Rinse the crucible and cap well with test water. Fill the crucible halfway with test water. Add 5 drops of reagent T1 and mix. Add 5 drops of T2 reagent and mix. Add reagent T3 drop by drop. After each drop, mix the sample so that the colour is the same throughout. Count the number of drops of reagent T3 added. Stop the determination if the colour changes from wine red or violet to bright blue throughout the volume. Wait a few seconds with the last drops until the colour has settled.