Your instructor will prepare other pieces of dialysis tubing filled with a) distilled water as negative control and b) glucose solution as positive control for the Benedict’s Test. These bags will also be rinsed, blotted dry, and weighed, and then placed in dishes of Lugol’s iodine solution for 30 minutes. Briefly explain the purpose of negative control and positive control: negative control in an experiment uses treatment that isn’t expected to produce results, while positive control in an experiment uses treatment that is known to produce results. Your instructor will weigh the control bags and post this data. Record the “time zero” data in the table above. You will need test tubes: While waiting for the incubation period (30 minutes), prepare glass test tubes as described in steps 15 & 16. Also label the 7th test tube ‘ED’ to collect the sample from the experimental dish. Wear goggles or glasses when handling Benedict’s and Burette solutions. These reagents contain copper which is poisonous. Upon conclusion of the 30-minute period, remove 10 ml of the dish solution from right next to your dialysis bag. Transfer it to an empty test tube labeled ‘ED’. Then remove your dialysis bag from the dish with fingers, forceps, or clamp. Be careful not to puncture the bag. Very gently blot the bag with a paper towel, removing any solution clinging to the outer surface. Record the color of the bag contents (note that the color of the contents will change slowly after removing from Lugol’s solution). Using the same balance that you previously used, weigh the bag. Record your data in your table along with the color of the experimental bag solution and the control bag solutions. Also, indicate the color of the solution in your dialysis dish after 30 minutes (record here: yellowing). Was there a change in the mass over the 30-minute period? If so, did the weight increase or decrease? Increased. What was the change in weight after 30-minute incubation? Unable to do LAB. Calculate the % change (or %A) in weight using the following formula and enter this data into Table 2 (above). This calculation was introduced in Lab Ex1. To what do you attribute this change in mass? Explain: molecules moved into the bag. Your instructor will weigh the control dialysis bags at 30 minutes and post the data on the board/screen/Canvas website. Record the weights in Table 1 on the preceding page.
The Correct Answer and Explanation is:
In this experiment, negative and positive controls are used to ensure that the results of the dialysis process are reliable and interpretable.
- Negative Control: The dialysis bag filled with distilled water is the negative control. The purpose of the negative control is to provide a baseline, ensuring that any changes observed in the experimental setup are not due to external factors or handling errors. Distilled water is not expected to have any active solutes that would interact with the iodine solution or the glucose solution inside the bag. Any changes in the mass or the solution color of the negative control can be attributed to factors other than the experimental variables, such as diffusion of water into or out of the bag due to osmosis.
- Positive Control: The dialysis bag filled with a glucose solution serves as the positive control. The positive control is used to confirm that the experimental setup works as expected and that the solution inside the dialysis bag can diffuse through the dialysis tubing. In this case, glucose will react with the Benedict’s solution and potentially show a color change if the glucose has diffused into the solution surrounding the bag. This confirms that the experimental system (including the dialysis tubing and the iodine solution) is functioning correctly. The glucose solution inside the dialysis bag should exhibit a significant change in mass (an increase), as water molecules would diffuse into the bag due to osmosis, making the bag swell.
After 30 minutes, the bags are carefully removed from the iodine solution and weighed. Any changes in mass are noted. For the glucose solution, the mass is expected to increase, as glucose molecules will diffuse through the dialysis tubing, drawing water with them. This is an example of osmosis and active transport. Similarly, the color of the solution inside the bag will change, indicating that the glucose has moved out of the bag, while iodine from the solution may have diffused into the bag and reacted with the starch, changing its color.
The percentage change in weight is calculated using the formula:%Change=Final Mass−Initial MassInitial Mass×100\% \text{Change} = \frac{\text{Final Mass} – \text{Initial Mass}}{\text{Initial Mass}} \times 100%Change=Initial MassFinal Mass−Initial Mass×100
The increase in mass suggests that water moved into the dialysis bag (due to osmosis), as glucose molecules are too large to pass through the dialysis membrane. This movement of water is what contributes to the weight change observed in the experiment.
