Residual most often refers to a pool's chlorine residual, which is the amount of measurable chlorine that remains in the water after it has been treated with a chlorine product.
The following types of chlorine residuals can be measured in pool water:
- Free available chlorine residual includes hypochlorous acid (HOCl), the strongest disinfectant form of chlorine, and hypochlorite ion (OCl-), which is much less effective as a disinfectant.
- Combined chlorine residual measures the chloramines in the water; these are roughly 100 times less effective as disinfectants than free available forms of chlorine.
- Total available chlorine residual is the sum of the free and combined chlorine residuals.
Test kits measure the free available chlorine and total available chlorine residuals. To calculate the combined chlorine residual, simply subtract the free available chlorine from the total available chlorine.
Note: the term "residual" can also sometimes refer to the water's remaining concentration of another chemical, such as cyanuric acid. Thus, it is important to determine which chemical's residual a text is referring to.