How to Use Chemical Oxygen Demand Calculator
The Chemical Oxygen Demand Calculator computes the COD of a water sample from standard back-titration data. Enter the four measured values and the result is displayed instantly with a full step-by-step substitution.
- Blank titration volume (V_blank) – The volume of titrant (usually FAS, ferrous ammonium sulphate) consumed by the blank (reagent-only) sample in mL.
- Sample titration volume (V_sample) – The volume of titrant consumed by the actual water sample in mL. This should be less than or equal to V_blank.
- Titrant concentration (N) – The normality of the titrant solution in mol/L (equivalents per litre).
- Water sample volume (V) – The volume of the water sample used for digestion in mL.
- Review the result – The Chemical Oxygen Demand Calculator shows the formula, substitution steps, and COD in mg/L O₂.
Formula & Theory - Chemical Oxygen Demand Calculator
The Chemical Oxygen Demand Calculator uses this core formula or rule from the dichromate back-titration method (ISO 6060 / APHA 5220):
COD = [(V_blank − V_sample) × N × 8000] / V| Symbol | Meaning | Unit |
|---|---|---|
| V_blank | Titrant volume for blank | mL |
| V_sample | Titrant volume for sample | mL |
| N | Titrant normality | mol/L (eq/L) |
| V | Volume of water sample | mL |
| 8000 | Milliequivalent weight of O₂ × 1000 | mg·mL/mol |
| COD | Chemical Oxygen Demand | mg/L O₂ |
The factor 8000 derives from the equivalent weight of oxygen (8 g/eq) multiplied by 1000 to convert grams to milligrams and litres to millilitres.
Assumptions and Limits
- This formula is specific to back-titration methods using potassium dichromate (K₂Cr₂O₇) as oxidant.
- V_blank must always be greater than or equal to V_sample; if V_sample > V_blank the measurement is invalid.
- Results are theoretical estimates. Certain compounds (e.g., chloride, volatile organics) may interfere with the dichromate oxidation.
Use Cases for Chemical Oxygen Demand Calculator
The Chemical Oxygen Demand Calculator is essential in water quality monitoring and environmental chemistry. Common uses include:
- Wastewater treatment plants – Assessing influent and effluent COD to monitor treatment efficiency.
- Environmental compliance – Verifying discharged effluent meets regulatory COD limits.
- Industrial process water – Monitoring COD in cooling water, process streams, and leachate.
- Laboratory experiments – Processing titration results quickly for environmental chemistry coursework.
By automating the substitution steps, the Chemical Oxygen Demand Calculator reduces arithmetic errors and speeds up routine water quality reporting.