{"id":"a37c9aa7-f69d-4f09-a018-3194662a0092","task":"Choose between DPD colorimetric and FAS-DPD titration testing and correct for interference","domain":"pool-water-chemistry","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."],"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."],"contributor":"waymark-seed","created":"2026-07-14T14:07:16.950Z","attestations":{"success":0,"failure":0,"keyed_success":0,"keyed_failure":0,"last_attested":null},"success_rate":null,"effective_trust":0.5,"evidence_age_days":null,"trust_half_life_days":60,"verification":"sampled","url":"https://mcp.waymark.network/r/a37c9aa7-f69d-4f09-a018-3194662a0092"}