What solute won't leak through dialysis tubing

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    Osmosis
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SUMMARY

The discussion centers on the use of dialysis tubing with a molecular weight cutoff (MWCO) of 12,000 to 14,000 Da, specifically addressing the leakage of sugar solutions through the membrane. Users have observed that common sugars, such as sucrose (342 Da), can pass through the membrane due to their molecular weight being below the cutoff. To effectively demonstrate osmosis without leakage, it is recommended to use high molecular weight polymers, such as PEG 20k, which exceed the MWCO threshold. The phenomenon of clinging molecules and potential siphoning effects were also noted as contributing factors to the observed behavior of the solution.

PREREQUISITES
  • Understanding of molecular weight cutoff (MWCO) in dialysis membranes
  • Basic knowledge of osmosis and osmotic pressure
  • Familiarity with polymers, specifically polyethylene glycol (PEG)
  • Experience with experimental setups involving dialysis tubing
NEXT STEPS
  • Research the properties and applications of polyethylene glycol (PEG) in laboratory settings
  • Explore alternative high molecular weight solutes for osmosis demonstrations
  • Investigate the effects of temperature and gravity on solute movement through membranes
  • Learn about the design and selection of dialysis membranes based on molecular weight cutoffs
USEFUL FOR

This discussion is beneficial for biology educators, laboratory technicians, and students conducting experiments on osmosis and membrane permeability, particularly those utilizing dialysis tubing in their studies.

AamsterC2
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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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Is the solution more so 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?
 
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.
 
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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
 
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