Understanding how chlorine works to sanitize and oxidze pool water. Terry Arko- Hasa Pool
Chlorine is the primary chemical used in swimming pools to keep them safe for swimmers. Did you know that there are different types of chlorine that can be formed in the pool water? I am not referring to the different compounds of chlorine that are available to be purchased at pool supply stores or hardware stores. Most pool owners and pool service techs know that there are different types of chlorine from liquid , tablets and granular. Also, these differing compounds are comprised of various by products. The three main types of chlorine compounds available for pool treatment are:
- Calcium hypochlorite- Also known as Cal-Hypo by product is calcium available in tablets and granular shock.
- Tri-Chlor Tablets - by product is cyanuric acid CYA - 6 ppm of CYA for every 10 ppm of chlorine.
- Liquid Sodium Hypochlorite 12.5% - by product is sodium chloride
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Whatever type of chlorine you choose to sanitize when added to water the immediate chemical reaction is the same. As a result of chlorine's reaction to water, other chemicals and contaminants many differing species of chlorine can exist in pool water. It is most important to know the three main test of chlorine that are needed in order to know how to obtain the best pool water quality. There are three forms of chlorine you need to be aware of when you test your pool water.
- Total Chlorine TC
- Free Chlorine FC 2-4 ppm
- Combined Chlorine CC 0-0.2ppm
Free chlorine FC is the type we focus on the most. FC consists of two main constituents, Hypochlorous acid HOCl and Hypochlorite ion -OCl. When chlorine is added to pool water it creates HOCl. This amazing molecule has 99% ability to kill germs and algae. Contact with water causes some HOCl to disassociate into Hydrogen H+ and OCl-. The OCl- is an ion and has only 1% ability to kill germs and algae. Together HOCl and OCL- make up FC. The standard recommendation for FC is 2-4 ppm. A free chlorine test measures both HOCl and OCl-. However, the FC test cannot tell the percentage HOCl or OCl- in the water. Since the primary purpose for chlorine is to keep the water safe from germs and algae the highest percent of FC should be HOCl. The most effective way to produce the highest percent of HOCl is by pH and temperature of the water. The lower the pH and water temperature the more HOCl produced. That is good. The higher the pH and water temperature the less HOCl and more OCl- produced. That is bad. At a pH of 7.5 and water temperature of 68 degrees there is 55% of HOCl produced. At a pH of 8 and water temperature of 86 degrees the HOCl goes down to 24 percent. Water temperature can’t always be controlled in the summer. This is one good reason to run the pH slightly lower in the hot season. You can produce 55% HOCl in water with a temperature of 86 degrees by adjusting pH down to 7.4.
Total chlorine TC is a measurement of both FC and CC. This test can indicate the presence of chlorine in the water, but it cannot tell you how much FC or CC is present. Relying on a TC test only when trying to maintain water quality is ineffective. An OTO test method is ineffective for determining whether the water in the pool is safe or not. The OTO only tells you there is chlorine present however it cannot tell you what form of chlorine is most prevalent. This is why it is important to have a way to test TC and FC both.
Combined Chlorine CC is another way of saying chloramines. This is chlorine that has combined with contaminants in the water to form nitrogen bound chlorine. Not good. CC is a poor disinfectant for pools and leads to irritating chlorine gas odors, particularly in indoor pools. This is why 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 0.2 ppm. One test method used to measure both FC and CC down to 0.2 ppm is a FAS-DPD test. There are also 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 present in the pool water. For the most ideal water quality the FC should be maintained ALWAYS higher than the CC.