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Author Topic: ionic strength and ion diffusion rate  (Read 169 times)

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Corribus

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ionic strength and ion diffusion rate
« on: December 05, 2017, 04:46:23 AM »

Consider the following thought experiment:

You have a 2 two-chambered vessels in which the two chambers of each vessel are separated by a membrane that is permeable to both sodium and chloride ions but nothing else. In vessel A, one chamber contains a sodium chloride solution. In the other chamber is distilled water. In vessel B, one chamber contains the same sodium chloride solution and the other chamber contains a solution of high ionic strength that includes neither sodium nor chloride. Would you expect the diffusion of sodium and chloride ions across the membrane to be faster in vessel A or vessel B? Would the equilibrium distribution of sodium and chloride be the same in both systems?
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Borek

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Re: ionic strength and ion diffusion rate
« Reply #1 on: December 05, 2017, 12:20:43 PM »

I wonder if the difference in osmolality won't play some role.

Osmosis is about solvent traveling, but if we allow only ions to pass the membrane, will they move to equalize pressures?
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Re: ionic strength and ion diffusion rate
« Reply #2 on: December 05, 2017, 04:43:30 PM »

My initial thought was that vessel B would exhibit slower rate of diffusion and/or have an equilibrium shifted more toward the chamber initially containing sodium chloride, in order to equalize the total number of ions in both chambers. However then I convinced myself that maybe this is not the case, that a high ionic strength solution is "more polar" and solubility of additional ions would be favored, such that vessel B may have the faster rate of diffusion. Now I can't decide which is more likely to be correct. :/

This does have some relevance to something I'm doing - I have some experimental data that suggest the first scenario may be the correct one. Well, anyway a high ionic strength in a solution reducing an ion diffusion rate was a hypothesis I created to explain some experimental data, but then after thinking about how ionic strength affects the solubility of solids (actually making them more soluble) I'm not sure. This is why I created the thought experiment... but it turned out not to be helpful because as mentioned above, I can talk myself into either answer.

Obviously it's hard to create a magic membrane like the one described, but I wonder if there's a more realistic system that could be designed to figure out the answer.


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