Operator terms · Water and wastewater
What is free chlorine?
Free chlorine is the chlorine still available to disinfect after demand has been met and before it combines with ammonia. It is the strongest, fastest-acting form of chlorine residual, so operators measure it to confirm the water still has disinfecting power.
Last reviewed 2026-08-03
Where it fits
When chlorine is added, some is used up as demand. What is left over is the residual. The part of that residual not bound to ammonia is free chlorine, and it does the strongest disinfecting work.
Free versus combined
Free chlorine acts fast and strong. Combined chlorine, chlorine bonded to ammonia as chloramine, acts more slowly but lasts longer in the distribution system. Total chlorine is the two added together.
Common questions
- Is free chlorine a residual?
- Yes, it is the available part of the chlorine residual that is not combined with ammonia.
- Which disinfects faster, free or combined?
- Free chlorine acts faster and stronger; combined chlorine lasts longer.
- What is total chlorine?
- Free chlorine plus combined chlorine added together.