2.2 Sanitation & disinfection (chlorine, bromine, breakpoint)

The Chemistry of Chlorine

Chlorine sanitizes by oxidizing and destroying microorganisms. When any chlorine source dissolves in water it forms hypochlorous acid (HOCl) and the hypochlorite ion (OCl⁻). Together these two make up free available chlorine (FAC) — the chlorine available to kill. They are not equal partners: HOCl is roughly 80-100 times more effective as a sanitizer than OCl⁻. Keeping as much of the free chlorine as possible in the HOCl form is therefore a central operator goal.

pH controls the HOCl / OCl⁻ split

The ratio between the two forms is set by pH. As pH rises, strong HOCl converts to the weak OCl⁻ ion:

At pH 7.5 only about half of your free chlorine is the strong killing form; at pH 8.0 barely a fifth is. This is the single best reason to hold pH at the ideal 7.4-7.6: it keeps sanitizer strong while staying comfortable for swimmers and gentle on surfaces. Chlorine allowed to sit at high pH is largely wasted, which is why chasing a chlorine ppm number without controlling pH is a common and costly mistake.

Free, combined, and total chlorine

• Free available chlorine (FAC): HOCl + OCl⁻ — chlorine still available to sanitize.
• Combined chlorine (CC): chlorine that has already reacted with ammonia and nitrogen compounds (from sweat, urine, and cosmetics) to form chloramines. Chloramines are weak sanitizers and cause the sharp "chlorine smell," red eyes, and irritation that swimmers wrongly blame on too much chlorine.
• Total chlorine (TC): FAC + CC. Therefore CC = TC − FAC. Combined chlorine should be kept below about 0.2-0.3 ppm.

Breakpoint chlorination

Chloramines are removed by oxidizing them — by pushing free chlorine high enough to burn through the combined chlorine. The rule of thumb is that you must raise free chlorine to roughly ten times the combined chlorine to reach the breakpoint, the point at which chloramines are destroyed and only free chlorine remains.

Worked example: a pool tests total chlorine 3.0 ppm and free chlorine 2.4 ppm. Combined chlorine = 3.0 − 2.4 = 0.6 ppm. To reach breakpoint you need free chlorine of about 10 × 0.6 = 6 ppm at the pool. Adding too little only creates more chloramines and worse odor, which is why partial dosing fails — you must clear the breakpoint in a single decisive push, not in timid increments.

Chlorine Sources

Different products deliver chlorine with different side effects on pH and cyanuric acid, and choosing a source is really about managing those side effects.

Trichlor and dichlor are stabilized — they carry cyanuric acid, so continuous use steadily raises CYA and can "lock" chlorine if CYA climbs too high. Cal-hypo and liquid are unstabilized and raise pH, so they are commonly paired with acid additions to hold pH in range.

Cyanuric acid — sunscreen for chlorine

Outdoor pools lose free chlorine rapidly to sunlight, so cyanuric acid (CYA) — also called stabilizer or conditioner — is added to shield it. CYA forms a loose bond with chlorine that blocks UV and dramatically slows that loss; the ideal range is 30-50 ppm. Below about 30 ppm, outdoor chlorine can burn off in a few hours; above roughly 50-100 ppm, CYA begins to slow chlorine's kill rate — the effect casually called "chlorine lock," where high stabilizer holds a chlorine reading but weakens its punch. Because trichlor and dichlor add CYA with every dose, stabilizer creeps upward over a season and is corrected only by partial draining and dilution. Always test CYA before assuming a low chlorine reading means the pool simply needs more chlorine.

Bromine for Spas

Hot spas break chlorine down quickly and drive off gas, so many spas use bromine. Bromine forms hypobromous acid (HOBr); unlike chloramines, bromamines remain effective sanitizers, so bromine tolerates bather waste and higher pH better than chlorine. Bromine is more stable in hot water and has a milder odor. Its drawback is that it is not protected by cyanuric acid and is destroyed quickly by sunlight, so it is used mainly on indoor spas.

ORP — Measuring Killing Power

Oxidation-reduction potential (ORP), read in millivolts, measures the oxidizing strength of the water rather than a chlorine concentration in ppm. A common target is 650-750 mV. Automated controllers dose sanitizer to hold an ORP setpoint, but because ORP is affected by pH and CYA it supplements — never replaces — direct ppm testing.