Free vs Total vs
Monochloramine
"Chlorine" on a report can mean several different things. Measuring the wrong species is one of the most common reasons a residual looks fine on paper but the system behaves otherwise. Here is the plain-English version.
The four terms, defined
Free chlorine
The chlorine available to disinfect — chemically, hypochlorous acid (HOCl) and hypochlorite ion (OCl⁻), plus dissolved Cl₂. It is the fastest, strongest disinfectant and is what most chlorinated systems target as a residual.
Combined chlorine
Chlorine that has reacted with ammonia or organics to form chloramines — monochloramine, dichloramine, trichloramine, and organic chloramines. It still has some disinfecting power but is weaker than free chlorine.
Total chlorine
Simply free chlorine plus combined chlorine — the sum of everything. Most analyzers measure free and total directly; combined is calculated as the difference between the two.
Monochloramine
A specific, well-defined chloramine (NH₂Cl) formed deliberately when a system uses chloramination. In many large chloraminated systems it is essentially the entire chlorine residual, with very little free chlorine present.
The one equation to remember
Total chlorine = Free chlorine + Combined chlorine
Analyzers typically measure free and total directly, then report combined chlorine as the difference. In a chloraminated system, almost all of that combined chlorine is monochloramine — so total chlorine, not free chlorine, is what reflects your disinfectant residual.
Breakpoint chlorination, briefly
When chlorine is added to water containing ammonia, it does not immediately produce a free residual. It first forms chloramines, so combined (and total) chlorine rises. Add more chlorine and those chloramines are progressively destroyed — total chlorine actually drops — until the demand is satisfied at the breakpoint. Only beyond the breakpoint does additional chlorine survive as free chlorine.
The practical catch: because the curve rises, dips, then rises again, the same total-chlorine reading can occur at three different chlorine doses. A single number, in isolation, can not tell you which side of the breakpoint you are on. That is why operators on chloramination watch more than one parameter.
Which one should you measure?
Free chlorine — for free-chlorine systems
No ammonia present, disinfecting with free chlorine: measure free chlorine directly.
Total chlorine or monochloramine — for chloraminated systems
The free-chlorine DPD reaction barely responds to monochloramine, so a free reading would read near zero. Measure total chlorine or monochloramine to see your real residual.
More than one species — for nitrification risk and transitions
Watching free, total, and monochloramine together helps detect nitrification, breakpoint excursions, and the loss of residual in distribution.
sensorsMeasuring all three from one sensor
The Halogen MP5 platform reports free chlorine, total chlorine, and monochloramine — along with pH, conductivity, temperature, and ORP — from a single amperometric sensor, which is useful where systems transition between free chlorine and chloramination or need to watch for nitrification.
Frequently asked questions
What is the difference between free chlorine and total chlorine?
Free chlorine is the disinfectant that has not yet reacted — hypochlorous acid and hypochlorite ion. Total chlorine is free chlorine plus combined chlorine (the chloramines formed when chlorine reacts with ammonia or organics). Combined chlorine is the difference: total minus free.
Is monochloramine the same as combined chlorine?
Monochloramine is the main and most desirable component of combined chlorine in a chloraminated system, but combined chlorine also includes dichloramine, trichloramine, and organic chloramines. In a well-run chloraminated system, monochloramine dominates and the other chloramines are minimized.
What is breakpoint chlorination?
As chlorine is added to water containing ammonia, it first forms chloramines, raising combined chlorine. Adding still more chlorine destroys those chloramines (the curve dips), until the demand is satisfied at the breakpoint. Past the breakpoint, additional chlorine remains as a free chlorine residual. Because total chlorine can read the same value at three different doses, a single total-chlorine reading alone can be ambiguous during chloramination.
Should I measure free chlorine or total chlorine?
If your system has no ammonia and disinfects with free chlorine, measure free chlorine. If you chloraminate, total chlorine (or a direct monochloramine measurement) reflects your residual, because the DPD free-chlorine reaction does not respond significantly to monochloramine. Measuring more than one species gives a clearer picture of where you are on the breakpoint curve.
Why do chloraminated systems show almost no free chlorine?
In a chloraminated system the chlorine is intentionally bound to ammonia as monochloramine, so very little free chlorine remains. A free-chlorine measurement will read near zero even though a substantial disinfectant residual is present as monochloramine — which is why the correct parameter matters.
References & sources
- 1.Understanding Breakpoint Chlorination — definitions of free, combined, and total chlorine; the breakpoint curve and its zones — Kuntze USA. https://www.kuntzeusa.com/knowledge/understanding-breakpoint-chlorination
- 2.Free Chlorine or Total Chlorine? A Technical Overview — monochloramine chemistry and why the free DPD reagent does not respond to monochloramine — Pyxis Lab. https://www.pyxis-lab.com/free-chlorine-or-total-chlorine/
- 3.Chloramination and the Breakpoint Chlorination Curve — why systems chloraminate (DBP reduction, longer-lasting residual) and how monochloramine forms — Hach Company. https://www.hach.com/industries/drinking-water/disinfection/chloramination
- 4.MP5 / MP-TOTAL Multiparameter Chlorine Analyzer — product documentation (free chlorine, total chlorine, and monochloramine from one amperometric sensor) — Halogen Systems Inc.. https://halogensys.com/chlorine-analyzer-mp5-system/
Related guides
Monitoring a chloraminated system?
See how one sensor can track free, total, and monochloramine together.