organic scintillators, to be specific the solvent is Linear Alkylbenzene (LAB) with 2,5-Diphenyloxazole (PPO). When this solution contains oxygen, the PPO is oxidized. My question is, whether this process is proportional over time or if the progress is of different nature.
Also, I am storing samples at a temperature of 80°C to excellarate the oxidation. What would be the best way to estimate the "actual" age of these samples when exposerd to this temperature?
Thank you

Offline pgk

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Re: Oxidation of organic liquid scintillators
« Reply #1 on: July 04, 2019, 12:31:10 PM »
All kind of chemical reactions are time dependent, including photoxidations like this one.
Anyway and although not absolutely coinciding to the reality, photoxidations can be considered to have zeroth order kinetics under constant flux irradiance (or constant irradiation intensity, if irradiation surface is constant) and constant oxygen concentration (not a degassed solvent).
dC/dt = K  →  C = Kt
The rate constant K is temperature dependent, according to the Arrhenius equation.
Thus, the "actual" age of your samples at 80°C, can be estimated if the Arrhenius constant A and reaction activation energy Ea are known.
But even if the Arrhenius constant A and reaction activation energy Ea are unknown, they can easily be calculated by a set of a few measurements (say a couple) at different temperatures.
PS: If flux irradiance (or irradiation intensity) or oxygen concentration are not constant, your photoxidation kinetics is more complex.
« Last Edit: July 04, 2019, 12:53:19 PM by pgk »

Offline pgk

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Re: Oxidation of organic liquid scintillators
« Reply #2 on: July 04, 2019, 03:50:12 PM »
Or else and more simply, you can apply the rule of thumb which states that “the reaction rate doubles with every 10°C rise in temperature”. But this does not always work and may lead to less accurate results.

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