osmosis - Why is dialysis tubing impermeable to sucrose when it should retain >12,000 Da?

While writing up a lab for biology, I realized that the dialysis tubing for sale online seemed to retain large molecules (12-14,000 Da) but didn't specify an upper bound. I learned that the tubing is impermeable to Sucrose, molecular weight 342 Da, because it's too big. This doesn't make sense to me; the site says the tubing "retains >12,000 Da molecules", but Sucrose is clearly smaller than that.

So either A) Sucrose does indeed pass through dialysis tubing and I misheard/mis-learned or B) my class used tubing with much smaller pores.

Answer

There exist many types of semi-permeable membranes (the ones used for dialysis tubing), with various pore sizes. One of the very common lab experiment on the topic of osmosis is using a sucrose solution (sucrose is cheap) and small-pores membrane, such that water and small ions (typically Na⁺ and Cl^–) can pass, but not sucrose. The one which you link to simply happens to be a variety with much wider pores.