Seawater at rest...

[Externet]

Sterile seawater left at total rest for a looong period of time in a tall sealed container with no air; would it have more ions concentration near the surface or near the bottom ?

I mean sterile to denote no influence of movement by microorganisms of any kind to affect the 'at rest status'

Edited August 9, 2010 by Externet

[Mr Skeptic]

I think there would be unnoticeably more ions at the bottom, since the density would be greater (both the density of the ions+water and the water itself due to compression). Just how tall a container are you proposing?

[Externet]

Any number of metres of your choice, it is just to enhance the question with perception of a greater differential.

[CaptainPanic]

I think that sea water does not de-mix (or un-mix)... however, large amounts of fresh water on top of salt water might also not mix.

 

I'd be curious if there is any article or other reference to suggest that salt water actually de-mixes when left at rest. That could be an interesting way to produce a perpetual motion device, as you can actually extract energy (electric) from two volumes of water with different amounts of salt.

[PaulS1950]

since salt water is a solution it will remaun mixed - never separating.

This changes if you remove heat or add heat to the equation.

If you place saltwater in a centrifuge (simulating long periods of high gravity) it remains salt water.

[John Cuthber]

It would separate out slightly; there would be a difference in the concentrations of ions at the top and bottom of the container.

I'd sooner have to calculate the difference, rather than measure it.

(incidentally, I'm generally quite good at measuring things- it's what I do for a living ; and I'm rather bad at maths)

[CaptainPanic]

It would separate out slightly; there would be a difference in the concentrations of ions at the top and bottom of the container.

I'd sooner have to calculate the difference, rather than measure it.

(incidentally, I'm generally quite good at measuring things- it's what I do for a living ; and I'm rather bad at maths)

 

Is that concentration difference due to the fact that (salt) water is still slightly compressible (although considered incompressible, like all liquids, there is a small but measurable compressibility)?

Or are you perhaps talking only about the bottom and top surfaces of the body of water, and not about its bulk? Different interfaces will of course influence the salt water differently.

 

I mentioned before that it's possible to generate electricity from a salinity gradient (reversed electrodialysis, for example). And if I look into a summary of the theory of electrodialysis, then I cannot see the gravity in that formula... which would mean that it's possible to generate small currents if you simply place two electrodes into the container with salt water at different heights. And that would make it a perpetual motion device... I remain skeptic.

[Mr Skeptic]

It doesn't work that way.

 

For example, given a pressure differential, you can use that pressure differential to create electricity. Since pressure depends on depth, this means that if you connect the surface to the bottom of the sea, you can generate lots of electricity due to the huge pressure difference, right? The trouble of course is that your connection will have the inverse effect.

[insane_alien]

Is that concentration difference due to the fact that (salt) water is still slightly compressible (although considered incompressible, like all liquids, there is a small but measurable compressibility)?

Or are you perhaps talking only about the bottom and top surfaces of the body of water, and not about its bulk? Different interfaces will of course influence the salt water differently.

 

higher concentrations of salt are more dense than lower concentrations. this means that there will be a seperation effect however the effectiveness is limited by the diffusion of the salt due to chemical potential. if you had a greater gravitational field you could get a greater concentration gradient and i suppose with enough you could get such a difference that the salt would crystalise out however this would require black hole levels of g.

[CaptainPanic]

higher concentrations of salt are more dense than lower concentrations. this means that there will be a seperation effect however the effectiveness is limited by the diffusion of the salt due to chemical potential. if you had a greater gravitational field you could get a greater concentration gradient and i suppose with enough you could get such a difference that the salt would crystalise out however this would require black hole levels of g.

 

I am still not convinced. Can anyone please provide any article or other reference for me to read about this phenomenon? As I understand your explanations correctly, any solution (not just salt in water) at rest would auto-separate, and the separation would only be countered by diffusion.

 

However, I cannot seem to find any reference at all regarding this phenomenon (although that is perhaps because I use the wrong keywords).

 

Assumptions

I assumed so far that we start our experiment with a container with water, in which we have dissolved an amount of salt adequate for the experiment. The salt completely dissolves, and no solids are left after the salt is dissolved. The salt is, at first, equally distributed as you may expect after vigorous stirring.

Then, with the salt equally distributed, there is no difference in concentration at all, and therefore no difference in density.

We then allow the solution to come to a complete rest, so that there is no movement other than the individual motion and interactions of the molecules in the solution.

 

What would be the driving force of the separation, given the situation that I just described?

 

insane_alien, I agree with your remark that, if there would be any separation effect due to a concentration difference, we would have to take diffusion into account as a phenomenon that will counter the separation. But, I still fail to see why there would be any separation in the first place.

[Mr Skeptic]

Diffusion is only relevant when there is a difference in concentration, so cannot function to keep the concentrations exactly equal. So, gravity pulls slightly harder on the ions than on the water, and makes a tiny tiny tiny difference, and any further difference is negated by diffusion. I doubt we'd be able to measure the difference.

[insane_alien]

well, the driving force is gravity for the seperation.

 

the distribution (even of a well mixed solution) of ions is not uniform but random. there will be small pockets where the concentration is slightly higher than that of is surroundings and vice versa, this is the nature of a random distribution.

 

due to this there will be separation by buoyancy.

 

it will nto be a large effect due to the small differences in density and the lifetime of pockets of higher concentration(not long at all) but it will be present.

 

you really don't stand a chance of even detecting this in a lab but if you made a column a few kilometers tall(need to be in a mine shaft for support) and filled it with salt water and let it sit for some time after mixing then i'm sure it could be measured.

 

iirc, haloclines are something like this.

Edited September 6, 2010 by insane_alien

[Externet]

... if you had a greater gravitational field you could get a greater concentration gradient and i suppose with enough you could get such a difference that the salt would crystalise out however this would require black hole levels of g.

 

Thanks.

Then instead of thinking on an absurdly tall column; what about feeding the salty water into a centrifuge ?

Would then the ions float or sink, yielding some separation of the solution concentration ?

[John Cuthber]

They do separate density gradients in centrifuges.

http://en.wikipedia.org/wiki/Isopycnic_centrifugation

 

Using roughly a million g might be thought of as a bit tricky.

 

I think haloclines"cheat".

The fresh water isn't on top because it diffused there. It got there from the rain (or river run off) onto an already salty sea.