Using iodine to follow starch digestion
One way to investigate a factor affecting digestive enzymes is to vary pH and measure its effect on the activity of amylase. Amylase digests starch, so the iodine test can be used to follow the disappearance of starch during the reaction.
| Iodine result | Interpretation |
|---|---|
| Strong blue-black colour | A high concentration of starch remains, indicating little or no amylase activity so far. |
| Weaker positive colour | Less starch remains, indicating that more digestion has occurred. |
| Iodine remains orange-brown | No starch is detected, so the endpoint has been reached. |
Method
- Place separate drops of iodine solution into the wells of a spotting tile.
- Label a test tube with the pH being investigated.
- Add 2 cm3 of amylase solution and 1 cm3 of the chosen buffer solution to the tube.
- Add 2 cm3 of starch solution, start the timer immediately and mix with a glass rod.
- After 10 seconds, transfer a drop of the reaction mixture to the first iodine drop.
- Clean the glass rod, then repeat the iodine test at 10-second intervals.
- Stop when the iodine remains orange-brown, showing that no starch is detected.
- Repeat the procedure using buffers at different pH values. A shorter time to the endpoint indicates faster starch digestion.
Keeping the investigation valid
Variables other than pH should be kept constant between tests. These include the volume and concentration of amylase, the volume and concentration of starch, the volume of buffer and the method of stirring. The glass rod should also be cleaned thoroughly between samples.
Judging exactly when the iodine stops changing colour can be subjective. A colorimeter can provide a more objective measure of changes during the reaction.
Exam Tip: When naming control variables involving solutions, specify both volume and concentration where appropriate rather than writing only “amount”.