The DPD tablet is the most widely used test for determining Free and Combined Chlorine in pool water. The DPD #3 Rapid tablet measures the level of Combined Chlorine. Combined Chlorine is a sanitizer that has reacted with impurities within the water and is, therefore, less capable of disinfecting pool water. It can cause eye irritation and a strong chlorine odor.
To test for Combined Chlorine, simply add a DPD #3 Rapid tablet to the Free Chlorine water sample. A measurable increase in sample color indicates undesirable Combined Chlorine, which should be treated with a chlorine or oxygen-based shock treatment.