Water Quality COD Determination: Experimental Steps, Procedures, and Testing Guide

Chemical Oxygen Demand (COD) is one of the most critical parameters in water quality analysis, quantifying the amount of oxygen required to chemically oxidize organic matter in a water sample. This technical article provides a comprehensive, step-by-step guide to COD determination in water, covering sample collection and preservation, reagent preparation, reflux digestion, titration, spectrophotometric methods, interference control, and result calculation in accordance with international standards such as ISO 6060, GB 11914, HJ 828, and ASTM D1252. Whether you operate an environmental monitoring laboratory, a wastewater treatment facility, or an industrial quality control program, this guide will help you perform accurate and reproducible COD measurements.

Technical Overview

Chemical Oxygen Demand (COD) is a standardized measurement of the oxygen equivalent consumed by oxidizable organic and inorganic matter in a water sample when subjected to chemical oxidation by a strong oxidizing agent, typically potassium dichromate, under strongly acidic conditions at elevated temperature. The result is expressed in milligrams of oxygen per liter (mg/L O₂) and serves as one of the most widely used aggregate indicators of organic pollution in water and wastewater.

The COD test is essential because it provides a rapid estimate of the total pollutant load in a sample, unlike the Biochemical Oxygen Demand (BOD) test, which requires a five-day incubation period. COD values quantify the oxygen-depleting potential of effluents discharged into receiving water bodies, making this parameter a cornerstone of discharge compliance, treatment process control, and environmental regulation worldwide.

Several recognized methods govern COD determination, including the classical reflux digestion dichromate titration method described in ISO 6060, GB 11914-89, and HJ 828-2017, as well as the closed-reflux colorimetric method outlined in ASTM D1252 and the rapid digestion spectrophotometric method specified in HJ/T 399-2007. The selection of method depends on the expected COD concentration, the chloride content of the sample, and the required turnaround time.

Testing Samples

Surface Water

Rivers, lakes, reservoirs, and streams typically exhibit low to moderate COD concentrations, generally below 30 mg/L. These samples are collected in clean glass containers and analyzed promptly to reflect in-situ conditions.

Municipal Wastewater

Raw domestic sewage and treated municipal effluent contain elevated concentrations of biodegradable and refractory organic compounds, with COD values commonly ranging from 100 mg/L to over 1000 mg/L. Dilution is frequently required before digestion.

Industrial Effluents

Discharges from chemical processing, textile manufacturing, pulp and paper production, food and beverage processing, pharmaceutical production, and petrochemical facilities may contain high-strength organic loads and interfering substances such as chlorides, sulfides, or heavy metals, requiring specific pretreatment or interference masking.

Treated and Reclaimed Water

Water that has passed through biological or advanced treatment processes is tested to verify compliance with discharge permits and reuse criteria, often requiring high-sensitivity analytical procedures.

Groundwater and Source Water

Groundwater samples are analyzed to establish baseline water quality and detect contamination from leachate, landfill seepage, or agricultural runoff.

Testing Items

The core analytical deliverable of this procedure is the COD concentration of the water sample, expressed in mg/L of oxygen consumed. Supporting determinations performed as part of a complete COD testing workflow include:

• Sample homogenization and representative subsampling, including suspension of settleable solids where required by the applicable standard
• Sample dilution factor verification for high-strength wastewaters
• Chloride concentration assessment to determine the need for interference masking
• Blank correction and calibration verification using standard reference substances such as potassium hydrogen phthalate
• Parallel duplicate testing to demonstrate precision and reproducibility
• Recovery checks on spiked samples to confirm analytical accuracy

Testing Methods

The following experimental steps describe the classical open reflux digestion with dichromate oxidation followed by ferrous ammonium sulfate titration, which is the reference procedure for COD determination. The rapid spectrophotometric alternative is also described.

Step 1: Sample Collection and Preservation

Collect representative samples in clean borosilicate glass bottles. Homogenize the sample thoroughly, and analyze the unsettled, homogeneous portion. If immediate analysis is not possible, preserve the sample by acidifying with concentrated sulfuric acid to a pH below 2 and store it at approximately 4 °C. Preserved samples should be analyzed as soon as practicable, preferably within 48 hours, to minimize changes in organic composition.

Step 2: Reagent and Apparatus Preparation

Prepare and standardize all reagents before beginning the digestion:

• Potassium dichromate (K₂Cr₂O₇) standard solution, typically 0.2500 mol/L (equivalent to 1/6 K₂Cr₂O₇) for standard-range samples or 0.0250 mol/L for low-range samples
• Ferrous ammonium sulfate ((NH₄)₂Fe(SO₄)₂·6H₂O) titrant at approximately 0.10 mol/L, standardized daily against the potassium dichromate solution
• Sulfuric acid–silver sulfate (Ag₂SO₄–H₂SO₄) catalyst solution, prepared by dissolving silver sulfate in concentrated sulfuric acid to promote oxidation of straight-chain aliphatic compounds
• Mercuric sulfate (HgSO₄) for masking chloride interference in saline samples
• Ferroin (1,10-phenanthroline ferrous sulfate complex) indicator solution for endpoint detection
• Potassium hydrogen phthalate (KHP) standard solution for calibration verification and quality control

Required apparatus includes a reflux apparatus with 250 mL ground-glass Erlenmeyer flasks and water-cooled condensers, a heating mantle or hotplate capable of maintaining steady boiling, a burette or automated titrator with 0.05 mL readability, analytical balance, and volumetric glassware.

Step 3: Digestion Flask Charging

Pipette 20.0 mL of the homogeneous sample, or an aliquot diluted to 20.0 mL with distilled water, into a clean reflux flask. If the sample contains more than approximately 30 mg/L of chloride, add about 0.4 g of mercuric sulfate to complex the chloride and prevent its oxidation. Add a few pumice stones or glass beads to prevent bumping. Pipette 10.00 mL of the potassium dichromate standard solution into the flask, followed by several drops of ferroin indicator is added later at titration stage; at this point, gently swirl to mix.

Step 4: Attachment of Condenser and Acid Addition

Attach the reflux flask to the condenser before adding the acid reagent to prevent loss of volatile organic compounds. Slowly and carefully add 30 mL of the sulfuric acid–silver sulfate solution through the open end of the condenser while continuously swirling the flask to ensure thorough mixing. The addition of concentrated acid to an aqueous solution is highly exothermic, so this step must be performed slowly with adequate mixing.

Step 5: Reflux Digestion

Apply heat and reflux the mixture for two hours from the onset of visible boiling. Maintain gentle, steady boiling without excessive foaming or bumping. The two-hour digestion period ensures complete oxidation of the oxidizable organic matter by the dichromate in the presence of the silver sulfate catalyst. After the digestion period is complete, turn off the heat and allow the apparatus to cool.

Step 6: Cooling and Dilution

Rinse the interior of the condenser with 20 to 30 mL of distilled water to recover any condensed material. Carefully disconnect the condenser, and dilute the digestate with distilled water to approximately 140 mL if following the open reflux protocol, ensuring the solution is at least seven-fold diluted relative to residual acid for a sharp titration endpoint. Allow the flask to cool to room temperature before titration.

Step 7: Titration

Add 2 to 3 drops of ferroin indicator to the cooled solution. Titrate with the standardized ferrous ammonium sulfate solution, swirling continuously, until the color changes sharply from yellow-green through blue-green to a red-brown (reddish-maroon) endpoint. Record the titrant volume consumed to the nearest 0.05 mL.

Step 8: Blank Determination

Repeat the entire digestion and titration procedure using 20.0 mL of distilled water in place of the sample. The reagent blank corrects for trace oxidizable impurities in the reagents and must be run with every batch of samples.

Step 9: Calculation of Results

Calculate the COD concentration using the following expression:

COD (mg/L) = [(V₀ − V₁) × c × 8000] / V

• V₀ = volume of ferrous ammonium sulfate titrant consumed for the blank, in mL
• V₁ = volume of ferrous ammonium sulfate titrant consumed for the sample, in mL
• c = molar concentration of the ferrous ammonium sulfate titrant (equivalent to 1 mol (NH₄)₂Fe(SO₄)₂ per mole Fe²⁺)
• 8000 = molar mass of oxygen divided by four, converted to milligrams (8 × 1000)
• V = volume of original sample aliquot used, in mL

Apply the appropriate dilution factor if the sample was diluted before digestion. Report results to three significant figures.

Alternative Method: Rapid Digestion Spectrophotometry

For high-throughput laboratories, the rapid digestion spectrophotometric method offers a faster alternative. A measured sample volume is dispensed into a sealed digestion tube containing premeasured dichromate reagent, mercuric sulfate, and silver sulfate in concentrated sulfuric acid. The tube is heated at 165 °C for approximately 20 minutes in a block digestor, cooled, and the absorbance of the reduced Cr³⁺ species is measured spectrophotometrically at 440 nm for the low range or 600 nm for the high range. The COD concentration is read directly from a calibration curve prepared using potassium hydrogen phthalate standards. This method requires significantly less reagent volume and produces minimal liquid waste.

Quality Control and Interference Management

• Chloride is the principal positive interference and is masked with mercuric sulfate; samples with very high salinity may require additional treatment or method modification
• Nitrite interference is eliminated by adding sulfamic acid when significant nitrite is present
• Volatile organics may be lost if acid is added before the condenser is attached, resulting in negative bias
• Calibration verification with KHP standard solutions should be performed with each analytical batch, with recovery typically required to fall within 90–110 percent
• Replicate determinations should agree within established precision limits, typically a relative deviation of less than 10 percent
• All acidic chromium-bearing waste must be collected and disposed of as hazardous waste in accordance with applicable regulations

Applications

COD determination is applied across a broad spectrum of environmental and industrial contexts:

• Compliance monitoring of municipal and industrial wastewater discharges against national and local discharge standards
• Operational control of biological wastewater treatment plants, including assessment of influent load, treatment efficiency, and effluent quality
• Environmental impact assessment and routine surveillance of rivers, lakes, reservoirs, and coastal waters by regulatory authorities
• Process optimization in chemical, petrochemical, pharmaceutical, food and beverage, pulp and paper, textile, and fermentation industries
• Verification of treatment system performance, including dissolved air flotation, anaerobic digestion, and advanced oxidation processes
• Support for academic research, pollution source apportionment, water quality modeling, and environmental forensics
• Acceptance testing and quality verification services offered by accredited third-party laboratories

FAQ

What is the difference between COD and BOD?

COD measures the total chemically oxidizable matter, both biodegradable and non-biodegradable, using a strong oxidant and yields results in about two and a half hours. BOD measures only the biodegradable fraction oxidized by microorganisms over a standard incubation period, typically five days. COD values are therefore generally equal to or higher than BOD values for the same sample.

Why is potassium dichromate used as the oxidant?

Potassium dichromate provides a high, well-defined oxidation potential in strongly acidic media, oxidizes a broad spectrum of organic compounds essentially quantitatively in the presence of a silver sulfate catalyst, and is available in high purity as a primary standard, allowing precise standardization of the titrant.

How is chloride interference handled?

Chloride ions are oxidized by dichromate and cause a positive bias. Mercuric sulfate is added to complex chloride and suppress this interference. For samples with exceptionally high chloride content, such as certain industrial brines or marine samples, a chloride correction or an alternative analytical approach may be necessary.

What is the detection range of the reflux titration method?

The classical open reflux method with a 0.2500 mol/L dichromate solution is applicable to samples with COD values from approximately 50 mg/L up to 700 mg/L when a 20 mL aliquot is used. Higher concentrations are accommodated by dilution, and lower concentrations can be measured using the 0.0250 mol/L dichromate reagent or the spectrophotometric method.

How long does a complete COD determination take?

The reflux digestion itself requires two hours of boiling, and the full procedure including cooling, titration, and blank correction typically takes about four hours from start to finish. The rapid spectrophotometric method reduces total analysis time to under one hour per batch.

Why must the sample be acidified during storage?

Acidification to a pH below 2 inhibits microbial activity that would otherwise continue to degrade organic matter during storage, ensuring the measured COD reflects the condition of the sample at the time of collection. Refrigeration at approximately 4 °C provides complementary preservation.

What safety precautions are required?

The procedure involves concentrated sulfuric acid, toxic mercury and chromium compounds, and high-temperature digestion. Analysts must wear appropriate chemical-resistant gloves, eye protection, and laboratory coats; perform acid additions and digestions in a fume hood; and collect all chromium- and mercury-containing waste streams for regulated hazardous waste disposal.

How are results verified for accuracy?

Accuracy is demonstrated through reagent blanks, duplicate analyses, calibration checks with potassium hydrogen phthalate standards, and matrix spikes with recovery within acceptance limits. Participation in interlaboratory proficiency testing programs further confirms the reliability of results reported by accredited third-party laboratories.