Water Chemistry
A measure of how acidic or basic pool water is on a 0–14 scale. Illinois IDPH requires semi-public pools to hold pH between 7.2 and 7.8, with 7.4–7.6 as the practical operating target.
Pool water is kept slightly basic (above 7.0) because that's where chlorine effectiveness and swimmer comfort balance out. Below 7.2, water becomes corrosive — it eats plaster, etches grout, and burns eyes. Above 7.8, chlorine's sanitizing effectiveness drops sharply and calcium precipitates as scale on tile and inside heater elements. pH is the fastest-moving reading on most pool service logs — bather load, rain, chemical dosing, and CO₂ off-gassing all push it around daily.
Low pH doesn't mean the pool needs chlorine — it means the pool needs pH up (sodium carbonate) or reduced acid dosing.
Chicago-area pools tend to climb in pH through summer because of fill-water alkalinity and CO₂ loss.
pH swings don't affect test accuracy alone — they invalidate the usefulness of the chlorine reading too, since chlorine efficacy is pH-dependent.
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Related terms · Water Chemistry
Cyanuric Acid (CYA / Stabilizer)
A pool additive that protects chlorine from UV degradation in outdoor pools. Essential in moderation, counterproductive above 70ppm.
Chlorine Lock
A state in which chlorine test readings appear normal but the chlorine is ineffective at actually sanitizing the pool. Usually caused by excessive cyanuric acid.
Combined Chlorine (Chloramines)
Chlorine that has already reacted with organic contaminants and lost most of its sanitizing value. The primary cause of the strong `chlorine smell` in pools.
Langelier Saturation Index (LSI)
A composite calculation (pH + temperature + calcium hardness + alkalinity + CYA adjustment) that measures whether pool water is corrosive, balanced, or scale-forming.
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