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What solute won't leak through dialysis tubing

  1. Dec 8, 2016 #1
    I purchased dialysis tubing from carolina medical supplies with a molecular weight cutoff of 12,000 to 14,000 MW. I've been using a sugar solution to demonstrate osmosis but I've noticed that if I leave it alone for an hour or two the sugar begins to leak through the membrane. Is there anything else I could use instead of sugar that wouldn't leach out while still being a good solute to demonstrate osmosis and osmotic pressure?
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  3. Dec 8, 2016 #2

    Fervent Freyja

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    Is the solution moreso clinging to the membrane than leaking/pouring out? I think there is a slight siphoning effect seen which is caused by gravity; the clinging or gathering of solution can sometimes plug pores and flow back into the membrane after a while. There is no real loss of solution if it is still clinging to the membrane. Have you measured the real loss, or are you going by appearance?
  4. Dec 8, 2016 #3


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    12,000 to 14,000 MW cut off means that molecules smaller than 12,000-14,000 Da will be able to pass through the membrane. Most sugars are well below this cutoff (e.g. table sugar [sucrose] is 342 Da). You would need to use some type of molecule (likely some type of polymer) with a molecular weight much greater than 14 kDa (the general guideline if you don't want your molecule to escape is to use a MWCO that is one third of the size of your macromolecule). For your purposes, something like a solution of PEG 20k or some other high MW polymer might work.
  5. Dec 8, 2016 #4
    I know it's leaking out because if I move the membrane I can see the sugar in the water (it distorts the light since it makes some of the water denser) and using a lot of table sugar changes the color of the water and when I wait longer I can see the fresh water starts to change color too. And yeah, it's strange because it does take a while to flow through (much less than the water) so I'm assuming it might be the water molecules clinging too it that makes the sugar in affect bigger when they're dissolved but still small enough to slowly make it through the membrane
    Last edited: Dec 8, 2016
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