Most test strips and kits report three numbers: free chlorine (FC), combined chlorine (CC), and total chlorine (TC = FC + CC). Each one tells you something different about the pool's sanitation.
The two active forms of chlorine
When you add chlorine to water, it splits into two forms depending on pH:
- HOCl (hypochlorous acid): the active, strong form. Kills pathogens quickly.
- OCl⁻ (hypochlorite ion): a much weaker form. Much slower to disinfect.
Both count as “free chlorine” on the test — but their kill-power differs by ~100×.
The pH/HOCl table (memorize this)
| pH | % HOCl (active) | % OCl⁻ (weak) |
|---|---|---|
| 6.5 | ~90% | ~10% |
| 7.0 | ~75% | ~25% |
| 7.4 | ~57% | ~43% |
| 7.5 | ~50% | ~50% |
| 7.8 | ~33% | ~67% |
| 8.0 | ~22% | ~78% |
This is why a pool with 3 ppm FC at pH 8.0 sanitizes worse than a pool with 2 ppm FC at pH 7.4. Same chlorine level — very different effectiveness.
Combined chlorine: what it actually is
When free chlorine bonds with ammonia (from urine, sweat, organic matter), it forms chloramines:
- Monochloramine (NH₂Cl): relatively stable, modest irritant
- Dichloramine (NHCl₂): stronger smell, strong irritant
- Trichloramine (NCl₃): the “chlorine smell” in public pools and the main respiratory irritant
All three register as combined chlorine on a DPD test. The “chlorine smell” people blame on “too much chlorine” is actually too little chlorine— there's not enough free chlorine to destroy the chloramines.
How to read a DPD test correctly
A standard DPD test block:
- Fill the sample vial to the line with pool water (from elbow depth, away from returns).
- Add DPD Reagent 1 drops. Cap and invert. Read color — this is free chlorine (FC).
- Add DPD Reagent 3 drops to the same sample. Cap and invert. Read new color — this is total chlorine (TC).
- Combined chlorine = TC − FC.
For precision, use FAS-DPD titration (e.g., Taylor K-2006) — reads to 0.2 ppm instead of 0.5 ppm color-block resolution.
Target numbers
- FC: 1–3 ppm (residential, unstabilized) / 3–5 ppm typical with CYA 30–50 ppm
- CC: < 0.2 ppm
- If CC is > 0.5 ppm: shock to breakpoint (10×CC)