☠️ Cyanides in water
In short
Determination of total and free cyanides in water after distillation — spectrophotometric method with pyridine and barbituric acid at 578 nm. The test is performed according to PN-EN ISO 6703:2021; Photometric range: 0.002–0.025 mg CN⁻ in sample. The procedure comprises 7 steps (about 1 h 50 min–2 h 20 min in total); it is used for: Drinking water quality control (standard: ≤ 50 µg/L CN⁻), Monitoring of electroplating and coking wastewaters, Control of metallurgical wastewaters.
At a glance
- Standard: PN-EN ISO 6703:2021
- Category: Physicochemistry
- Procedure steps: 7
- Total time of stages: 1 h 50 min–2 h 20 min
- Photometric range: 0.002–0.025 mg CN⁻ in sample
- Titrimetric range: > 0.05 mg CN⁻
- Limit of quantification: ~5 µg/L (photometry)
Overview
Cyanides are among the most highly toxic water contaminants. The cyanide ion CN⁻ blocks cytochrome oxidase in mitochondria, inhibiting cellular respiration. The lethal dose of KCN for humans is only 1–3 mg/kg body weight. The permissible cyanide concentration in drinking water is 50 µg/L (Journal of Laws 2017, item 2294).
ISO 6703 standard comprises three parts: Part 1 — determination of total cyanide, Part 2 — free cyanides (easily liberatable), Part 3 — cyanogen chloride. The method includes reflux distillation in acidic environment to release HCN from cyanide complexes, absorption in NaOH, and subsequent spectrophotometric determination.
Cyanides enter waters from electroplating, coking, metallurgical wastewaters, gold extraction processes (cyanide leaching), and plastics and pesticide production. Cyanide monitoring is required in water permits of industrial facilities.
Method principle
The method comprises two stages: (1) distillation and (2) spectrophotometric determination.
Distillation stage: The sample is acidified with phosphoric acid to pH < 2, which releases hydrogen cyanide (HCN) from cyanide compounds (including complexes with metals). HCN is distilled from the sample and absorbed in NaOH solution (0.4 mol/L), where it converts to ionic CN⁻.
Determination stage: Cyanides in the distillate react with chloramine-T forming cyanogen chloride (CNCl) at pH < 8. Then CNCl reacts with pyridine-barbituric acid forming a red-violet colored complex. Absorbance is measured spectrophotometrically at 578 nm (with barbituric acid) or 620 nm (with pyridine-pyrazolone).
The photometric method allows determination of 0.002–0.025 mg CN⁻, titration method (Tyndall effect) > 0.005 mg, and titration method with indicator > 0.05 mg.
Applications
- Drinking water quality control (standard: ≤ 50 µg/L CN⁻)
- Monitoring of electroplating and coking wastewaters
- Control of metallurgical wastewaters
- Monitoring of waters near gold mines
- Analysis of plastic production wastewaters
- Monitoring of surface waters near chemical plants
Key parameters
| Parameter | Value |
|---|---|
| Photometric range | 0.002–0.025 mg CN⁻ in sample |
| Titrimetric range | > 0.05 mg CN⁻ |
| Limit of quantification | ~5 µg/L (photometry) |
| Wavelength | 578 nm (pyridine-barbituric acid) or 620 nm (pyridine-pyrazolone) |
| Sample max concentration | < 100 mg CN⁻/L (without dilution) |
| Interferences | Sulfides, aldehydes (formaldehyde), nitrites, thiocyanates |
Standard
- Standard number
- PN-EN ISO 6703:2021
- Title (PL)
- Jakość wody — Oznaczanie cyjanków
- Title (EN)
- Water quality — Determination of cyanide
Step-by-step procedure
-
Sample preservation
Acidification is FORBIDDEN (would release HCN!). Add NaOH to pH > 12. Store in dark, 4°C. Test for sulfides (lead paper) — if present, add PbCO₃.
-
Distillation
Transfer 500 mL sample to round-bottom flask. Add H₃PO₄ to pH < 2. Connect to condenser and absorber (25 mL NaOH 0.4 mol/L). Distill under reflux until ~450 mL distillate collected.
-
Reagent preparation
Prepare fresh chloramine-T solution (daily). Check pyridine-barbituric acid reagent (stored at 4°C, valid ~1 week).
-
Color reaction
To 50 mL distillate add 0.5 mL chloramine-T. Mix 1 min. Add 5 mL pyridine-barbituric acid solution. Mix. Wait 8 min for color development.
-
Spectrophotometric measurement
Measure absorbance at 578 nm in 1 cm cuvette (or 5 cm for low concentrations). Read concentration from calibration curve. Reading within 15 min of adding coloring reagent.
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Calibration
KCN standard solutions: 0; 0.02; 0.05; 0.1; 0.2; 0.5 mg/L CN⁻ (distilled like samples). Calibration curve: A = f(c). R² > 0.998.
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Quality control
Blank sample (dist. water through distillation). CRM per series. Duplicate every 10 samples. Recovery 85–115%.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Reflux distillation apparatus | Glass set for cyanide distillation (1L round-bottom flask + condenser + absorber) | 3 000–8 000 PLN |
| UV-Vis spectrophotometer | Hach DR6000, Shimadzu UV-1900, Thermo Evolution 300 | 15 000–80 000 PLN |
| Hot plate or heating mantle | IKA, Heidolph — boiling temperature control | 2 000–5 000 PLN |
| Volumetric flasks and pipettes | Class A flasks 100, 250, 500 mL; automatic pipettes 1–10 mL | 50–500 PLN/pc |
| Spectrophotometric cuvettes | Quartz or glass cuvettes 1 cm, 5 cm | 100–500 PLN/pc |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Phosphoric acid (H₃PO₄) | 7664-38-2 | For acidifying sample during distillation (HCN release) |
| Sodium hydroxide (NaOH) | 1310-73-2 | 0.4 mol/L, absorption solution for capturing HCN from distillate |
| Chloramine-T | 127-65-1 | 0.014 mol/L (0.40 g/100 mL). For CN⁻ → CNCl conversion. Prepare fresh daily |
| Barbituric acid | 67-52-7 | 15 g barbituric acid + 75 mL pyridine + 15 mL conc. HCl to 1 L. Store at 4°C |
| Pyridine | 110-86-1 | Component of coloring reagent, analytical grade purity |
| KCN standard solution | 151-50-8 | 1000 mg/L CN⁻. Deadly toxic! Store in NaOH, away from acids |
Health and safety (OHS)
- KCN and HCN — DEADLY TOXIC! Lethal dose <3 mg/kg. Never acidify CN samples/solutions outside fume hood
- Work only in fume hood with exhaust on. HCN sensor available in room
- Pyridine — toxic, acts on CNS. Inhalation of vapors dangerous. Work in fume hood
- Chloramine-T — strong oxidizer, irritating. Gloves and safety goggles
- First aid for CN poisoning: antidote kit (4-DMAP, sodium thiosulfate, amyl nitrite)
- Cyanide waste — oxidize with sodium hypochlorite (NaOCl) before disposal. Hazardous waste
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 6703:2021 — “Water quality — Determination of cyanide”.
How does this method work?
The method comprises two stages: (1) distillation and (2) spectrophotometric determination. Distillation stage: The sample is acidified with phosphoric acid to pH < 2, which releases hydrogen cyanide (HCN) from cyanide compounds (including complexes with metals).
What is the measuring range and accuracy?
Photometric range: 0.002–0.025 mg CN⁻ in sample; Titrimetric range: > 0.05 mg CN⁻; Limit of quantification: ~5 µg/L (photometry); Wavelength: 578 nm (pyridine-barbituric acid) or 620 nm (pyridine-pyrazolone).
How long does the test take?
The times given for the individual stages add up to approximately 1 h 50 min–2 h 20 min. The procedure comprises 7 steps.
What equipment is required?
Reflux distillation apparatus, UV-Vis spectrophotometer, Hot plate or heating mantle, Volumetric flasks and pipettes, Spectrophotometric cuvettes. Indicative cost of the core instrument (Reflux distillation apparatus): 3 000–8 000 PLN.
Where is this test used?
Drinking water quality control (standard: ≤ 50 µg/L CN⁻); Monitoring of electroplating and coking wastewaters; Control of metallurgical wastewaters; Monitoring of waters near gold mines; Analysis of plastic production wastewaters; Monitoring of surface waters near chemical plants.
What safety precautions apply?
KCN and HCN — DEADLY TOXIC! Lethal dose <3 mg/kg. Never acidify CN samples/solutions outside fume hood; Work only in fume hood with exhaust on. HCN sensor available in room; Pyridine — toxic, acts on CNS. Inhalation of vapors dangerous. Work in fume hood; Chloramine-T — strong oxidizer, irritating. Gloves and safety goggles; First aid for CN poisoning: antidote kit (4-DMAP, sodium thiosulfate, amyl nitrite); Cyanide waste — oxidize with sodium hypochlorite (NaOCl) before disposal. Hazardous waste.
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 6703.