Pool water chemistry is often taught one parameter at a time: pH this week, alkalinity next week, chlorine after that. The result is technicians who know how to adjust each number individually but who don't understand how all six parameters interact as a system — why fixing one often moves another, why the order of adjustments matters, and why a pool can have every individual parameter "in range" and still have problems.
This guide treats pool chemistry the way it actually works: as a unified system with interconnected parameters. Master all six together, and pools under your care will stay clear, balanced, and complaint-free week after week.
| Parameter | Ideal Range | What Happens Out of Range |
|---|---|---|
| Free Chlorine (FC) | 1–3 ppm (varies by CYA) | Low: bacteria/algae bloom. High: irritation, bleaching. |
| pH | 7.4–7.6 | Low: corrosion, irritation. High: chlorine ineffective, scale. |
| Total Alkalinity (TA) | 80–120 ppm | Low: pH crashes. High: pH drifts high, cloudy water. |
| Calcium Hardness (CH) | 200–400 ppm | Low: etching. High: scale, cloudy water. |
| Cyanuric Acid (CYA) | 20–50 ppm (liquid Cl) / 60–80 ppm (SWG) | Low: chlorine burns off fast. High: chlorine locked up. |
| LSI (Langelier Index) | −0.3 to +0.3 | Negative: corrosive. Positive: scale-forming. |
Free chlorine is the active sanitizer in the water — the hypochlorous acid (HOCl) and hypochlorite ion (OCl⁻) that kill bacteria, oxidize organic matter, and prevent algae. HOCl is the effective form; at pH 7.4, roughly 65% of chlorine is HOCl. At pH 8.0, only 23% is HOCl. This is the most critical reason pH control matters.
The CYA relationship: Cyanuric acid binds a significant portion of free chlorine, releasing it slowly. The bound chlorine is chemically active but at a much reduced rate. For chlorine to be effective in a stabilized pool, you must maintain FC as a percentage of CYA — not as an absolute number. The minimum effective FC is 7.5% of CYA for routine maintenance; 10%+ for active problems.
Raise FC: Liquid sodium hypochlorite (10–12.5% pool chlorine or 6% household bleach), calcium hypochlorite (cal-hypo), or dichlor. Each source has different effects on other parameters — see our chlorine comparison guide.
Lower FC: Dilution (partial drain and refill), or simply wait for natural degradation. In emergencies, sodium thiosulfate neutralizes chlorine rapidly but must be used carefully.
pH measures hydrogen ion concentration on a logarithmic scale. For pool water, pH 7.4–7.6 is the sweet spot because it balances three competing needs: chlorine effectiveness (prefers lower pH), swimmer comfort (eyes are pH 7.4), and surface protection (very low pH is corrosive).
The chlorine effectiveness curve: At pH 7.0: 73% HOCl. At pH 7.4: 63% HOCl. At pH 7.6: 48% HOCl. At pH 8.0: 23% HOCl. At pH 8.5: only 9% HOCl. A pool at pH 8.2 needs roughly five times more chlorine to achieve the same sanitation as a pool at pH 7.2.
Raise pH: Sodium carbonate (soda ash) for fast raises, sodium bicarbonate (baking soda) for slower raises with TA impact. Aeration also raises pH by offgassing CO₂. See our full pH raising guide.
Lower pH: Muriatic acid (hydrochloric acid) is the standard. Dry acid (sodium bisulfate) is an alternative where liquid handling is a concern. See our pH lowering guide and muriatic acid guide.
Total alkalinity measures the water's capacity to resist pH change — its buffering ability. It is primarily made up of bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions. Without adequate TA, pH swings wildly with every chemical addition or environmental change. With too much TA, pH becomes stubbornly high and resistant to acid adjustment.
The pH/TA relationship: TA anchors pH. This is why you always adjust TA before adjusting pH. Adding acid to lower pH also lowers TA. Adding sodium bicarbonate to raise TA also raises pH slightly. These parameters cannot be adjusted in isolation.
Raise TA: Sodium bicarbonate (baking soda / Alkalinity Up). Dose at 1.4 lbs per 10,000 gallons to raise TA by approximately 10 ppm. See our complete TA guide.
Lower TA: Muriatic acid additions combined with aeration (to recover pH without raising TA). This is the acid-aeration method: dose acid to drop both pH and TA, then run fountains or bubblers to raise pH back through CO₂ exchange, leaving TA lower.
Calcium hardness measures the concentration of dissolved calcium ions (Ca²⁺) in the water. This parameter directly affects the LSI — too low and the water becomes corrosive, aggressively dissolving calcium carbonate from plaster, grout, and metal surfaces; too high and scale deposits form on surfaces and equipment.
Surface type matters: Plaster pools need higher CH because the plaster surface can donate calcium if the water is undersaturated. Vinyl and fiberglass pools have non-porous surfaces and tolerate lower CH without surface damage, though equipment (heaters, pumps) still needs protection.
Raise CH: Calcium chloride (calcium chloride flake or granular). Adds calcium without significantly affecting pH or TA. Pre-dissolve in a bucket of water before adding to the pool — calcium chloride dissolution is highly exothermic (gets very hot). See our full calcium hardness guide.
Lower CH: Requires partial drain and refill with softer water. There is no chemical that removes calcium from water in a practical service context. In hard water regions (CH of fill water exceeds 300 ppm), annual or semi-annual draining is often the only viable long-term strategy.
Cyanuric acid (CYA, isocyanuric acid) bonds with free chlorine and protects it from UV photolysis — the process by which sunlight destroys chlorine. Without CYA, an outdoor pool loses most of its chlorine in 2–4 hours of direct summer sunlight. With CYA at 30–50 ppm, chlorine half-life extends to several days.
The tradeoff: CYA protection comes at the cost of reduced chlorine activity. At higher CYA levels, you need proportionally higher FC to maintain the same sanitation capacity. This is the CYA-FC relationship that every pool professional must understand: as CYA rises, the minimum required FC rises with it. See our CYA complete guide.
Raise CYA: Cyanuric acid granules (stabilizer/conditioner) dissolved in a sock in the skimmer, or trichlor pucks (which add CYA as a byproduct). Dichlor also adds CYA.
Lower CYA: Partial or complete drain and refill. There is no chemical that removes CYA. Dilution is the only practical solution.
The LSI is not a directly testable parameter — it's a calculated composite of pH, temperature, calcium hardness, total alkalinity, and TDS. It quantifies whether pool water is corrosive (negative LSI) or scale-forming (positive LSI). A balanced LSI means the water is in equilibrium with calcium carbonate and will neither dissolve surfaces nor deposit scale.
The formula: LSI = pH − pHs, where pHs is the saturation pH calculated from temperature, CH, TA, and TDS factors. Apps like PoolLens calculate this automatically.
See our dedicated LSI guide for full formula breakdown, temperature effects, and correction strategies.
When multiple parameters are out of range, the order of adjustment determines whether you create new problems while fixing existing ones. The proven sequence:
| Parameter | Test Frequency | Test Method |
|---|---|---|
| Free Chlorine (FC) | Every visit | DPD FAS-DPD (Taylor K-2006) |
| Combined Chlorine (CC) | Every visit | DPD FAS-DPD (same test) |
| pH | Every visit | DPD phenol red or digital meter |
| Total Alkalinity | Every visit (or every other) | Titrimetric (Taylor K-2006) |
| Calcium Hardness | Monthly | Titrimetric (Taylor K-2006) |
| Cyanuric Acid | Monthly | Turbidimetric (melamine reagent) |
| LSI | Quarterly or when diagnosing issues | Calculated from above inputs |
| TDS | Quarterly | TDS meter or conductivity probe |
Pool chemistry is a system, not a checklist. Understanding the key interactions separates good technicians from great ones:
The most valuable thing a service professional can do after understanding these parameters is to document them consistently across time. A single test reading tells you where you are today. A six-month trend shows you:
PoolLens is built specifically for this workflow — log all six parameters per stop, review trends per account, and work offline without signal at the equipment pad. A complete chemistry record is your professional reputation on paper.
PoolLens is the offline-first chemistry log built for working pool service professionals. Enter readings at the pool, review trends by account, and never lose a test result to spotty signal again.
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