🧪 Phenolic index of water
Determination of total volatile phenols in water after distillation — color reaction with 4-aminoantipyrine in presence of potassium hexacyanoferrate(III). Measurement at 510 nm.
Overview
The phenolic index determines the sum of volatile phenolic compounds (phenol, cresols, xylenols, chlorophenols and others) in water that react with 4-aminoantipyrine (4-AAP) under reaction conditions. It is not a complete sum of phenols — some compounds (e.g., phenols with para substituents) do not react with 4-AAP.
Phenols in water cause unpleasant taste and odor (especially chlorophenols — detection threshold 0.1–1 µg/L), are toxic to aquatic organisms and may indicate industrial pollution. The permissible concentration in drinking water is not directly regulated for phenolic index, but phenol as a chemical substance has a limit of 0.5 µg/L in some regulations.
Sources of phenols in waters include: coking industry, oil refineries, chemical plants, phenol-formaldehyde resin production, pharmaceuticals, and natural decomposition processes of organic matter. Phenolic index is a routine parameter in industrial wastewater monitoring.
Method principle
The method includes preliminary distillation of volatile phenols from a sample acidified with phosphoric acid, followed by color reaction with 4-aminoantipyrine (4-AAP).
Distillation stage: The sample is acidified with H₃PO₄ and distilled to separate volatile phenols from the matrix. Phenols pass into the distillate.
Color reaction stage: In alkaline environment (pH 10, ammonia buffer), phenols react with 4-aminoantipyrine in the presence of oxidizer — potassium hexacyanoferrate(III) K₃[Fe(CN)₆] — forming a colored antipyrine complex (red-amber):
Phenol + 4-AAP + K₃[Fe(CN)₆] → colored antipyrine complex
Direct method: absorbance measurement at 510 nm (range 0.1–5 mg/L). Extraction method (with chloroform): colored complex extracted to CHCl₃, measurement at 460 nm — higher sensitivity (range 0.002–0.5 mg/L).
Applications
- Monitoring of industrial wastewater (coking plants, refineries, chemical plants)
- Control of drinking water quality and its taste/odor
- Monitoring of surface waters (industrial pollution)
- Control of wastewater treatment plant efficiency
- Study of groundwater in industrial areas
- Monitoring of industrial waste landfills
Key parameters
| Parameter | Value |
|---|---|
| Range (direct method) | 0.1–5 mg/L phenol |
| Range (extraction method) | 0.002–0.5 mg/L phenol |
| Wavelength | 510 nm (direct), 460 nm (extraction with CHCl₃) |
| Limit of quantification | ~0.002 mg/L (extraction method) |
| Repeatability | RSD < 5–10% |
| Interferences | Oxidizers, sulfides, aldehydes, oils |
Standard
- Standard number
- PN-ISO 6439:1994
- Title (PL)
- Jakość wody — Oznaczanie indeksu fenolowego — Metody spektrometryczne z 4-aminoantypiryną po destylacji
- Title (EN)
- Water quality — Determination of phenol index — 4-Aminoantipyrine spectrometric methods after distillation
Step-by-step procedure
1. Sample preservation
Add H₃PO₄ to pH < 4 (inhibits biological degradation of phenols). Add CuSO₄ (1 g/L) as preservative. Analysis within 24h, store at 4°C.
2. Distillation
Transfer 500 mL sample to flask. Add H₃PO₄ to pH ~4. Distill collecting ~450 mL distillate. If distillate turbid — redistill.
3. Reaction pH adjustment
To 50 mL distillate add ammonia buffer to pH 10±0.2. Mix well.
4. Color reaction
Add 1 mL 4-aminoantipyrine solution. Mix. Add 1 mL K₃[Fe(CN)₆]. Mix. Wait 15 min for color development.
5. Extraction (extraction method)
Transfer to separatory funnel. Add 25 mL CHCl₃. Shake 10 times. Separate phases. Repeat extraction. Collect chloroform phase.
6. Absorbance measurement
Direct method: measure at 510 nm. Extraction method: measure CHCl₃ phase at 460 nm. Cuvette 1 cm or 5 cm.
7. Calibration and calculations
Standards: 0; 0.01; 0.05; 0.1; 0.5 mg/L phenol (distilled like samples). Calibration curve. Result as mg phenol/L.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Distillation apparatus | Glass set: 1 L round-bottom flask, Liebig condenser, receiver | 2 000–5 000 PLN |
| UV-Vis spectrophotometer | Hach DR6000, Shimadzu UV-1900, PerkinElmer Lambda 365 | 15 000–80 000 PLN |
| Separatory funnel for extraction | 250 mL borosilicate glass separatory funnels with PTFE stopcock | 150–400 PLN/pc |
| Spectrophotometric cuvettes | Glass cuvettes 1 cm and 5 cm path length | 100–500 PLN/pc |
| Water bath | For distillation temperature control | 3 000–8 000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| 4-Aminoantipyrine (4-AAP) | 83-07-8 | 0.13 g/200 mL water. Coloring reagent — prepare fresh daily |
| Potassium hexacyanoferrate(III) K₃[Fe(CN)₆] | 13746-66-2 | 1.0 g/100 mL. Oxidizer — initiates coupling reaction with 4-AAP |
| Phosphoric acid (H₃PO₄) | 7664-38-2 | For acidifying sample before distillation |
| Ammonia buffer pH 10 | — | NH₃/NH₄Cl to set color reaction pH to 10±0.2 |
| Chloroform (CHCl₃) | 67-66-3 | For extraction of antipyrine complex (extraction method). 25 mL per sample |
| Phenol standard solution | 108-95-2 | 1000 mg/L in water. Phenol is toxic — weigh in fume hood |
Health and safety (OHS)
- Phenol — toxic, corrosive, causes chemical burns to skin. Absorbs through skin! Nitrile gloves
- Chloroform — harmful when inhaled, possible carcinogenic effect. Work only in fume hood
- Pyridine (if used) — toxic, irritates respiratory tract. Fume hood
- K₃[Fe(CN)₆] — harmless under normal conditions, but in acidic environment can release HCN!
- Chloroform waste — organic waste, collect separately
- Safety goggles, nitrile gloves, lab coat mandatory