🧪 Phenolic index of water

Physicochemistry PN-EN ISO 6439

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

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.

   ⏱ Time: 5 min

2. Distillation

   Transfer 500 mL sample to flask. Add H₃PO₄ to pH ~4. Distill collecting ~450 mL distillate. If distillate turbid — redistill.

   ⏱ Time: 60–90 min • 🌡 Temperature: ~100°C

3. Reaction pH adjustment

   To 50 mL distillate add ammonia buffer to pH 10±0.2. Mix well.

   ⏱ Time: 2 min

4. Color reaction

   Add 1 mL 4-aminoantipyrine solution. Mix. Add 1 mL K₃[Fe(CN)₆]. Mix. Wait 15 min for color development.

   ⏱ Time: 15 min

5. Extraction (extraction method)

   Transfer to separatory funnel. Add 25 mL CHCl₃. Shake 10 times. Separate phases. Repeat extraction. Collect chloroform phase.

   ⏱ Time: 15 min

6. Absorbance measurement

   Direct method: measure at 510 nm. Extraction method: measure CHCl₃ phase at 460 nm. Cuvette 1 cm or 5 cm.

   ⏱ Time: 5 min

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.

   ⏱ Time: 10 min

Required equipment and apparatus

Reagents, media and consumables

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

Frequently asked questions