The balance of water chemistry depends on two closely related, but distinct, measurements: pH and total alkalinity. When a system presents with high pH alongside low total alkalinity, it indicates an unstable condition requiring a specific, sequential corrective strategy. Understanding this dual imbalance is the first step toward restoring the water to a stable, comfortable, and chemically effective state.
What are pH and Total Alkalinity
The pH measurement indicates the concentration of hydrogen ions, determining how acidic or basic the water is. Measured on a logarithmic scale from 0 to 14, below 7 is acidic, 7 is neutral, and above 7 is basic. The ideal range for managed water systems is between 7.2 and 7.6, which allows sanitizers to work efficiently. A change of one whole number represents a tenfold change in the concentration of hydrogen ions.
Total Alkalinity (TA), measured in parts per million (ppm), measures the water’s capacity to resist changes in pH. This capability is called the water’s buffering capacity and is provided by dissolved compounds like bicarbonates, carbonates, and hydroxides. The recommended range for TA is between 80 and 120 ppm. Low alkalinity means there are too few buffering compounds to neutralize acids or bases, resulting in an unstable pH reading.
The Consequences of High pH and Low Alkalinity
The high pH portion of this imbalance creates immediate problems by hindering the effectiveness of sanitizing agents like chlorine. As the pH rises above the ideal range, a smaller percentage of the chlorine converts into its most potent form, hypochlorous acid, which is responsible for disinfection. This reduced efficiency means the sanitizer is not working as hard, potentially leading to issues with water clarity and sanitation. Furthermore, elevated pH can cause calcium and other minerals to precipitate out of the water, resulting in the formation of scale on surfaces and equipment.
The low total alkalinity introduces the problem of instability, often described as pH “bounce.” With a low concentration of buffering agents, the pH level can swing rapidly up or down with even minor changes to the system. Simple environmental factors, such as rain, organic debris, or the addition of a chemical, can cause the pH to shift unpredictably. This erratic movement makes continuous water balance nearly impossible to maintain.
Sequential Steps to Restore Balance
The presence of high pH and low alkalinity demands a specific, sequential strategy, as fixing one often impacts the other. The priority is always to restore the water’s stability by addressing the low total alkalinity first. Sodium bicarbonate, commonly known as alkalinity increaser, is the chemical typically used to raise the TA level. This step is performed first because the water cannot maintain a set pH until its buffering capacity is properly established.
Adding sodium bicarbonate will raise the total alkalinity into the desired 80–120 ppm range, which will also result in a slight increase to the already high pH. This temporary pH increase is acceptable because stability is the primary goal at this stage. Once the TA level is confirmed through retesting, the next step is to address the high pH. The now-stable high pH is lowered using a pH decreaser, usually sodium bisulfate or muriatic acid.
The application of this acid-based product will reduce the pH into the optimal range of 7.2 to 7.6. This addition of acid also slightly consumes the bicarbonate compounds, which may cause a minor drop in the TA level. The sequential process is necessary because if one were to try to lower the pH first, the low alkalinity would allow the pH to drop too quickly, resulting in a corrosive water state that could damage equipment and surfaces. Multiple rounds of testing and small, gradual chemical additions are often needed to dial in both levels.
Why This Imbalance Occurs
The combination of high pH and low alkalinity frequently occurs due to the process of aeration, which happens when water is vigorously agitated. Features like waterfalls, fountains, or simple splashing increase the surface area of the water exposed to the atmosphere. This agitation encourages the release of dissolved carbon dioxide gas from the water. The loss of carbon dioxide disrupts the carbonic acid equilibrium in the water, which in turn causes the pH to rise.
Another common contributor is the regular use of certain sanitizers that have a naturally high pH, such as liquid chlorine or sodium hypochlorite. These products consistently drive the pH upward, requiring frequent additions of acid to compensate. If the acid is added without monitoring the total alkalinity, the acid can consume the already low concentration of buffers, causing the alkalinity to drop further while the high-pH sanitizer continues to push the pH up. Maintaining a balanced system requires consistent testing, which allows for small, proactive adjustments to both the TA and pH.