Core Practical: Investigate the effect of pH on enzyme activity

The idea

This core practical tests how changing pH affects the rate of an enzyme-catalysed reaction. A common method uses amylase to break down starch: set up buffered solutions of different pH, add amylase and starch, and test samples at intervals with iodine solution. Iodine turns blue-black while starch remains and stays orange-brown once starch is gone, giving a time to completion; a shorter time means a faster rate. Control variables include temperature, volumes and concentrations of starch and enzyme, and the same time intervals. Record results in a table, calculate a rate such as 1 ÷ time, plot rate against pH and identify the optimum. Explain the pattern using active-site shape.

What earns the marks

• Identify the independent variable as pH, controlled using buffer solutions, and the dependent variable as a measure of rate such as time taken for starch to be digested.
• Describe a valid method: add enzyme to buffered substrate, sample at regular intervals and test with iodine, recording the time when the iodine no longer turns blue-black.
• Identify control variables such as temperature, volumes and concentrations of enzyme and substrate, and keep them constant to make the comparison fair.
• Process results by calculating a rate (for example 1 ÷ time) and plotting a graph of rate against pH to identify the optimum pH.
• Explain the results in terms of active-site shape: activity is highest at the optimum pH and falls as pH moves away because the active site is altered.

Common mistakes and how to put them right

• Treating pH as the dependent variable.

  Put it right by stating that pH is changed deliberately (independent) and the rate or time is measured (dependent).

• Failing to control temperature.

  Put it right by keeping temperature constant, since temperature also affects enzyme activity and would confound the pH result.

• Recording 'time to clear' without explaining what is measured.

  Put it right by stating that iodine no longer turning blue-black shows starch has been digested.

Exam tips

• State clearly which variable is changed, which is measured and which are controlled before describing the steps.
• Explain how the iodine result is interpreted, linking colour change to the presence or absence of starch.
• When analysing data, calculate a rate and describe the trend, identifying the optimum pH from the graph.