Water Chemistry and Disinfection
Know the five control numbers
Free chlorine (the active residual), combined chlorine (the spent, irritating residual), pH, total alkalinity and cyanuric acid drive nearly every water-quality decision. Hypochlorous acid is the killing form of chlorine, and its share of the free chlorine residual falls sharply as pH climbs - which is why roughly 7.2–7.8 is the working band even where code allows up to 8.0. Total alkalinity (about 80–120 ppm) buffers pH against swings. Calcium hardness (about 200–400 ppm) and the Langelier Saturation Index tell you whether the water will scale (positive index) or corrode (negative index).
Traps that show up in questions
Combined chlorine is not measured directly by routine testing - it is total chlorine minus free chlorine. "Strong chlorine smell and red eyes" means chloramines are high, so the fix is oxidation, not less chlorine. High cyanuric acid ties up chlorine: the residual reads fine but disinfection is poor, and the only real fix is dilution. Watch for questions that confuse alkalinity with pH, or that treat test-kit readings as infallible when reagents are expired or the sample was taken at the surface.
Worked example
A test shows free chlorine 1.0 ppm and total chlorine 1.8 ppm. Combined chlorine = 1.8 − 1.0 = 0.8 ppm, well above the usual 0.2 ppm action level. Breakpoint chlorination uses roughly 10 × the combined chlorine figure, so plan to add about 8 ppm of free chlorine, then re-test before reopening.
Rule of thumb: if the math says shock, oxidize to breakpoint and confirm with a fresh DPD test - never trust the smell test.