Last updated: July 3, 2026
Chemical Oxygen Demand Calculator
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Quick Answer
Chemical Oxygen Demand (COD) by dichromate titration is calculated as ((blank FAS volume minus sample FAS volume) × FAS normality × 8000) divided by sample volume in mL. The result is reported as mg/L O₂ and estimates the oxygen equivalent of chemically oxidizable material in water or wastewater.
Chemical oxygen demand equals the blank FAS titre minus the sample FAS titre, times the FAS normality, times 8000, divided by the sample volume. The answer is reported in milligrams per litre as oxygen.
Key Takeaways
- COD by titration equals ((V_blank − V_sample) × N × 8000) divided by sample volume.
- The result is reported as mg/L O₂, an oxygen-equivalent measure of chemically oxidizable material.
- A valid dichromate titration normally has V_blank greater than V_sample.
- The 8000 factor combines 8 mg/meq oxygen with 1000 mL/L.
- Control chloride interference, standardize FAS, and run blanks for reliable COD data.
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Formula
COD (mg/L O2) = ((V_blank − V_sample) × N × 8000) / sample volume
Where:
- COD=Chemical oxygen demand(mg/L O₂)
- V_blank=FAS volume used by reagent blank(mL)
- V_sample=FAS volume used by digested sample(mL)
- N=Normality of ferrous ammonium sulfate titrant(eq/L)
- 8000=Milliequivalent weight of oxygen (8) × 1000 mL/L(mg·mL/eq·L)
- V_sample volume=Original sample aliquot volume(mL)
Worked Examples
Municipal wastewater COD
A 20 mL digested sample has blank and sample FAS titres of 12.0 mL and 6.0 mL with 0.100 N FAS.
- 1Find the titre difference: V_blank − V_sample = 12.0 − 6.0 = 6.0 mL.
- 2Multiply by normality and oxygen factor: 6.0 × 0.100 × 8000 = 4800.
- 3Divide by sample volume: 4800 ÷ 20 mL = 240 mg/L O₂.
High-strength effluent COD
A 25 mL aliquot uses a 10.5 mL blank titre and 4.5 mL sample titre with 0.250 N FAS.
- 1Titre difference = 10.5 − 4.5 = 6.0 mL.
- 2Oxygen equivalent term = 6.0 × 0.250 × 8000 = 12000.
- 3COD = 12000 ÷ 25 = 480 mg/L O₂.
Small-volume COD digestion
A 10 mL sample gives a 15.0 mL blank titre and a 3.0 mL sample titre with 0.050 N FAS.
- 1Titre difference = 15.0 − 3.0 = 12.0 mL.
- 2Numerator = 12.0 × 0.050 × 8000 = 4800.
- 3COD = 4800 ÷ 10 = 480 mg/L O₂.
Introduction
Chemical Oxygen Demand (COD) estimates the oxygen equivalent needed to chemically oxidize organic and oxidizable inorganic material in water. In the classical closed-reflux dichromate method, excess dichromate digests the sample in strong acid, then the remaining dichromate is back-titrated with ferrous ammonium sulfate (FAS). This calculator applies the standard COD equation used in environmental analysis and complements titration, molarity, and solution concentration calculations. Method details align with Standard Methods and the US EPA COD method.
What is chemical oxygen demand?
COD is reported as milligrams of oxygen per litre (mg/L O₂). It does not measure oxygen directly; it expresses the amount of dichromate consumed by the sample as an equivalent amount of oxygen. Higher COD generally means a water sample contains more oxidizable organic matter, reducing agents, or industrial contamination.
COD is faster than a five-day BOD test and is widely used for wastewater process control.
It measures chemically oxidizable material, so it may include substances not biodegradable by microbes.
Results are usually compared across consistent methods, digestion times, and chloride controls.
Use pH context from the pH calculator when evaluating acidic or alkaline waste streams.
COD titration formula explained
The working equation is COD = ((V_blank − V_sample) × N × 8000) / sample volume. The blank contains reagents but no sample, so it represents the total oxidant originally present. The sample consumes dichromate during digestion, leaving less oxidant to titrate; therefore the blank titre is larger than the sample titre. The factor 8000 is 8 mg per milliequivalent of oxygen times 1000 mL per litre.
If V_sample is greater than V_blank, the entered titres are inconsistent for this method; recheck burette readings, blank preparation, and titrant normality.
How to calculate COD step by step
Run the reagent blank and sample through the same digestion and titration procedure. Record the FAS volume for each, subtract the sample titre from the blank titre, multiply by FAS normality and 8000, then divide by the original sample volume in millilitres. This calculator keeps all volume terms in mL, matching the standard equation.
Digest the blank and sample with excess potassium dichromate under acidic reflux conditions.
Back-titrate remaining dichromate with standardized FAS.
Compute ΔV = V_blank − V_sample.
Compute COD = ΔV × N × 8000 ÷ sample volume.
Report the result as mg/L O₂ with dilution factors if the sample was diluted before digestion.
Quality control and interferences
Accurate COD requires standardized FAS, clean glassware, reagent blanks, duplicate samples, and control of chloride interference. Chloride can be oxidized by dichromate and falsely raise COD; mercuric sulfate is commonly used to complex chloride in approved methods. Nitrite, reduced sulfur compounds, and some inorganic reducing agents can also contribute to COD.
Use matrix spikes, duplicates, and certified reference materials when COD results support regulatory reporting.
Interpreting COD values
COD values vary widely by sample type. Clean surface water may be below 20 mg/L, treated effluent often falls in tens of mg/L, municipal wastewater influent may be a few hundred mg/L, and industrial wastewater can be thousands of mg/L. Always interpret COD together with flow, BOD, total organic carbon, suspended solids, and treatment objectives.
| Sample type | Typical COD range | Comment |
|---|---|---|
| Clean surface water | < 20 mg/L | Low oxidizable load |
| Treated municipal effluent | 20–100 mg/L | Depends on permit and plant performance |
| Raw municipal wastewater | 250–1000 mg/L | Strongly site dependent |
| Industrial effluent | > 1000 mg/L possible | Dilution often required |
Safety notes for dichromate COD
Dichromate COD uses strong sulfuric acid, hot digestion, silver catalyst, and hexavalent chromium reagents. Handle reagents in a fume hood with appropriate PPE and dispose of chromium-containing waste as hazardous waste. See general laboratory safety guidance from OSHA and method-specific instructions from Hach COD procedures.
Quick Reference Card
Chemical Oxygen Demand — Quick Reference
Quick reference • Chemical Oxygen Demand Calculator
COD = ((V_blank − V_sample) × N × 8000) / sample volumeValid range: Best when blank titre exceeds sample titre and sample COD lies within the validated titration range; dilute high-strength samples.
Common Values
⚠ Watch Out
- •Do not use the formula if the sample titre is greater than the blank titre without investigating the run.
- •Chloride can cause positive interference unless controlled by the approved method.
- •Dichromate and mercury reagents are hazardous and require proper waste handling.
- •Apply any sample dilution factor after calculating COD from the entered aliquot.
Pro Tips
- →Standardize FAS close to the time of analysis because ferrous titrant can oxidize in air.
- →Run duplicate digestions and repeat if relative percent difference exceeds your quality objective.
- →Choose a sample volume that gives a clear titre difference within the method range.
- →Record initial and final burette readings for both blank and sample instead of only calculated volumes.
FAQs
What does COD measure?
COD measures the oxygen equivalent of material in a sample that can be oxidized chemically by dichromate under acidic digestion conditions. It is reported as mg/L O₂.
Why is the blank titre subtracted from the sample titre?
The calculation uses V_blank minus V_sample because the sample consumes some dichromate during digestion. Less residual dichromate remains for back-titration, so the sample FAS titre is lower than the blank titre.
What are the units of the 8000 factor?
The 8000 factor combines the milliequivalent weight of oxygen, 8 mg/meq, with 1000 mL/L so the final result is mg/L as oxygen when titres and sample volume are in millilitres.
Can COD be negative?
A real COD result should not be negative. If the sample titre exceeds the blank titre, check the blank, endpoint, FAS standardization, sample dilution, and transcription of burette readings.
Is COD the same as BOD?
No. COD is a chemical oxidation test that can be completed quickly, while BOD measures biological oxygen consumption over several days. COD is usually equal to or higher than BOD for the same wastewater.
When should I dilute a COD sample?
Dilute samples when expected COD exceeds the working range of the method or when the titre difference would be too large for accurate titration. Multiply the calculated result by the dilution factor when applicable.