Determination of free and residual chlorine based on DIN EN ISO 7393-1 and APHA 4500-Cl

Significance of the topic

This bulletin addresses accurate determination of free and residual chlorine in water and wastewater, critical for public health protection and regulatory compliance. Chlorine is a widely used disinfectant that prevents waterborne diseases but can form toxic by-products if not properly monitored. Reliable measurement methods support safe drinking water, effective wastewater treatment, and environmental protection.

Objectives and study overview

The document reviews and compares three established methods for quantifying free and total chlorine: potentiometric titration according to DIN EN ISO 7393-1, iodometric titration following APHA 4500-Cl Method B, and an ion-selective electrode approach per APHA 4500-Cl Method I. Key goals include evaluating detection alternatives (Optrode, Pt-Titrode, starch indicator), optimizing reagent selection (avoiding toxic agents), and confirming method suitability over 0.3–5 mg/L chlorine concentration range.

Methodology and instrumentation

Potentiometric titrations (DIN EN ISO 7393-1)

• Titrator with MET U mode, 20 mL dosing unit
• Optrode and Pt-Titrode sensors
• Reagents: ammonium iron(II) sulfate, DPD indicator, phosphate buffer pH 6.5

Iodometric titration (APHA 4500-Cl Method B)

• Titrator with DET U and MET U modes
• Optrode or Pt-Titrode indicators, starch indicator
• Reagents: KI, KIO3 standard, acetic/sulfuric acids, Na2S2O3 titrant

Ion-selective electrode technique (APHA 4500-Cl Method I)

• pH/mV meter or titrator capable of ion measurement
• Platinum rod, iodide ISE
• Reagents: KI, acetate buffer pH 4.0, KIO3 calibration standards

Sample volumes ranged from 25 mL to 500 mL depending on concentration. Titrant titers are determined daily using potassium permanganate (DIN EN ISO) or iodate (APHA methods), avoiding hazardous dichromate and mercuric salts.

Main results and discussion

– Equivalence and break-point detection with Optrode or Pt-Titrode yielded equivalent chlorine values within analytical uncertainty.
– Substituting permanganate for dichromate in titer determination removed toxic chromium handling without performance loss.
– In iodometric titration, starch photometric endpoints correlate closely with potentiometric detection using ISE or Pt electrode.
– Chloramine titrations require slightly longer reaction times due to slower chlorine release.
– ISE method demonstrated linear response over 0.2–5 mg/L chlorine, with slope around –27.4 mV per decade.

Benefits and practical applications

• Flexible detection: laboratories can choose potentiometric or photometric endpoints without compromising precision.
• Reduced hazardous reagents: elimination of mercury(II) chloride and dichromate promotes safer workflows.
• Daily titer checks ensure consistent accuracy in routine water analysis and QA/QC operations.
• Applicable to drinking water, wastewater, environmental monitoring, and industrial process control.

Future trends and potential applications

Advances may include:

• Miniaturized, field-deployable sensors for on-site chlorine monitoring.
• Automated flow-through titration systems integrated with data-logging and remote reporting.
• Development of alternative, non-toxic indicators and reagents to further reduce laboratory hazards.
• Coupling with machine learning for real-time prediction of disinfection by-product formation.

Conclusion

The comparative evaluation confirms that DIN EN ISO and APHA methods deliver robust, interchangeable approaches for free and total chlorine analysis. By modernizing titration reagents and offering sensor flexibility, Metrohm’s protocols enhance safety and maintain high accuracy across water quality applications.

References

• DIN EN ISO 7393-1: Water quality – Determination of free chlorine and total chlorine – Titrimetric method using N,N-diethyl-1,4-phenylenediamine
• APHA Standard Methods 4500-Cl: Residual Chlorine