Chlorine tablets (trichlor) are the most popular sanitizer choice for residential pools — and for good reason. They are convenient, slow-dissolving, and require minimal daily attention. But liquid chlorine (sodium hypochlorite) is the professional standard for a reason too. Understanding when to use each — and the real trade-offs — is one of the most practical things a pool tech can know.
Liquid chlorine is a water solution of sodium hypochlorite (NaOCl), typically sold at 10–12.5% available chlorine for pool use. It is essentially concentrated bleach — the same chemistry as household bleach but at higher concentration. No stabilizer, no added calcium, no pH buffers — just chlorine and water. Its natural pH is approximately 11–13.
Trichlor tablets are a solid chlorine compound — trichloroisocyanuric acid (Cl₃CY) — typically 90% available chlorine by weight. But the remaining chemistry matters enormously: trichlor is approximately 52–57% cyanuric acid by formula weight. The tablet dissolves slowly in a floater or inline feeder, releasing both chlorine and CYA simultaneously. Its pH in solution is approximately 2.8–3.1 — strongly acidic.
| Factor | Liquid Chlorine | Trichlor Tablets |
|---|---|---|
| Available chlorine | 10–12.5% | ~90% |
| Cost per lb of Cl₂ | $0.30–0.50 | $1.00–1.50 |
| Adds CYA? | No | Yes (~6 ppm per 10 ppm Cl₂) |
| pH effect | Slightly raises pH (pH 11+) | Significantly lowers pH (pH 2.8) |
| Adds calcium? | No | No |
| Adds sodium? | Yes (trace amounts) | No |
| Shelf life (unopened) | 6–12 months (degrades ~5%/mo at room temp) | 2–5 years if stored dry and cool |
| Ease of use | Requires handling (gloves, goggles) | Very convenient — floater or feeder |
| Safe for SWG pools? | Yes — preferred | Caution — CYA accumulation |
| Indoor pools | Yes — preferred | No — CYA serves no purpose indoors |
This is the issue that trips up the most homeowners — and some service techs. Every 3-inch trichlor tablet (typically 8 oz) dissolves and releases approximately 0.5 oz of CYA alongside its chlorine. In a 10,000-gallon pool using one tablet per week, that adds roughly 2–3 ppm of CYA per week, or 40–60 ppm per 20-week swimming season.
In the first year, this is actually helpful — the pool builds up needed CYA protection. By years two and three without any dilution, CYA can reach 100–150 ppm, at which point:
Trichlor's pH of 2.8 means every tablet that dissolves is pushing pool pH downward. This seems like it might offset the natural pH rise from aeration and off-gassing of CO₂ — and in practice, it often does. Many tablet-fed pools hold pH remarkably stable without acid additions.
But this can create a false sense of stability. The pH is being maintained by two competing forces (natural rise vs trichlor acidity) rather than by proper chemistry. When one force changes — higher bather load, a rainstorm, reduced sun — pH can swing unpredictably.
Liquid chlorine's pH of ~12 pushes pH slightly upward, but the effect per normal dose is minor. A 1-gallon dose of 12.5% liquid chlorine in a 15,000-gallon pool raises pH by approximately 0.1 or less.
Liquid chlorine degrades significantly with time and heat. At 77°F (25°C), sodium hypochlorite loses approximately 5% of its potency per month. At 95°F (35°C, common in a pool shed in summer), the degradation rate doubles. Liquid chlorine bought in spring and stored until fall may be down to 50–60% labeled strength by August.
Trichlor tablets are dramatically more stable. Properly stored dry and away from heat, tablets maintain 90%+ potency for 2–5 years. This makes them advantageous for customers in cold-climate areas who buy chemicals in bulk.
SplashLens lets you note each pool's chlorine source and track CYA trends over time. Know which accounts are approaching the CYA ceiling before they get there. Works offline at every stop.
Open SplashLens Free →Liquid chlorine is better for pools with already-high CYA levels, salt pools, indoor pools, and any situation requiring precise FC control. Trichlor tablets are more convenient for maintenance but continuously add CYA and lower pH — they require more monitoring.
Trichlor tablets are approximately 52–57% cyanuric acid by weight. Each pound of trichlor adds about 6 ppm of CYA to a 10,000-gallon pool. A pool using one 3-inch tablet per week through a 22-week season can accumulate 50–80 ppm of CYA from tablets alone.
Liquid chlorine (sodium hypochlorite) has a pH of approximately 11–13. When added to pool water, it raises pH slightly. For normal doses in a properly buffered pool, the effect is minor — typically 0.1 or less per gallon added to a 15,000-gallon pool.
Liquid chlorine added to an outdoor pool without CYA stabilizer can lose 50–90% of its potency in 2 hours of direct sun exposure. With CYA at 30–50 ppm, residual typically lasts 6–12 hours in full sun. Liquid chlorine in the jug degrades at approximately 5% per month at room temperature, faster in heat.