The pH scale measures how acidic or basic the water is, with a reading of 7.0 being neutral. Pool water should ideally be maintained in a slightly basic range, between 7.4 and 7.6. Maintaining this narrow window is important for swimmer comfort, as a high pH can cause skin and eye irritation. When pH rises above 7.8, chlorine sanitizer effectiveness decreases dramatically, and the water becomes prone to cloudiness and scale formation on surfaces and equipment.
The Role of Aeration
The primary physical cause for a rising pH is the process of aeration, which leads to the loss of dissolved carbon dioxide (\(\text{CO}_2\)) from the water. Water naturally contains \(\text{CO}_2\), which reacts to form carbonic acid (\(\text{H}_2\text{CO}_3\)), a weak acid that helps keep the pH level lower. The pool water is often intentionally “over-carbonated,” meaning it holds more dissolved \(\text{CO}_2\) than the air above it.
Whenever the water is agitated, such as by waterfalls, fountains, spillways, high-speed return jets, or heavy splashing from swimmers, this dissolved \(\text{CO}_2\) escapes into the atmosphere. The removal of this acidic component causes the water to become more basic, or alkaline, and the pH level rises.
A saltwater chlorine generator also contributes to this process. The electrolysis process in the salt cell creates hydrogen gas bubbles (\(\text{H}_2\)), and the resulting turbulence forces the dissolved \(\text{CO}_2\) to off-gas at an accelerated rate. This physical agitation is a constant, unavoidable factor that contributes to the upward drift of the pool’s pH.
High Total Alkalinity
While aeration is the mechanism that raises the pH, high Total Alkalinity (TA) is the underlying chemical condition that makes the problem persistent and difficult to manage. TA is a measure of the alkaline substances in the water, primarily bicarbonates, carbonates, and carbonic acid, which function as a buffer to stabilize the pH. When TA is too high—above the recommended range of 80 to 120 parts per million (ppm)—it creates a condition often described as “pH lock”.
A high TA level means the pool water resists having its pH lowered. When acid is added to lower the pH, the acid is primarily consumed by the high concentration of alkaline substances (the buffer) rather than directly lowering the pH. This buffering action is what makes the pH quickly drift back up to its high starting point after an acid treatment.
The elevated alkalinity level also accelerates the off-gassing of \(\text{CO}_2\). When the water’s buffering capacity is high, the \(\text{CO}_2\) is driven out more rapidly, leading to a faster pH increase than would occur in a pool with balanced TA. The TA level must be correctly managed first, as it dictates the stability and responsiveness of the pH.
Chemical Additives and Pool Surface Leaching
Certain common pool maintenance chemicals inherently introduce high alkalinity into the water, contributing directly to a rising pH. Chemicals used for sanitizing and shocking, such as Calcium Hypochlorite (Cal Hypo) and Lithium Hypochlorite, are highly alkaline and will noticeably increase the pH with each dose. Similarly, \(\text{pH}\) Increaser (sodium carbonate) or alkalinity increaser (sodium bicarbonate) intentionally add alkaline substances that raise both \(\text{pH}\) and TA.
A temporary but significant factor is surface leaching, particularly in new pools with plaster or concrete finishes. During the first year, the plaster surface is still curing and releases calcium hydroxide into the water. This highly basic substance dissolves into the pool water, causing a persistent upward spike in \(\text{pH}\) that requires frequent acid additions to control until the surface is fully cured.
Strategies for pH Stabilization
The most effective method for long-term \(\text{pH}\) stabilization involves controlling the Total Alkalinity. Since acid lowers both \(\text{pH}\) and TA, the goal is to use repeated applications of an acid, such as muriatic acid or sodium bisulfate, to reduce the TA level until it is within the optimal range of 80 to 120 ppm.
To reduce TA effectively, acid is typically added by pouring it into one deep area of the pool, allowing it to sink and reduce the alkalinity in that localized area before it fully mixes. Once the TA is lowered to the acceptable range, the water’s buffering capacity is reduced, and \(\text{pH}\) fluctuations become much easier to manage with smaller, less frequent acid doses.
Another corrective measure is to adjust equipment that promotes aeration. Temporarily turning off water features, waterfalls, or decorative spillways will reduce the physical agitation that drives off \(\text{CO}_2\). Minimizing aeration will slow the rate at which the \(\text{pH}\) climbs. Regular testing of both \(\text{pH}\) and TA is necessary to maintain balance, as addressing the root cause of high TA is the only way to prevent chronic issue of rising \(\text{pH}\).