Free chlorine (FC) test
FC is the test experts focus on the most. It consists of two main constituents, hypochlorous acid (HOCl) and hypochlorite ion (OCl-). When chlorine is added to pool water, it creates HOCl. This molecule can kill 99 per cent germs and algae. Contact with water causes some HOCl to disassociate into hydrogen (H+) and OCl-, the latter is an ion with only one per cent ability to kill germs and algae.
Together HOCl and OCl- make up FC. The standard recommendation for FC is 2 to 4 ppm. The FC test measures both HOCl and OCl-; however, the test cannot tell the percentage of either in the water. Since the primary purpose for chlorine is to keep the water safe from germs and algae, the highest per cent of FC should be HOCl.
The most effective way to produce the highest per cent of HOCl is by maintaining the pH and temperature of the water. The lower the pH and water temperature, the more HOCl is produced, which is good. The higher the pH and water temperature, the less HOCl and more OCl- is produced, which is bad.
At a pH of 7.5 and water temperature at 20 C (68 F), 55 per cent of HOCl is produced. At a pH of 8.0 and water temperature at 30 C (86 F), HOCl goes down to 24 per cent. Water temperature cannot always be controlled in the summer, which is good enough reason to run the pH slightly lower in the hot season. Pool owners can also produce 55 per cent HOCl in water with a temperature of 30 C (86 F), by adjusting pH down to 7.4.
Combined chlorine (CC) test
CC is another way of saying chloramines. This chlorine is combined with contaminants in the water to form a nitrogen-bound chlorine. It is a poor disinfectant for pools and leads to irritating chlorine gas odours, particularly in indoor pools. Therefore, it is vital to test both TC and FC. By doing so, the level of CC in the water is determined. Subtracting FC from TC gives the number for CC in the pool. The ideal level of CC in pools is zero; the maximum acceptable level is 0.4 ppm. A more preferred level is no more than 0.2 ppm.
One test method used to measure both FC and CC down to 0.2 ppm is a FAS-DPD test. There is also a photometric test for determining TC, FC, and CC. For the safety of swimmers, and to keep algae from growing, it is vital to know what type of chlorine is dominant in the pool water.
