Calculated and Measured Forms of Chlorine in Bleach at Different pH Values, A Coupled Equilibrium Experiment

An experiment is presented that uses commercial bleach to teach the concept of coupled equilibrium. The predicted concentrations of five forms of chlorine in a solution of commercial bleach are calculated as a function of pH using a spreadsheet. Students then prepare diluted bleach solutions and adjust the pH of the solutions to values ranging from about 1 to 12. Students learn how changing the pH shifts the series of coupled chemical reactions. Students then measure the concentrations of three of the forms of chlorine, Cl2 , HClO, and ClO- at different pH values by ultraviolet (UV) spectrophotometry. Since the UV spectra of the different forms of chlorine overlap, a spreadsheet is used to perform a least squares calculation to quantify the different forms of chlorine.