Adobe stock

In order to protect customers from harmful microorganisms, pool-care experts and pool owners alike should have an understanding of sanitization. Sanitizers protect swimmers by killing pathogens introduced from sources such as rain, source water, swimmers, and nearby vegetation.

Chlorine is the most commonly preferred among these products. Demand for granular and tableted types will likely remain high in 2021. To meet this need while working with expected shortages, improving chlorine’s efficiency will be essential to satisfying customers. Professionals can help maximize chlorine’s efficiency through such factors as chemical balance, considering the characteristics of the sanitizer program of choice, and incorporating supplemental products into maintenance routines will. If this is done correctly, customers can safely enjoy their backyard investments.

Why chlorine is effective

Chlorine has been the preferred disinfectant for drinking and recreational water since the early 1900s. It kills quickly because it is toxic to many organisms.

When chlorine is added to pool or spa water, hypochlorous acid (HOCl) forms, along with a negatively charged hypochlorite ion (OCl-). Hypochlorous acid, the most active killing form of chlorine, has a neutral charge, allowing it to easily penetrate the negatively charged cell walls of most microorganisms.

The most popular testing methods available to pool professionals will detect both the beneficial hypochlorous acid and the less desirable hypochlorite. Keeping free chlorine available at 1 to 4 parts per million is the most important task to ensure safe water. It takes effort, but there are strategies worth examining that will be effective.

Water balance

Balanced water is not only important for protecting pool surfaces, but is also important to chlorine’s efficiency.

When chlorine is administered to the water, it dissociates into hypochlorous acid and hypochlorite.

Hypochlorous acid is the killing form needed to sanitize and oxidize.

Chlorine is most effective when maintained in neutral environments with a pH range of 7.4 to 7.6. Water with a lower pH shifts the equilibrium to the left side of the equation, producing more hypochlorous acid. If the pH exceeds 7.6, the equilibrium shifts to generate more hypochlorite, which is about 80-100 times less effective at sanitizing. Negatively charged microorganisms are repelled by the less effective hypochlorite’s negative charge and won’t be effectively killed.

Influence of Total Alkalinity and Sanitizer Type

As seen in the chart, keeping the pH in range is very important to chlorine’s effectiveness. Total alkalinity helps to control pH balance more effectively. A total alkalinity below 40 ppm causes difficult-to-correct pH swings. It also can decrease the pH, which impacts swimmer comfort and can be detrimental to pool surfaces and equipment.

These effects on alkalinity are important when evaluating the options for a chlorine-based sanitization program. The sodium hydroxide by-product introduced by salt chlorinators or sodium hypochlorite creates an acid demand, resulting in cloudy water if not corrected quickly. Acid will be needed to account for this change and keep the water balanced. At the opposite end of the spectrum, trichlor’s acidity over time erodes total alkalinity depending on the amount applied and the pH balance of source and rain water.

Total alkalinity should be measured weekly and maintained at 80-150 ppm to maintain pH between 7.2 and 7.6. This range helps keep the proper amount of hypochlorous acid available for disinfection and maximize its efficiency in the pool water.

Liquid Chlorine

It is anticipated that the 2021 pool season will see a rise in the use of liquid chlorine by pool service professionals and consumers, because it is easy to use and economically advantageous.

In a 10,000-gallon pool, 10.7 fluid ounces of standard 12% sodium hypochlorite increases free chlorine by 1 ppm. Using stabilizer will help protect chlorine from rapid degradation caused by sunlight and ultraviolet rays. Unstabilized chlorine has a half-life of only 35 minutes. Properly stabilizing a pool using liquid chlorine will allow free chlorine to last six times longer. Keeping an adequate stabilizer residual reduces the need for constant replenishment to maintain a proper free chlorine residual.

To further get the most out of your sanitizer, climate control of stored chemicals will be crucial, because sodium hypochlorite’s shelf life is adversely affected by heat. With just a 10o F temperature increase, 10-15% sodium hypochlorite decreases its potency almost 3 times as fast. Higher concentrations of liquid chlorine also degrade faster, an effect that is further accelerated by increased summer temperatures. Paying close attention to this during transportation and storage will be critical to insuring that maximum-strength liquid chlorine is applied every time.

Liquid chlorine’s typical pH of approximately 13 raises the water’s pH. This doesn’t just diminish hypochlorous acid output, but it also increases the propensity of calcium carbonate precipitation when added to the pool, which can result in prolonged cloudy water or scale deposits on pool surfaces and equipment in areas of hard water. To account for this pH shift, standard application of liquid chlorine into 10,000 gallons will likely require 10 to 16 fl. oz. of muriatic acid, or 12 to 20 oz. of dry acid, accompanied with a scale inhibitor for maximum protection.

Oxidation Importance

The addition of a weekly maintenance oxidizer is another important tool to maximizing sanitization efficiency.

Applying a weekly shock to prevent buildup will garner better customer satisfaction, and cost less overall. Some professionals only apply oxidizers to remediate major issues, such as an algae outbreak or cloudy water. But, as with most things, prevention is important for pool care success. During oxidation, electrons transfer between hypochlorous acid and its target. Each atom has a preferred oxidation state based on its periodic table location, which controls the reaction. To oxidize effectively, both chlorine and its target will need to change their oxidation states. Hypochlorous acid generates chlorine at a +1 state and needs to decrease to a state of -1, which happens as hypochlorous acid interacts with most organisms.

Nitrogen and many nitrogenous compounds are a common target for chlorine because they react with chlorine so readily. To prevent nitrogenous compounds from remaining in a form that will continue to react with the free chlorine needed for proper sanitization, maintenance shocking should be performed weekly, as well as after rain storms or heavy use to enable chlorine to primarily focus on sanitization.

Prevention is an easy way to avoid fighting an uphill and likely expensive battle with superchlorination or water dilutions.

Ancillary Products


Most algae growth is suppressed under an adequate chlorine residual, however algae stresses chlorine and diminishes sanitization efficacy.

Because of this, it is important to use a weekly algaecide to prevent growth, especially as temperatures rise during pool season. Quat algaecides behave like detergents and are cationic antimicrobial agents that bind to negatively charged membranes of algae’s cell walls to destroy the cell. Quats are economical but can cause foaming if overdosed, or in pools with added agitation from spillover fountains or attached spas. In these situations, algaecides containing chelated copper or polyquats become more appropriate. Polyquats behave similarly to quats but do not foam.

Eustigmatos, a stubborn form of green algae, contains the powerful antioxidant beta-carotene. Beta-carotene’s antioxidant properties protect the cell from chlorine oxidation. Using a more specialized algaecide with copper circumvents this challenge.

It’s always a better practice to incorporate a preventative algaecide on top of thorough brushing in regular maintenance. This alleviates some of chlorine’s burden, and lessens the demand on chlorine so it can focus on sanitizing.


To maximize chlorine efficiency, professionals should also consider ways to prevent water-quality problems before they develop.

Chlorine and adequate filtration will remove visible contaminants from the water, but oils and dirt inevitably deposit along the waterline. An enzyme is a naturally derived catalytic protein used to break organic matter down into more manageable forms. Enzymes can be highly specialized and often picked for certain wastes. One type of enzyme, protease, breaks down nitrogenous amino acids. Another enzyme, lipase, hydrolyzes fats and oils. Often, multiple enzymes are paired together to target more than one type of problem. For better performance, enzyme and surfactants are often a part of dual-action formulas, as they have a synergistic relationship. With surfactants, a hydrophobic “tail” attaches to oils at the surface, while a hydrophilic “head” lifts oils from the surface where they enter the water to be broken down and removed by filtration. These also help prevent oily buildup in the filter. The elimination of these oxidizable non-living wastes reduces the demand on chlorine and improves the overall water quality.


After the challenges experienced globally throughout 2020 with increased demand on pool supplies and products, it is important to plan for similar conditions in the upcoming season.

With COVID-19 still a major influence on the pool industry and homeowners alike, finding ways to add efficiencies into pool maintenance routines is key to delivering on the expectations of a homeowner’s backyard investment. Understanding how the basics of pool care can impact water quality and what additional technologies can maximize sanitizer performance will be important, as it is typically more successful and less costly to remove the things that present challenges before they become major problems.