Titration experiment. Hand adjusting a burette tap to control the flow of potassium dichromate (VI) solution (orange) into a conical flask containing iron (II) ions. The potassium dichromate is being used to find out the amount of iron (II) ions in the flask. The reaction of iron (II) and potassium dichromate (VI) produces the green colour seen in the flask. The endpoint of the reaction (when all the iron (II) ions are used up) is detected using a special indicator (not seen). The volume of the potassium dichromate (VI) solution used is noted. The other concentrations and volumes are known and allow calculation of the iron (II) concentration.