Pool Water Chemistry Guide: pH, Alkalinity, Chlorine & Cyanuric Acid

Pool chemistry isn't complicated once you understand how the parameters interact. Get these right and your pool stays clear, your equipment lasts, and swimmers don't come out with red eyes.

pH: The Foundation

Target range: 7.4–7.6

pH is the most important number in pool chemistry because it affects everything else. At 7.4–7.6, chlorine works at peak efficiency. Below 7.2, water becomes corrosive — etches plaster, corrodes metal fittings, irritates eyes and skin. Above 7.8, chlorine becomes dramatically less effective: at pH 8.0, chlorine is only about 20% active vs. 75% at pH 7.4.

To lower pH: muriatic acid (liquid) or sodium bisulfate (dry). To raise pH: sodium carbonate (soda ash). Add chemicals with the pump running; wait 4+ hours before retesting.

Total Alkalinity: The Buffer

Target range: 80–120 ppm

Think of alkalinity as pH's shock absorber. High alkalinity makes pH resistant to change ("pH lock") — usually in the 8.0–8.4 range. Low alkalinity makes pH erratic, swinging dramatically with each chemical addition or rainstorm.

To raise alkalinity: sodium bicarbonate (baking soda). To lower alkalinity: muriatic acid, added in increments with the pump off (add to the deep end, wait 1 hour, then circulate). Alkalinity changes are slow — adjust in 10–20 ppm increments.

Free Chlorine: The Sanitizer

Target range: 2–4 ppm (outdoor pools without stabilizer: 1–3 ppm)

Free chlorine (FC) is the active sanitizer. "Combined chlorine" is FC that's already reacted with ammonia compounds from sweat and urine — it's what creates that "pool smell" and eye irritation. Total chlorine = FC + CC. You want CC <0.5 ppm.

If your pool smells like chlorine, it's not because there's too much — it's usually because there's too little free chlorine and too much combined. Shock the pool (raise FC to 10+ ppm) to burn off combined chlorine.

Liquid chlorine (sodium hypochlorite, 10–12.5%) is the cleanest option — no cyanuric acid buildup, no stabilizer. Trichlor tablets (3-inch pucks) are convenient but add cyanuric acid with every dose. Dichlor is similar. Cal-hypo granules (calcium hypochlorite) raise calcium hardness.

Cyanuric Acid: The Double-Edged Stabilizer

Target range: 30–50 ppm (outdoor pools). Max: 100 ppm

Cyanuric acid (CYA, or stabilizer/conditioner) protects chlorine from UV degradation. Without it, direct sunlight destroys 75–90% of your free chlorine within hours. With CYA at 30–50 ppm, chlorine lasts several days outdoors.

The problem: CYA "holds onto" chlorine. As CYA rises, you need higher free chlorine levels to maintain the same sanitizing power. At 100 ppm CYA, you need 7.5 ppm FC to match the sanitation of 2 ppm FC at 30 ppm CYA. This is called chlorine lock.

CYA doesn't degrade — it only dilutes. The only way to lower it is to drain and refill part of the pool. If you've been using trichlor tabs for years without any draining, test your CYA. Values above 80 ppm mean your chlorine is working at a fraction of its advertised strength.

Calcium Hardness

Target range: 200–400 ppm (plaster pools: 200–400; vinyl/fiberglass: 150–250)

Too low: water becomes "hungry" and leaches calcium from plaster surfaces, causing pitting. Too high: calcium precipitates out of solution and deposits on surfaces and equipment (scale). To raise: calcium chloride. To lower: dilution (partial drain and refill). Southern California tap water often runs 300–400 ppm calcium already.

Testing Frequency and Method

Test pH and FC at minimum twice per week in summer, once weekly in cooler months. Full panel (pH, FC, TC, TA, CYA, calcium) monthly or when troubleshooting. For accuracy: a Taylor K-2006 drop test kit beats test strips on every parameter that matters. Strips are fine for quick pH/chlorine checks but lose accuracy for TA and CYA.

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