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This question was posed a couple years ago but not answered adequately. You can view the older thread here. The cellulose membrane commonly used in high school and university labs is widely available from suppliers such as Wards, Carolina, Flinn, PASCO, etc. The specifications are either available online or were obtained by contacting the companies and in each case the molecular weight cut off (MWCO) was between 6,000 and 16,000 Da. Regardless of the specifications these products are used to conduct osmosis demonstrations by placing 0.2-1.0M sucrose on one side of the membrane and to create water potential. In addition to having conducted this exercise myself the results are validated in countless examples online. The mass/pressure results from osmosis and a benedicts test indicate sucrose is not moving across the membrane.

Considering Sucrose has a MW of ~342 Da the previous explanations seem insufficient;

1) The pore size of the membrane is variable. True, but according to specs at least 10% of the pores are larger than the specification and even with variation of 0-6,000 Da (worst case) it would still be like throwing a tennis ball through a garage door 10% of the time.

2) Interactions between solute molecules, shape, and solute-solvent causing a larger solvodynamic radius. Sucrose doesn't have any charges I'm aware of, it has a simple structure, and even covered in water molecules it would still be much smaller than the pores.

Any suggestions or alternative explanations?

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Benedict's reagent detects reducing sugars (free aldehyde group) by reducing soluble blue Cu(II) to insoluble red Cu(I) oxide. Sucrose aquoous solution is inert unless it is pre-hydrolyzed by heating with strong acid catalyst.

That certainly is a poser. One possibility (SWAG, not fact) is regenerated crosslinked cellulose is overall not permeable to neutral solutes. Its size selection is active toward dissociated solutes and proteins that are charged. One can test this with anti-freeze. The sodium fluorescene color should freely diffuse. Tiny molecule ethylene glycol (EG) should be retained.

https://www.chemetrics.com/Glycol
Test kit and how it works: Iodate cleavage of 1,2-diol to aldehyde, then aldehyde detection with Purpald.

Purpald aldehyde detection

One could iodate cleave permeated EG (or propylene glycol, glycerin, etc.) then use Benedict's solution. Serial dilution of standard EG solution for control and calibration. One expects any dialysis membrane to leak somewhat. Major passage of uncharged 1,2-dihydroxylic solutes smaller and larger than than sucrose is the SWAG's falsifying observation.

http://chemsee-foods.com/antifreeze-detection
http://www.chemworld.com/Ethylene-Glycol-Test-Kit-p/k-4815.htm
http://www.hach.com/glycol-visual-test-kit-model-eg-1/product?id=7640218461
EG in food and fluids tests.
http://www.astm.org/Standards/E202.htm
Testing standards

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