This reagent reacts with chlorine, causing the water to turn red. It's a reagent that indicates when to replace a water purifier (cartridge). It serves as a guide for when to replace the cartridge. This reagent can be used for tap water and the chlorine (free residual chlorine) in water purifiers. Measure by putting one whole packet into 10cc of water. Water from a water purifier: After adding the reagent and stirring ⇒ If it turns pink to pink-red, it contains residual chlorine, indicating it's time to replace the cartridge. This reagent can be used for tap water and the chlorine (free residual chlorine) in water purifiers. Tap water: After running tap water for about 30 seconds, add the reagent and stir ⇒ If it turns pink to pink-red, it's okay. You can check the chlorine concentration of your tap water and bath water.