Pool chemistry looks intimidating on paper — logarithmic pH scales, ppm targets, carbonate equilibria — but in practice, it comes down to six measurable parameters that interact in predictable ways. Understand these six, know their ideal ranges and what goes wrong outside them, and you can diagnose and treat 95% of pool chemistry problems you will encounter in the field. This guide is written for new technicians and anyone who wants to build a solid foundation before diving into advanced topics.
| Parameter | Ideal Range | Test Frequency | Primary Risk If Off |
|---|---|---|---|
| Free Chlorine (FC) | 1–4 ppm (chlorine) / 2–6 ppm (SWG) | Every visit | Algae, pathogens |
| pH | 7.2–7.8 | Every visit | Chlorine inefficiency; corrosion or scaling |
| Total Alkalinity (TA) | 80–120 ppm | Monthly | pH bounce (low) or pH lock (high) |
| Calcium Hardness (CH) | 200–400 ppm | Monthly | Etching (low) or scaling (high) |
| Cyanuric Acid (CYA) | 30–50 ppm outdoor / 60–80 ppm SWG | Monthly | UV destruction (low) / chlorine lock (high) |
| Combined Chlorine (CC) | Less than 0.5 ppm | Every visit | Chloramine odor, eye irritation, bather risk |
Free chlorine is the active sanitizing agent in a pool — specifically, hypochlorous acid (HOCl), a weak acid that penetrates cell walls and destroys bacteria and algae. It is what your test kit measures as "free chlorine."
Free chlorine is consumed by UV light, organic matter (bathers, leaves, debris), and chemical reactions. Without maintenance, it degrades continuously. The job of pool chemistry is to keep it in the effective range at all times.
pH measures how acidic or basic the water is on a logarithmic scale from 0–14. Pool water should stay between 7.2 and 7.8. This range matters for two reasons: swimmer comfort (7.2–7.8 closely matches the human eye's natural pH) and chlorine effectiveness.
The fraction of free chlorine that exists as active HOCl is highly pH-dependent:
| pH | Active HOCl % | Practical Implication |
|---|---|---|
| 7.0 | 73% | Very effective; slightly acidic for swimmers |
| 7.2 | 66% | Good — recommended lower bound |
| 7.4 | 55% | Ideal for comfort and efficacy balance |
| 7.6 | 45% | Still acceptable |
| 7.8 | 33% | Upper bound — chlorine noticeably less effective |
| 8.0 | 22% | Problematic; feels "low chlorine" even when it's not |
| 8.4 | 10% | Dangerous — pool appears balanced but poorly sanitized |
Total alkalinity is the water's buffering capacity — its ability to resist pH change. Think of it as pH's shock absorber. Water with proper alkalinity (80–120 ppm) holds pH stable between acid additions and chlorine dosing. Water with low alkalinity experiences "pH bounce" — large pH swings in response to small inputs. Water with very high alkalinity becomes "pH locked" — hard to adjust with normal acid doses.
TA is controlled with:
Calcium hardness measures dissolved calcium in the water. At the correct level, water is "satisfied" — it doesn't need to leach calcium from surfaces or deposit excess calcium as scale. Below 200 ppm, water aggressively attacks plaster, grout, and concrete. Above 400 ppm, the water deposits calcium carbonate as white scale on tile, equipment, and heater tubes.
CH is raised with calcium chloride (CaCl₂) and can only be lowered by dilution (partial drain-and-refill) or reverse osmosis filtration.
CYA is the stabilizer that protects chlorine from UV destruction. In outdoor pools without CYA, sunlight can destroy 50–90% of free chlorine in a few hours. With proper CYA (30–50 ppm), UV protection extends chlorine residual through the entire day.
Combined chlorine (also called chloramines) forms when free chlorine reacts with ammonia from bather waste — sweat, urine, and body oils. Chloramines are much weaker sanitizers than HOCl but create the characteristic "pool smell" that most people associate with "too much chlorine." Paradoxically, strong chlorine odor usually means there is too little free chlorine relative to bather load — not too much.
CC above 0.5 ppm requires superchlorination (shock) to break down chloramines. The target is always below 0.5 ppm; ideally, CC reads zero at every test.
The order you test and add chemicals matters. A wrong sequence causes interactions between chemicals and inaccurate readings.
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Open SplashLens Free →The six essential pool chemistry parameters are: (1) Free Chlorine (FC), (2) pH, (3) Total Alkalinity (TA), (4) Calcium Hardness (CH), (5) Cyanuric Acid (CYA), and (6) Combined Chlorine (CC). Each plays a distinct role in water safety, surface protection, and equipment longevity.
Test in this order: (1) Free Chlorine, (2) Combined Chlorine/Total Chlorine, (3) pH, (4) Total Alkalinity, (5) Calcium Hardness, (6) CYA. Test before making any chemical additions, and test again 30–60 minutes after major adjustments.
Add chemicals in this sequence: (1) Alkalinity adjuster first, (2) Wait 30–60 min and retest, (3) pH adjuster, (4) Wait 30 min, (5) Chlorine last. Never add two chemicals simultaneously. Always wait at least 15–30 minutes between different chemical additions with the pump running.
pH controls chlorine effectiveness more than any other parameter. At pH 7.0, approximately 73% of free chlorine is the active HOCl form. At pH 7.8, only 33% is active. At pH 8.2, only 10% is active. High pH causes apparent chlorine deficiency even when FC readings look fine.