Question

I have a salt water pool (sodium chloride). These pools are equipped with an electric cell that turns the salt in the pool into chlorine. Since the chlorine evaporates every day, why do I not have to regularly add salt to the water? Is the chlorine produced in this pool the same as the chlorine in a traditional pool?

Answer

As mentioned in your question, the salt added to the pool contains sodium chloride.  The chloride (Cl-), from sodium chloride, is necessary in order for the electric cell to convert or oxidize the chloride to chlorine (Cl2).  The chlorine then reacts with the water so at the appropriate pH it forms a  low concentration of disinfectants called hypochlorous acid (HClO) and hypochlorite (ClO-).   Once these disinfectants are used up with organics, splashed out of the pool, or some of the chlorine dissipates, then more chloride has to be added to the water as the new “chlorine source”.  In regular swimming pools that use tablets, the tablets generate the same disinfectants, hypochlorous acid (HClO) and hypochlorite (ClO-).  By the way, hypochlorite is also the active ingredient in bleach.

- Chadron Friesen