⚗️ Chemical oxygen demand (COD-Cr) — oxidation with potassium dichromate under a reflux condenser (2 h, 148 °C) and titration with ammonium iron(II) sulfate

Physicochemistry PN-ISO 6060

Overview

ISO 6060:1989 specifies a method for determining the chemical oxygen demand (COD) of water with dichromate; the COD is the mass concentration of oxygen equivalent to the amount of dichromate consumed by the dissolved and suspended substances when this oxidant acts under defined conditions. The method applies to waters with a COD of 30–700 mg/l (preferably 300–600 mg/l; higher values after dilution; below 30 mg/l the result is reported as "< 30 mg/l") and a chloride content of up to 1000 mg/l (above that — a modified procedure). Organic compounds are oxidised to a large extent (the exceptions include the pyridine ring, quaternary nitrogen compounds and aromatic hydrocarbons); volatile hydrophobic substances may evaporate; bromide, iodide, nitrite, certain sulfur compounds and metals are also oxidised. The dichromate COD is an approximate measure of the theoretical oxygen demand — a realistic one for municipal wastewater, a poor one for certain industrial effluents containing substances that are difficult to oxidise. Reproducibility: the standard deviation between laboratories is below 10 mg/l; at 500 mg/l it is 20 mg/l for potassium hydrogen phthalate and 25 mg/l for the effluent of a pulp mill; for industrial effluents of 140–160 mg/l it is about 14 mg/l. The 2019 regulation (Dz.U. 2019 poz. 1311 [Journal of Laws 2019, item 1311]) lists PN-ISO 6060 (alongside PN-ISO 15705) as the reference method for COD in wastewater. In Poland it is in force as PN-ISO 6060:2006.

The manufacturers of cuvette tests describe the same chemistry: the COD is more accurate than the BOD (a relative standard deviation of 5–10 %) and faster (2 h of digestion as against 5 days), and chloride is the main cause of interference — the cuvettes with the dichromate method contain mercury sulfate, which eliminates the effect of chloride up to the level defined in the procedure. In the cuvette version used in the PWr laboratory course, 2.5 cm³ of water is added to the digestion solution (potassium dichromate, mercury sulfate and silver sulfate in concentrated sulfuric acid), it is heated in a thermoreactor at 148 °C for 1 h and measured spectrophotometrically.

Method principle

A portion of the sample is heated under a reflux condenser in the presence of mercury(II) sulfate with a known amount of potassium dichromate and a silver catalyst in strong sulfuric acid for a fixed time; part of the dichromate is reduced by the oxidisable substances. The remaining dichromate is titrated with ammonium iron(II) sulfate and the COD is calculated from the amount of dichromate reduced (1 mol of Cr₂O₇²⁻ ≡ 1.5 mol of O₂). Reagents: sulfuric acid 4 mol/l (220 ml of H₂SO₄ of 1.84 g/ml made up to 1000 ml); silver sulfate in sulfuric acid (10 g of Ag₂SO₄ in 35 ml of water + 965 ml of H₂SO₄; dissolving takes 1–2 days); standard potassium dichromate 0.040 mol/l with a mercury salt (80 g of HgSO₄ in 800 ml of water + 100 ml of H₂SO₄, and after cooling 11.768 g of K₂Cr₂O₇ dried for 2 h at 105 °C, made up to 1000 ml; stable for at least 1 month; alternatively without the mercury salt, in which case 0.4 g of HgSO₄ is added to the portion before the dichromate); ammonium iron(II) sulfate of about 0.12 mol/l (47.0 g of the hexahydrate salt + 20 ml of H₂SO₄ made up to 1000 ml), standardised every day against 10.0 ml of the dichromate diluted to 100 ml with sulfuric acid with 2–3 drops of ferroin: c = 2.4/V; standard potassium hydrogen phthalate 2.0824 mmol/l (0.4251 g dried at 105 °C made up to 1000 ml; theoretical COD 500 mg/l, stable for at least 1 week at about 4 °C); ferroin (0.7 g of FeSO₄·7H₂O or 1 g of Mohr's salt + 1.50 g of 1,10-phenanthroline monohydrate in 100 ml). The quality of the distilled water is checked by blanks with and without boiling: a difference in titrant consumption above 0.5 ml (0.2 ml where the COD is below 100 mg/l) indicates poor water. Apparatus: an assembly for heating under a reflux condenser (a flask or a reaction tube of 250 ml with a ground joint, without significant losses of volatile substances), a heating mantle or a plate that brings the mixture to the boil within 10 min without local overheating, a precision burette of 10 ml graduated in 0.02 ml, glass beads of 2–3 mm; glassware reserved solely for COD and not cleaned with detergents. The samples (preferably in glass bottles) are analysed as soon as possible, at the latest 5 days after sampling; for storage 10 ml of H₂SO₄ 4 mol/l per litre is added and they are kept at 0–5 °C; they are homogenised before the portion is taken. Result: COD = 8000·c·(V₁ − V₂)/V₀ [mg O₂/l], where c is the concentration of the ammonium iron(II) sulfate, V₀ the volume of the portion before dilution and V₁ and V₂ the volumes of titrant for the blank and for the sample; rounded to 1 mg/l.

Applications

• Municipal and industrial wastewater (inlet/outlet of a treatment plant) — the reference method for COD under Dz.U. 2019 poz. 1311
• Polluted surface waters and landfill leachate (COD 30–700 mg/l, above that after dilution)
• Control of treatment processes and balancing of the organic load (the COD/BOD5 ratio)
• Wastewater with a chloride content of up to 1000 mg/l (above that — a modified procedure)
• Laboratories that run the reference method alongside cuvette tests (PN-ISO 15705)

Key parameters

Standard

Standard number

PN-ISO 6060:2006 (ISO 6060:1989)

Title (PL)

Jakość wody — Oznaczanie chemicznego zapotrzebowania tlenu

Title (EN)

Water quality — Determination of the chemical oxygen demand

Step-by-step procedure

1. Sample

   Take it preferably into a glass bottle; analyse it as soon as possible (within 5 days). For storage add 10 ml of H₂SO₄ 4 mol/l per litre and keep at 0–5 °C; shake and homogenise before taking the portion. Dilute samples with a COD above 700 mg/l.

2. Reaction

   Transfer 10.0 ml of the sample (diluted if necessary) into the reaction flask, add 5.00 ± 0.01 ml of the potassium dichromate solution and a few boiling beads, and mix. Slowly add 15 ml of the silver sulfate in sulfuric acid and connect the condenser at once.

3. Heating

   Bring the mixture to the boil within 10 min and boil it gently for a further 110 min (temperature 148 °C ± 3 °C); the boiling must be free of bumping — bumping indicates local overheating and falsifies the result.

4. Cooling and dilution

   Cool the flask at once in cold water to about 60 °C, rinse the condenser with a small amount of water, remove the condenser, dilute the mixture to about 75 ml and cool it to room temperature.

5. Titration

   Add 1–2 drops of ferroin (keeping the amount as constant as possible) and titrate the excess dichromate with ammonium iron(II) sulfate to the first sharp change of colour from blue-green to reddish brown (the blue-green colour may return after a few minutes).

6. Blanks and check test

   With each series run in parallel two blanks with 10.0 ml of water and a test with 10.0 ml of potassium hydrogen phthalate (result ≥ 96 % of 500 mg/l). Standardise the titrant every day: 10.0 ml of the dichromate diluted to about 100 ml with sulfuric acid 4 mol/l, 2–3 drops of ferroin; c = 2.4/V.

7. Calculation

   COD = 8000·c·(V₁ − V₂)/V₀ [mg O₂/l]; report to 1 mg/l; values below 30 mg/l as "< 30 mg/l". In the test report: the reference to the standard, the identification of the sample, the result and any deviations.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Boiling concentrated solutions of sulfuric acid with dichromate — protective clothing, gloves and a full face shield; rinse any spillage at once with plenty of water
• Add the sulfuric acid to the water carefully, swirling the contents of the flask gently
• Silver sulfate and mercury sulfate are toxic; the spent reagents contain mercury, silver and chromium — dispose of them in accordance with the national regulations (ISO 5790, annex B)
• Potassium dichromate is carcinogenic — avoid the dust and contact with the skin

Frequently asked questions