A Kinetic Experiment for Measuring the Concentration of Hypochlorite Used to Treat Water for Humanitarian Relief

A kinetic experiment with three applications is presented. First, the experiment introduces a simple and accurate method to measure the hypochlorite concentration in sodium hypochlorite solutions. Sodium hypochlorite solutions are widely used to chlorinate water in both low-income nations and for emergency disaster management use in developed nations. However, there is currently no inexpensive or practical way in low-income countries to measure the hypochlorite concentration of concentrated sodium hypochlorite solutions before they are diluted into drinking water. In this method, blue food coloring, baking soda and washing soda are added to a sodium hypochlorite solution. The time required for the color to fade to a specific color reference (visually detected) is recorded. The reaction time and temperature are read into a calibration table which gives hypochlorite concentrations. The results of this method typically agree with thiosulfate titration results to within about five percent. The %RSD of students’ measurements was typically two percent. A second application for this experiment is a high school-level laboratory exercise which requires minimal equipment and chemicals. The exercise demonstrates how to make a calibration curve for a hypochlorite generator and to calculate the volume of bleach needed to properly chlorinate water. The exercise has practical use in schools in both developed and low-income nations. A third application of the experiment is a more in-depth version, designed for advanced chemistry classes. In this version, students measure rate constants and use a second-order integrated rate law equation to calculate concentrations at any time.