Students should use a continuous sampling technique to determine the time…

The idea

This required practical investigates how pH affects the rate at which amylase digests starch. You set up a series of buffered starch–amylase mixtures at different pH values, keep temperature constant, and remove a small sample every 30 seconds. Each sample is tested with iodine reagent on a spotting tile. While starch remains, iodine turns blue-black; once digestion is complete, the iodine stays orange-brown. Continuous sampling means testing repeatedly at fixed intervals rather than testing only at the start and end, so you can identify the time when starch first disappears. That time is the end point for that pH. Comparing end points across pH values shows the optimum pH for amylase. A control without enzyme confirms that starch does not break down on its own.

What earns the marks

• State that amylase breaks down starch into simpler sugars, so starch concentration falls over time.
• Describe adding a fixed volume of starch solution and a fixed volume of amylase solution to buffer solutions of known pH values.
• Explain that a water bath or electric heater keeps temperature constant because enzyme activity is affected by temperature.
• Describe removing a sample at regular intervals, such as every 30 seconds, and testing it with iodine reagent on a spotting tile.
• Record the time at which iodine no longer turns blue-black as the time taken for starch to be completely digested.
• Compare end-point times across pH values and identify the pH giving the shortest time as the optimum.
• Include a control or reference sample to show that starch does not disappear without amylase.

Common mistakes and how to put them right

• Testing only at the start and end of the experiment.

  Put it right by taking samples at regular intervals throughout so the end point can be detected.

• Assuming the blue-black colour means digestion is complete.

  Put it right by stating that blue-black shows starch is still present, while orange-brown shows starch is absent.

• Using different volumes or concentrations of starch or amylase at each pH.

  Put it right by keeping these variables the same so pH is the only independent variable.

Exam tips

• Name the independent variable as pH, the dependent variable as time taken for starch to be completely digested, and list the control variables clearly.
• Explain why continuous sampling is used: it allows the exact end point to be detected rather than only showing whether starch remains at the end.
• When describing results, link shorter digestion time to faster enzyme activity and identify the optimum pH from the shortest time.