To 150 mL of saturated chlorine water in a 500 mL conical flask, add some glass beads, and pipette 5 mL of the Wijs solution into the flask. Shake, heat to boiling, and boil briskly for 10 min. Cool, add 30 mL of 2% sulfuric acid and 15 mL of 15% potassium iodide solution, and mix well.