The acidity or alkalinity of pool water is measured by its pH level, which operates on a scale of 0 to 14. A reading of 7 is neutral, while anything above 7 is alkaline, and anything below 7 is acidic. Maintaining a pH level within the ideal range of 7.4 to 7.6 is necessary for proper water balance and swimmer comfort. When the water becomes too alkaline (high pH), specific acid-based chemicals must be introduced to safely decrease the pH back into the acceptable range.
Why Maintaining Proper pH is Essential
Allowing the pool’s pH to remain elevated, typically above 7.8, has several detrimental effects on both the water quality and the swimming environment. High pH significantly reduces the efficiency of chlorine, the primary sanitizer used to keep the water safe. As the pH rises, more of the chlorine converts to a less effective form. This shift means substantially more chlorine must be used to achieve the same level of sanitation, leading to unnecessary chemical costs and potentially poor water quality.
Swimmer comfort is also compromised when the pH is too high, as alkaline water can cause irritation to the eyes and skin. A consistently high pH level also encourages the formation of scale, which is the buildup of calcium carbonate on pool surfaces and equipment. Scaling can appear as cloudy water or rough deposits on the walls. Over time, this buildup can clog filters and circulating pipes, damaging the pool’s infrastructure.
The Chemicals Used to Lower pH
Two primary acid-based chemicals are used by pool owners to effectively lower the water’s pH and total alkalinity: muriatic acid and sodium bisulfate. Muriatic acid, also known as hydrochloric acid, is a powerful liquid acid that is highly effective and fast-acting. It is the most cost-efficient option for reducing both pH and total alkalinity, making it a popular choice for larger adjustments and in pools with salt chlorine generators.
The powerful nature of muriatic acid necessitates extreme caution during handling, as it releases strong, corrosive fumes and can cause severe chemical burns if it contacts skin or eyes. Sodium bisulfate, often sold as “dry acid” or “pH decreaser,” is a granular alternative to the liquid acid. It is considered much safer to store and handle because it does not produce hazardous fumes or pose the same immediate splash hazard as the liquid form.
While sodium bisulfate is safer and preferred for minor adjustments or in spas, it is also more expensive per application than muriatic acid. The granular product is slower to dissolve, especially in cooler water, and a larger quantity is required to achieve the same pH reduction. Both chemicals function by introducing hydrogen ions into the water, which neutralizes the alkaline compounds and lowers the overall pH.
Step-by-Step Application and Safety
Before adding any chemical, it is necessary to test the water to determine the current pH and total alkalinity levels. Knowing the exact readings allows for the precise calculation of the dosage needed to bring the pH into the target range of 7.4 to 7.6. It is better to add a conservative amount and retest later, rather than overshooting the target.
Safety protocols must be strictly followed, particularly when handling muriatic acid, due to its corrosive nature. Personal protective equipment, including acid-resistant gloves, safety goggles, and protective clothing, should be worn to guard against splashes and fumes. The chemical should be handled in a well-ventilated area, and a neutralizing agent like baking soda should be kept nearby in case of spills.
For muriatic acid, it is recommended to pre-dilute it by slowly adding the measured acid to a plastic bucket that already contains water, using a ratio of about one part acid to ten parts water. The rule is always “add acid to water,” never the reverse, to prevent a dangerous chemical reaction. With the pool’s circulation system running, the diluted acid should be carefully poured into the deep end or near a return jet to ensure rapid mixing and avoid concentrated areas.
Dry acid (sodium bisulfate) should also be pre-dissolved in a bucket of water before application, following the manufacturer’s recommended dosage. After the chemical is added, the pump must run for at least four to six hours to ensure the acid is fully circulated and mixed. A follow-up test must be conducted after the circulation period to confirm the pH has been successfully lowered to the desired level.
Common Causes of Rising pH
The pH level in a pool tends to naturally drift upward over time due to several common factors. One of the most frequent causes is aeration, which occurs when water features like waterfalls, slides, or heavy splashing create turbulence. This agitation causes dissolved carbon dioxide to escape from the water and into the atmosphere, a process known as off-gassing. Because dissolved carbon dioxide forms a weak acid in water, its loss makes the water less acidic, resulting in a corresponding increase in pH.
High total alkalinity (TA) also contributes to consistently rising pH, as TA acts as a buffer that stabilizes the pH and resists change. If the TA is too high, it becomes difficult to lower the pH. Furthermore, the addition of certain common pool chemicals can directly elevate the pH, such as liquid chlorine or calcium hypochlorite shock, which are highly alkaline. Using a new plaster or pebble finish can also cause the pH to be consistently high for the first year after installation.