For liquid phase reactions, a small amount of the reaction mixture is withdrawn and the reaction is stopped either by quenching the reaction with a suitable solvent, or freezing the sample in a freezing mixture. This method is repeated with all the samples that have to be collected at intervals of time. Then the concentrations in the samples are determined with a suitable analytical technique that monitors any of the properties mentioned above. For e.g., hydrolysis of ethyl acetate in acidic medium results in the formation of acetic acid and ethanol as given by the equation,
For this reaction, the increase in the concentration of acetic acid is determined by measuring the pH of the solution (sample). The concentration values obtained at different times are then plotted. From the graph of concentration versus time, the average rate and instantaneous rates of the reaction are obtained.
