The chemical oxygen demand calculator above converts the titrant volumes from a closed-reflux dichromate digestion into a COD expressed as milligrams of oxygen per litre. It also reports the ratio against a five-day BOD result on the same sample, the share of the demand that the biological test detects, and the daily mass load implied by a stated flow. The arithmetic is short; what the number does and does not represent is where the care is needed, and that is the subject of most of this page.
Arb Digital publishes free calculators across chemistry and process work. COD is described here as a laboratory calculation defined by a published method. The reference procedures are the dichromate reflux methods in Standard Methods for the Examination of Water and Wastewater, method 5220, and the corresponding EPA methods 410.3 for titrimetric determination and 410.4 for semi-automated colorimetry, both of which appear on the EPA's approved list under the Clean Water Act.
What This COD Calculator Does
You supply the volume of ferrous ammonium sulfate titrant used to back-titrate the blank, the volume used for the digested sample, the titrant molarity and the sample volume. The page returns the COD in milligrams of oxygen per litre, applying any dilution factor you used before digestion. It is the standard titrimetric calculation and nothing more; it does not evaluate the result against any limit.
Two optional inputs extend it. Entering a five-day BOD figure for the same sample gives the COD to BOD ratio and the share of the chemical demand that the biological test also detects, which is the usual way of characterising how readily degradable an effluent is. Entering a flow converts the concentration into a mass of oxygen demand per day, which is the form used when describing a load rather than a sample.
This page sits alongside several others that describe different properties of the same water. The total dissolved solids calculator covers dissolved mineral content, the water hardness calculator covers calcium and magnesium, and the wastewater loading calculator covers loading rates across a treatment process. COD is specifically a measure of oxidisable material, and it is not interchangeable with any of them.
How to Use It
- Record both titrations. The blank and the sample must be digested and titrated in the same batch with the same reagents, or the difference between them means nothing.
- Enter the standardised titrant molarity as determined on the day. Ferrous ammonium sulfate oxidises in storage, so its molarity is standardised against the dichromate rather than assumed from the label.
- Enter the sample volume actually taken for digestion, as specified by the method variant you are running.
- Enter the dilution factor if the sample was diluted before digestion, which is normal for a strong effluent that would otherwise consume all the dichromate.
- Add the BOD and the flow if you have them, to get the ratio and the daily load. Both are optional and neither affects the COD figure itself.
The Formula and How It Is Calculated
The titrimetric COD calculation is COD (mg O₂/L) = (A − B) × M × 8000 / V, where A is the millilitres of ferrous ammonium sulfate used for the blank, B the millilitres used for the sample, M the titrant molarity and V the sample volume in millilitres. Any dilution factor multiplies the result.
Working the default: the blank took 9.60 mL and the sample 4.30 mL, so the difference is 5.30 mL. Multiplying by a titrant molarity of 0.100 gives 0.530 millimoles of ferrous ion, which is the amount of dichromate the sample consumed expressed in electron equivalents. Multiplying by 8000 and dividing by the 50.0 mL sample gives 84.8 mg O₂/L. At a flow of 2,000 cubic metres per day that is 169.6 kilograms of oxygen demand per day, and against a five-day BOD of 40 mg/L the ratio is 2.12.
The method is one of the approved procedures listed by the EPA under section 304(h) of the Clean Water Act; both the titrimetric and the colorimetric determinations appear in the list of approved CWA chemical test methods, with the full regulatory framework at 40 CFR Part 136.
Where the Factor 8000 Comes From
The constant looks arbitrary and is not. It is the product of two conversions. The first is the equivalent weight of oxygen, which is 8 grams per equivalent: an oxygen molecule accepts four electrons when it is reduced, and its molar mass of 32 divided by four equivalents gives 8. The second is a factor of 1,000 to convert grams to milligrams and to move from millilitres to litres.
Understanding this makes the rest of the calculation intelligible. COD is not a measurement of oxygen at all; nothing in the test involves oxygen. It measures how many electrons the sample's oxidisable material will give up to dichromate under acid digestion, and then expresses that electron count as the mass of oxygen that would have accepted the same number. It is a deliberate unit conversion into a form that can be compared with the biological test, which does measure oxygen.
This is also why the titration is a back-titration. Dichromate is added in known excess and the sample is digested with it; the ferrous ammonium sulfate then determines how much dichromate is left over. The blank establishes the starting excess. The difference between the two titrations is the dichromate the sample consumed, and everything else in the formula is unit conversion. The titration calculator covers the general form of this kind of determination, and the molarity calculator covers standardising the titrant.
COD, BOD and What the Ratio Says
COD and BOD measure overlapping but different things. COD is a chemical oxidation under strongly acidic conditions with a powerful oxidant at high temperature, and it oxidises very nearly everything that can be oxidised. BOD measures what micro-organisms consume over five days at a controlled temperature, so it detects only what is biologically available in that window. COD is therefore almost always the larger number, and a COD below the BOD on the same sample indicates an analytical problem rather than an unusual effluent.
The ratio between them is used as a characterisation of the material present. A ratio close to one means most of the oxidisable material is readily biodegradable. A large ratio means much of the demand comes from substances the organisms could not use within five days, which may be because they are recalcitrant, because they are toxic to the culture, or because the seed was not acclimated. What the ratio cannot do is identify which of those explanations applies; that requires knowing what is in the water.
The practical attraction of COD is speed. The digestion takes about two hours against five days for a BOD, so it is usable for process control where a five-day answer arrives too late to act on. Where a facility has established a stable relationship between the two for its own effluent, COD is often run routinely and BOD periodically to confirm the relationship still holds. That relationship is specific to the effluent and cannot be transferred from another site.
Interferences and Why the Blank Matters
The best-known interference is chloride, which dichromate oxidises just as it oxidises organic matter, so a saline sample returns a COD inflated by an amount that has nothing to do with the organic load. The published methods address this with mercuric sulfate, which complexes chloride and removes it from the reaction, and each method states the chloride concentration above which the correction is no longer adequate. A high-chloride sample analysed without that step gives a number that is wrong rather than merely imprecise.
Reduced inorganic species are the second group. Nitrite, sulfide and ferrous iron are all oxidised by dichromate and all contribute to the measured demand. Their contribution is real oxygen demand in the sense that the receiving water would also have to supply it, but it is not organic demand, and interpreting a COD as an organic measurement when a reduced inorganic species is present will overstate the organic content.
The blank carries more weight than its brief mention in a procedure suggests. Reagent water is not free of oxidisable material, glassware contributes, and the reagents themselves consume a small amount of dichromate. Because the result is a difference between two titrations, any error in the blank transfers directly and at full size into the reported COD. A blank from a different reagent batch, or one that has stood for a different time, is not a valid reference.
What a COD Number Is and Is Not
A COD result is a bulk parameter. It says how much oxidisable material a sample contains, expressed in oxygen equivalents, and it says nothing whatever about what that material is. Two samples with identical COD values can contain entirely different substances with entirely different environmental behaviour, and no COD figure distinguishes a readily degradable sugar from a persistent synthetic compound.
Nor is it a compliance verdict. Discharge limits for oxygen demand parameters are set in individual permits, which in the United States are issued under the National Pollutant Discharge Elimination System; the EPA's summary of NPDES permit basics explains that each permit carries its own discharge limits, monitoring requirements and reporting obligations. What applies to a given discharge is what that permit says, issued by the permitting authority for that jurisdiction, and a calculated number from any tool is not a determination of compliance with it. Regulatory reporting requires analysis by an accredited laboratory using an approved method with its own quality control.
Within a treatment works the figure is used alongside others rather than alone. Loading calculations use it with flow and volume, as in the wastewater loading calculator; residence time comes from the detention time calculator; and the biomass available to treat the load is characterised through the MLVSS calculator. The oxygen equivalence that underlies the factor of 8000 comes from the molar mass of oxygen, which the molar mass calculator derives from a formula.
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Browse All Free Tools Suggest a ToolCommon Mistakes to Avoid
- Swapping the blank and sample volumes — the blank always consumes more titrant, so a negative difference means the two have been entered the wrong way round.
- Using the nominal titrant molarity — ferrous ammonium sulfate oxidises in storage and must be standardised on the day of use.
- Forgetting the dilution factor — a strong effluent diluted before digestion returns a result that must be multiplied back up.
- Ignoring chloride — without the correction specified in the method, a saline sample returns a COD that is inflated for reasons unrelated to organic load.
- Reading COD as an organic carbon measurement — reduced inorganic species contribute to the demand, and the test identifies nothing about what is present.
Related Free Tools From Arb Digital
For the rest of a water characterisation, see the total dissolved solids calculator and the water hardness calculator. On the process side, the wastewater loading calculator, the detention time calculator and the MLVSS calculator cover loading, residence time and biomass. For the laboratory arithmetic behind the titration, use the titration calculator, the molarity calculator and the molar mass calculator. The full free online tools hub lists everything else.
Frequently Asked Questions
It is a measure of how much oxidisable material a water sample contains, expressed as the mass of oxygen that would be needed to oxidise it. It is determined by digesting the sample with dichromate rather than by measuring oxygen directly.
It combines the equivalent weight of oxygen, which is 8 grams per equivalent because an oxygen molecule accepts four electrons, with a factor of 1,000 that converts grams to milligrams and millilitres to litres.
Because dichromate is added in excess and the sample consumes part of it during digestion. The back-titration measures what is left, so less remains for the sample and the difference is what the sample used.
COD is a chemical oxidation that reaches almost everything oxidisable in about two hours. BOD measures only what micro-organisms consume over five days. COD is therefore usually the larger figure, and the ratio between them indicates how readily degradable the material is.
Yes, substantially. Dichromate oxidises chloride as well as organic matter, so a saline sample returns an inflated COD unless the chloride is complexed as the published method specifies, and each method states the concentration above which that correction is insufficient.
No. It is a bulk parameter reported in oxygen equivalents. Two samples with the same COD can contain completely different substances, and identifying them requires a different analysis entirely.
No. Discharge limits are set in the permit issued by the relevant authority, and reporting against them requires analysis by an accredited laboratory using an approved method with its own quality control.
This calculator is provided for education and general reference. It performs the published titrimetric arithmetic only and is not a water quality determination, a compliance assessment or laboratory guidance. Discharge limits are set by the water authority or permitting body with jurisdiction, and regulatory results must come from an accredited laboratory using an approved method.
