⚗️ Total and free cyanide in water — continuous flow analysis (CFA) with UV decomposition, distillation or gas diffusion and photometric detection (chloramine-T, pyridine-4-carboxylic acid, 1,3-dimethylbarbituric acid)
In short
In the CFA system cyanide bound in complexes is decomposed with a UV-B lamp (312–400 nm) at pH 3. The test is performed according to PN-EN ISO 14403-2:2012 (ISO 14403-2:2012); Range: usually 2–500 µg/l CN⁻ (10–100 µg/l described); changed by dilution or by the length of the cell. The procedure comprises 6 steps; it is used for: Industrial wastewater (electroplating shops, coking plants, the chemical and metallurgical industries) — free and bound cyanide against the limits of the regulation (CFA reference method), Groundwater and leachate from landfills and post-industrial sites, Drinking water and surface water — control of cyanide at the µg/l level.
At a glance
- Standard: PN-EN ISO 14403-2:2012 (ISO 14403-2:2012)
- Category: Physicochemistry
- Procedure steps: 6
- Range: usually 2–500 µg/l CN⁻ (10–100 µg/l described); changed by dilution or by the length of the cell
- Decomposition of the complexes: UV-B 312–400 nm at pH 3.8, filter below 290 nm (protection against the conversion of SCN⁻); lamp switched off for free cyanide
- Separation of the HCN: in-line distillation at 125 °C or gas diffusion at 30–40 °C through a hydrophobic membrane into NaOH 0.2 mol/l
Overview
ISO 14403-2:2012 specifies methods for determining cyanide in various types of water (groundwater, drinking water, surface water, leachate, wastewater) at concentrations usually from 2 µg/l to 500 µg/l in the undiluted sample (the standard describes the range 10–100 µg/l; the range is changed by dilution or by the optical path length of the cell); sea water can be analysed after the reagents and standards have been adjusted to the salinity. Free cyanide (easily liberatable cyanide) is the sum of the cyanide ions and of the cyanide in weak metal complexes that release HCN at pH 3.8; total cyanide additionally covers the stronger metal-cyanide complexes with the exception of those of gold, cobalt, platinum, ruthenium and rhodium (partial recovery); thiocyanates, organic cyanides and cyanates do not fall within this definition (where organic cyanides are suspected, gas diffusion should be used). The 2019 regulation (Dz.U. 2019 poz. 1311 [Journal of Laws 2019, item 1311]) names PN-EN ISO 14403 (continuous flow analysis) alongside PN-C-04603-1 (spectrophotometry) as the reference methods for cyanide in wastewater, for which the limits are 0.1 mg CN/l (free) and 5 mg CN/l (bound). In Poland it is in force as PN-EN ISO 14403-2:2012 (English version).
Interferences: oxidising agents (chlorine) decompose cyanide — the sample is tested with iodide-starch paper and treated with sodium thiosulfate (crystals added until there is no reaction), after which 0.6 g of ascorbic acid per 1000 ml is added (only if the analysis is carried out within 24 h); sulfides from 100 mg/l upwards interfere with the colorimetry, especially with gas diffusion and amperometry — 25 ml of the stabilised sample (pH above 12) is treated with powdered lead carbonate until there is no reaction on lead acetate paper and then filtered (avoid an excess of lead and a long contact time); aldehydes and ketones bind cyanide (add ethylenediamine), and during the distillation aldehydes convert cyanide into nitrite (removal with silver nitrate changes the proportions of the forms); nitrites above 5 mg/l — sulfamic acid in the pH 3.8 buffer; sulfites above 1 mg/l interfere; particles block the tubing (homogenisation, filtration above 0.1 mm); salts above 10 g/l block the distillation coil (dilute or use diffusion); thiocyanates may give slightly high results. The classical colorimetric method (PWr textbook "Chemia wody" [Water chemistry], PN-80/C-04603/01) is based on the same chemistry after distillation: the cyanogen chloride from chloramine-T with pyridine gives glutaconic aldehyde, which with barbituric acid turns into a red-violet compound (578 nm), range 0.005–1.0 mg/dm³.
Method principle
Total cyanide: the cyanide bound in complexes is decomposed with UV light at pH 3.8 — a UV-B lamp (312–400 nm) and a coil of borosilicate glass, quartz or PTFE; the UV system must filter out light below 290 nm so that thiocyanate is not converted into cyanide. The hydrogen cyanide present at pH 3.8 is separated by in-line distillation at 125 °C or by gas diffusion at 30–40 °C through a hydrophobic membrane into a sodium hydroxide solution. The HCN is determined photometrically: the cyanide reacts with chloramine-T to give cyanogen chloride, which with pyridine-4-carboxylic acid and 1,3-dimethylbarbituric acid gives a red dye whose absorption is proportional to the concentration. Free cyanide: the same procedure with the UV lamp switched off; with distillation, zinc sulfate solution is added to the sample stream, precipitating the ferrocyanides as the zinc complex; with diffusion, 50 µl of tetraethylenepentamine solution per 30 ml of sample is added to release the cyanide from the nickel complex. Alternatively, free and total cyanide can be determined after gas diffusion with an amperometric detector (annex C). Reagents (all of analytical grade; KCN and K₂Zn(CN)₄ are toxic): grade 1 water, HCl 1 mol/l, NaOH 0.4, 1.0, 0.2 (receiving solution with diffusion) and 0.01 mol/l (rinsing, diluent for the standards), polyoxyethylene lauryl ether surfactant 30 g/100 ml, citric acid, ascorbic acid, sulfamic acid, tetraethylenepentamine (0.75 g per 250 ml, stable for 1 month), zinc sulfate heptahydrate (10 g/l), potassium hydrogen phthalate, chloramine-T trihydrate, 1,3-dimethylbarbituric acid, pyridine-4-carboxylic acid, KSCN, K₃Fe(CN)₆; standards: KCN 1000 mg/l CN (2500 mg of KCN in NaOH 0.01 mol/l made up to 1000 ml; 3 months at 1–8 °C; titre according to annex B) or commercial potassium tetracyanozincate 1000 mg/l; a solution of 10 mg/l (1 week), a solution of 1 mg/l and at least 5 calibration standards spread evenly over the working range (e.g. 1, 3, 5, 6, 8, 10 ml of the 1 mg/l solution made up to 100 ml → 10, 30, 50, 60, 80, 100 µg/l; stable for 2 days at 1–5 °C). Buffer pH 3.8 for the distillation/diffusion: 10 g of citric acid in about 350 ml of water + 50 ml of NaOH 0.4 mol/l (adjusted to pH 3.8) + 12.5 ml of HCl 1 mol/l, made up to 500 ml (after the HCl has been added the pH is about 3.4, and after mixing with the sample 3.8; stable for 3 months at 1–5 °C); buffer for the detection with distillation: 2.3 g of NaOH + 20.5 g of potassium hydrogen phthalate, pH 5.2, 1 ml of surfactant, made up to 1000 ml (1 month in a dark bottle). Calibration according to ISO 8466-1/-2.
Applications
- Industrial wastewater (electroplating shops, coking plants, the chemical and metallurgical industries) — free and bound cyanide against the limits of the regulation (CFA reference method)
- Groundwater and leachate from landfills and post-industrial sites
- Drinking water and surface water — control of cyanide at the µg/l level
- Laboratories with a large number of samples — automatic flow analysis with in-line UV decomposition and distillation/diffusion
- Samples with organic cyanides or of high salinity — the gas diffusion variant; the amperometric variant (annex C)
Key parameters
| Parameter | Value |
|---|---|
| Range | usually 2–500 µg/l CN⁻ (10–100 µg/l described); changed by dilution or by the length of the cell |
| Decomposition of the complexes | UV-B 312–400 nm at pH 3.8, filter below 290 nm (protection against the conversion of SCN⁻); lamp switched off for free cyanide |
| Separation of the HCN | in-line distillation at 125 °C or gas diffusion at 30–40 °C through a hydrophobic membrane into NaOH 0.2 mol/l |
| Detection | chloramine-T → ClCN; pyridine-4-carboxylic acid + 1,3-dimethylbarbituric acid → a red dye; buffer pH 5.2 (hydrogen phthalate) |
| Interferences | Cl₂ (thiosulfate + 0.6 g/l of ascorbic acid), S²⁻ ≥ 100 mg/l (PbCO₃), NO₂⁻ above 5 mg/l (sulfamic acid), SO₃²⁻ above 1 mg/l, aldehydes/ketones, salts above 10 g/l, particles above 0.1 mm |
| Standards | KCN 1000 mg/l in NaOH 0.01 mol/l (iodometric titre, annex B) or K₂Zn(CN)₄; at least 5 standards, e.g. 10–100 µg/l, stable for 2 days |
Standard
- Standard number
- PN-EN ISO 14403-2:2012 (ISO 14403-2:2012)
- Title (PL)
- Jakość wody — Oznaczanie cyjanków ogólnych i wolnych przy zastosowaniu analizy przepływowej (FIA i CFA) — Część 2: Metoda analizy przepływowej ciągłej (CFA)
- Title (EN)
- Water quality — Determination of total cyanide and free cyanide using flow analysis (FIA and CFA) — Part 2: Method using continuous flow analysis (CFA)
Step-by-step procedure
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Preservation and removal of interferences
Test a drop of the sample on iodide-starch paper — if it turns blue, add crystals of sodium thiosulfate until there is no reaction, then 0.6 g of ascorbic acid per 1000 ml (only if the analysis is carried out within 24 h). Where sulfides are present (lead acetate paper), treat a further 25 ml of the stabilised sample (pH above 12) with powdered PbCO₃ until there is no reaction and filter it through a dry filter. Homogenise and remove particles above 0.1 mm; dilute samples with more than 10 g/l of salts or use diffusion.
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Standards
From the KCN solution of 1000 mg/l (2500 mg of KCN in NaOH 0.01 mol/l; titre according to annex B) or from K₂Zn(CN)₄ prepare a solution of 10 mg/l, then one of 1 mg/l and at least 5 standards within the working range (e.g. 10, 30, 50, 60, 80, 100 µg/l in NaOH 0.01 mol/l); store at 1–5 °C for up to 2 days.
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CFA system — total cyanide
Assemble the manifold (figure A.1 for distillation or A.2 for diffusion): sample + buffer pH 3.8 → UV coil (UV-B lamp switched on, filter below 290 nm) → in-line distillation at 125 °C or diffusion at 30–40 °C into NaOH 0.2 mol/l → reagents: chloramine-T, pyridine-4-carboxylic acid and 1,3-dimethylbarbituric acid (with distillation, with the pH 5.2 buffer) → photometric detector. Stabilise the baseline and check the reagent blanks.
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Free cyanide
Repeat with the UV lamp switched off; with distillation add zinc sulfate solution to the sample stream (precipitation of the ferrocyanides as the zinc complex); with diffusion add 50 µl of tetraethylenepentamine solution per 30 ml of sample (release of the cyanide from the nickel complex).
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Calibration and measurement
Measure the standards and the blank, establish the calibration function according to ISO 8466-1 or -2; measure the samples in the same series; with amperometry (annex C) add a few mg of PbCO₃ per 10 ml of sample and filter or decant before placing them in the tray.
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Result
Report the concentration of total and free cyanide as CN⁻ in µg/l (mg/l), taking the dilution into account; record the variant (distillation/diffusion, UV), the preservation and the interferences removed; results in the presence of thiocyanates may be slightly high.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Continuous flow analyser (CFA) with an autosampler, a peristaltic pump and bubble segmentation | modules according to figures A.1 (distillation) and A.2 (diffusion) | — |
| UV module: a UV-B lamp 312–400 nm with a filter below 290 nm and a coil of borosilicate glass/quartz/PTFE | decomposition of the cyanide complexes at pH 3.8 | — |
| In-line distillation module (125 °C) or gas diffusion module (30–40 °C) with a hydrophobic membrane | separation of the HCN | — |
| Photometric detector with a flow cell | measurement of the red dye; alternatively an amperometric detector (annex C) | — |
| Iodide-starch and lead acetate papers, filters | tests for oxidising agents and sulfides, removal of particles | — |
| Refrigerator 1–8 °C, volumetric flasks of 100 and 1000 ml | storage of the standards and the samples | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Buffer pH 3.8 (citric acid 10 g + NaOH 0.4 mol/l 50 ml + HCl 1 mol/l 12.5 ml per 500 ml) | — | medium for the UV decomposition and the liberation of HCN (with sulfamic acid where nitrites are present) |
| Sodium hydroxide 0.2 mol/l (receiving) and 0.01 mol/l (rinsing, standards) | — | absorption of the HCN with diffusion; stabilisation of the cyanide |
| Chloramine-T trihydrate, pyridine-4-carboxylic acid, 1,3-dimethylbarbituric acid | — | colour reaction (red dye) |
| Buffer pH 5.2 (NaOH 2.3 g + potassium hydrogen phthalate 20.5 g/l) with surfactant | — | buffer for the photometric detection (distillation) |
| Zinc sulfate 10 g/l; tetraethylenepentamine 0.75 g per 250 ml | — | free cyanide: precipitation of the ferrocyanides (distillation) / release from the Ni complex (diffusion) |
| Standards KCN 1000 mg/l or K₂Zn(CN)₄ 1000 mg/l; sodium thiosulfate, ascorbic acid, lead carbonate | — | calibration; removal of oxidising agents and sulfides |
Health and safety (OHS)
- Potassium cyanide, potassium tetracyanozincate, their solutions and the waste are strongly toxic — work in a fume hood, gloves, no acidification of the solutions outside a closed system (HCN), cyanide waste to be disposed of after alkaline oxidation
- UV-B lamp — shielding against the radiation; hot distillation module at 125 °C
- Lead carbonate, silver nitrate and ethylenediamine — toxic/irritant; dispense them carefully
- Chloramine-T and the acids — goggles, gloves
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 14403-2:2012 (ISO 14403-2:2012) — “Water quality — Determination of total cyanide and free cyanide using flow analysis (FIA and CFA) — Part 2: Method using continuous flow analysis (CFA)”.
How does this method work?
Total cyanide: the cyanide bound in complexes is decomposed with UV light at pH 3.8 — a UV-B lamp (312–400 nm) and a coil of borosilicate glass, quartz or PTFE; the UV system must filter out light below 290 nm so that thiocyanate is not converted into cyanide.
What is the measuring range and accuracy?
Range: usually 2–500 µg/l CN⁻ (10–100 µg/l described); changed by dilution or by the length of the cell; Decomposition of the complexes: UV-B 312–400 nm at pH 3.8, filter below 290 nm (protection against the conversion of SCN⁻); lamp switched off for free cyanide; Separation of the HCN: in-line distillation at 125 °C or gas diffusion at 30–40 °C through a hydrophobic membrane into NaOH 0.2 mol/l; Detection: chloramine-T → ClCN; pyridine-4-carboxylic acid + 1,3-dimethylbarbituric acid → a red dye; buffer pH 5.2 (hydrogen phthalate).
How long does the test take?
The procedure comprises 6 steps. The standard does not give a duration for every stage — the laboratory's own procedure decides.
What equipment is required?
Continuous flow analyser (CFA) with an autosampler, a peristaltic pump and bubble segmentation, UV module: a UV-B lamp 312–400 nm with a filter below 290 nm and a coil of borosilicate glass/quartz/PTFE, In-line distillation module (125 °C) or gas diffusion module (30–40 °C) with a hydrophobic membrane, Photometric detector with a flow cell, Iodide-starch and lead acetate papers, filters, Refrigerator 1–8 °C, volumetric flasks of 100 and 1000 ml.
Where is this test used?
Industrial wastewater (electroplating shops, coking plants, the chemical and metallurgical industries) — free and bound cyanide against the limits of the regulation (CFA reference method); Groundwater and leachate from landfills and post-industrial sites; Drinking water and surface water — control of cyanide at the µg/l level; Laboratories with a large number of samples — automatic flow analysis with in-line UV decomposition and distillation/diffusion; Samples with organic cyanides or of high salinity — the gas diffusion variant; the amperometric variant (annex C).
What safety precautions apply?
Potassium cyanide, potassium tetracyanozincate, their solutions and the waste are strongly toxic — work in a fume hood, gloves, no acidification of the solutions outside a closed system (HCN), cyanide waste to be disposed of after alkaline oxidation; UV-B lamp — shielding against the radiation; hot distillation module at 125 °C; Lead carbonate, silver nitrate and ethylenediamine — toxic/irritant; dispense them carefully; Chloramine-T and the acids — goggles, gloves.
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 14403-2.