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I understand that there is a pressure gradient between an aqueous solution, e.g. salty water, and, say, pure water due to osmosis. My question is, is it possible to match or almost match, the osmotic pressure between two solutions of different solutes, but using a solution with a different solute. For example, would it be possible to make the osmotic pressure gradient between salty water and some other solution zero, assuming both are at the same temperature and pressure?

I am not a chemist, so purely qualitative answers are helpful.

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  • $\begingroup$ Just to clarify - the solvent would be the same for both solutions? $\endgroup$
    – Todd Minehardt
    Jul 31, 2015 at 13:19
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    $\begingroup$ Hi Todd,yes, the solvent would be the same (just made a nonsense comment saying the opposite and had to edit it) $\endgroup$
    – crobar
    Jul 31, 2015 at 13:34
  • $\begingroup$ Does the isotonic case mentioned here illustrate what I think you're looking for? The example is biological in nature, and is that of a blood cell membrane that is selectively permeable to one of the solutes and the water, but not the other. If so, I'll go ahead and edit the answer I was working on and post it. I want to make sure we're on the right track. $\endgroup$
    – Todd Minehardt
    Jul 31, 2015 at 14:47
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    $\begingroup$ I think that is pretty much the situation I'm thinking of, yes. $\endgroup$
    – crobar
    Jul 31, 2015 at 14:53

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Yes, I think so. Based on our comments above, I believe you want to prepare two solutions that are isotonic with each other (have the same concentrations of different solutes) and allow them to communicate across a membrane that is selectively-permeable to water only.

Tonicity is described here as:

Tonicity is a measure of the effective osmotic pressure gradient (as defined by the water potential of the two solutions) of two solutions separated by a semipermeable membrane. In other words, tonicity is the relative concentration of solutions that determine the direction and extent of diffusion. It is commonly used when describing the response of cells immersed in an external solution. Unlike osmotic pressure, tonicity is influenced only by solutes that cannot cross the membrane, as only these exert an effective osmotic pressure. Solutes able to freely cross the membrane do not affect tonicity because they will always be in equal concentrations on both sides of the membrane.

I found this nice explanation with illustrations and one example where it shows that you could prepare a 10 mM solution of albumin and a 10 mM solution of glycine, separated by a water-permeable membrane, and no net flow of water occurs in either direction.

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  • $\begingroup$ Could this principle somehow be used to desalinate water? $\endgroup$
    – iad22agp
    Jul 31, 2015 at 20:59
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    $\begingroup$ I believe not: you have to get the salts out of the water, and they're the species which create osmotic pressure if they're not able to cross a membrane. Hence the use of reverse osmosis, where the osmotic pressure of the salt water is exceeded by external pressure on the system (at a very high energy cost). $\endgroup$
    – Todd Minehardt
    Jul 31, 2015 at 22:40

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