This is a testing liquid for the measurement of sanitizer using the Titration (drops) method.
DPD Reagent #1 is the first reagent in a series of three used to determine chlorine and bromine levels. Maintaining your hot tub at the correct chlorine and bromine level keeps your water clear, free of bacteria and helps extend the life of your hot tub equipment.

The optimum bromine level is 3–5 parts per million (ppm) and the chlorine level should stay between 1.0 and 3.0 parts per million (ppm).
DPD Reagent #1 is used as part 1 of the measurement of sanitizer in the Care Free test kit.

To properly titrate (apply drops) while testing, reagent bottles need to be fully vertical during titration to ensure that the correct drop is produced.