Why Is Pool pH Important? Ideal Levels and Adjustment Tips

What Is Pool pH?

pH is used to measure the concentration of hydrogen ions in water. It uses a logarithmic scale, which means that each unit represents a tenfold change in hydrogen ion (H⁺) concentration. Water with a pH of 6 is ten times more acidic than water with a pH of 7, and 100 times more acidic than water with a pH of 8. Therefore, seemingly small changes in the reading can have a disproportionate effect on the water’s properties.

The pH scale usually ranges from 0 to 14. A pH below 7 indicates acidic water, while a pH above 7 indicates alkaline water. In swimming pools, maintaining a balanced pH is essential for ensuring the effectiveness of disinfectants, protecting pool materials, and enhancing swimming comfort.

What causes pH fluctuations in a swimming pool?

If your pool’s pH remains consistently unbalanced, it is crucial to understand the causes. The most significant factors include:

1. Chlorine-based disinfectants: For example, if a pool uses disinfectants containing trichloroisocyanuric acid, simply adding them will lower the water’s pH because the aqueous solution of trichloroisocyanuric acid is acidic; thus, long-term use will cause the pH to drop; If the pool uses disinfectants such as bleach, long-term use will cause the water’s pH to rise.
2. Carbon dioxide loss: When water circulation increases—such as when fountains are running or spa jets are in use—carbon dioxide escapes, reducing carbonic acid and potentially raising the pH.
3. Total Alkalinity: If the total alkalinity is too low, the pH will become unstable.
4. Water Source Quality and Fresh Water Replenishment: The alkalinity and hardness of tap water vary significantly across different regions.
5. Swimmers’ Sweat and Body Fluids: Sweat, lotions, creams, and skin oils can be transferred into the water, altering the pH.
6. Rainfall: Rainwater is typically acidic and can rapidly disrupt the water balance in outdoor pools.
7. Inadequate ventilation systems: In indoor pools, the accumulation of gases can cause changes in the water’s pH.

How Does Pool pH Affect Chlorine Disinfection?

Effects of Excessively High Pool pH

1. Reduced Chlorine Disinfection Efficiency: As pool water pH increases, a smaller proportion of free chlorine is present as hypochlorous acid (HOCl), the more effective form of free chlorine for disinfection under typical pool conditions. More chlorine is present as hypochlorite ions (OCl⁻), which generally disinfect less efficiently. This can reduce disinfection performance, especially when pH rises above the recommended range. Pool operators should monitor both pH and free chlorine levels to maintain effective water disinfection.
2. Surface Deposits and Scaling: Calcium in the water combines with carbonates to form scale.
   • This is common along pool edges, where water comes into contact with dust and dirt; over time, the water gradually turns black.
   • Calcium carbonate precipitates on the pool’s filter, preventing it from capturing debris from the water.
   • The water becomes cloudy, and transparency decreases.
3. Health Effects: In water with high pH, as well as intense thirst and a slippery feeling; these conditions also occur at the bottom of the pool, increasing the risk of slips and falls.

Effects of Too Low Pool pH

1. Reduced Disinfection Effectiveness: When the same amount of chlorine-based pool disinfectant is added to the pool, the lower the pH, the higher its relative active concentration. However, instability accelerates decomposition, leading to excessive formation of hypochlorous acid in a short period, and rapid chlorine consumption, which prevents sustained disinfection.
2. It gradually erodes pool equipments (tiles, liners, chlorinated rubber coatings) and metal components (stainless steel steps, handrails, drains, pipes, and pumps), affecting the normal operation of equipment and increasing replacement costs.
3. The pool water will begin to dissolve certain minerals and metallic materials, thereby indirectly creating an environment highly conducive to algae growth.
4. As corrosion occurs on the pool walls and metal components, these metal ions are released from the water and adhere to the pool walls and floor, forming black stains.
5. When swimming, you may experience a burning or stinging sensation in your eyes and nose, your swimwear may fade, and your skin may become dry and itchy.

How to Maintain a Swimming Pool Based on pH Levels

Measuring and Adjusting pH Levels

1. Measure pH and Alkalinity: To obtain accurate readings, always take samples at a depth of 30–40 centimeters, away from the edges and skimmers.
2. If the total alkalinity is too low, increase it.
   • Reason: Total alkalinity helps maintain stable pH levels. Without this buffering effect, pH levels will fluctuate wildly. When total alkalinity is too low, the pH becomes unstable.
   • Standard Dosage: Approximately 38.000 grams of bicarbonate per 680 liters of water will raise the alkalinity by 10 ppm. Adjust according to your pool’s actual volume.
3. Adjusting pH
   • pH above 7.8: Add an acidic substance, such as sodium bisulfate or hydrochloric acid;
   • pH below 7.2: Add an alkaline substance, such as sodium carbonate (soda ash) or sodium bicarbonate (baking soda);

Follow the principle of adding small amounts frequently and evenly. First dissolve the adjusting agent in a bucket, stir thoroughly, then pour it evenly along the pool’s circulation inlets to avoid localized concentration spikes caused by direct addition. Keep the circulation running for 6 to 12 hours, then measure the pH again.

Regular Testing

Ensure the pool pH is maintained between: