Easy and Rapid Application for Residual Chlorine Analysis in Water Samples

Rapid, Automated Analysis of Residual Chlorine in Swimming‑pool Water Using the Gallery Discrete Analyzer (STAT‑Protocol)

Significance of the topic

Monitoring residual chlorine (free and combined) in recreational and process waters is essential to ensure effective disinfection while minimising formation of harmful disinfection by‑products (e.g., chloroform, chlorinated acetic acids, trichloramines, cyanogen chloride). Free residual chlorine (FRC) provides the primary disinfectant activity; combined/total residual chlorine (TRC) includes less effective chloramines formed by reactions with nitrogenous compounds. Because active chlorine species are labile (loss by volatilisation, reaction with matrix components and light), rapid, accurate analysis is necessary for reliable control and regulatory compliance.

Objectives and overview of the study

• Assess the Thermo Scientific Gallery discrete analyser (STAT‑Protocol) for rapid measurement of FRC and TRC in swimming‑pool water.
• Compare Gallery results to a manual DPD spectrophotometric reference method based on EPA 330.5 and SFS‑EN ISO 7393‑2:2000.
• Evaluate method performance across low and high concentration ranges, quality control recoveries, calibration procedures, and practical workflow considerations (reagent stability, sample handling).

Methodology

Analytical principle

• Colorimetric DPD chemistry: FRC oxidizes N,N‑diphenyl‑p‑phenylene diamine (DPD), producing a pink/red product measured at ~510 nm. TRC measurement uses potassium iodide (KI) to liberate iodine from iodide; the liberated iodine then reacts with DPD for a total residual chlorine measurement.
• Combined residual chlorine (CRC) is obtained by automated subtraction of FRC from TRC results.

Instrumentation and reagents (workflow)

• Thermo Scientific Gallery discrete analyser using a STAT‑Protocol to minimise sample exposure to air and process samples immediately after insertion.
• Reagents: RC R1 (phosphate buffer), RC R2 (DPD reagent), RC R3 (potassium iodide solution for TRC).
• Calibration: potassium permanganate (KMnO4) stock calibrator (0.02 M KMnO4 equivalent to 3547 mg/L Cl2) used to prepare working standards. Working standards for low and high ranges (e.g., 1.5 mg/L for low, 12 mg/L for high-equivalent) prepared by dilution. Calibration performed manually or automatically by the analyser.
• Key operational parameters: TRC incubation uses elevated temperature (37 °C) and short incubation times (e.g., 60 s after final reagent addition). Measurement wavelength: 510 nm (some comparisons at 515 nm in standards).

Samples and quality control

• Samples taken from multiple public swimming pools; natural FRC range 0.12–0.58 mg/L and TRC range 0.11–0.89 mg/L. Spiked samples extended the range to ~0.1–3.0 mg/L (FRC spiked up to 2.94 mg/L; TRC up to 2.89 mg/L).
• QC materials prepared from sodium hypochlorite and verified by sodium thiosulphate titration.

Instrumentation used

• Thermo Scientific Gallery discrete analyser (STAT‑Protocol). Alternative compatible platforms: Gallery Plus, Aquakem analyser.
• Thermo Scientific Multiskan GO spectrophotometer used for manual DPD reference measurements (510 nm).

Main results and discussion

Method comparison and correlation

• FRC (low range): regression y = 0.946x − 0.009, r² = 0.992 — strong agreement with manual DPD reference, slight low bias in low concentrations.
• FRC (high range): regression y = 1.005x − 0.025, r² = 0.998 — excellent agreement across higher concentrations.
• TRC (low range): regression y = 1.034x − 0.014, r² = 0.986 — very good correlation with modest positive bias.
• TRC (high range): regression y = 1.049x − 0.034, r² = 0.997 — excellent agreement across the high range.

Bias and recoveries

• Average bias for TRC: ~2.1% in the 0.1–0.5 mg/L range; ~4.9% in the 0.5–3.0 mg/L range — acceptable analytical agreement for routine monitoring.
• Average bias for FRC: −5.4% in the low range (0.1 mg/L area) and 0.5% in the high range; however, individual low‑concentration results occasionally deviated considerably (up to −26%), attributed mainly to analyte instability and sample handling (evaporation, delays).
• QC recoveries: FRC 92–98% (except smallest QC at 0.1 mg/L showing ~82% recovery); TRC 97–106%. Lower recoveries at the lowest QC level are likely linked to volatility/instability of active chlorine and sample evaporation.

Operational observations and interferences

• STAT‑Protocol (immediate analysis after sample insertion) and minimising sample exposure to air reduced losses and improved agreement with reference values; inserting one sample at a time is recommended for the most labile samples.
• Reagent handling: RC R1 and RC R2 refrigerated; RC R2 is light sensitive and showed signs of ageing in two weeks if not protected. RC R3 prepared daily.
• KMnO4 calibrator is a convenient and stable oxidant for calibration (stable up to 12 months if refrigerated and stored in amber bottles), but it is light sensitive.
• Potential interferences: oxidizing agents (rare at typical pool concentrations) and monochloramine can affect FRC determinations; the method can be adapted (e.g., use of arsenite or thioacetamide step) for problematic matrices. Contamination from sample matrixes or reagents was not systematically studied in this work.
• Routine maintenance: a hypochlorite wash and tubing re‑flush as part of daily analyser maintenance are recommended to remove residual analyte and avoid carryover.

Benefits and practical applications of the method

• Fast, automated discrete analysis using the STAT‑Protocol reduces sample exposure and operator workload while enabling near‑real‑time measurement of labile chlorine species.
• Automated TRC/FRC/CRC workflows and built‑in calibration reduce manual handling and standardise routine monitoring in pool and water testing laboratories.
• High correlation with established manual DPD reference methods demonstrates suitability for operational and compliance testing across typical pool concentration ranges (0.1–3.0 mg/L).

Limitations and practical caveats

• FRC instability: very low concentrations near the quantitation limit are vulnerable to evaporation and handling delays; single‑sample STAT runs or minimised headspace sample tubes (e.g., 10 mL tubes filled to one‑third) are recommended to reduce loss.
• Small QC at 0.1 mg/L returned lower recoveries; labs should validate low‑end performance and consider additional controls or lower detection methods if required.
• The study did not fully explore matrix interferences or cross‑contamination between samples and reagents; users should validate method performance for specific sample types and mixed runs.

Future trends and potential developments

• Further automation and integration with on‑line sensors and SCADA/LIMS for continuous monitoring and automated corrective actions.
• Improved reagent formulations (more stable DPD reagents, light‑protected kits) to extend shelf life and reduce maintenance frequency.
• Enhanced approaches to address interferences (monochloramine, other oxidants) and lower quantitation limits using refined chemistries or preconcentration techniques.
• Standardisation of STAT‑style protocols across discrete analyser platforms to ensure inter‑laboratory comparability and regulatory acceptance.

Conclusion

The Gallery discrete analyser using the STAT‑Protocol provides a rapid, reliable automated approach for measuring free, total and combined residual chlorine in swimming‑pool water. Results correlate strongly with EPA 330.5 and SFS‑EN ISO 7393‑2:2000 manual DPD methods across low and high concentration ranges. Practical recommendations include minimising sample exposure (STAT runs or single‑sample insertion), routine hypochlorite servicing, careful reagent storage, and validation for specific sample matrices. The method is well suited for routine laboratory monitoring where speed, throughput and standardised reporting are priorities, though attention is needed at the low concentration limit due to analyte instability.

Reference

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 ed. 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 ed. 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 ed. 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. 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.