My niece has been assigned an experiment in Junior High that requires an aquairum divided in two (2) using a semipermeable membrane. Knowing her uncle is an aquarium hobbiest (read that "junkie"), she called me up today to ask me where she can find such a membrane big enough for a 5-10 gal tank (her dad and her hadn't found anything in the online aqaurium sites), and to ask me to help her as her mentor in this Fall Semester-long experiment.

Does anyone here know where I could find a semipermeable mebrane-tank divider, or some such material that can be used in that capacity?

She also doesn't know yet what porosity it is suppose to be, so if you know of ones in a selection of "grades" / porosity, that would be even more wonderful.

Thank you VERY much for your help!

I've seen them at local fish stores. Its just a piece of semi-clear plastic with lots of small holes.

A semipermeable membrane will not have any visible holes in it. It will have pores just large enough for certain gases to pass thru but too small for water molecules to make it thru. Or, it could have pores large enough for water molecules but not for calcium or magnesium ions, etc. That means extremely small "holes". But, I don't know where to get the stuff - a google search might help find a place.

I'm really thinking this will be a tough one. the only thing that comes to mind is the Kordon Breather Bag box liner. Not sure of availability or how to seal it to the tank glass.

One way to do it may be to use a standard tank divider (or eggcrate or whatever -- its the frame you care about), then adhere the Kordon Breather box liner to it, sealing only on the edges of the bags and where they overlap. This way you can easily attach the divider to the glass, and work around the size limitation of the breather bags.

It seems to me it all depends on the purpose for the membrane. If it is to allow gas exhange, but not water exchange, that is different from allowing certain ions to exchange, but not others. I would be very interested in learning more about the experiment that will be done.

CO2 exchange membranes are slllloooowwww vs active systems.

Regards,
Tom Barr