Water chemistry

Combined Chlorine and Breakpoint Chlorination

Measure combined chlorine, calculate the breakpoint FC target at 10 times CC and dose it, with a chart and a worked 15,000 gallon example.

Combined chlorine (CC) is chlorine that has already reacted with swimmer waste and organics. It smells sharp, stings eyes and does little sanitizing. The goal is under 0.2 ppm. When it climbs, you raise free chlorine (FC) high enough to burn it off. That step is called breakpoint chlorination.

Measure CC

CC = total chlorine (TC) - free chlorine (FC).

1. Run the DPD test for FC and record it.
2. Run the DPD test for TC and record it.
3. Subtract FC from TC.

Example: FC 1.5, TC 2.3, so CC = 0.8 ppm. That is above the 0.2 goal.

Two testing traps:

• At very high FC, DPD color can bleach out and read low or even zero. A clear sample after the reagent does not mean zero chlorine. Dilute the sample with chlorine-free water and multiply the result.
• Non-chlorine shock products can read like chlorine on a DPD test. Follow the label's wait time before you test.

The breakpoint rule

Raise FC to about 10 times CC. Hold it until CC drops.

The dose column uses 10.2 fl oz of 12.5% liquid chlorine per ppm per 10,000 gallons. Scale it by gallons divided by 10,000 and adjust for your actual starting FC.

Worked example: 15,000 gallon pool

Readings: FC 1.5, TC 2.3, CC 0.8, CYA 50. The multiplier is 15,000 / 10,000 = 1.5.

1. Breakpoint target: 10 x 0.8 = 8 ppm FC.
2. FC to add: 8 - 1.5 = 6.5 ppm.
3. Liquid chlorine, 12.5%: 10.2 fl oz x 6.5 x 1.5 = about 99 fl oz, a little under a gallon.
4. Cal hypo, 65%: 2 oz x 6.5 x 1.5 = about 19.5 oz by weight, around 1.2 lb. Cross-check: 1 lb raises FC about 7.8 ppm per 10,000 gallons, which is about 5.2 ppm in 15,000, so 6.5 ppm takes about 1.25 lb.

Steps at the pool:

1. Add the chlorine at dusk with the pump running. Dissolve cal hypo in a bucket of water first, adding the product to the water.
2. Let the pool circulate, then retest FC. Top it up to 8 ppm if needed.
3. Check FC again in the morning. If FC fell only a little overnight and the water is clear, CC should be dropping.
4. Retest CC. Run another cycle if it is still above 0.2.

When 10 times CC is not enough

A SLAM-style shock level is about 40% of CYA. At CYA 50 that is 20 ppm. If the pool has algae, cloudy water or a persistent smell, shock to that level instead of 10 times CC. If CC is high and the water looks clean, 10 times CC is usually the working number. For algae and green water, see green pool cleanup.

If CYA is high, a SLAM-style shock gets expensive, and dilution may come first. See high CYA and what to do.

Decide fast

• CC under 0.2 ppm: no action. Keep FC at the CYA minimum.
• CC above 0.2 ppm and the water is clear: breakpoint at about 10 times CC.
• CC above 0.2 ppm with algae, cloudy water or a strong smell: shock to about 40% of CYA.

What builds CC

• Swimmers, sweat, sunscreen and oils.
• Leaves, pollen and organic debris.
• FC kept too low for the CYA, so chlorine cannot keep up.
• Heavy use after a party without a follow-up dose.

A sharp smell tells you CC is high. A pool with plenty of FC and clean water has little smell.

Keep CC low

• Hold FC at about 7.5% of CYA or more. See chlorine types compared if the sanitizer is part of the problem.
• Dose after a heavy-use day.
• Keep the filter clean and the pump running long enough.
• Brush, skim and remove debris.
• Test TC and FC together at least on pools that smell.

Mistakes

• Reading TC as if it were FC.
• Adding chlorine without testing, then guessing CC from the smell.
• Using 10 times CC on a pool that needs a full algae shock.
• Testing a very high FC sample without dilution.
• Skipping the overnight check.
• Letting swimmers in while FC sits at breakpoint levels. Follow the label and local code for when swimming can resume.

Safety

Follow the product label and SDS. Never mix chemicals, especially chlorine with acid or with other chlorine products. Add chemical to water, never water to chemical. Keep swimmers out until FC returns to a safe level per the label and local code.

The chlorine dose tool does the volume math for any target.