It is free, available chlorine that does the hard work of killing bacteria and oxidizing contaminants. When the free chlorine combines with these contaminants, such as ammonia, soap and other nitrogen-containing organic compounds, we call it combined chlorine, or chloramines. In pool and spa water, this form of chlorine has very little sanitizing ability, and no oxidizing ability. You can think of combined chlorine as a spent bullet.
Water treatment plants often use chloramines to treat drinking water because chloramines are more stable than free chlorine, and they last longer. Those chloramines are specially developed for treating drinking water, and as a result they display very little odor.
However, free chlorine has a much bigger job in swimming pools and spas because of all the nasty things that can make their way into the water. When free chlorine combines with different contaminants like human waste, the resulting chloramines put out a foul odor. Combined chlorine can also irritate the skin and eyes of bathers and swimmers. Therefore, when the combined chlorine concentration is greater than 0.2 ppm (parts per million), you need to shock (superchlorinate) the pool or spa. Superchlorinating will oxidize the combined chlorine and get rid of the contaminants for good.
There is no convenient, direct testing method that measures combined chlorine. Instead, we often have to start with total chlorine, which we can easily measure. Total chlorine is just the sum of combined chlorine and free chlorine. In other words,
(combined chlorine) = (total chlorine) minus (free chlorine)
For example, a test that yields a total chlorine level of 3 ppm and a free chlorine level of 1 ppm indicates a combined chlorine level of 2 ppm (3 ppm total - 1 ppm free = 2 ppm combined chlorine). In the immortal words of Dr. H. Tueau, "Time to shock that sucker!"
There are other indicators that suggest the presence of combined chlorine. Your nose is a good place to start. As we mentioned above, combined chlorine can emit an odor bad enough to make your eyes water. This odor is often misinterpreted. Many swimmers and pool operators assume that the "strong chlorine smell" means there is too much chlorine in the water. In reality, that smell probably indicates that the pool needs more active chlorine, not less. Consider the smell of household bleach, which contains a greater concentration of chlorine than any swimming pool or spa should have. The bleach still smells fresh and not so strong. On the other hand, the odor of chloramine is so strong and unpleasant that your nose will complain many yards away from an average swimming pool.
In addition to the awful smell, your body may detect the presence of combined chlorine in the form of skin and eye irritation. Some people who have experienced this irritation have thought they were allergic to chlorine. Instead, it is more likely that they were sensitive to the buildup of chloramines. There was probably not enough free chlorine present to sanitize the water and oxidize the combined chlorine. This is especially true in spas and hot tubs where the sanitizer residual is used up quickly.
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)