# Donnan equilibrium and electroneutrality

## Main Question or Discussion Point

Hello,
I've heard that when you are considering a Gibbs-Donnan equilibrium, that is if you have for example a situation like this:

There are two compartments 1 and 2 divided by a membrane, and at the beginning you have:
In compartment 1 a neutral solution of permeable ions H+ and Cl-;
In compartment 2 a neutral solution of permeable ions H+ and an anion that is impermeable to the membrane;
water can pass through the membrane.

I've read that not only at the beginning but also at equilibrium each compartment is electrically neutral.

Let's suppose now to have another situation: you have two solutions of NaCl at different concentrations, and between them a membrane that lets through only Na+ and not water nor Cl-.
In this case does the electroneutrality hold when the system is at equilibrium too? if not, why?
Thank you

Borek
Mentor
I am not sure I understand what you are asking about. In such systems we observe a potential difference between separated cells, this potential difference has it source in a charge separation, so solutions are not neutral. However, charges are very small, so depending on what you are trying to calculate, assumption that solutions are electroneutral can be right - or wrong.

That is the same i thought: if there is potential difference, the two compartments must be charged; but i read in many places that in a Donnan equilibrium the potential difference is given only by microscopic charge distribution, in a order of a Debye length, while the whole compartments are neutral...I really can't understand...

Borek
Mentor
No contradiction here - solution is bulk is electroneutral, as charge is present only very close to the membrane. So if you look at whole volume - it is charged. If you look at a thin layer of the solution close to the membrane, it is charged, as that's where the charge resides. But if you look at the whole volume minus this layer, it is electroneutral.

Ok thank you. So what you just said applies to both Nernst and Donnan equilibria?

I've just asked a university teacher and he gave me an answer that I think is very explanatory and clear.
He said that, since there is a potential difference between the compartments, actually a charge do exist inside each compartment, but it is a so small amount that it can be ignored. Effectively, the electroneutrality principle is used to calculate molar concentrations of ions , but the excess (or lack) of ions needed to build up the commonly considered potential differences is many (tens) orders of magnitude below any concentration we use in our calculations...

Borek
Mentor
I have a feeling he said exactly the same thing I wrote

You are right: you said it quite clearly in the first post you wrote. I've realized just now. I'm sorry, but for some reason I don't know, I missed the point of your explanation the first time I read it..