Why is it necessary to have positive and negative controls in this experiment?
An indicator is a substance that changes color when in the presence of the substance it indicates. In this experiment, IKI will be used an indicator to test for the presence of starch and glucose.
(5) 100 mL Beakers
*You Must Provide
- Measure and pour 50 mL of water into a 100 mL beaker. Cut a piece of dialysis tubing 15.0 cm long. Submerge the dialysis tubing in the water for at least 10 minutes.
- Measure and pour 82 mL water into a second 100 mL beaker. This is the beaker you will put the filled dialysis bag into in Step 9.
- While the dialysis bag is still soaking, make the glucose/sucrose mixture. Use a graduated pipette to add five mL of glucose solution to a third beaker and label it “Dialysis bag solution”. Use a different graduated pipette to add five mL of starch solution to the same beaker. Mix by pipetting the solution up and down the pipette six times.
- Using the same pipette that you used to mix the dialysis bag solution, remove two mL of that solution and place it in a clean beaker. This sample will serve as your positive control for glucose and starch.
- Dip one of the glucose test strips into the two mL of glucose/starch solution in the third beaker. After one minute has passed, record the final color of the glucose test strip in Table 3. This is your positive control for glucose.
- Use a pipette to transfer approximately 0.5 mL of IKI to into the two mL of glucose/starch solution in the third beaker. After one minute has passed, record the final color of the glucose/starch solution in the beaker in Table 3. This is your positive control for starch.
- Using a clean pipette, remove two mL of water from the 82 mL of water you placed in a beaker in Step 2 and place it in a clean beaker. This sample will serve as your negative control for glucose and starch.
- Dip one of the glucose test strips into the two mL of water in the beaker. After one minute has passed, record the final color of the glucose test strip in Table 3. This is your negative control for glucose.
- Use a pipette to transfer approximately 0.5 mL of IKI to into the two mL of water in the beaker. After one minute has passed, record the final color of the water in the beaker in Table 3. This is your negative control for starch.
Note: The color results of these controls determine the indicator reagent key. You must use these results to interpret the rest of your results.
- After at least 10 minutes have passed, remove the dialysis tube and close one end by folding over 3.0 cm of one end (bottom). Fold it again and secure with a rubber band (use two rubber bands if necessary).
- Make sure the closed end will not allow a solution to leak out. You can test this by drying off the outside of the dialysis bag with a cloth or paper towel, adding a small amount of water to the bag, and examining the rubber band seal for leakage. Be sure to remove the water from the inside of the bag before continuing.
- Using the same pipette which was used to mix the solution in Step 3, transfer eight mL of the solution from the Dialysis Bag Solution beaker to the prepared dialysis bag.
|Figure 4:Step 9 reference.|
- Place the filled dialysis tube in beaker filled with 80 mL of water with the open end draped over the edge of the beaker as shown in Figure 4.
- Allow the solution to sit for 60 minutes. Clean and dry all materials except the beaker with the dialysis bag.
- After the solution has diffused for 60 minutes, remove the dialysis tube from the beaker and empty the contents into a clean, dry beaker. Label it dialysis bag solution.
- Test the dialysis bag solution for the presence of glucose and starch. Test for the presence of glucose by dipping one glucose test strip into the dialysis bag directly. Again, wait one minute before reading the results of the test strips. Record your results for the presence of glucose and starch in Table 4. Test for the presence of starch by adding two mL IKI. Record the final color in Table 4 after one minute has passed.
- Test the solution in the beaker for glucose and starch. Use a pipette to transfer eight mL of the solution in the beaker to a clean beaker. Test for the presence of glucose by dipping one glucose test strip into the beaker. Wait one minute before reading the results of the test strip and record the results in Table 4. Add two mL of IKI to the beaker water and record the final color of the beaker solution in Table 4.
|Table 3: Indicator Reagent Data|
|IKI Solution||Dark Purple||Black||n/a||n/a|
|Glucose Test Strip||n/a||n/a||Light Green||Yellow|
|Table 4: Diffusion of Starch and Glucose Over Time|
|Indicator||Dialysis Bag After 1 Hour||Beaker Water After 1 Hour|
|Glucose Test Strip||Purple|
1. Why is it necessary to have positive and negative controls in this experiment?
2. Draw a diagram of the experimental set-up. Use arrows to depict the movement of each substance in the dialysis bag and the beaker.
3. Which substance(s) crossed the dialysis membrane? Support your response with data-based evidence.
4. Which molecules remained inside of the dialysis bag?