Pool and spa maintenance relies heavily on keeping the water chemistry in a precise balance. A common concern for pool owners is the relationship between the chlorine used for sanitation and the water’s pH level. The effect on pH is not caused by the amount of chlorine itself, but rather by the specific chemical composition of the chlorine product introduced into the water.
The Chemical Impact of Different Chlorine Types on pH
The chemical reaction chlorine undergoes when added to water, known as hydrolysis, always results in the formation of hypochlorous acid, the primary sanitizing agent. The impact on the water’s pH is determined by the other chemical compounds present in the chlorine product itself. Unstabilized chlorine products, such as sodium hypochlorite (liquid chlorine) and calcium hypochlorite, are highly alkaline solutions. Liquid chlorine often has a pH between 12 and 13, while calcium hypochlorite is between 10 and 12, causing an immediate rise in pool pH upon addition.
When these hypochlorites are consumed as they sanitize, they convert into a mild acid, which almost entirely neutralizes the initial alkaline effect. The net effect of using these products is often considered nearly pH-neutral over the long term. This long-term pH drift is more often attributed to the outgassing of carbon dioxide from the water, a process accelerated by high total alkalinity or aeration features like waterfalls.
Conversely, stabilized chlorines, specifically trichloroisocyanuric acid (Trichlor) and sodium dichloroisocyanurate (Dichlor), are acidic. Trichlor is highly acidic, with a pH between 2.7 and 3.3, and consistently lowers both the pool’s pH and total alkalinity. Constant use of trichlor tablets requires the regular addition of a pH-increasing chemical to counteract the introduced acid.
Dichlor is moderately acidic, with a near-neutral pH between 5.5 and 7.0, and has a less dramatic lowering effect on the pH compared to trichlor. Both trichlor and dichlor contain cyanuric acid (CYA), which acts as a stabilizer protecting the chlorine from ultraviolet light degradation. The difference in their pH impact results directly from their base chemical composition, not just the presence of chlorine.
Consequences of Imbalanced pH Levels
Maintaining the water’s pH within the ideal range of 7.2 to 7.8 is necessary for sanitation and bather comfort. When the pH level rises above this range, the effectiveness of the chlorine sanitizer is significantly reduced. This reduction occurs because high pH shifts the balance of hypochlorous acid (HOCl) to the less effective hypochlorite ion (\(OCl^-\)). For instance, at a pH of 7.5, chlorine is about 50% effective, but if the pH climbs to 8.0, its sanitizing power can drop to 25%.
A high pH also has physical consequences for the water and pool equipment. High alkalinity causes calcium to precipitate out of the water, leading to cloudiness and the formation of scale deposits on pool surfaces, filters, and heaters. Water that is too basic can also cause irritation to the eyes and skin of swimmers, a discomfort often mistakenly blamed on excessive chlorine levels.
Identifying and Correcting Sources of High pH
While some chlorine products contribute to high pH, the primary driver of persistent pH rise is often the outgassing of carbon dioxide from the water. Water features, such as fountains, spillways, and waterfalls, increase aeration. This aeration drives carbon dioxide out of the water, causing the pH to increase. This process is accelerated when the water has a high total alkalinity (TA), which measures the concentration of alkaline substances like bicarbonates and carbonates.
Total alkalinity acts as a buffer that resists changes in pH. If TA is too high, it makes the pH difficult to manage and prone to upward drift. A recommended TA target is between 80 and 120 parts per million (ppm), though some pools may need to target a lower range, such as 70 to 90 ppm, to stabilize the pH. Regularly testing both the pH and TA levels is the first step in diagnosing the issue.
To reduce a persistently high pH, pool owners must introduce an acid into the water. The two most common chemicals used are muriatic acid and sodium bisulfate (dry acid). Muriatic acid is a highly effective liquid form, but it requires careful handling and slow addition to the deepest area of the pool. Sodium bisulfate is a granular form that is easier to handle and is pre-dissolved before being added to the water. Adjustments should be made gradually, allowing the chemicals to circulate and balance before retesting the water.