Chemical Oxygen Demand Calculator – COD from Titration Data
The Chemical Oxygen Demand (COD) Calculator computes the COD value of a water or wastewater sample from titration data. Enter the sample volume, titrant volume used, titrant normality, and blank titrant volume — and get COD in mg/L, an oxygen demand category, and a water quality assessment. Useful for environmental engineers, water quality laboratory technicians, and wastewater treatment professionals. Formula based on the standard dichromate titration COD test method. Results should be interpreted alongside full water quality analysis.
Formula
This calculator transforms the provided inputs into the requested outputs using standard domain equations.
Quick Tip
Change one input at a time to see which variable influences the result most.
Have titration data from a COD test and need to calculate the result? Enter your sample volume, titrant figures, and normality — and this tool returns COD in mg/L with a water quality category instantly.
Featured Answer
Q: How do I calculate Chemical Oxygen Demand (COD) from titration?
A: COD in mg/L = (A − B) × N × 8000 / V, where A is the titrant volume for the blank (mL), B is the titrant volume for the sample (mL), N is the normality of the titrant, and V is the sample volume (mL). For a 20 mL sample with a 6.5 mL blank titrant, 4.8 mL sample titrant, and 0.1N solution: COD = (6.5 − 4.8) × 0.1 × 8000 / 20 = 68 mg/L.
How to Use Chemical Oxygen Demand Calculator
- Enter the sample volume in mL — the volume of water or wastewater used in the COD test.
- Enter the titrant volume used for the sample in mL — from the back-titration endpoint.
- Enter the titrant normality — the normality of the ferrous ammonium sulfate (FAS) solution used.
- Enter the blank titrant volume in mL — from the blank (reagent-only) titration run alongside the sample.
What is Chemical Oxygen Demand (COD)?
Chemical Oxygen Demand (COD) is a measure of the amount of oxygen required to oxidise all organic and inorganic matter in a water sample chemically. It is one of the most important parameters in water and wastewater quality analysis.
COD is expressed in mg/L (milligrams of oxygen per litre) and gives a fast, indirect indication of organic pollution in water — the higher the COD, the more polluted the water.
The standard test method uses potassium dichromate (K₂Cr₂O₇) as the oxidising agent under acidic conditions. After digestion, the remaining dichromate is back-titrated using ferrous ammonium sulfate (FAS), and the titration volumes are used to calculate COD.
The standard formula: COD (mg/L) = (A − B) × N × 8000 / V
Where A = blank titrant volume (mL), B = sample titrant volume (mL), N = titrant normality, V = sample volume (mL), and 8000 = equivalent weight of oxygen × 1000.
The water quality assessment output classifies the COD result against standard threshold categories.
Example: Sample volume 20 mL, blank titrant 7.2 mL, sample titrant 5.0 mL, normality 0.25N.
| Field | Value |
|---|---|
| COD (mg/L) | (7.2 − 5.0) × 0.25 × 8000 / 20 = 220 mg/L |
| Oxygen Demand Category | Moderate pollution |
| Water Quality Assessment | Suitable for secondary treatment; not potable |
COD Calculation: From Titration Data to Water Quality Assessment
Why Chemical Oxygen Demand Calculator Matters
COD is a cornerstone parameter in water quality testing — required for industrial effluent monitoring, wastewater treatment plant operation, environmental compliance reporting, and river/lake quality assessment. Calculating it correctly from raw titration data is a routine but error-prone task.
The formula involves four measured values that must be entered correctly and in consistent units. A simple arithmetic error in the calculation can lead to significantly misreported COD values — which affects regulatory compliance and treatment decisions.
This calculator makes the calculation immediate and reliable: enter your four values, get the COD in mg/L, and see it benchmarked against standard water quality categories.
COD Calculation Formula Explained
The dichromate back-titration COD formula:
COD (mg/L) = (A − B) × N × 8000 / V
- A = volume of FAS used for blank titration (mL)
- B = volume of FAS used for sample titration (mL)
- N = normality of FAS solution
- V = volume of water sample (mL)
- 8000 = milliequivalent weight of oxygen (8 g/eq) × 1000 mL/L
The blank titration accounts for reagent oxygen demand (background) — subtracting B from A gives the net oxygen consumed by the sample.
Water Quality COD Benchmarks
| COD (mg/L) | Category | Typical Source |
|---|---|---|
| < 20 | Very clean | Drinking water, pristine rivers |
| 20–50 | Clean | Surface water with minimal loading |
| 50–150 | Slightly polluted | Municipal runoff, treated effluent |
| 150–300 | Moderately polluted | Domestic sewage, secondary effluent |
| 300–600 | Heavily polluted | Industrial wastewater, raw sewage |
| > 600 | Severely polluted | High-strength industrial effluent |
Common Mistakes to Avoid
- Swapping A and B — A is the blank, B is the sample. The blank typically requires more FAS because no organic material has consumed the dichromate. If B > A, recheck the titration.
- Using the wrong normality — FAS normality is often prepared fresh and standardised daily. Use the actual standardised normality, not the target preparation normality.
- Inconsistent unit handling — all volumes must be in mL. If your sample volume is recorded in litres, convert to mL before entering.
- Not running a blank — the blank corrects for reagent background demand. Omitting it produces systematically inflated COD values.
- Misidentifying the endpoint — the ferroin indicator changes from blue-green to reddish-brown at endpoint. A faint endpoint or delayed colour change leads to titration error.
When to Use This Calculator
Use this tool in the laboratory immediately after completing a COD titration — to calculate the result from raw data before recording it in the logbook. Also useful for checking previously recorded calculations or training new laboratory technicians.
For related water chemistry, the Chemical Equation Balancer can assist with balancing the dichromate oxidation reaction. For general chemistry naming and formula work, the Chemical Name Calculator is useful.
Important Assumptions and Limitations
This calculator implements the standard dichromate back-titration COD formula as described in Standard Methods for the Examination of Water and Wastewater (APHA). It assumes ferrous ammonium sulfate (FAS) is used as the back-titrant with ferroin indicator. Results are only as accurate as the titration data entered. Calculation method reviewed against APHA Standard Methods for COD determination references.
Results should be interpreted alongside full water quality analysis by a qualified environmental professional.
Frequently Asked Questions
Find answers to common questions about Chemical Oxygen Demand Calculator
Chemical Oxygen Demand (COD) is a water quality parameter measuring the amount of oxygen required to chemically oxidise all organic and inorganic matter in a water sample. Expressed in mg/L, it is widely used to assess organic pollution in wastewater, industrial effluent, and surface water. Higher COD indicates greater pollution and higher treatment load.
Use the formula: COD (mg/L) = (A − B) × N × 8000 / V, where A = blank titrant volume (mL), B = sample titrant volume (mL), N = titrant normality, and V = sample volume (mL). For example: (7.0 − 5.2) × 0.25 × 8000 / 20 = 180 mg/L. This calculator computes the result when you enter your four titration values.
The calculator is mathematically accurate for the standard dichromate back-titration method. COD result accuracy depends on the precision of the laboratory titration — errors in endpoint detection, FAS standardisation, or sample preparation are not compensated by the formula. For reliable environmental data, standard laboratory protocols should be followed alongside this calculation.
The oxygen demand category classifies the calculated COD value against standard water quality benchmarks — from very clean (under 20 mg/L) to severely polluted (over 600 mg/L). The category provides immediate context for the numerical result, indicating whether the sample is within acceptable limits for treated effluent discharge, requires further treatment, or represents a high-strength industrial waste.
Use it during or immediately after laboratory COD testing to compute the result from titration data. It is also useful for quality checking previously recorded calculations, training lab personnel on the formula, or preparing environmental compliance reports that require COD values from raw analytical data.
The Central Pollution Control Board (CPCB) of India prescribes COD discharge limits that vary by industry and receiving water body. For general industrial effluent discharged to inland surface water, the standard limit is typically 250 mg/L. For discharge to land, it may be up to 250–400 mg/L. Specific industry categories have individual limits — always check the applicable CPCB or State PCB standard.
COD (Chemical Oxygen Demand) measures the total oxygen needed to chemically oxidise all matter, including non-biodegradable compounds, and gives results within hours. BOD (Biological Oxygen Demand) measures the oxygen consumed by microorganisms to biodegrade organic matter over 5 days (BOD₅). COD is always higher than or equal to BOD. COD is preferred for rapid testing; BOD reflects the biologically active portion of organic pollution.
The blank titration measures the oxygen demand of the reagents themselves — the background that exists even without any sample pollutants. Subtracting the sample titrant volume from the blank corrects for this background demand, isolating the oxygen demand attributable to the sample alone. Without the blank correction, all COD results would be systematically inflated by the reagent background.