Measuring Rate of Reaction

The rate of an enzyme-controlled reaction is the change in the amount of substrate or product per unit time. It can be measured by following either:
• how quickly a product appears, for example collecting the oxygen released when catalase breaks down hydrogen peroxide, using a gas syringe or an upturned measuring cylinder
• how quickly a substrate disappears, for example the loss of starch when amylase acts, tested with iodine solution (blue-black to orange-brown), or measured with a colorimeterIf the amount of product is plotted against time, the curve is steepest at the start and then levels off. The rate falls because substrate is used up, so its concentration is highest at the start. For this reason we compare the initial rate.To find the initial rate, draw a tangent to the curve at time zero and calculate its gradient (change in y ÷ change in x).If the time taken to reach an end point (such as starch disappearing) is measured, the rate can be expressed as 1/t.You should control variables, such as temperature (water bath), pH (buffer) and volumes and concentrations of solutions, and repeat each measurement.Worked example: 24 cm³ of oxygen is collected in 60 s. Mean rate = 24 ÷ 60 = 0.4 cm³ s⁻¹. If starch disappears in 50 s, rate = 1/50 = 0.02 s⁻¹.