Chemical Oxygen Demand (COD) is a fundamental parameter for assessing the degree of organic pollution in surface water bodies such as rivers, lakes, and reservoirs. This article provides a comprehensive, step-by-step technical guide to COD determination in surface water based on internationally recognized standards such as ISO 6060, ASTM D1252, and the widely applied dichromate method. Environmental engineers, laboratory analysts, and water quality professionals will find detailed procedures covering sample collection, digestion, titration, calculation, and quality control.
Technical Overview
Chemical Oxygen Demand (COD) quantifies the mass of oxygen required to chemically oxidize organic and oxidizable inorganic matter in a water sample under strongly acidic conditions using a powerful oxidizing agent, typically potassium dichromate. In surface water monitoring, COD serves as a primary indicator of anthropogenic organic pollution load, complementing parameters such as biochemical oxygen demand (BOD), dissolved oxygen (DO), and total organic carbon (TOC). Because surface water typically contains relatively low concentrations of organic matter compared with wastewater, analysts must select the appropriate method, digestion scale, and detection range to obtain reliable results.
The internationally recognized reference method is the dichromate reflux method, described in ISO 6060 and ASTM D1252, and in many national standards such as the GB and HJ series used in China (for example, HJ 828-2017, which superseded GB 11914-89). For cleaner surface waters with expected COD below approximately 30 mg/L, the permanganate index method (ISO 8467, GB 11892-89) is frequently preferred because potassium permanganate offers milder oxidation and better sensitivity at low concentrations. This article focuses primarily on the dichromate method while also addressing the permanganate index as a complementary approach for low-pollution surface waters.
Testing Samples
River Water
River water samples are collected at designated monitoring cross-sections, typically at mid-depth and mid-channel where flow is well mixed. Rivers receiving municipal or industrial discharges may exhibit COD values ranging from 10 mg/L to several hundred mg/L, so pre-dilution assessment is often necessary. Sampling frequency usually follows hydrological regimes, with increased sampling after storm events or discharge incidents.
Lake and Reservoir Water
Lakes and reservoirs are stratified systems; samples should be collected at multiple depths (epilimnion, thermocline, and hypolimnion) during stratified periods. COD values in these water bodies usually fall between 2 and 20 mg/L, making the permanganate index or low-range digestion procedures more appropriate. Eutrophic lakes may show elevated COD due to algal biomass and humic substances.
Estuarine and Coastal Surface Water
Brackish and saline samples require special attention because high chloride concentrations interfere with the dichromate method. Chloride is oxidized by dichromate and consumes silver sulfate catalyst, causing positive bias. The addition of mercuric sulfate as a complexing agent, or the use of chlorine-corrected calculations, must be applied when chloride exceeds 100 mg/L.
Reference and Quality Control Samples
Certified reference materials (CRMs), laboratory-prepared potassium hydrogen phthalate (KHP) standard solutions, and procedural blanks are analyzed alongside field samples to verify accuracy, precision, and recovery. A KHP solution with a theoretical COD of 500 mg/L is the most common calibration verification standard.
Testing Items
The principal testing items related to surface water oxygen demand determination include:
- Chemical Oxygen Demand (CODCr) by the dichromate method, applicable range typically 10–700 mg/L without dilution
- Permanganate Index (CODMn) for low-pollution surface waters, applicable range approximately 0.5–10 mg/L as oxygen consumed
- Blank verification and spike recovery to confirm method performance
- Chloride interference assessment for saline or estuarine samples
- Method detection limit (MDL) confirmation, typically 4 mg/L for standard dichromate titration and approximately 0.5 mg/L for permanganate index
Testing Methods
Principle of the Dichromate Method
A measured volume of sample is refluxed in a strongly acidic medium with a known excess of potassium dichromate (K2Cr2O7). Silver sulfate (Ag2SO4) catalyzes the oxidation of straight-chain organic compounds, while mercuric sulfate (HgSO4) complexes chloride ions to suppress interference. After digestion, the remaining unreduced dichromate is titrated with standardized ferrous ammonium sulfate (FAS) using ferroin (1,10-phenanthroline ferrous sulfate complex) as the indicator. The endpoint is a sharp color change from blue-green to reddish-brown. The difference between the blank titration volume and the sample titration volume, combined with the FAS standardization factor, yields the COD value.
Step 1: Sample Collection and Preservation
Collect at least 100 mL of surface water in clean glass bottles. Homogenize the sample thoroughly, and analyze promptly. If immediate analysis is not possible, acidify the sample to pH ≤ 2 with concentrated sulfuric acid and store at 4 °C for a maximum of 7 days. Never use plastic containers for extended storage of trace organic samples, and avoid headspace in bottles intended for replicated analyses.
Step 2: Reagent Preparation
- Potassium dichromate standard solution (0.2500 mol/L for standard range, or 0.0250 mol/L for low range): dry K2Cr2O7 at 105 °C for 2 hours before weighing
- Ferrous ammonium sulfate titrant (approximately 0.05 mol/L or 0.005 mol/L): standardize daily against the dichromate standard
- Silver sulfate–sulfuric acid catalyst: dissolve 10 g Ag2SO4 in 1 L concentrated H2SO4, with stirring over one to two days
- Mercuric sulfate: add 0.4 g per 20 mL sample when chloride concentration is significant
- Ferroin indicator solution: commercially available or prepared from 1,10-phenanthroline monohydrate and ferrous sulfate
Step 3: Digestion Procedure
Place 20.0 mL of the homogenized sample (or an aliquot diluted to 20.0 mL) into a 250 mL reflux flask. Add mercuric sulfate if required for chloride complexation, then slowly add 10.0 mL of potassium dichromate standard solution and several glass beads. Attach the flask to a condenser and carefully add 30 mL of silver sulfate–sulfuric acid solution through the condenser opening while mixing. Reflux the mixture for 2 hours at approximately 146 °C (the reflux temperature of the acid mixture). Alternatively, sealed-tube digestion at 165 °C for 15–20 minutes, followed by spectrophotometric measurement at 600 nm (for high range) or 440 nm (for low range), may be used where validated against the reflux reference method.
Step 4: Titration
After cooling, rinse down the condenser with distilled water (typically 45–75 mL total rinse). Add 2–3 drops of ferroin indicator and titrate the residual dichromate with standardized ferrous ammonium sulfate until the color changes sharply from blue-green to reddish-brown, persisting for at least 30 seconds. Conduct a full blank determination using 20.0 mL of reagent water in place of the sample, following identical procedures.
Step 5: Calculation
Calculate the COD concentration using the formula: COD (mg/L) = [(V0 − V1) × c × 8 × 1000] / Vs, where V0 is the blank titration volume (mL), V1 is the sample titration volume (mL), c is the molar concentration of the standardized FAS solution, 8 is the milligram-equivalent mass of oxygen per millimole, and Vs is the sample volume (mL). Report results to three significant figures.
Step 6: Permanganate Index Alternative for Clean Surface Water
For pristine surface waters, add a known excess of potassium permanganate to an acidified sample, boil for a fixed period (typically 10 minutes in a boiling water bath), then reduce the remaining permanganate with excess sodium oxalate and back-titrate with permanganate solution. This gentler oxidation better reflects readily oxidizable organic matter in oligotrophic waters.
Quality Control Requirements
- Analyze a blank with every batch of 20 samples or fewer
- Verify titrant standardization at the start and end of each analytical session
- Run duplicate samples at a rate of at least 10 percent, with relative deviation within 10 percent
- Perform spike recovery checks using KHP, targeting recovery between 90 and 110 percent
- Dilute samples expected to exceed the upper working range and re-digest
- Maintain complete records of reagent lot numbers, titration volumes, and endpoint observations
Applications
COD determination in surface water supports a wide range of environmental management activities:
- Routine national and regional surface water quality monitoring programs and compliance assessment against water quality standards such as GB 3838 environmental quality standards for surface water
- Environmental impact assessments for industrial, municipal, and infrastructure projects
- Water pollution source apportionment and total pollutant load control (total emission cap programs)
- Calibration and validation of online COD monitors deployed at discharge outfalls and automatic monitoring stations
- Lake eutrophication studies and watershed management planning
- Emergency response to accidental spills of organic pollutants into rivers and reservoirs
- Verification of wastewater treatment plant effluent compliance before discharge to receiving surface waters
FAQ
Why is the dichromate method preferred over the permanganate method for polluted water?
Potassium dichromate oxidizes a substantially higher proportion (typically 90–100 percent) of organic compounds present in water compared with potassium permanganate, which oxidizes only about 60 percent. Therefore, the dichromate method provides a more complete and reproducible measure of the total chemically oxidizable load, making it the reference method for regulatory COD limits.
How is chloride interference managed?
Chloride above approximately 100 mg/L is oxidized by dichromate and causes falsely elevated results. Adding mercuric sulfate at a ratio of 10:1 (HgSO4:Cl−) complexes the chloride. For highly saline samples, alternative approaches such as the chlorine correction or manganese-catalyzed methods may be required, and mercury-free alternatives should be considered for environmental safety reasons.
What is the typical detection limit of the dichromate titration method?
The method detection limit of the standard reflux titration procedure is approximately 4 mg/L, with a practical lower quantification limit of 10 mg/L. For cleaner surface waters, the low-range digestion (0.025 mol/L dichromate) or the permanganate index should be used.
Why must the silver sulfate catalyst be added slowly through the condenser?
Concentrated sulfuric acid has a high heat of dilution with water. Slow addition through the condenser while the mixture is agitated prevents localized boiling and splashing, ensures safe operation, and maintains complete mixing of the acid with the aqueous sample.
Can sealed-tube (Hach-style) spectrophotometric COD tests replace the reflux method?
Sealed-tube digestion with spectrophotometric readout is widely accepted for routine monitoring after validation against the reference reflux method, particularly when equipped with certified reagent vials. However, for regulatory disputes and certification purposes, results should be confirmed by the titrimetric reference method specified in ISO 6060, ASTM D1252, or equivalent national standards.
How should results be reported for surface water classification?
Results should be reported in mg/L of oxygen consumed, rounded appropriately, and interpreted against the applicable surface water quality classification thresholds defined by the governing environmental standard in the jurisdiction of the monitoring program.