How salty is a half salty solution? I mean, only one kind of ion

Studiot

Sodium ions are much smaller than chloride ions so
I could imagine a container, separated into two by a semi permeable membrane, such that only the sodium ions (edit and possibly solvent ions) could pass through.

Put pure water into one half and salt solution into the other.
The chloride ions would remain in one half of the container, but the pure water half would soon contain some sodium ions that had passed through so you would have some solution containing only one species of solute ion.

However as I already noted in post#12 there would also be other ions in the pure water half, due to some ionic dissociation of the solvent water.

Can you guess what else would happen to maintain electrical neutrality?

Weissritter

As said before, electrons would be attracted, and that's how much I can guess. Electron jumping from s sublevel to higher ones sounds too much to be possible, so the best I can guess is the electron-leeching effect.

Borek

Electrons would stick to their ions, so they would not change the charge distribution.

If anything I would expect to see changes in the concentrations of other ions in the solution.

Please remember that moving ions through the membrane as Studiot suggested means creating charge separation. Charge separation means potential difference - and any measurable change in concentrations would require pretty high voltages.

Studiot

Borek, ( or any practising chemist) you got there first and if you have better information please post it. Particularly about the practical availability of suitable molecular sieves/membranes.

Electrons are not ions, nor would they exist in a free state in solution.

I don't know if you are aware but ionic solvents work by splitting into ions (dissociation).
The solvent starts out electrically neutral so the resulting charges on tha anions (-ve) and cations (+ve) it splits into balance out, amintaining the electrical neutrality.

Two such solvents for sodium chloride are ammonia and water, the salt being about three times as soluble in water as ammonia.

When an (electrically neutral) salt dissolves in an ionic solvent it also splits up into ions.
In the same way electrical neutrality is maintained by balancing out the charges of the anions and cations created from the solvent molecules.

I have tried to show this in the attachment.

If we have our container with a watertight door over the membrane initially and fill each side up as shown in Fig 1 we will have only solvent ions in the left hand side and solvent ions plus solute ions in the right.

I have appended the approximate sizes of the ions so you can see that the sodium and chloride ions are much bigger than the sovent ions from the water.

So the solvent ions can pass freely through the membrane, as can the sodium ions. However the membrane is such that the chloride ions (about twice the size of the sodium) are too big to pass the membrane.

Thus in Fig2 when we open the door the sodium ions (but not the chloride ions) will spread (diffuse) into the left hand chamber.
Because cations (Na⁺) are spreading left, they will displace some solvent cations in the left hand chamber to the right hand one.
So you will end up with only sodium and solvent ions in the left hand chamber. You cannot avoid the solvent ions since the salt would not dissolve without them.

It is this movement of solvent ions that maintains overall electrical neutrality.

This is a very simplistic description of what happens that I'm sure will horrify the pure chemists but I hope it serves to promote understanding.

Whilst we are on the subject can you say why the cations are in general smaller than anions?

Attached Thumbnails

 

Borek

Can't say I disagree. The main problem is that what you wrote seems to be suggesting all ions get separated the way you wrote them, while in fact only some small amounts will move through the membrane. But on some general level you are right about processes taking place.

Studiot

You are right I meant to mention at the end that the diffusion would not proceed very far, but forgot.

DrDu

http://en.wikipedia.org/wiki/Gibbs%E...3Donnan_effect

Weissritter

You guys are really helpful. I won't say that an only-one ion solution exists, and having you all clarified this, the diffusion chat may proceed.
It is sad to see the diffusion would create the sodium-and-hydronium-and-hydroxide to a very limited extent, but that's how it works. Thank you Studiot for your diagram. Thank you all.
Even though my doubts on this subjects have been currently clarified, I, with all my magnanimity will allow further posts with relevance to the thread

Studiot

Dissociation creates the ions.

Diffusion moves them about.

The hydroxyl (negative) and hydroxonium (english) or hydronium (american) (positive) are always present to some extent. The addition of other substances changes the amount present.

I noted in my first post that the conditions under which you could isolate a single ionic species are not the sort of conditions where you would go taste sampling them.

Weissritter

Yes, both of us have issues about having plasma in the mouth. We'd need a volunteer to know what it could taste like for a human. If micromass hears about this, I think I know who he'd nominate.