Free Chlorine-Key to Clean Pool Water

The most popular pool and spa sanitizer is chlorine. All chlorine does the same thing when it is added to the water, regardless of the form of chlorine added. It forms hypochlorous acid and hypochlorite ions. Together these two are considered free available chlorine. Hypochlorous acid is the form of chlorine that kills bacteria, algae and disease-causing organisms. Hypochlorous acid is the attack dog that guards your pool against microbiotic intruders. (In general, you wouldn’t want a dog in the pool, but this is an exception.) Hypochlorite ions are more like benevolent night watchmen.

Given that hypochlorous acid protects the pool more vigorously, we might ask ourselves how to increase its proportion relative to hypochlorite ions. The pH of the water determines the ratio of hypochlorous acid to hypochlorite ions. Lower pH levels would be the most effective in producing a higher level of hypochlorous acid, making the chlorine more efficient. However, very low pH levels would destroy the equipment and irritate swimmers. (Lower pH means the water is more acidic.) Therefore, you need to maintain pH at an appropriate level (7.2 – 7.6 is ideal) to balance the water. The closer the water is to the low end of the ideal range (7.2), the more effective the chlorine will be because there is more hypochlorous acid present.

You must maintain free chlorine at a sufficient level to disinfect potential contaminants on contact. The more chlorine in the water, the more it can sanitize and oxidize the water. (Remember that sanitizing and oxidizing are the processes that chlorine uses to keep the water clear and clean.) However, if the free chlorine level gets too high, it can make the water uncomfortable for swimmers. The trick is to keep the free chlorine level in the ideal range. In a swimming pool, keep free chlorine at a minimum of 1 ppm (parts per million) and a maximum of 10 ppm, with an ideal concentration of 1 to 3 ppm.

In spas the level needs to be maintained at a slightly higher level due to the smaller volume and higher temperature. (Chlorine is less stable in hot water and bright sunlight. That means it dissipates more rapidly from a spa. Chlorine will also dissipate from a pool, albeit more slowly.) The minimum level should be 2 ppm in a spa, again no higher than 10 ppm, and ideally 3 to 5 ppm.

Table 1: Recommended range (in parts per million)