This pH Phenol Red Indicator Solution is for use with Taylor Residential Series Comparators (sold separately). It measures levels of pH. Proper pH maintenance helps to maintain water, keeping swimmers comfortable in the pool and protecting pool equipment. 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.