Combined chlorine and cyanuric-acid correction

Reviewed by the Pool Operator Prep editorial team. Worked from public water-chemistry and hydraulics facts. Last verified: 2026-07-24.

How do you calculate combined chlorine and the cyanurate alkalinity correction?

Combined chlorine is total chlorine minus free chlorine; 0.2 ppm or more usually calls for a shock. For the saturation index, subtract the cyanurate contribution from total alkalinity: cyanuric acid times a pH factor (about 0.32 at pH 7.5) gives the cyanurate alkalinity, and the remainder is the carbonate alkalinity you actually use.

Combined chlorine = total chlorine - free chlorine; at or above about 0.2 to 0.4 ppm it signals a shock. For the saturation index, carbonate alkalinity = total alkalinity - (cyanuric acid x a pH factor), because cyanurate contributes to the measured total alkalinity but not to the carbonate balance.

The formula, worked

Formula - combined chlorine and corrected alkalinity

CC = TC − FC ; carbonate alk = TA − ( CYA × factor )

Worked example

Total chlorine 3.2 ppm, free chlorine 2.8 ppm -> CC = 3.2 − 2.8 = 0.4 ppm (shock)Total alkalinity 110 ppm, cyanuric acid 40 ppm, pH 7.5 -> factor 0.32Cyanurate alkalinity: 40 × 0.32 = 12.8 ppmCarbonate alkalinity: 110 − 12.8 = 97.2 ppm

= CC 0.4 ppm (shock); use 97 ppm carbonate alkalinity in the LSI

Common slip Subtracting free from total in the wrong order (a negative combined chlorine is impossible), or skipping the cyanurate correction so the alkalinity factor is overstated and the saturation index reads more scale-forming than the water really is. The pH factor rises with pH; interpolate between the table points.

Two overlapping circles representing free and combined chlorine, with a coral shield above the overlap standing in for the cyanuric-acid correction applied to the reading. — Combined chlorine is the overlap; the coral shield is the CYA correction the exam expects you to apply.

Where this shows up

Combined chlorine sets the breakpoint target, and the corrected carbonate alkalinity feeds straight into the saturation index. The pool chemistry calculator applies the same cyanurate correction from tool-data.json before it computes the LSI.

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Problem set: 6 worked problems

Each problem is original, authored from the underlying non-copyrightable relationship, with the full worked solution and the exact slip that produces each wrong answer. Work the problem before opening the solution. For a timed, randomized run with a per-topic score, use the free knowledge practice test.

Watch for Two subtractions trip operators here. Combined chlorine is total minus free, never the reverse, so a negative answer is impossible. And the saturation index needs carbonate alkalinity, which is total alkalinity minus the cyanurate contribution; skipping that correction reads the water as more scale-forming than it is.

Total chlorine is 3.0 ppm and free chlorine is 2.6 ppm. What is the combined chlorine?
- A 5.6 ppm
- B 2.6 ppm
- C 0.0 ppm
- D 0.4 ppm
Show the worked solution

Correct answer: D. 0.4 ppm

Combined chlorine = total - free = 3.0 - 2.6 = 0.4 ppm, which is at the shock threshold. Adding the two gives 5.6, a common misread; combined chlorine is always the difference, not the sum.

Source: Public relationship: combined chlorine = total chlorine - free chlorine
Total chlorine is 2.8 ppm and free chlorine is 2.8 ppm. What is the combined chlorine?
- A 5.6 ppm
- B 2.8 ppm
- C -2.8 ppm
- D 0.0 ppm (no combined chlorine)
Show the worked solution

Correct answer: D. 0.0 ppm (no combined chlorine)

2.8 - 2.8 = 0.0 ppm, meaning all the chlorine is free and none is tied up as chloramine, the ideal. Total chlorine can never be less than free chlorine, so a negative combined value signals the readings were entered in the wrong order.

Source: Public relationship: combined chlorine = total chlorine - free chlorine
Total alkalinity 110 ppm, cyanuric acid 40 ppm, pH 7.5 (cyanurate factor 0.32). What carbonate alkalinity goes into the LSI?
- A 110 ppm
- B 70 ppm
- C 123 ppm
- D About 97 ppm
Show the worked solution

Correct answer: D. About 97 ppm

Cyanurate alkalinity = 40 x 0.32 = 12.8 ppm; carbonate alkalinity = 110 - 12.8 = 97 ppm. Using the full 110 ppm overstates the alkalinity factor; subtracting the whole 40 ppm cyanuric acid instead of the corrected 12.8 gives 70 ppm.

Source: Public cyanurate correction: carbonate alkalinity = total alkalinity - (cyanuric acid x pH factor)
Total alkalinity 90 ppm, cyanuric acid 30 ppm, pH 7.6 (cyanurate factor 0.35). What carbonate alkalinity goes into the LSI?
- A 90 ppm
- B 60 ppm
- C 100 ppm
- D About 80 ppm
Show the worked solution

Correct answer: D. About 80 ppm

Cyanurate alkalinity = 30 x 0.35 = 10.5 ppm; carbonate alkalinity = 90 - 10.5 = 79.5, about 80 ppm. The factor rises with pH, so the same cyanuric acid subtracts a little more at 7.6 than at 7.5.

Source: Public cyanurate correction: carbonate alkalinity = total alkalinity - (cyanuric acid x pH factor)
Which alkalinity value belongs in the Langelier Saturation Index?
- A The total alkalinity as measured
- B The combined chlorine
- C The calcium hardness
- D The carbonate alkalinity, after subtracting the cyanurate contribution
Show the worked solution

Correct answer: D. The carbonate alkalinity, after subtracting the cyanurate contribution

The saturation index balances carbonate chemistry, so it uses carbonate alkalinity, which is total alkalinity minus the part contributed by cyanurate. Plugging in the raw total alkalinity overstates the alkalinity factor.

Source: Public Langelier method: the alkalinity term is carbonate alkalinity, not total alkalinity
An operator skips the cyanurate correction and uses the full total alkalinity in the LSI. How does that bias the result?
- A It understates the LSI, hiding scale risk
- B It has no effect on the LSI
- C It changes the combined chlorine
- D It overstates the alkalinity factor, so the LSI reads more scale-forming than the water is
Show the worked solution

Correct answer: D. It overstates the alkalinity factor, so the LSI reads more scale-forming than the water is

The alkalinity factor is a logarithm of carbonate alkalinity, so using a value that is too high inflates the factor and pushes the whole index up, toward scale-forming. The bias is always in the scale-forming direction because the correction only ever subtracts.

Source: Public Langelier method: an overstated alkalinity term raises the LSI

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