Why cant I have a solution of just anions? or Cations?

Main Question or Discussion Point

Why not?

if I had a solution of K+(aq) and Cl(-) and placed it within an electric field, my ions would separate to different sides correct?

Why can't I just take a beaker and scoop of the liquid toward one end of the electric field? Wouldn't that contain mostly anions/cations and no longer be electroneutral? I would have a monopole, correct?

Is there anything wrong with my idea? Why can't I have this solution? I really need it.

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hilbert2
Gold Member
The Coulomb force between charges is terribly strong, if you had a significant amount of positive and negative ions separated in that way, they would attract each other with like, say, a force of million newtons even if they were kept a mile away from each other. Also, if you had a few grams of positive K+ ions kept inside a 1-liter container, the ions would repel each other so strongly that the pressure inside the container would probably be larger than that at the center of the Sun. There's practically no way how you could separate KCl to its ions in that manner.

DrClaude
Mentor
[V];4771786 said:
if I had a solution of K+(aq) and Cl(-) and placed it within an electric field, my ions would separate to different sides correct?
is incorrect.

While cations will accumulate near the cathode, and anions near the anode, only a thin layer of one type of ions will be found close to the electrode. As they accumulate, they will attract counter-ions, so the imbalance in solution is very localized. See http://en.wikipedia.org/wiki/Double_layer_(interfacial [Broken])

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I see. What about a solution with a very low concentration of one ion? Would this be possible by blasting gaseous ions through a magnetic field to separate them? I imagine eventually it would stop flowing to their respective container since the coulumbic force is so great. But there has to be a range of stable concentrations for such a solution (must be fairly low)!

DrClaude
Mentor
[V];4772142 said:
I see. What about a solution with a very low concentration of one ion? Would this be possible by blasting gaseous ions through a magnetic field to separate them? I imagine eventually it would stop flowing to their respective container since the coulumbic force is so great. But there has to be a range of stable concentrations for such a solution (must be fairly low)!
That would work until the accumulated charge was big enough to create a spark and discharge with something else, which would probably occur before the Coulombic repulsion would be strong enough.

For a cool illustration, check out Lord Kelvin’s Thunderstorm.

dbh
You can indeed produce a solution with an ionic content of unbalanced charge. This is the case in droplets produced in an electrostatically produced aerosol.
The stability of a charged droplet is disrupted when the excess charge, which accumulates around the surface of the droplet, overcomes the cohesive forces behind the liquid's surface tension. This can occur as the droplet accumulates charge, say from an electric field and a nearby plasma discharge, or as the droplet shrinks due to evaporation. At some point the droplet will distort and spray off the excess charge in an event I called droplet electrospray. The aerosol produced by these spraying droplets contains highly charged droplets of much smaller dimensions. This was the topic of my thesis. I have time-lapse video of these events captured through a microscope.
Ions can also pop off the surface of the droplet and drift away. These can be detected and analyzed by mass spectrometry and has formed the basis of one type of atmospheric pressure mass spectrometry ion source.
http://pubs.acs.org/doi/abs/10.1021/ac00094a015

Relating to that topic, I have one more question.

Now, if I used a generator (rotating magnetic field) to generate electricity to say, REDUCE an unbalanced solution (that is mostly positive in character) or even found a way to reduce the aerosol spray in air (maybe through ionizing the air?) Would the generator's total current and voltage decrease over time?

The reason I ask, is because every circuit we see is a closed loop, so essentially all electrons are returned to the source (the coil within the rotating field). Reducing an unbalanced solution should ensure that not ALL electrons are necessarily returned to the source.

Is there an reasons why my observations would be different than what I expect?

dbh
The short answer is it depends on how you define your system. An electrospray is part of a circuit. There are ion-molecule reactions, oxidation/reduction reactions, etc. all contributing to the net current flow. You can deflect and trap charged particles and ions, accumulate and even store them for an indefinite period of time, creating a localized charge but all the charges will balance in the bigger picture - it's just a matter of accounting for them.