Diffusion and Osmosis

In this experiment you will measure diffusion of small molecules through dialysis tubing, an example of a selectively permeable membrane. Small solute molecules and water molecules can move freely through a selectively permeable membrane, but larger molecules will pass through more slowly, or perhaps not at all. The movement of a solute through a selectively permeable membrane is called dialysis. The size of the minute pores in the dialysis tubing determines which substances can pass through the membrane.

A solution of glucose and starch will be placed inside a bag of dialysis tubing. Distilled water will be placed in a beaker, outside the dialysis bag. After 30 minutes have passed, the solution inside the dialysis tubing and the solution in the beaker will be tested for glucose and starch. The presence of glucose will be tested with Benedict's solution. The presence of starch will be tested with Lugol's solution, (Iodine Potassium-Iodide or IKI). For the Lugol test, a dark blue color is a positive result.

Procedure

1. Obtain a 10 cm piece of 2.5-cm dialysis tubing that has been soaking in water. Twist one end of the tubing and fold it over, then tie off that end with string to form a bag. To open the other end of the bag, rub the end between your thumb and forefinger until the sides separate. The bag might be stubborn so be persistent.

**IMPORTANT** You will be testing both the contents of the bag and the beaker for both starch and glucose at the beginning of the 20 minute experiment and at the end. To complete the data table, the term "initial" refers to conditions before the 20 minutes while "final" refers to conditions after the 20 minutes.

2. Use a graduated cylinder to obtain approximately 10 mL of the 15% glucose/1% starch solution and place it in the bag. Tie off the other end of the bag, leaving a bit of space for the expansion of the contents in the bag. Record the color of the solution in Table 1 as initial color of the Lugol's test.

3. Use the Benedict's test to confirm the presence of glucose in the 15% glucose/1% starch solution.

- place 5 mL Benedict's reagent in a small test tube
- add 8 drops of sample
- boil vigorously for 1-2 min
- the formation of a precipitate means that glucose is present.

Record the results in Table 1 as initial color of the Benedict's test.

4. Place approximately 100 mL of distilled water in a 250 mL beaker. Add approximately 3 mL of Lugol's solution to the distilled water and record the color in Table 1 as initial color of Lugol's test. Test this solution for glucose and record the results in Table 1 as initial color of Benedict's test.

5. Immerse the bag in the beaker of solution.

6. Allow your set-up to stand for approximately 20 minutes or until you see a distinct color change in the bag or in the beaker.

7. Withdraw a sample from the beaker and test it for glucose. Record the results in Table 1 as final color under Benedict's test. Figure out a way to test for the presence of starch and record the results.

8. Remove the bag and cut it open at the top. Test the solution inside for glucose and record the results in Table 1 as final color of Benedict's test. Figure out a way to test for the presence of starch and record the results.

Table 1