Breakpoint chlorination is the process of adding enough free chlorine to chemically destroy all combined chlorine (chloramines) in pool water. The required dose is at least 10 times the measured combined chlorine level.
Key Facts
- The breakpoint dose is 10 times the combined chlorine level, measured in ppm.
- Below breakpoint, adding chlorine actually increases combined chlorine before it starts decreasing it.
- Breakpoint must be reached in one dose — partial shock treatments leave more combined chlorine behind.
- Water should not be re-entered until free chlorine drops below 5 ppm after breakpoint treatment.
The Chemistry Behind Breakpoint
When free chlorine reacts with ammonia in pool water, it forms chloramines in stages. At low chlorine-to-ammonia ratios, monochloramine forms and then dichloramine. As the ratio increases further, a "breakpoint" is reached where further chlorine addition converts chloramines into nitrogen gas, which escapes the pool. The breakpoint occurs at a molar ratio of about 7.6:1 (Cl:N), which approximately corresponds to 10x the combined chlorine level in ppm. Beyond the breakpoint, the pool is clear of chloramines and free chlorine rises sharply.
Calculating the Dose
Step 1: Test both free chlorine and total chlorine using a DPD test kit. Step 2: Calculate combined chlorine (CC = Total - Free). Step 3: Multiply CC by 10 to find the ppm of free chlorine you need to add. Step 4: Add the current free chlorine reading to get the target free chlorine level. Step 5: Use the pool shock calculator to determine how many pounds of shock product are needed to raise free chlorine to that target level based on your pool volume. Always use a non-stabilised shock (calcium hypochlorite or lithium hypochlorite) for breakpoint chlorination.
Executing Breakpoint Correctly
Add the full calculated shock dose after sunset — this prevents UV from destroying the high chlorine concentration before it reaches breakpoint. Pre-dissolve granular shock in a bucket of pool water before adding, or distribute liquid chlorine evenly around the pool. Run the pump and filter continuously for at least 8 hours. Test free chlorine the following morning. If it has dropped below 5 ppm, the pool is safe to enter. If it is still above 5 ppm, wait and test again before allowing swimming.
Examples
Pool test results: FC 2.0 ppm, TC 2.8 ppm. CC = 2.8 - 2.0 = 0.8 ppm. Breakpoint target = 0.8 x 10 = 8 ppm of FC to add. Target FC level = existing 2.0 + 8.0 = 10.0 ppm. For a 15,000-gallon pool, the shock calculator shows approximately 4.5 lbs of calcium hypochlorite (65% concentration) will raise FC by 8 ppm. Add after dark, run the pump overnight, and test in the morning.
Common Mistakes to Avoid
- Using a stabilised chlorine shock (trichlor or dichlor) for breakpoint — these add CYA and will not reach the required concentration as efficiently.
- Adding shock in multiple small doses over several days, which drives combined chlorine through the intermediate peak without reaching the true breakpoint.
- Not testing total chlorine before shocking — without knowing the CC level, you cannot calculate the correct dose.
- Pool & Hot Tub Alliance — Pool & Spa Operator Handbook, 2022
- Taylor Technologies — Pool/Spa Water Chemistry Reference