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Potentiometry
Posted 18.05.2015, 16:08 GMT-4 Version 4.4, Version 5.0 3 Replies
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I feel like my issue should not be that hard, but I am struggling to model this.
I have a system in which there is a very nonpolar (organic) membrane between two aqueous solutions. I am trying to model the flux (diffusion) of a charged species out of the membrane into one of the aqueous solutions due to a change in thickness of the Nernstian diffusion layer governed by the stirring rate of the system.
One of the biggest considerations is that there is no net current change, just a change in potential (phase boundary potential differences). This makes it difficult to justify with other ion transfer models out there.
Any help would be appreciated!
I have a system in which there is a very nonpolar (organic) membrane between two aqueous solutions. I am trying to model the flux (diffusion) of a charged species out of the membrane into one of the aqueous solutions due to a change in thickness of the Nernstian diffusion layer governed by the stirring rate of the system.
One of the biggest considerations is that there is no net current change, just a change in potential (phase boundary potential differences). This makes it difficult to justify with other ion transfer models out there.
Any help would be appreciated!
3 Replies Last Post 20.05.2015, 04:14 GMT-4
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Posted:
9 years ago
Hi
I do not quite understand your question. What is the quantity you wish to calculate? You are talking about diffusion, so how is potential related to the problem? What is the chemistry of the system? Stirring has an effect on the total permeability (P_tot) of the membrane like this:
1/P_tot = 1/P_m + 1/P_aq
where P_m is the permeability of the membrane (D_m/h_m) and P_aq that of the unstirred layer (D_aq/h_aq); h's are the thicknessess of the respective layers.
How are you modeling stirring?
best egards
Lasse
I do not quite understand your question. What is the quantity you wish to calculate? You are talking about diffusion, so how is potential related to the problem? What is the chemistry of the system? Stirring has an effect on the total permeability (P_tot) of the membrane like this:
1/P_tot = 1/P_m + 1/P_aq
where P_m is the permeability of the membrane (D_m/h_m) and P_aq that of the unstirred layer (D_aq/h_aq); h's are the thicknessess of the respective layers.
How are you modeling stirring?
best egards
Lasse
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Posted:
9 years ago
I currently do not have a good way to model the stirring. That is one issue I have.
I am trying to calculate the change in the concentration of an ion as it diffuses out of the membrane with a change in thickness of the unstirred layer. That change should relate that to the change in potential as I am dealing with potentiometry and phase boundary potentials.
I am trying to calculate the change in the concentration of an ion as it diffuses out of the membrane with a change in thickness of the unstirred layer. That change should relate that to the change in potential as I am dealing with potentiometry and phase boundary potentials.
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Posted:
9 years ago
I have simulated stirring so that I have defined a unstirred layer with some thickness h where I use the aqueous diffusion coefficient and beyond that (in the bulk) I use an unrealistically high diffusion coeffient which ensures that concentration differences are leveled immediately. A potentiometric sensor measures the concentration in the bulk via Nernst equation, it does not see any potential profiles. And if electric current is zero, there is no potential profile, only very close to the membrane surface if it has a fixed charge density (ion selective membrane). Potential profiel close to a charged membrane is solved with the Poisson-Boltzmann equation (see Guoy-Chapman theory).
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Lasse
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Lasse