A student is investigating the effect of pH on the rate of reaction between sodium thiosulfate solution and hydrogen peroxide solution. They are using a timer to measure the time taken for the reaction to go to completion. Explain the importance of controlling the pH of the hydrogen peroxide solution in this investigation.

Controlling the pH of the hydrogen peroxide solution is essential because the rate of the reaction with sodium thiosulfate is highly dependent on the proton concentration. In acidic solution the peroxide is protonated to form H₂O₂⁺, which reacts more readily with thiosulfate, giving a faster rate. In basic solution the reaction is slower because the peroxide is deprotonated and the thiosulfate is less nucleophilic. By keeping the pH constant the student ensures that any change in reaction time is due to the variable being studied (e.g. concentration) rather than an unintended change in proton concentration. This allows the rate data to be compared accurately and the kinetic law to be determined reliably.