Question

Suppose you have a solution containing both salt (NaCl) and glucose,and dialysis tubing with a pore size that allow the passage of Na+ and Cl- ions,but doesn't allow the passage of glucose. How could you remove essentially all of salt from the solution?

Answer 1

Use the fact that in solution molecules or ions will tend to travel down their concentration gradient. Run your solution through the tubing, while passing another solution in the opposite direction (as countercurrent flow will maximize the exchange rate) and make sure your other solution (your dialysate) has a similar osmotic pressure (isotonic) so fluid volume is not lost, but make sure that it has no Na or Cl so these ions will tend to travel down their concentration gradient. Continuously pump fresh dialysate through the system so that you are constantly getting rid of the Na and Cl you remove and keeping the concentration gradient moving from hi to lo away from your solution and into the dialysate.

Answer 2

Can you explain more?

Answer 3

Short explanation;
2 main ideas: Diffusion of dissolved molecules (down concentration gradients) and Osmosis (movement of the solvent)
*the dissolved components will want to flow down their concentration gradient (From high amounts of stuff to low amounts of stuff) if they can. Each type of ion (glucose, Na, K etc) will have its own concentration gradient and its own equilibrium point.
*the total amount of dissolved stuff contributes to the concentration gradient of the solvent (basically how dilute everything is) so the solvent will also want to flow down its concentration gradient until everything is equal on both sides of the tubing, which creates osmotic pressure (the force driving the flow of the solvent).

Detailed explanation:
From the problem we can assume that the small ions Na and Cl will fit through the small holes in your dialysis tubing but the larger glucose molecules will not. We can also assume the solvent (the liquid that everything is dissolved in, maybe water) is also able to travel freely through the pores. A fundamental principle of fluid dynamics is that when you bring 2 fluid systems into contact they are going to want to reach an equilibrium state where the amount of stuff (energy or temperature, ions or molecules etc) is shared equally throughout the 2 fluids. So as soon as you bring the fluid inside the tubing into an environment with a different fluid outside the tubing, the system (everything both inside and outside) will want to start moving toward an equilibrium state.

IF you only had Na, K and water (no glucose) then pretty quickly there would be no difference between the fluid in the tubing or outside the tubing, if it was left sitting for a while. But if you kept dumping out the fluid that was outside the tubing and replacing it with fresh water, then each time, more Na and K will leak out, and you will keep dumping it out, so eventually it will be present at very dilute concentrations inside the tubing.

The presence of the glucose trapped inside the tubing means that there will be a permanent amount of glucose particles inside the tubing, and on average there will be less water inside the tubing. It might seem like the glucose will push all the water out of the tubing (like people getting pushed off a crowded bus) but that is NOT what happens at the molecular level. Instead, the solvent (ie water) wants to be present at the same concentration everywhere in the system, so if there is more glucose in the tube, the water will want to travel IN toward the glucose so that the contents of the tube are just as dilute as the fluid outside the tube. So glucose contributes to fluid entering the tubing, changing the total volume of fluid inside the tubing. The opposite could also happen - if you start floating your tube in a fluid that has a lot more stuff dissolved in it (say a sugar solution) then water will flow out of the tubing so that it can try to make everything have the same concentration. Note that this desire of the solvent to move toward a concentration equilibrium is called an osmotic pressure. Why do you care about keeping the osmotic pressure balanced? If you don't pay attention to this, you can end up reducing the water content in something by drawing all the water out of it (which is what happens when you put sugar on strawberries and they make a syrup) or expanding the water content inside of it, which is what happens if you put red blood cells into a pure water solution - they swell up and burst (lyse) from water rushing in to 'dilute' their insides. We don't want the amount of fluid inside your dialysis tubing to swell up or shrivel.

At any rate, by setting up your system so that you have continuously fresh fluid moving outside the tubing, you will draw out the salt ions that are able to move without altering the amount of glucose. A counter-current (fluid flowing past in opposite directions) simply makes this process more efficient. By balancing the osmotic pressure so that there is no net flow of solvent (ie water) between compartments, you will keep the volume of fluid inside the tubing at a fixed amount, which is good because you probably don't want to end up making your glucose solution more or less concentrated.