Understanding Chemical Oxygen Demand
Chemical oxygen demand represents the oxygen deficit created by organic and inorganic reducing agents present in water. It reflects the overall organic load, ranging from simple sugars to complex polymers and industrial byproducts. A COD result in milligrams per litre (mg/L) tells you directly how much dissolved oxygen would be consumed if all pollutants were oxidised.
COD differs fundamentally from biochemical oxygen demand (BOD). While BOD measures only the fraction of organics that microorganisms can decompose over five days, COD measures everything oxidisable—including recalcitrant compounds that bacteria cannot break down. This makes COD a more comprehensive indicator of total organic contamination.
COD levels classify water quality:
- Very clean water: <5 mg/L
- Moderately polluted: 5–20 mg/L
- Heavily contaminated: >100 mg/L
Industrial effluents and sewage typically show COD values between 200–1000 mg/L before treatment.
COD Calculation Method
The dichromate titrimetric method is the standard approach for laboratory COD determination. After oxidising the sample with potassium dichromate under acidic conditions, you titrate excess dichromate with ferrous ammonium sulfate (FAS). The difference in FAS consumption between a blank and the sample reveals the organic content.
COD (mg/L) = (A − B) × N × 8000 ÷ Sample volume (mL)
A— Volume of FAS (mL) needed to titrate the blank run (containing no sample)B— Volume of FAS (mL) needed to titrate the sample after dichromate reactionN— Normality of the FAS solution (mol/L equivalent)Sample volume— Volume of water sample used in the dichromate oxidation step (mL)
Laboratory Procedure and Practical Considerations
COD determination involves heating the sample with excess potassium dichromate in concentrated sulfuric acid for two hours at 150 °C. This oxidation step breaks down virtually all organic matter. After cooling, you add diphenylamine indicator and back-titrate with standardised FAS solution until the colour shifts from blue to colourless.
Several factors influence accuracy:
- Sample dilution: High-COD samples must be diluted to keep titration volumes within 20–50 mL
- Chloride interference: Samples containing >1000 mg/L Cl− require mercuric sulfate masking to prevent oxidation errors
- Temperature stability: Dichromate solutions degrade in sunlight; store in amber bottles away from heat
- FAS standardisation: Re-standardise ferrous ammonium sulfate weekly against certified dichromate to ensure reliable normality
Replicate each sample at least twice and average the results to account for titration variability.
Applications in Water Quality and Treatment Monitoring
Environmental agencies and utilities measure COD to assess pollution severity and design treatment strategies. Wastewater treatment plants track COD removal efficiency across primary, secondary, and tertiary stages—a reduction of 80–90% is typical for conventional activated sludge systems.
COD serves as an indirect measure of treatment efficacy because it does not distinguish between biodegradable and non-biodegradable carbon. Combining COD with BOD gives a fuller picture: a low BOD but high COD suggests the presence of resistant synthetic compounds or industrial chemicals. This helps operators optimise aeration times and adjust chemical doses.
Regulatory limits vary by jurisdiction. Many countries enforce COD discharge limits of 50–250 mg/L for industrial wastewater depending on sector. Drinking water standards typically expect <10 mg/L, though COD alone does not guarantee potability—other contaminants and microbial hazards remain relevant.
Common Errors and Best Practices
Accurate COD measurement requires attention to detail at every step of the titrimetric method.
- Avoid over-dilution of high-COD samples — Excessive dilution amplifies the relative error in the blank titration. If your sample COD exceeds 600 mg/L, dilute it to bring the expected FAS titre into the 30–45 mL range, then multiply the result by the dilution factor.
- Control dichromate heating temperature precisely — Temperature below 140 °C leaves organic compounds incompletely oxidised; above 160 °C, the dichromate itself may decompose. Use a certified heating block or reflux apparatus and maintain 150 °C throughout the two-hour reaction.
- Use fresh FAS solution and verify normality weekly — Ferrous ammonium sulfate oxidises readily in air. Prepare FAS solutions in deoxygenated water, store under nitrogen or argon if possible, and standardise against dichromate to catch degradation before it skews results by 5–10%.
- Distinguish sample COD from blank drift — If your blank titre (A) shifts significantly between replicates (typically >0.5 mL), suspect dichromate contamination or improper reagent preparation. Discard old stock and prepare fresh dichromate solution to ensure reliable background subtraction.