Chemical Oxygen Demand (COD) Calculator
Enter the FAS (ferrous ammonium sulfate) volumes used for the blank and sample, the normality of your FAS solution, and the original sample volume. The calculator applies the standard dichromate back-titration formula (Standard Methods 5220 D) to return COD in mg O2/L or g O2/L, with a water-quality classification, step-by-step working, and a reference table of typical COD ranges for different water types.
Formula
Worked example
Blank titration (A) = 15.5 mL, sample titration (B) = 10.2 mL, FAS normality (N) = 0.1 eq/L, sample volume (V) = 20 mL. Difference = 15.5 - 10.2 = 5.3 mL. COD = (5.3 x 0.1 x 8000) / 20 = 4240 / 20 = 212 mg O2/L, placing the sample in the primary effluent / raw domestic wastewater range.
What is Chemical Oxygen Demand?
Chemical Oxygen Demand (COD) is a key water quality parameter that measures the total quantity of oxygen required to chemically oxidise all organic matter (and some inorganic species) in a water sample to carbon dioxide and water. Unlike Biological Oxygen Demand (BOD), which counts only the biodegradable fraction that bacteria can break down over five days, COD captures the full oxidisable load, including compounds that resist biological treatment. The test uses a strong chemical oxidant, typically potassium dichromate (K2Cr2O7) under acidic conditions and heat, to force the reaction to completion. COD results are expressed as milligrams of oxygen equivalent per litre (mg O2/L). The test is faster than BOD (2-3 hours versus 5 days) and is therefore the primary routine control parameter for wastewater treatment plants, industrial discharge monitoring, and environmental compliance.
How the dichromate back-titration method works
The closed-reflux dichromate method (Standard Methods 5220 D) is the most widely used COD procedure. A measured volume of sample is digested with excess potassium dichromate, sulfuric acid, and a silver sulfate catalyst at 150 degrees Celsius for two hours. Organic matter reduces dichromate (Cr6+ to Cr3+). The remaining, unreacted dichromate is then back-titrated with a standardised ferrous ammonium sulfate (FAS) solution to find how much dichromate was consumed. A parallel blank (reagent only, no sample) is run under identical conditions to account for background oxidant demand. The difference in FAS volumes between blank (A) and sample (B), multiplied by the FAS normality and the conversion constant 8000 (the milliequivalent weight of oxygen, 8 g/eq, times 1000 mL/L), divided by the sample volume in mL, gives COD in mg O2/L. Chloride ions interfere by consuming dichromate directly; mercuric sulfate is added to complex chloride before digestion.
Interpreting your result and regulatory context
COD benchmarks differ by application. Drinking water sources generally have COD below 5 mg/L; values above 10 mg/L indicate pollution. Surface waters typically range from 5 to 20 mg/L. After secondary biological treatment, COD usually falls to 20-60 mg/L. The EU Urban Wastewater Treatment Directive sets a discharge consent of 125 mg O2/L for treated effluent from plants serving more than 2,000 population equivalents. Raw domestic sewage typically runs 250-1,000 mg/L, while industrial streams can reach tens of thousands of mg/L. The COD/BOD ratio is a useful biodegradability indicator: a ratio of 1.5-3.0 is typical of conventional municipal wastewater and suggests a good response to biological treatment; ratios above 3.5 indicate that a significant fraction is non-biodegradable, warranting chemical or physical pre-treatment.
Quality control and common sources of error
Accurate COD results depend on careful technique and reagent preparation. Verify the FAS normality daily by titrating against a standard potassium dichromate solution, because FAS oxidises slowly in air and its effective normality drifts. Run at least one blank and one standard check solution (for example, potassium hydrogen phthalate at a certified COD value) with every batch. Chloride interference is the most common analytical problem: add mercuric sulfate at 10:1 mass ratio relative to expected chloride before digestion. Volatile organic compounds can escape during digestion if the vials are not sealed properly - use the closed-reflux approach. Very high COD samples (above about 900 mg/L with standard reagents) may consume all the dichromate; dilute the sample and retest. Precipitated solids should be homogenised before sampling to avoid under-reporting particle-bound COD.
Typical COD ranges by water type
| Water type | Typical COD (mg O₂/L) | Significance |
|---|---|---|
| Clean drinking water | < 5 | Excellent quality |
| Surface water | 5 - 20 | Good; suitable for many uses |
| Secondary treated effluent | 20 - 60 | Typical biological treatment output |
| EU discharge limit | < 125 | Urban Wastewater Treatment Directive |
| Primary treated effluent | 150 - 400 | Needs further treatment |
| Raw domestic wastewater | 250 - 1000 | Untreated sewage |
| Industrial wastewater | 500 - 50,000+ | Highly variable by industry |
Reference values for chemical oxygen demand in different water types. Values are approximate and vary by source, season, and local conditions.
Frequently asked questions
What is the difference between COD and BOD?
COD (Chemical Oxygen Demand) measures the total oxygen equivalent of all oxidisable matter - biodegradable and non-biodegradable - using a chemical oxidant in a 2-3 hour test. BOD (Biological Oxygen Demand) measures only the oxygen that bacteria actually consume when breaking down biodegradable organic matter over a 5-day incubation. COD is always higher than BOD for the same sample. The COD/BOD ratio reveals biodegradability: a ratio near 1.5-2.0 means most COD is biodegradable; ratios above 3.5 point to significant non-biodegradable content that biological treatment will not remove.
Why is 8000 used in the COD formula?
The constant 8000 combines two conversion factors. Oxygen has an equivalent weight of 8 g/eq (molecular weight 32 g/mol divided by 4, because each O2 molecule accepts 4 electrons in the oxidation half-reaction). Multiplying by 1000 converts grams per litre to milligrams per litre and mL to L. The combined factor 8 x 1000 = 8000 converts the milliequivalents of oxidant consumed directly into mg O2/L, which is the conventional unit for reporting COD.
What should I do if my sample COD titrant volume (B) is greater than the blank (A)?
A sample volume greater than the blank is physically impossible in a correctly run dichromate test and indicates an error. The most likely causes are: the wrong vial was labelled blank or sample, the FAS solution degraded (recheck its normality), there was a transcription error in recording titration volumes, or the sample was over-diluted so that almost no dichromate was consumed in either vial. Repeat the analysis with fresh reagents and careful labelling.
How do chloride ions interfere with COD, and how do I correct for them?
Chloride ions are themselves oxidised by potassium dichromate during digestion (Cl- to Cl2), artificially inflating the COD reading. The standard correction is to add mercuric sulfate (HgSO4) to the digestion mixture at approximately a 10:1 mass ratio relative to the expected chloride concentration. Mercuric sulfate forms a stable complex with chloride, preventing it from reacting with dichromate. Standard Methods and EPA Method 410.4 both specify this approach. For samples with very high chloride (above about 2000 mg/L), dilution before analysis may also be necessary.
What is a typical COD value for treated wastewater?
After effective secondary biological treatment (activated sludge or trickling filter), COD typically falls to 20-60 mg O2/L. The EU Urban Wastewater Treatment Directive requires COD below 125 mg O2/L for most consented discharges, with many treatment plants targeting 60-80 mg O2/L as a process control goal. Advanced tertiary treatment (filtration, UV, or membrane processes) can reduce COD further, often to below 30 mg O2/L.
Can I use molarity instead of normality in the COD formula?
Yes. For FAS (ferrous ammonium sulfate hexahydrate), each mole of Fe2+ donates exactly one electron, so the normality equals the molarity. If your balance or reagent certificate gives molarity (mol/L), you can use it directly as N in the formula. Confirm this equivalence whenever you use a different reagent; for other reductants the equivalent factor may differ.
What volume of sample should I use for COD analysis?
Standard Methods 5220 D recommends using 20 mL of sample for the standard COD range (up to about 900 mg/L with standard dichromate reagent strength). For high-strength industrial samples expected to exceed 900 mg/L, dilute the sample before digestion and multiply the result by the dilution factor. For very low-strength samples (below about 50 mg/L), increasing the sample volume to 50 mL and adjusting reagent volumes proportionally can improve sensitivity.