💧 Free and total chlorine in water
In short
Determination of free and total chlorine in water by DPD colorimetric method — color reaction with N,N-diethyl-1,4-phenylenediamine, measurement at 515 nm. The test is performed according to PN-EN ISO 7393-2:2018; Measurement range: 0.03–5 mg/L Cl₂. The procedure comprises 6 steps (about 41 min in total); it is used for: Routine control of free chlorine in drinking water, Disinfection monitoring in water treatment plants, Control of chlorine in pool water (0.3–0.6 mg/L).
At a glance
- Standard: PN-EN ISO 7393-2:2018
- Category: Physicochemistry
- Procedure steps: 6
- Total time of stages: 41 min
- Measurement range: 0.03–5 mg/L Cl₂
- Limit of quantification: ~0.02 mg/L Cl₂
- Wavelength: 515 nm (photometer) or 510–530 nm (spectrophotometer)
Overview
Chlorine is the most commonly used disinfectant in drinking and pool water treatment. A distinction is made between free chlorine (HOCl + OCl⁻ — active disinfecting form) and combined chlorine (chloramines — NH₂Cl, NHCl₂, NCl₃ — formed from reaction of chlorine with ammonia). The sum of free and combined chlorine is total chlorine.
Monitoring free chlorine concentration is critical: too little — ineffective disinfection (infection risk), too much — unpleasant taste/odor and risk of toxic disinfection by-products (THM, HAA). Permissible free chlorine concentration in drinking water in Poland: max 0.3 mg/L (in network) to 0.5 mg/L (at intake).
The DPD (N,N-diethyl-1,4-phenylenediamine) method according to ISO 7393-2 is the most widely used method for chlorine control in water. It is a colorimetric method for routine control purposes — fast, simple, and suitable for both laboratory and field measurements.
Method principle
Free chlorine reacts directly with DPD reagent (N,N-dialkyl-1,4-phenylenediamine) at pH range 6.2–6.5, forming a red-pink Würster dye (semiquinonoid diamine). Color intensity is proportional to free chlorine concentration.
HOCl/OCl⁻ + DPD → Würster dye (magenta/pink)
Free chlorine measurement: DPD reacts immediately with free chlorine (HOCl and OCl⁻).
Total chlorine measurement: After adding potassium iodide (KI) to the sample with DPD, chloramines (combined chlorine) oxidize iodide to iodine, which in turn reacts with DPD giving additional color.
Total chlorine = free chlorine + combined chlorine (after KI addition)
Absorbance is measured photometrically at 515 nm (filter) or 510–530 nm (spectrophotometer) in 1 cm cuvette. Alternatively, visual color comparison with disk standard is used.
Measurement range: 0.03–5 mg/L Cl₂ (typically 0.02–0.07 mmol/L).
Applications
- Routine control of free chlorine in drinking water
- Disinfection monitoring in water treatment plants
- Control of chlorine in pool water (0.3–0.6 mg/L)
- Monitoring of water distribution network (residual chlorine)
- Disinfection control in wastewater treatment plants
- Field measurements using portable photometers
Key parameters
| Parameter | Value |
|---|---|
| Measurement range | 0.03–5 mg/L Cl₂ |
| Limit of quantification | ~0.02 mg/L Cl₂ |
| Wavelength | 515 nm (photometer) or 510–530 nm (spectrophotometer) |
| Reaction pH | 6.2–6.5 (phosphate buffer) |
| Reaction time | Immediate (free chlorine), 2 min (total chlorine) |
| Repeatability | RSD < 3–5% |
Standard
- Standard number
- PN-EN ISO 7393-2:2018
- Title (PL)
- Jakość wody — Oznaczanie chloru wolnego i chloru całkowitego — Część 2: Metoda kolorymetryczna z N,N-dialkilem-1,4-fenylenodiaminą, do celów kontroli rutynowej
- Title (EN)
- Water quality — Determination of free chlorine and total chlorine — Part 2: Colorimetric method using N,N-dialkyl-1,4-phenylenediamine, for routine control purposes
Step-by-step procedure
-
Sampling
Collect sample in glass container. Analyze IMMEDIATELY — free chlorine undergoes rapid decomposition (measure sample within 15 min of collection!). Do not preserve.
-
Free chlorine measurement
To dry cuvette add phosphate buffer and DPD Reagent No.1 (or DPD 1 tablet). Add 10 mL sample. Mix by inversion. Immediately measure absorbance at 515 nm.
-
Total chlorine measurement
To new cuvette add buffer + DPD No.1 + sample (as above). Then add approx. 1 g KI (or DPD No.3 tablet). Mix. Wait 2 min. Measure absorbance.
-
Combined chlorine calculation
Combined chlorine = Total chlorine − Free chlorine. Result in mg/L Cl₂.
-
Calibration
Prepare chlorine standard solutions (iodometric standardization): 0; 0.1; 0.5; 1.0; 2.0; 5.0 mg/L Cl₂. Calibration curve A = f(c) at 515 nm. Calibrate min. monthly.
-
Quality control
Blank sample (deionized water + DPD). Control standard every 10 samples. Comparison with titrimetric method (ISO 7393-1) during validation.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Photometer/spectrophotometer | Hach DR300/DR900 (field), Hach DR6000 (laboratory), Palintest Photometer 7500 | 2 000–15 000 PLN (field), 30 000–80 000 PLN (laboratory) |
| DPD comparator (visual) | Palintest Pooltester, Lovibond Comparator 2000+ with chlorine disks | 500–2 000 PLN |
| Measurement cuvettes | Glass cuvettes 1 cm (spectrophotometer) or dedicated to photometer | 50–300 PLN/pc |
| Automatic pipettes | For measuring 10 mL sample and reagents | 800–2 000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| DPD Reagent No.1 (free chlorine) | — | N,N-diethyl-1,4-phenylenediamine in phosphate buffer pH 6.2–6.5. Tablets (Palintest, Lovibond) or solution |
| DPD Reagent No.3 (total chlorine) | — | DPD No.1 + potassium iodide (KI). For determining total chlorine (free + combined) |
| Phosphate buffer solution pH 6.5 | — | Na₂HPO₄/KH₂PO₄ with EDTA — pH buffering and metal masking (Fe, Mn) |
| Chlorine standard solution | — | Sodium hypochlorite (NaOCl) solution standardized iodometrically. Or certified chlorine CRM |
| Potassium iodide (KI) | 7681-11-0 | Crystalline, analytical grade. For DPD No.3 reaction (total chlorine) |
| DPD tablets (option) | — | DPD 1 + DPD 3 in tablets — convenient for field measurements (Palintest, Lovibond) |
Health and safety (OHS)
- DPD (N,N-diethylphenylenediamine) — sensitizing, irritates skin. Nitrile gloves
- Chlorine solutions (NaOCl) — corrosive, oxidizing. Avoid contact with skin and eyes
- DPD tablets — do not swallow, keep away from children
- KI — relatively safe, but avoid eye contact
- Safety goggles and gloves when preparing reagents
- Low overall risk — one of the safest analytical methods
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 7393-2:2018 — “Water quality — Determination of free chlorine and total chlorine — Part 2: Colorimetric method using N,N-dialkyl-1,4-phenylenediamine, for routine control purposes”.
How does this method work?
Free chlorine reacts directly with DPD reagent (N,N-dialkyl-1,4-phenylenediamine) at pH range 6.2–6.
What is the measuring range and accuracy?
Measurement range: 0.03–5 mg/L Cl₂; Limit of quantification: ~0.02 mg/L Cl₂; Wavelength: 515 nm (photometer) or 510–530 nm (spectrophotometer); Reaction pH: 6.2–6.5 (phosphate buffer).
How long does the test take?
The times given for the individual stages add up to approximately 41 min. The procedure comprises 6 steps.
What equipment is required?
Photometer/spectrophotometer, DPD comparator (visual), Measurement cuvettes, Automatic pipettes. Indicative cost of the core instrument (Photometer/spectrophotometer): 2 000–15 000 PLN (field), 30 000–80 000 PLN (laboratory).
Where is this test used?
Routine control of free chlorine in drinking water; Disinfection monitoring in water treatment plants; Control of chlorine in pool water (0.3–0.6 mg/L); Monitoring of water distribution network (residual chlorine); Disinfection control in wastewater treatment plants; Field measurements using portable photometers.
What safety precautions apply?
DPD (N,N-diethylphenylenediamine) — sensitizing, irritates skin. Nitrile gloves; Chlorine solutions (NaOCl) — corrosive, oxidizing. Avoid contact with skin and eyes; DPD tablets — do not swallow, keep away from children; KI — relatively safe, but avoid eye contact; Safety goggles and gloves when preparing reagents; Low overall risk — one of the safest analytical methods.
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number PN-EN ISO 7393-2.