☠️ Cyanides in water

Physicochemistry ISO 6703-1

Overview

STATUS (as of 29.10.2026). ISO 6703-1:1984 (first edition, 01.08.1984) is published in the ISO catalogue, with no newer edition. The standard has not been adopted into the set of Polish Standards: the PKN search for "ISO 6703" returns only PN-ISO 16703:2009 and PN-EN ISO 16703:2011 (hydrocarbons — a different subject), so a reference to "PN-ISO 6703" or "PN-EN ISO 6703" points to a document that does not exist. In the ISO 6703 series, part 2 deals with easily liberatable cyanide and part 3 with cyanogen chloride (both of 1984, published), while part 4 (determination of cyanide by diffusion at pH 6, 1985) is withdrawn. Polish regulations specify other documents for cyanides (see LEGAL REQUIREMENTS) — the method described below is not interchangeable with them.

SCOPE (clause 1). Three methods for determining total cyanide in water containing less than 100 mg of cyanide per litre; higher concentrations — after dilution of the sample. Ranges of cyanide mass in the portion: photometric method with pyridine/barbituric acid — 0.002–0.025 mg, titrimetric method using the Tyndall effect — more than 0.005 mg, titrimetric method using an indicator — more than 0.05 mg. According to the introduction, the methods of parts 1–3 are suitable for controlling the quality of water and for examining municipal sewage and industrial effluents.

TOTAL CYANIDE (clause 2). Simple and complex bound cyanides, including organic compounds containing cyanogen groups that form hydrogen cyanide under the conditions of the method; cyanohydrins are detected in part. Mononitriles (R-CN), cyanate and thiocyanate ions and cyanogen chloride are not included. Complex cobalt cyanides, depending on their concentration, are decomposed only to the extent of 5–15 %, as are some organocyanide compounds (clause 3, note 1).

INTERFERENCES (clause 1, Table 1). Many ions and compounds interfere; Table 1 (not exhaustive) gives the concentrations up to which they — singly or in combination — do not interfere with the separation of hydrogen cyanide, e.g. thiosulfate, thiocyanate, carbonate and cyanate ions 1000 mg/l, nitrate and nitrite ions 500 mg/l, ammonium ions 2000 mg/l, iron(II) and iron(III) ions 5000 mg/l, silver and mercury ions 50 mg/l, chromate ions 300 mg/l, elemental chlorine 250 mg/l, hydrogen peroxide and perborate ions 10 mg/l. If any of these concentrations is likely to be exceeded, the sample is diluted with distilled water before stabilization. Aldehydes, e.g. formaldehyde, give low results (formation of cyanohydrins); fatty acids and elemental sulfur may also interfere.

LEGAL REQUIREMENTS. Water intended for human consumption: cyanides 50 µg/l (Regulation of the Minister of Health of 22 May 2026, Journal of Laws 2026, item 748, Annex 1 Part B item 16); measurement uncertainty up to 30 % of the parametric value, and the method is to determine the total amount of cyanide in all forms (Annex 4 Part B item 21 and note 3). Waste water: the reference methods for cyanides are PN-C-04603-1 (spectrophotometry), PN-C-04603-2 (titrimetry) and PN-EN ISO 14403 (continuous flow analysis) — ISO 6703-1 is not referenced there (Journal of Laws 2019, item 1311, Annex 12 item 20). Monitoring of surface and ground waters: for free and bound cyanides the regulation lists PN-80/C-04603.01, ISO 14403 and laboratory procedures (Journal of Laws 2021, item 1576, Annex 8, indicators 3.6.12–3.6.13).

WHAT WE DO NOT GIVE HERE. The publicly available text of the standard ends at the beginning of clause 6 (sampling and preservation). The rest of the preservation, the course of hydrogen cyanide liberation (clause 7) and the reagents, wavelengths, calculations and precision data of the three final determination methods (sections two to four) lie outside it, so we do not give them. For comparison: in part 3 of the same standard (cyanogen chloride) the pyridine/barbituric acid dye is measured at 578 nm.

Method principle

Liberation (section one, clause 3): the sample is heated with hydrochloric acid in the presence of copper(I) ions; the liberated hydrogen cyanide is carried by a current of air into an absorption vessel containing sodium hydroxide solution. Tin(II) chloride is added as a reducing agent, a zinc salt to provide stable zinc hexacyanoferrates, and cadmium salts as a sulfide acceptor and because of their bactericidal effect (footnote to clause 4).

Determination of cyanide ions in the absorption solution — one of three methods, because none is applicable in all cases (introduction): photometric with pyridine/barbituric acid (section two), titrimetric using the Tyndall effect (section three) or titrimetric using an indicator (section four).

Applications

• Water — total cyanide at concentrations below 100 mg/l, higher after dilution (clause 1)
• Municipal sewage and industrial effluents — according to the introduction to the standard
• Water intended for human consumption — cyanides 50 µg/l, the method is to cover cyanide in all forms (Journal of Laws 2026, item 748)

Key parameters

Standard

Standard number

ISO 6703-1:1984

Title (PL)

Jakość wody — Oznaczanie cyjanków — Część 1: Oznaczanie cyjanków ogólnych

Title (EN)

Water quality — Determination of cyanide — Part 1: Determination of total cyanide

Step-by-step procedure

1. Sampling and preservation

   If the sample contains undissolved cyanides, ensure a homogeneous distribution of the suspended matter in the sample and its dilutions. Immediately after sampling add to each litre of sample (or diluted sample) 5 ml of 5 mol/l sodium hydroxide, 10 ml of phenolphthalein solution and 5 ml of tin(II) chloride solution; further preservation steps are described in the part of clause 6 outside the available text.

2. Dilution in the case of interferences

   If the concentration of any substance in Table 1 may exceed its limiting value, the sample is diluted with distilled water before stabilization (clause 1).

3. Liberation of hydrogen cyanide

   In the apparatus of Figure 1: heating the sample with hydrochloric acid in the presence of copper(I) ions and transfer of hydrogen cyanide by a current of air into the absorption vessel with sodium hydroxide solution (clause 3); volumes, flow and time — clause 7, outside the available text.

4. Determination of cyanide ions

   In the absorption solution by one of three methods, depending on the expected mass of cyanide: photometric with pyridine/barbituric acid (0.002–0.025 mg), titrimetric using the Tyndall effect (more than 0.005 mg) or titrimetric using an indicator (more than 0.05 mg) (clause 1).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Cyanides are highly toxic — carry out all operations in a fume cupboard, avoid contact with skin and eyes, always pipette with a safety pipette (pipette by bulb) (warning of the standard)
• Samples and solutions containing cyanides or heavy metals are detoxified in accordance with local official regulations (warning of the standard)
• Other reagents of the standard are also hazardous, e.g. pyridine; cadmium salt solutions and chloroform — according to the safety data sheets

Frequently asked questions