Breakpoint chlorination refers to achieving a chlorine level where the disinfection demand is satisfied, meaning all undesirable substances in the pool water have been oxidized or neutralized.

Think of as a tipping point, at which oxidation is complete, and further additions of shock chlorine or other oxidizer are unnecessary and unfruitful.

For pool owners, breakpoint chlorination is a threshold level of free chlorine that is sufficient to fully remove chloramines, or combined chlorine (CC) from the pool water. If you don’t quite reach the threshold, many chloramines and other contaminants will survive, and if you overshoot the mark, you’ll have used more oxidizer than is necessary.

Chloramines form when free chlorine combines with nitrogen, ammonia, or other substances, rendering it ineffective as a sanitizer. These compounds are responsible for the strong "chlorine" odor in pools; while free chlorine itself has little smell, chloramines are foul-smelling, irritate the eyes, and do not disinfect. Therefore, they need to be removed regularly.

The generally accepted formula for breakpoint chlorination is 10x the level of Chloramines in the pool to reach the breakpoint threshold. For example, if your tested level of CC is 0.5 ppm, you would add enough shock to reach 5.0 ppm – and if your CC is 1.2 ppm, you would shock the pool to a level of 12.0 ppm. Ten times the level of chloramines, as tested by a DPD test kit.

However, if your pool also has severe algae or bacterial infection, or has a large amount of organic debris; the amount will need to be increased to account for the loss of residual used to fight contaminants other than chloramines.

In cases of severe water contamination, a factor of 20x or 30x the level of CC should be used, or the pool can be tested for combined chlorine afterwards, and treated again if necessary. For clean and clear pools, a factor of 10x the level of Chloramines is normally sufficient to reach breakpoint chlorination.

That depends on which type of chlorine you use, and what the level of Chloramines in the water, and also other contaminants like algae or bacteria ‑ but for example, in 10,000 gallons of pool water – in a clean, clear and balanced pool:

• 1 gallon of 6% Bleach will add 6.2 ppm of FC
• 1 pound of 56% Dichlor will add 6.6 ppm of FC
• 1 pound of 65% Cal Hypo will add 7.7 ppm of FC

It’s always best to add more pool shock than you think is needed to reach breakpoint; just to be sure that you have killed contaminants and broken chloramine bonds, and even more if your pool has a level of cyanuric acid, or if you use Trichlor chlorine tablets.

Cyanuric acid is used to protect the chlorine molecule from degradation by UV rays. In many sunny pools, operating without cyanuric acid would require 3-4x more chlorine than a pool with a residual of 30-50 ppm of cyanuric acid.

Cyanuric acid protects chlorine from the sun by limiting its activity, but in this process the chlorine molecule becomes a less effective killing organism. A percentage of efficacy or killing power is lost due to cyanuric acids influence. How much is lost depends on how much cyanuric acid is in the water, but something in the magnitude of 20-50% of chlorine is suppressed by CYA.

To compensate for this, it is recommended to increase pool shock dosages, to reach breakpoint chlorination in a pool using cyanuric acid. A good rule of thumb is to increase the percentage by the residual of cyanuric acid. For example, if your cyanuric acid level is 30, increase your shock dosage by 30%, and if CYA=50, increase shock dosage by 50%.

Pool water pH dramatically affects chlorine's power. In high pH conditions, more than half of the chlorine you add can become sluggish and ineffective, essentially wasting it. This reduced chlorine activity substantially increases the amount of oxidizer needed for breakpoint chlorination.

Before shocking the pool, always check pH and alkalinity levels and adjust alkalinity to 80-100 ppm and pH to 7.2. After breakpoint chlorination has been reached, or chloramine levels have been reduced to zero, raise the pH to a slightly more basic level of 7.4 for normal operation.

• Pool Chlorine Tablets
• Pool Shock
• Cyanuric Acid
• Cal Hypo Shock
• Dichlor Shock

Swimming Pool Shock User Guide