Overview
ISO 7393-3:1990 - "Water quality - Determination of free chlorine and total chlorine - Part 3: Iodometric titration method for the determination of total chlorine" specifies an iodometric titration procedure for measuring total chlorine in water. The method determines chlorine concentrations expressed as Cl2 from 0.01 to 0.21 mmol/l (0.71–15 mg/L). A direct titration variant (Annex B) is included for higher concentrations commonly found in treated drinking water (lower limit ~7 µmol/l or 0.5 mg/L).
Key topics and technical requirements
- Principle: Oxidation of iodide to iodine by free and combined chlorine in acid solution, followed by reduction and titration using sodium thiosulfate and back-titration with potassium iodate where applicable.
- Analytical range: 0.01–0.21 mmol/l (0.71–15 mg/L) for the standard procedure; Annex B direct titration usually applied above ~7 µmol/l (0.5 mg/L).
- Reagents and solutions: potassium iodide (KI), phosphoric acid, starch indicator, potassium iodate standard solution (10 mmol/l), sodium thiosulfate standard volumetric solution (10 mmol/l), and water free from chlorine and reducing substances.
- Apparatus & sampling: standard volumetric glassware and chlorine-demand–free glassware (preparation by controlled chlorination then dechlorination). Sampling guidance references ISO 5667 series.
- Interferences: method is only applicable in the absence of other oxidizing agents and species that oxidize iodide, including bromine/iodine, bromo-/iodoamines, ozone, hydrogen peroxide, bromate, chromate, chlorine dioxide, chlorite, nitrite and certain metal ions (Fe(III), Cu(II), Mn(III)), among others.
- Precision examples: interlaboratory and agency data reported in the standard show good repeatability for clean matrices (relative standard deviations as low as ~0.2–0.8% for distilled water), moderate precision for drinking water (~5.2% in one example) and poorer precision for polluted surface waters (up to ~9.7% at low concentrations). EPA interlaboratory data for direct titration show RSDs of ~2.2–3.7% at ~28 µmol/l and ~0.4–0.65% at ~282 µmol/l.
Applications and users
- Who uses it: municipal and industrial water laboratories, drinking water utilities, wastewater treatment facilities, environmental monitoring agencies, and accredited testing labs that require a standardized volumetric method for total chlorine.
- Practical uses: routine compliance testing, process control in chlorination and disinfection sites, verification of residual chlorine in treated drinking water, and method cross-checking where colorimetric methods are unsuitable.
- Advantages: well-established, low-instrumentation volumetric method with documented performance and a direct titration option for higher residuals.
Related standards
- ISO 7393-1: titrimetric method using N,N-diethyl-1,4-phenylenediamine (DPD)
- ISO 7393-2: calorimetric (DPD) method for routine control
- ISO 5667-1 and ISO 5667-2: guidance on water sampling design and techniques
Keywords: ISO 7393-3, iodometric titration, total chlorine determination, water quality, chlorine measurement, potassium iodide, sodium thiosulfate, standard method.
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