I purchased dialysis tubing from the Carolina company.

The molecular cut off is 12,000-14,000 Daltons and I've been using table sugar for my experiments with it to raise columns of water. While the sugar does work it slowly leaks out of the membrane over time if it's left to sit for an hour or so. What should I use if I want near 100% of the solute to stay within the dialysis tubing? Preferably something cheap and non-toxic that will also dissolve in water. Or maybe a different type of membrane I could purchase that would retain the sugar better?


1 Answer 1


Polyethylene oxide, AKA polyethylene glycol or PEG has MW > 100,000. You can find higher-MW PEG in disposable diapers and similar absorbent products.

You could also use agar, pectin or gelatin, available in groceries and from lab supply companies.

One of my favorite demos was to make an agar solution with a bit of phenolphthalein added and put it in sausage casing (AKA dialysis tubing). Holding the hotdog over an open bottle of ammonium hydroxide turned it red, and over hydrochloric acid, back to a clear frank.

  • $\begingroup$ Thank you very much for the suggestions! Could they be used for the experiment to raise a column of water? $\endgroup$ Commented Dec 9, 2016 at 3:34
  • $\begingroup$ Sure... but for thick solutions, the rate is slow, and might need a day or two undisturbed. Also, remember that larger molecules have less osmotic effect because, for the same weight of solute, there will be fewer mols. $\endgroup$ Commented Dec 11, 2016 at 1:00
  • $\begingroup$ Then is there any membrane I could use for sugar, salt or other small molecules? I'd like it to work relatively quickly. $\endgroup$ Commented Dec 12, 2016 at 1:13
  • $\begingroup$ There are membranes capable of separating water from salts, used for reverse osmosis desalination, e.g. appliedmembranes.com/reverse-osmosis-membranes.html, and there's a guide for selecting higher-mw membranes: spectrumlabs.com/dialysis/dtParms.html. These may be a bit pricey compared to cellophane sausage casing, though. Try experimenting with the membrane you have before investing a lot in others. $\endgroup$ Commented Dec 12, 2016 at 1:29
  • $\begingroup$ Thank you for the resources! I found that website before but didn't realize they had roles of membrane, and the other website is great as well. Thank you for answering all of my questions $\endgroup$ Commented Dec 13, 2016 at 2:40

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