Maintaining a pool reduces to four core measurements: free chlorine, pH, total alkalinity, and cyanuric acid. Each has a target range, and each affects the others. This guide walks through what each one does and why it matters.
Key Facts
- Free chlorine should be 1–3 ppm for pools and 3–5 ppm for hot tubs.
- pH between 7.2 and 7.6 is essential for both swimmer comfort and chlorine effectiveness.
- Total alkalinity between 80 and 120 ppm stabilises pH and prevents rapid swings.
- Cyanuric acid above 80 ppm significantly reduces chlorine effectiveness even when the reading looks correct.
Free Chlorine: The Active Sanitizer
Free chlorine (FC) is the active disinfectant in pool water. It kills bacteria, viruses, and algae by oxidising their cell walls on contact. The CDC recommends a minimum of 1 ppm for pools, but most operators target 2–3 ppm as a safety buffer against rain dilution, bather load, and UV degradation. Free chlorine exists in two forms: hypochlorous acid (HOCl) and hypochlorite ion (OCl-). Hypochlorous acid is 40–80 times more effective, and its proportion is determined almost entirely by pH — which is why chlorine and pH management cannot be separated.
pH: The Master Control
pH measures whether water is acidic or alkaline on a scale of 0–14, with 7.0 being neutral. Pool water should sit between 7.2 and 7.6. At this range, chlorine is highly effective, swimmer eyes are comfortable, and equipment corrosion is minimised. Below 7.2, water becomes aggressive and corrodes plaster, grout, and metal. Above 7.8, chlorine effectiveness drops sharply — at pH 8.0, active chlorine is only about 22% of what the test shows.
Alkalinity and Stabilizer
Total alkalinity (TA) acts as a pH buffer. Without adequate alkalinity (target 80–120 ppm), pH swings wildly with every chemical addition or rainstorm. Cyanuric acid (CYA) protects outdoor chlorine from UV degradation — without it, direct sunlight can deplete chlorine in as little as two hours. Target CYA for standard outdoor pools is 30–50 ppm. Above 80 ppm, CYA suppresses chlorine activity enough that even a 3 ppm reading may provide inadequate sanitation.
Examples
Test shows FC 0.5 ppm, pH 7.8, TA 140 ppm, CYA 60 ppm. Low FC is the immediate problem, but the high pH explains why it depleted so quickly — chlorine is only about 30% effective at pH 7.8. Bring pH down first (which also lowers TA slightly), then add chlorine. This order means your chlorine dose works at full strength immediately rather than fighting an uphill battle.
Common Mistakes to Avoid
- Adding chlorine without checking pH first — at pH 8.0, you may need triple the dose to achieve the same effect.
- Treating only one parameter without checking how it affects the others.
- Ignoring CYA buildup until algae appears, by which point normal chlorine doses have become ineffective.
- Pool & Hot Tub Alliance — Pool & Spa Operator Handbook, 2022
- CDC — Healthy Swimming Guidelines