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Commercial Pool DPD Titration to ANSI/APSP/ICC-11 Standards

SOP Compliance Updated Jun 19, 2026 ยท Pool Service

Why this matters

Health departments inspecting public, semi-public, and commercial pools accept FAS-DPD titration as the reference method for free and combined chlorine. The colorimetric DPD-1/DPD-3 tablet method that most residential techs run reads only to roughly 5 ppm before bleach-out, which is unusable when shock residuals or commercial high-bather-load chemistry pushes free chlorine above that ceiling. ANSI/APSP/ICC-11 Standard for Water Quality in Public Pools and Spas requires accurate measurement across the operating range; FAS-DPD titration delivers 0.2 ppm resolution from 0 to roughly 20 ppm with no bleach-out. A misread on a commercial spa during a pre-opening inspection can trigger a same-day closure under the local health code.

Equipment and reagents

  • Titrator: graduated 25 mL or 50 mL burette, or a calibrated drop bottle dispensing 0.2 ppm per drop on a 25 mL sample (Taylor K-2006 / K-2106 commercial kits use this geometry).
  • FAS (ferrous ammonium sulfate) titrant standardized to deliver 0.2 ppm Cl as Cl2 per drop on a 25 mL sample, or 0.5 ppm per drop on a 10 mL sample.
  • DPD Powder #1 (free chlorine indicator), DPD Powder #3 (total chlorine indicator after iodide reaction).
  • 25 mL sample tube with white reference background.
  • Sample bottle rinsed three times with source water before the working aliquot.

Free chlorine titration procedure

  1. Rinse the sample tube three times with the water being tested.
  2. Fill to the 25 mL line, level read at the meniscus bottom.
  3. Add one level dipper of DPD Powder #1, swirl until dissolved. Pink color indicates free available chlorine; deeper pink indicates higher residual.
  4. Hold the FAS titrant dropper vertically; add one drop at a time, swirl after each drop.
  5. Count drops until the pink color fades to colorless against the white background. The first drop that produces a stable colorless endpoint is the count. Do not over-titrate; the endpoint is sharp.
  6. Multiply drops by the kit factor (0.2 ppm per drop on 25 mL, 0.5 ppm per drop on 10 mL). Record as Free Chlorine in ppm.

Combined chlorine and total chlorine

  1. Without dumping the titrated sample, add one level dipper of DPD Powder #3 to the now-colorless tube. The pink color returns proportional to combined chlorine (chloramines plus organic-N chloramines).
  2. Continue titrating with FAS, one drop at a time, swirling between drops.
  3. Count only the additional drops to a stable colorless endpoint. Multiply by the kit factor. This is Combined Chlorine.
  4. Total Chlorine = Free + Combined. ANSI/APSP/ICC-11 sets combined chlorine at 0.4 ppm or less in indoor pools and spas; many state codes require closure or superchlorination above this threshold.

Calibration, QC, and recordkeeping

  • Standardize the FAS dropper against a fresh chlorine standard each season; replace reagents annually or at the date stamped on the bottle, whichever is earlier. FAS degrades faster in hot vehicle compartments.
  • Run a duplicate titration on every fifth sample on a high-bather-load day; results should agree within 0.2 ppm.
  • Log free chlorine, combined chlorine, pH, temperature, total alkalinity, calcium hardness, cyanuric acid, ORP setpoint, and titration time in the bound pool log required by most state public-pool codes. Many jurisdictions require entries every two hours during operating hours.

Do not substitute DPD colorimetric strips or colorwheel comparators for FAS-DPD titration on a public-pool inspection day. The colorimetric methods bleach out above roughly 5 ppm free chlorine and read low when combined chlorine is high. Recording an inaccurate residual that triggers a swimmer illness investigation is a regulatory and insurance exposure that the technician personally signs the log for.

Troubleshooting endpoint problems

  • No pink color after DPD #1: chlorine is zero, or the sample was over-chlorinated to the point of immediate bleach-out. Dilute the sample 1:1 with chlorine-demand-free water, retest, and double the result.
  • Color returns after the endpoint: chloramines are reconverting; complete the combined-chlorine step.
  • Yellow-green tint instead of pink: copper interference (algaecide carryover) or very high chloramines. Add EDTA or use a copper-masking reagent per kit instructions.
  • Cloudy sample: filter through a #1 filter paper before titration; turbidity hides the endpoint.

References

  • ANSI/APSP/ICC-11 American National Standard for Water Quality in Public Pools and Spas.
  • Pool & Hot Tub Alliance / NSPF Pool & Spa Operator Handbook, current edition, FAS-DPD titration chapter.
  • Taylor Technologies K-2006 / K-2106 Pool & Spa Water Test Kit instruction sheet.
  • CDC Model Aquatic Health Code 2023 Edition, Section 5.7 Water Quality Testing and Monitoring.

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