Easy and Rapid Application for Residual Chlorine Analysis in Water Samples

Significance of the topic

Accurate and rapid measurement of residual chlorine (free and combined) in water is essential for ensuring effective disinfection while minimizing formation of harmful disinfection by-products. In recreational waters such as swimming pools, monitoring free residual chlorine (FRC) and combined/total residual chlorine (CRC/TRC) prevents under- or overdosing, reduces user irritation and odor problems, and limits formation of chloramines and other toxic species. The unstable nature of free chlorine requires analytical workflows that minimize exposure to air and delays between sampling and measurement.

Objectives and overview of the study

This work evaluated Thermo Scientific Gallery discrete analyzer applications (STAT-Protocol) for rapid measurement of free residual chlorine (FRC), total residual chlorine (TRC) and combined residual chlorine (CRC) in pool water. The Gallery results were compared with manual DPD spectrophotometric reference methods based on EPA 330.5 and SFS-EN ISO 7393-2:2000. Key aims were to assess method correlation, precision and recovery across low and high concentration ranges and to document practical operational considerations for routine laboratory use.

Methodology

• Analytical principle: DPD (N,N-diphenyl-p-phenylenediamine) colorimetric reaction producing a red color measured at ~510 nm (some methods report 515 nm for specific protocols). FRC is measured directly (no KI). TRC measurement requires potassium iodide (KI) to liberate iodine from oxidants; liberated iodine reacts with DPD.
• Gallery STAT-Protocol: discrete analyzer minimizes sample exposure to air by automatic, immediate analysis after sample insertion. Reagents RC R1 (phosphate buffer), RC R2 (DPD reagent) and RC R3 (potassium iodide solution for TRC) are dispensed into disposable cuvettes, sample is added, and absorbance measured at 510 nm.
• Calibration: a 0.02 M KMnO4 stock (equivalent 3547 mg/L Cl2) was used as stable calibrator; working standards prepared to cover low (0.1–0.5 mg/L) and high (0.5–3.0 mg/L) ranges. Calibration can be performed manually or automatically by the analyzer.
• Incubation: TRC protocols include a short incubation; in Gallery TRC High/Low the final incubation was ~60 s at 37°C (shorter than standard methods due to elevated temperature), with order of reagent/sample addition varying between low and high protocols.
• Samples and QC: field samples were collected from several public pools (FRC 0.12–0.58 mg/L; TRC 0.11–0.89 mg/L). Spiked samples were prepared to extend range up to ~3 mg/L. Quality control materials were prepared from sodium hypochlorite and verified by titration.

Used instrumentation

• Thermo Scientific Gallery discrete analyzer (Gallery, Gallery Plus, or Aquakem analyzer platforms) running STAT-Protocol applications for FRC, TRC and CRC.
• Thermo Scientific Multiskan GO spectrophotometer used for manual DPD reference measurements (510 nm).
• Reagents: DPD reagents (RC R2), phosphate buffer (RC R1), potassium iodide solution (RC R3) and KMnO4 calibrator stock.

Main results and discussion

• Method correlation: strong agreement between Gallery applications and manual DPD reference methods across tested ranges with regression results reported as follows:
  
  
  • FRC Low: y = 0.946 x − 0.009, r² = 0.992
  • FRC High: y = 1.005 x − 0.025, r² = 0.998
  • TRC Low: y = 1.034 x − 0.014, r² = 0.986
  • TRC High: y = 1.049 x − 0.034, r² = 0.997
• Bias and recovery: average bias for TRC was modest (≈2.1% in 0.1–0.5 mg/L; ≈4.9% in 0.5–3.0 mg/L). QC recoveries across both methods were generally good: FRC recoveries 92–98% (except the 0.1 mg/L QC at ~82%), TRC recoveries 97–106%.
• Low-concentration challenges: the smallest QC (0.1 mg/L) approached the quantitation limit and showed reduced recovery (82% for FRC; reference method also showed ~82–90%), likely due to analyte volatility/evaporation and handling delays. Evaporation from low-level samples was observed and led authors to recommend minimizing headspace and handling time (e.g., using 10 mL test tubes filled to ~1/3 volume) and using the STAT-Protocol to reduce delay between sampling and analysis.
• Reagent stability and maintenance: KMnO4 calibrator stock is stable up to 12 months when tightly sealed, refrigerated and protected from light. Some Gallery reagent (RC R2) showed visible aging (pink color) after two weeks in storage—daily replacement and refrigeration of reagents is advised. Routine hypochlorite wash and tubing re-flush recommended for the discrete analyzer to prevent carryover.
• Operational advantages: automated, immediate analysis via STAT-Protocol reduced atmospheric exposure and allowed high-throughput, continuous runs with automatic CRC calculation (TRC − FRC = CRC).

Practical benefits and applications

• High-throughput laboratory monitoring of pool and spa water for regulatory compliance and public health.
• Reliable TRC and FRC determination across low and high ranges appropriate for routine QA/QC and operational control.
• Automated workflows reduce operator variability and delay-associated loss of free chlorine signal, improving comparability to manual DPD methods.

Limitations and operational considerations

• Free chlorine instability requires rapid, low-headspace handling; STAT-Protocol and immediate analysis recommended to limit bias at low concentrations.
• Interferences: oxidizing agents and matrix color/turbidity can affect colorimetric response. High monochloramine concentrations can complicate FRC determination; application design can mitigate some interferences (e.g., reagent selection, reaction-stopping agents).
• Reagent aging and light sensitivity (KMnO4, DPD reagents) necessitate controlled storage and daily reagent checks.

Future trends and potential applications

• Broader automation and integration of discrete analyzers with LIMS and real-time reporting to support rapid corrective actions in pool management and water treatment plants.
• Development of more stable reagents or stabilizing additives to improve low-concentration FRC measurement and extend reagent shelf life.
• On-line, in situ sensors and hybrid systems combining electrochemical and optical detection for continuous monitoring and speciation (distinguishing free chlorine, monochloramine and combined chlorines).
• Enhanced methods for simultaneous detection of disinfection by-products (DBPs) and nitrogenous precursors to better manage health risks related to chlorination chemistry.

Conclusion

The Gallery discrete analyzer STAT-Protocol provides a rapid, automated and well-correlated approach for measuring FRC, TRC and CRC in pool water compared to established manual DPD methods (EPA 330.5, SFS-EN ISO 7393-2). Results demonstrate good linearity and recoveries across the target ranges, with anticipated challenges at the low end due to analyte instability. Best practice recommendations include minimizing sample headspace and delay, routine instrument cleaning (hypochlorite wash), proper reagent storage and routine QC to ensure reliable operation in routine laboratory environments.

References

1. US EPA. Methods for Chemical Analysis of Water and Wastes, Method 330.5: Chlorine, Total Residual (Spectrophotometric, DPD). 1978.
2. Rice EW, Baird RB, Eaton AD, editors. Standard Methods for the Examination of Water and Wastewater. 22nd Edition. 4500-CL (A) Introduction. American Public Health Association; 2012.
3. Rice EW, Baird RB, Eaton AD, editors. Standard Methods for the Examination of Water and Wastewater. 22nd Edition. 4500-CL (F) DPD Ferrous Titrimetric Method. American Public Health Association; 2012.
4. Rice EW, Baird RB, Eaton AD, editors. Standard Methods for the Examination of Water and Wastewater. 22nd Edition. 4500-CL (G) DPD Colorimetric Method. American Public Health Association; 2012.
5. Wendelken SC, Losh DE, Fair PS. Method 334.0: Determination of Residual Chlorine in Drinking Water using an On-line Chlorine Analyzer. Office of Ground Water and Drinking Water, US EPA; 2009.
6. Leasca S. Peeing in the Pool: So Wrong—and Bad for our Health, Study Says. LA Times; March 11, 2014.
7. Krans B. Why peeing in the pool is chemical warfare. Healthline News; March 31, 2014.
8. Chloramines—Combined Chlorine Problems. Pool Wizard; 2011.