This Iron Reagent measures levels of iron. High levels of iron can stain pool water and pool equipment and reduce chlorine effectiveness. Maintain a safe and clean swimming environment with Iron Reagent. Easy to follow instructions allow for hassle free testing. Taylor formulates its reagents to remain effective for at least one year, with only very few exceptions (molybdenum indicator in liquid form is one; after four months old it should be tested against a standard periodically). As a general precaution, replace all reagents more than one year old, or at the beginning of a new testing season.