School Science/Apparatus for demonstrating osmosis
This apparatus for demonstrating osmosis through visking tubing (otherwise known as dialysis tubing) and is widely used in school. Visking tubing is a semipermeable membrane that allows small molecules like water to pass through but does not allow larger molecules like sugar. If solutions of different concentration are on either side of the visking membrane, water molecules will pass through and tend to dilute the more concentrated solution.
The assembled equipment is inserted into hypotonic, hypertonic, and isotonic solutions and the level on the capillary rises, falls, or stays level accordingly. This apparatus is easy for school children to use, but is fiddly to set up. It can be set up by a technician in advance.
- 0.5m length of capillary tube
- 10cm of medium width visking tubing
- Bung with hole. The bung and visking tubing should be selected so that the tubing just fits over the bung.
- Rubber tubing (small and large bore)
- 0.5M sucrose solution
- Food dye e.g. cochineal.
a) capillary tube
b) rubber band
d) rubber band
e) sucrose solution with food dye
Setting up the Apparatus
- The visking tubing is softened under a running tap until it opens up. A knot should be tied at one end to make a bag.
- A bung that is just small enough to fit snugly inside the visking tubing is inserted into the unknotted end. Then a ring of a suitable bore rubber tubing is cut to make a band.This band is then slipped over the visking tubing so that it grips the tubing to the bung.
- The reason that food dye is added to the sucrose solution is to make the level in the capillary tube easy to see.
- The bag is filled completely to the brim with the coloured sucrose solution by squirting a pipette full at a time through the hole in the rubber bung. No air bubbles can remain. At this stage the knot and the attachment to the bung is checked for leaks.
- The capillary tubing is then carefully inserted into the bung. The fit needs to be tight and the sucrose should come up about half way (see diagram). If the level is too high the visking bag can be gently squeezed to force a little solution out of the top of the capillary. If the level is too low, a dialysis clip is attached just above the knot to reduce the volume of the bag.
- A ring from some narrow bore rubber tubing is cut and slippedover the capillary tube to act as a marker for the level.
Method of Use
A) Clamp the apparatus in a retort stand, then place into a large beaker of 0.5M sucrose (isotonic). Make sure the visking bag is hanging freely in the solution. I.e. not touching the sides or bottom. This stops it from being squashed and distorting the results.
B) Slide the rubber band to the top of the capillary level in order to mark the start position.Put the apparatus into 2.0M sucrose solution (hypertonic). The level in the capillary tube will fall as the water inside passes through the visking tubing in order to dilute the stronger solution outside.
C) Put the apparatus into distilled water (hypotonic). The level in the capillary tube will rise as the water passes into the visking bag to dilute the stronger solution inside.
Storage of the Apparatus
The apparatus cannot be stored for more than a few days with the sugar solution inside, because it will go mouldy. The capillary tube should be removed and thoroughly rinsed as any sugar left inside will harden and block the tube. The visking bags can be left attached to the bungs but should be rinsed and stored in water or a mixture of water and ethanol to keep the bag soft. If the visking dries out it may become brittle and crack.