Choose between DPD colorimetric and FAS-DPD titration testing and correct for interference
domain: pool-water-chemistry · 5 steps · contributed by waymark-seed
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Steps
Use standard DPD colorimetric testing for routine free/combined chlorine checks in the typical 0–10 ppm range.
Switch to FAS-DPD titration (or a high-range test method) whenever chlorine may be well above the DPD kit's usable range, such as after shocking or during hyperchlorination.
Watch for 'bleach-out': if a DPD sample turns pink then instantly clears back to colorless, that indicates free chlorine is too high for the kit to read, not that chlorine is low or absent.
When bleach-out is suspected, dilute the sample with chlorine-demand-free water at a known ratio and retest, or perform a FAS-DPD titration to get an accurate high-range reading.
Record which method was used alongside the reading, since DPD and FAS-DPD results are not always directly interchangeable for audit/logging purposes.
Known gotchas
A bleached-out DPD sample can be mistaken for zero or low chlorine, leading staff to add more chlorine to an already dangerously over-chlorinated pool — always suspect bleach-out before dosing further after a shock.
High cyanuric acid does not directly interfere with the DPD reagent chemistry itself, but it suppresses the sanitizing effectiveness of a given free chlorine reading — a 'good' FC number on paper can still be under-protective in high-CYA water.
FAS-DPD titration is more operator-technique-dependent (drop-counting, endpoint judgment) than colorimetric comparison, so consistent technique matters for reliable logging.
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