Six types of chlorine, or chlorine compounds, are used in the sanitization of swimming pool water. They are sodium hypochlorite, calcium hypochlorite, lithium hypochlorite, chlorine gas, and stabilized compounds trichlor and dichlor.

“There isn’t one that is necessarily the ‘best,’” says Alison Osinski, Ph.D., owner of Aquatic Consulting Services in San Diego. “Choices should be made on costs, storage capabilities and so forth. But the one important determining factor should be the source water. For example, you don’t want to use calcium hypochlorite in an area where the water is rich in calcium, such as California.”

Here is an at-a-glance look at each type:

  • Sodium hypochlorite
  • Often referred to as “bleach” by service technicians, sodium hypochlorite is a liquid and the most common pool disinfectant. It has a pH of 13, so the water often will need correcting by adding muriatic acid to balance its impact. This form of chlorine also will increase the water’s total dissolved solids due to its sodium chloride content (salt), but it has no negative impact on disinfection.

    Sodium hypochlorite is not stable and eventually will lose strength over a period of time. It’s recommended that the product be kept in a cool, dark place. “It’s relatively safe to handle, but where you store it is important,” Osinski says. “You should also wear protective clothing when handling it.”

    Sodium hypochlorite is a liquid, so it must have secondary containment. This may make it bulky and problematic if you have limited storage area. Due to its high sodium content, it also can drive up TDS levels in the pool.

  • Calcium hypochlorite
  • Another popular pool sanitizing choice, calcium hypochlorite is a dry chlorine available in granular and tablet form. Techs often refer to it as “cal hypo.” Because of its high-chlorine content, it’s often used to superchlorinate pool water. Its pH is also fairly high (8.5 to 11).

    Osinski recommends mixing it with water first (a commercial pool requirement) rather than broadcasting the granules into residential pools. Cal hypo is not that soluble, and can settle on pool bottoms and walls, causing damage.

    One drawback is that its high calcium level can dramatically increase calcium in pools and spas. This creates scale on surfaces, as well as in filter and heating components. In addition, it’s considered a Class 3 oxidizer by the National Fire Protection Association, and can accelerate combustion and be ignited if heated or contaminated.

    “You don’t want to get it wet when you are storing it,” Osinski says. “Manufacturers are trying to develop it with additives that will keep it from burning. If you follow the guidelines, you will be OK. But with an untrained staff, this is not necessarily the product I want them working with.”

  • Lithium hypochlorite
  • A granular product, lithium hypochlorite’s solubility makes it an ideal choice for superchlorinating. However, its low chlorine content and high cost make it a poor choice for many service companies, especially those charged with maintaining commercial pools.

    “It’s true that the cost is high,” Osinski says. “But it’s a good product, about 35 percent available chlorine. You can also broadcast it in residential pools, so it’s often used in vinyl pools.”

  • Chlorine gas
  • Chlorine gas has 100 percent available chlorine, which is considered a pro and a con.

    It is an excellent disinfectant and has a relatively low price. But the cost of maintaining equipment, rising insurance premiums, training requirements and high toxicity levels has significantly reduced its use.

    Of the items on this list, it’s the only one that is true elemental chlorine. The rest are compounds.

    “You need a high level of training to handle chlorine gas,” Osinski says. “It’s disappearing because of the regulations associated with its transportation and use. It’s acidic with a pH of 1 or less, so you’ll be raising the pH all the time.

    “On the other hand, you won’t have any other inert ingredients added to mess up the water,” she adds.

  • Trichlor
  • Trichlor is available in tablet or stick form, and usually used in conjunction with an erosion feeder. It has a high level of available chlorine. But with a pH between 2.8 and 3.5, it will lower the pool water’s pH and total alkalinity. Sodium carbonate will be needed to correct the problem.

    Trichlor, which is pre-stabilized and has slow dissolution properties, is one of the most popular compounds for sanitizing residential pools. However, Osinski says the pre-stabilization factor is a blessing and curse. The chlorine residues will last longer, but also will increase CYA levels, which will need to be reduced by partial draining and refilling.

    “It shouldn’t be used indoors or in high bather load pools,” she says. “Pre-stabilization slows down the chlorine’s reaction, and you don’t want that in busy pools. It is, however, easy to store and has a long shelf life.”

  • Dichlor
  • Unlike other forms of pool chlorines, sodium dichlor has nearly neutral pH of 6.7. Dichlor, which is a salt, is soluble and often used in superchlorinating vinyl-liner pools.

    This compound comes in two forms: anhydrous, which has a higher level of available chlorine and therefore is more hazardous for storing (Class 3 oxidizer), and dehydrate, which has a lower level of available chlorine. This merits a lower NFPA hazard classification (Class 1 oxidizer).

    “A positive is that it is instantly soluble, so there are no residues or cloudiness,” Osinski says. “The problems arise in improper applications. Don’t use it in pressurized erosion feeders. It dissolves rapidly and that could cause an explosion. In residential pools, it’s best mixing it up with water and then adding it,” she notes. “But remember, chemicals always go in the water for mixing, not the other way around.”


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