Chlorine
Chlorine is the most widely used anti-microbial for pools. The following table lists the most commonly used forms of chlorine and some of their characteristics:
CHLORINE PRODUCT FORM CONTENT pH
Sodium Hypochlorite liquid bleach, liquid chlorine Liquid 10-12% 13-14
Dichlor granular stabilized chlorine Granular 56-62% 6-7
Trichlor tableted stabilized chlorine Tablets, Pucks & Sticks 90% 2-3
Calcium Hypochlorite granular chlorine, unstabilized Granular 47-75% 11-13
Free Chlorine is the most desirable form and is the form responsible for the actual sanitization activity in the water. It is measured using a free chlorine test kit and its level is critical in the pool. If this form is not present, little or no sanitizing can take place. Free chlorine is composed of two types of compounds: HOCl (hypochlorous acid) and OCl¯ (hypochlorite ion). They exist together in equilibrium. This means that together they make up 100% of the free chlorine content, but that content consists of some of each. For example, if 25% of the free chlorine is HOCl, then the OCl¯ level will be the other 75%. It is important to note that only the HOCl component is effective as a sanitizer. Therefore, it seems logical that we would want as much of the free chlorine as possible made up of the HOCl. However, the level of HOCl and OCl¯ present is dependent upon the pH. This is one of the critical reasons that the proper pH level in pool water is so important.
As the pH goes up or down, the relative amount of HOCl vs. OCl¯ also increases or decreases. The following chart shows how much of each of these two compounds are present at different pH levels.
pH %HOCl % of OCI¯
6.0 97 3
7.0 75 25
7.5 50 50
8.0 23 77
9.0 3 97
As the chart shows, at pH 7.5 only about half of the free chlorine exists in the desirable form of HOCl. The level of HOCl will increase as the pH goes down and it must also be pointed out that as the pH decreases, so does the stability of the chlorine. As pH rises, the stability of the free chlorine will increase, but its activity as a sanitizer diminishes. In order to get the most effective and economic benefit of chlorine, keep it in the desirable pH range of 7.2-7.8. Lower pH will be detrimental to pool surfaces and equipment, while higher levels will render chlorine ineffective as a sanitizer.
Combined Chlorine
Free chlorine is highly reactive and once added to water, quickly attacks bacteria as well as bather and other wastes. When this occurs, the chlorine is no longer considered free chlorine but rather its form has changed and is now referred to as combined chlorine. Bather and other wastes are largely made up of ammonia and nitrogen compounds. (These gasses are sometimes misunderstood to be too much chlorine). Combined chlorine is very stable, but has little or no sanitizing ability. Not only is combined chlorine a very poor sanitizer, it is the agent responsible for eye burn and skin irritation and results in the unpleasant chlorine odor often referred to as a pool with “too much chlorine”. It is therefore critical for bather health and comfort that combined chlorine be controlled and kept to a minimum. It is preferable that combined chlorine levels are kept to a maximum of 0.2 ppm.
Total Chlorine
Free chlorine + Combined chlorine = Total chlorine.
Total Chlorine – Free Chlorine = Combined Chlorine.