ISO 7393-1:1985

Overview

ISO 7393-1:1985 specifies a titrimetric DPD method for the determination of free chlorine and total chlorine in water. The method uses N,N-diethyl-1,4-phenylenediamine (DPD) to form a red oxidation product at pH ~6.2–6.5 that is titrated to a colourless end‑point with a standardized ammonium iron(II) sulfate solution. Applicable concentration range is 0.0004 to 0.07 mmol/L (≈0.03 to 5 mg/L) expressed as Cl2; higher concentrations can be measured after dilution. Seawater and waters with bromide/iodide require special procedures.

Key topics and technical requirements

• Principle: Direct reaction of DPD with free chlorine to produce a red compound; titration with Fe(II) to disappearance of colour. For total chlorine, potassium iodide (KI) is added before titration to liberate combined chlorine as oxidizing species that also oxidize DPD.
• Reagents & solutions: DPD reagent (≈1.1 g/L), phosphate buffer pH 6.5, ammonium iron(II) sulfate (stock and working standard solutions), potassium dichromate for standardization, potassium iodide, and optional sodium arsenite or thioacetamide to neutralize oxidants (used in manganese correction).
• Sample handling: Analyze immediately after sampling; avoid bright light, heat and agitation. Use chlorine‑demand‑free glassware (pre‑treated with hypochlorite then rinsed).
• Interferences: Bromine, iodine, bromamines/iodoamines, ozone, hydrogen peroxide, chromate, oxidized manganese, nitrite, Fe(III) and Cu(II) can cause positive or negative bias. The standard describes corrective steps (e.g., arsenite/thioacetamide for oxidized manganese; chromate removal by barium chloride).
• Quality control: Standardize Fe(II) solution against K2Cr2O7. The method’s repeatability and reproducibility have been evaluated (examples from EPA and UK studies provided in the standard).
• Reporting: Include reference to ISO 7393-1, full sample ID, results and expression units, and any non‑routine operations or circumstances affecting results.

Applications and users

• Routine water quality monitoring and compliance testing for drinking water, distribution systems, wastewater disinfection control, and environmental monitoring.
• Primary users: municipal and private water laboratories, environmental agencies, utilities, wastewater treatment operators, and contract analytical labs performing residual chlorine analysis.
• Useful for process control where rapid, volumetric determination of free and combined chlorine is required.

Related standards

• ISO 7393-2 - DPD colorimetric method (routine control)
• ISO 7393-3 - Iodometric titration method (for higher chlorine concentrations)
• Complementary guidance on reagent handling, waste disposal and analytical titration methods appears in the ISO bibliographic references cited in the standard.

Keywords: ISO 7393-1, DPD titrimetric method, free chlorine, total chlorine, water quality, N,N-diethyl-1,4-phenylenediamine, ammonium iron(II) sulfate, potassium iodide, chlorine determination.