You can investigate the effect of pH on an enzyme controlled reaction by carrying out a reaction at different pH's and timing how long it takes for the product to form. The practical detailed in the specification involves the breakdown of starch to maltose by amylase. It uses iodine, which turns blue-black in the presence of starch.

• A drop of iodine is put in each well in a spotting tile
• Using a water bath or electric heater, warm a solution of amylase, starch and a buffer solution (this is the independent variable, so is changed each time you repeat the experiment)
• At regular points in the experiment, take drops of the solution and place in the wells
• The starch is no longer present and has been completely broken down when the iodine solution remains brown as opposed to blue-black
• The time for this to occur is recorded and the rate is calculated from the equations: 1000/time
• The experiment should be repeated at different pH values while controlling all other factors which may affect the rate, such as temperature

In experiments where you are measuring how much of a product forms over time or how much of a reactant is used up, you should calculate the rate using the equation: rate = change/time