Maintaining a safe swimming environment relies heavily on a delicate chemical balance. Chlorine is the industry-standard sanitizer used to keep recreational water free of harmful microorganisms. However, its effectiveness is directly influenced by the water’s pH level, which measures how acidic or basic the water is. Even small changes in pH can drastically alter chlorine’s ability to kill pathogens and oxidize contaminants. Understanding this relationship is foundational for proper pool maintenance.
The Chemical Forms of Active Chlorine
The sanitizing action in pool water comes from free chlorine, which exists simultaneously in two chemical forms. When chlorine is added to water, it rapidly hydrolyzes to form a mixture of hypochlorous acid (HOCl) and hypochlorite ion (OCl-). These two molecules collectively make up the total free available chlorine responsible for disinfection.
Hypochlorous acid (HOCl) is the highly potent, fast-acting sanitizer. Its neutral electrical charge allows it to easily penetrate the negatively charged cell walls of pathogens, destroying them from the inside. HOCl can be up to 120 times more effective at killing microbes than its counterpart.
The hypochlorite ion (OCl-), in contrast, is a much slower and significantly less effective disinfectant. This molecule carries a negative electrical charge, causing it to be repelled by the similarly negative charge on microorganism cell walls. The proportion of these two forms is governed entirely by the water’s pH.
How pH Controls Chlorine Effectiveness
The concentration of hypochlorous acid is inversely proportional to the water’s pH level. When the pH is low, the water is more acidic, and the chemical equilibrium favors the formation of HOCl, maximizing sanitation power. As the pH rises, the equilibrium shifts, converting more active HOCl into the less potent OCl- form.
A pool with a pH of 7.0 contains approximately 76% HOCl, providing rapid disinfection. Raising the pH to 7.5 causes the HOCl concentration to fall to roughly 50% of the total free chlorine. At a pH of 8.0, the effectiveness drops further, leaving only about 24% HOCl available.
The optimal pH range for swimming pools is between 7.2 and 7.6. This range is chosen as a compromise to maintain effective HOCl concentration while ensuring swimmer comfort. A pH of 7.4 is often cited as ideal because it closely matches the pH of human eyes and mucous membranes, minimizing irritation.
Secondary Effects of pH Imbalance
Moving outside the optimal pH range creates problems beyond reduced sanitization speed. High pH levels, typically above 7.8, can lead to the precipitation of dissolved calcium minerals. This causes the water to become cloudy and results in the formation of scale, a chalky white deposit that builds up on pool surfaces and equipment like heaters. High pH also contributes to eye and skin irritation for swimmers and accelerates the development of chloramines, which cause the strong, unpleasant chlorine odor.
Conversely, low pH levels, below 7.2, make the water corrosive and acidic. This acidic environment aggressively attacks metal components, causing corrosion damage to pool equipment. Low pH water can also cause etching on plaster and grout, and it tends to make vinyl liners brittle. Swimmers often experience stinging eyes and nasal passages, as well as dry skin and hair.
Methods for Adjusting Water pH
Maintaining the correct pH requires regular testing using test strips or a liquid testing kit. Total alkalinity, which helps buffer the pH, should always be measured and adjusted before attempting to change the pH level. The recommended total alkalinity range is typically 80 to 120 parts per million (ppm).
To raise a low pH and make the water more alkaline, pool owners use sodium carbonate, commonly sold as soda ash or pH Increaser. This chemical is added directly to the water while the circulation system is running to ensure even dispersal.
For lowering a high pH, the most common chemicals are muriatic acid or sodium bisulfate, often labeled as pH Reducer or pH Minus. Muriatic acid is a strong liquid acid that requires careful handling due to its corrosive nature. Sodium bisulfate is a granular form, sometimes called dry acid, that is generally considered safer to handle.
When adding any chemical, it is important to follow the manufacturer’s instructions regarding dosage, and never mix different chemicals together. After adding a pH adjustment chemical, the water should be retested after several hours to confirm the level is within the desired 7.2 to 7.6 range.