🧪 COD — Chemical Oxygen Demand

Physicochemistry PN-ISO 15705

Overview

STATUS (as of 28.10.2026). PN-ISO 15705:2005 — Polish version, published 05.05.2005, implements ISO 15705:2002 [IDT]. The standard is current — no withdrawal note.

SCOPE (abstract of the standard). The determination is empirical and is applicable to any water sample, including all domestic and industrial waste waters. It applies to undiluted samples with an ST-COD of up to 1000 mg/l and a chloride concentration not exceeding 1000 mg/l. Samples with a higher ST-COD are diluted before the determination, and samples with a high chloride concentration so that the chloride concentration is about 1000 mg/l or less. For samples with a low COD the precision is reduced and the detection limit is poorer. The method oxidizes almost all types of organic compounds and most inorganic reducing substances.

DETECTION LIMIT AND READING (abstract of the standard). 6 mg/l for photometric detection at 600 nm and 15 mg/l for titrimetric detection — as reported by one laboratory that compared both techniques using a commercial tube kit with a range of up to 1000 mg/l (the detection limit is 4.65 times the within-batch standard deviation of a blank or a very low standard). Titrimetric reading is used for samples that show an atypical colour or turbidity after digestion.

ANNEXES OF THE STANDARD. Comparison with the full-scale method ISO 6060 (Annex A), discussion of possible hazards (Annex B), information on commercial tube kits (Annex C), use of the method over a reduced range (Annexes D and E) and checking of the chloride concentration (Annex F).

WASTE WATER — LEGAL REQUIREMENTS. Regulation of the Minister of Maritime Economy and Inland Navigation of 12 July 2019 (Journal of Laws 2019, item 1311): for COD determined by the dichromate method (CODCr) in waste water from treatment plants, the maximum permissible value is 150 mg O₂/l for a population equivalent (PE) below 2000, and 125 mg O₂/l or a minimum reduction of 75 % for a PE of 2000 or more (Annexes 2 and 3, 24-hour composite samples). As reference methods for COD the regulation lists PN-ISO 6060 and PN-ISO 15705 (Annex 12, item 14).

WHAT WE DO NOT GIVE HERE. We do not have the text of the standard beyond its scope and the list of annexes, so we do not state the composition and concentrations of the reagents, the sample volume in the tube, the digestion conditions required by the standard, the calibration, the wavelength for the low range, control criteria or precision data. Ready-made tube kits are used according to the manufacturer's instructions.

Method principle

Dichromate method (CODCr): the sample is heated with potassium dichromate in sulfuric acid, in the presence of silver sulfate as a catalyst. Chloride — the most common interferent — is bound with mercury(II) sulfate into a soluble complex. As it oxidizes the substances in the sample, chromium(VI) from the dichromate is converted into chromium(III); the result is reported in mg O₂/l.

In the small-scale version the sample and reagents are sealed in a tube, which after digestion also serves as the cell. In ASTM D1252 (Test Method B) and Standard Methods 5220 D the tubes are heated at 150 °C for 2 h, and then the increase of chromium(III) is measured at about 600 nm (higher range) or the loss of chromium(VI) at about 420 nm (low range). ISO 15705 specifies photometric detection at 600 nm or titrimetric detection — we do not quote its digestion conditions here.

Applications

• Waste water from treatment plants — COD determined by the dichromate method: 150 mg O₂/l (PE below 2000) and 125 mg O₂/l or 75 % reduction (PE of 2000 or more), in 24-hour composite samples (Journal of Laws 2019, item 1311, Annexes 2 and 3)
• Reference method for COD in waste water testing, alongside PN-ISO 6060 (Journal of Laws 2019, item 1311, Annex 12)
• Any water sample, including domestic and industrial waste waters — without dilution up to 1000 mg/l ST-COD and 1000 mg/l chloride
• Samples with an atypical colour or turbidity after digestion — titrimetric reading

Key parameters

Standard

Standard number

PN-ISO 15705:2005 (ISO 15705:2002)

Title (PL)

Jakość wody — Oznaczanie indeksu chemicznego zapotrzebowania tlenu (SP-ChZT) — Metoda zminiaturyzowana z zastosowaniem szczelnych probówek

Title (EN)

Water quality — Determination of the chemical oxygen demand index (ST-COD) — Small-scale sealed-tube method

Step-by-step procedure

1. Sampling and sample preparation

   Samples preferably in glass bottles; unstable samples are tested without delay, and if a delay is unavoidable they are acidified with concentrated sulfuric acid to pH ≤ 2; samples with suspended solids are homogenized (Standard Methods 5220 D). For waste water from treatment plants the regulation requires analysis of homogenized, undecanted and unfiltered samples (Journal of Laws 2019, item 1311, Annex 2, explanatory note 2).

2. Dilution

   A sample with an ST-COD above 1000 mg/l is diluted before the determination; a sample with a high chloride concentration — to about 1000 mg/l chloride or less (abstract of the standard).

3. Digestion in a sealed tube

   The sample and reagents are sealed in the tube and heated; in ASTM D1252 B — at 150 °C for 2 h in a heating block, oven or sand bath. We do not quote the conditions of ISO 15705 here.

4. Reading

   Photometrically at 600 nm, in the tube used as the cell; samples with an atypical colour or turbidity after digestion — titrimetrically (abstract of the standard).

5. Result

   ST-COD in mg/l (mg O₂/l), taking into account the dilution of the sample.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• The reagents contain potassium dichromate (chromium(VI)), concentrated sulfuric acid and a mercury salt; hazards are discussed in Annex B of the standard — the safety data sheets of the reagents and kits apply
• Used tubes (chromium, mercury, silver) are handled according to the safety data sheet of the kit

Frequently asked questions