💧 Free and total chlorine in water
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.
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
1. Sampling
Collect sample in glass container. Analyze IMMEDIATELY — free chlorine undergoes rapid decomposition (measure sample within 15 min of collection!). Do not preserve.
2. 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.
3. 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.
4. Combined chlorine calculation
Combined chlorine = Total chlorine − Free chlorine. Result in mg/L Cl₂.
5. 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.
6. 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