pH is a logarithmic scale measuring water acidity or alkalinity. Pool water must be maintained between 7.2 and 7.6 to ensure effective chlorine sanitation, swimmer comfort, and protection of pool surfaces and equipment.
Key Facts
- At pH 7.2, approximately 67% of free chlorine is in its most active form (hypochlorous acid).
- At pH 8.0, only 22% of free chlorine is active — three times more chlorine is needed for the same effect.
- pH naturally drifts upward in pools as CO2 escapes the water surface.
- Total alkalinity must be balanced first — it directly affects how stable pH will be.
What pH Measures
pH measures the concentration of hydrogen ions in water on a logarithmic scale from 0 (most acidic) to 14 (most alkaline). Pure water at 7.0 is neutral. Pool water at 7.2–7.6 is slightly alkaline — the range where chlorine is highly active, swimmer eyes are comfortable (human tears are pH 7.4), and pool surfaces are neither being corroded nor scaled. Each full pH unit represents a 10-fold change in acidity, which is why even small changes (0.2–0.3 units) have measurable effects on chlorine effectiveness.
Why pH Controls Chlorine
Free chlorine exists in two forms in water: hypochlorous acid (HOCl) and hypochlorite ion (OCl-). HOCl is roughly 80 times more effective as a disinfectant. At pH 7.2, about 67% of free chlorine is HOCl. At pH 7.4, it drops to about 55%. At pH 7.8, it drops to 28%. At pH 8.0, only 22% remains as HOCl. This means a pool with 2 ppm FC at pH 7.2 has roughly the same effective sanitation as a pool with 6 ppm FC at pH 8.0. High pH is one of the most common causes of chronic chlorine problems.
Adjusting pH
To lower pH: add muriatic acid (hydrochloric acid) or dry acid (sodium bisulfate). Muriatic acid is more economical and faster; dry acid is safer to handle. To raise pH: add sodium carbonate (soda ash) or sodium bicarbonate (baking soda). Soda ash raises pH quickly with minimal alkalinity change. Sodium bicarbonate primarily raises alkalinity but also raises pH modestly. Always add pH adjustment chemicals to the deep end with the pump running, never near the skimmer, and allow 30–60 minutes for full dispersion before retesting. Make incremental adjustments — overshoot is common with large doses.
Examples
A pool maintains 3 ppm FC but keeps developing algae spots. pH testing shows 8.0. At pH 8.0, only 22% of FC is active HOCl — effectively 0.66 ppm active chlorine despite the 3 ppm reading. Lowering pH to 7.4 immediately increases active chlorine to approximately 1.65 ppm without adding any chlorine product. The algae spots resolve within days. The fix cost a few dollars of acid instead of more chlorine that would not have helped at the high pH.
Common Mistakes to Avoid
- Adjusting pH without first correcting alkalinity — alkalinity controls how stable pH will be after adjustment.
- Adding pH increaser or decreaser in one large dose — overshoot is common and requires a corrective addition in the opposite direction.
- Not accounting for carbon dioxide in aerated pools — waterfalls and fountains off-gas CO2 rapidly, driving pH up regardless of how much acid you add.
- Pool & Hot Tub Alliance — Pool & Spa Operator Handbook, 2022
- Taylor Technologies — Pool/Spa Water Chemistry Reference