Chemical Oxygen Demand Calculator
COD — chemical oxygen demand — is wastewater’s headline number: the oxygen equivalent of everything in the water that a strong oxidant can burn. The standard measurement is a dichromate reflux followed by a back-titration with ferrous ammonium sulfate (FAS), and the result falls out of one formula built on the difference between a blank titration and the sample’s. This calculator evaluates that formula from your bench numbers — blank titre, sample titre, FAS normality, sample volume — and reports COD in mg/L, with the theory of the 8000 in the formula explained rather than assumed.
The Method in One Passage, and the Formula It Leaves Behind
The standard dichromate method (APHA Standard Methods 5220, ISO 6060) refluxes a measured sample volume for two hours in concentrated sulfuric acid with a known excess of potassium dichromate, silver sulfate as catalyst and mercuric sulfate to mask chloride. The organics (and other reductants) consume part of the dichromate; the analyst then titrates what is left with ferrous ammonium sulfate, using ferroin indicator. A reagent blank — everything but the sample — is refluxed alongside and titrated too: the blank says how much dichromate the FAS titre corresponds to with nothing consumed, the sample's smaller titre records what the sample's load ate, and the difference is the consumption. Converting that difference to oxygen equivalents gives the working formula:
COD (mg/L) = (Vblank − Vsample) × NFAS × 8000 ÷ Vsample taken (mL)
The 8000 is not a magic constant: oxygen's equivalent weight is 8 g per equivalent, and the factor bundles the conversions (equivalents → grams of O₂ → milligrams, millilitres of sample → litres) into one integer. The calculator above is that formula and nothing else — every COD figure a titrimetric lab reports passes through it.
Worked Example: A Mid-Strength Effluent
The defaults are a realistic bench record: blank titre 24.0 mL, sample titre 18.5 mL (the sample consumed the oxidant equivalent of 5.5 mL of titrant), FAS at 0.25 N, sample volume 20 mL. COD = 5.5 × 0.25 × 8000 / 20 = 550 mg/L — squarely in untreated municipal wastewater territory (typical raw sewage runs 250–1,000 mg/L; treated effluent consents often sit at 75–125 mg/L; clean river water reads under 20). Read the result's sensitivities the way an analyst does: a 0.1 mL titration error moves this answer by 10 mg/L; the FAS normality, which drifts as the reagent ages and must be standardised daily against standard dichromate, scales every result until it is caught — which is why the method's quality control (a potassium hydrogen phthalate standard, theoretical COD 500 mg/L at 425 mg/L of KHP, recovered within a few percent) is run beside samples rather than believed from last week.
What COD Counts, and What It Refuses to See
COD's strength is its near-total oxidation: hot acid dichromate mineralises roughly 95–100% of most organic compounds, so COD approximates the theoretical oxygen demand more closely than the five-day BOD test, which only counts what microbes manage to eat in five days at 20 °C. The BOD/COD ratio is therefore a biodegradability diagnosis in one division: near 0.5–0.7, a waste is readily biodegradable (domestic sewage); below 0.2, much of the load resists biology (many industrial effluents) and treatment design changes accordingly. The blind spots are equally definite. Some structures resist dichromate too (pyridine and related ring compounds, volatile organics lost before reflux seals). Chloride is oxidised and masquerades as COD unless the mercuric sulfate masking is adequate — brackish and marine samples live on that correction. And inorganic reductants (sulfide, ferrous iron, nitrite) register faithfully, which is why COD is a demand measurement and not an organics measurement; TOC is the neighbouring instrument when carbon itself, rather than its oxygen appetite, is the question.
Ranges, Dilutions and Bench Discipline
The titrimetric method in this form serves roughly 50–900 mg/L undiluted; stronger wastes are diluted into range (and the calculator's sample volume and result then refer to the diluted aliquot — multiply back by the dilution factor), while low-level variants with more dilute reagents reach down toward 5 mg/L for treated effluents and receiving waters. The discipline that keeps the formula honest: digest blanks in every batch (the blank titre is a measurement, not a constant); standardise the FAS on the day (ferrous solutions oxidise in air, and a 2% normality drift is a 2% COD drift across every sample); respect the chloride ceiling of the masking; and report with the method's name attached, because COD by dichromate, by permanganate (the "oxygen absorbed" of older and some national standards) and by photometric vial tests are cousins, not twins. Numbers from this page carry the dichromate pedigree — the one permits and textbooks mean when they say COD unqualified.
A Theory Check: The COD of Pure Glucose
The formula can be audited against pure chemistry, because a single compound's theoretical oxygen demand is stoichiometry, not analysis. Glucose burns as C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O: 180 g of glucose demand 192 g of oxygen, a ratio of 1.067 g O₂ per gram. A 300 mg/L glucose solution should therefore read 300 × 1.067 ≈ 320 mg/L COD — and dichromate reflux returns very nearly that, glucose being one of the compounds the method oxidises essentially completely. Ethanol audits the same way: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O prices 46 g of ethanol against 96 g of oxygen (2.09 g/g), so a 1,000 mg/L ethanol waste carries a theoretical demand near 2,087 mg/L — and distillery effluents, measured in the tens of thousands, are that arithmetic at industrial concentration. Laboratories exploit exactly this auditability with the KHP standard of the worked example, and the same computation, run for any pure substrate through the Molar Ratio Calculator, gives the theoretical ceiling a measured COD can be graded against: recovery far below theory means refractory structure; a reading above theory means interference — chloride first among suspects — and both diagnoses start from the arithmetic on this page.
Dilution Arithmetic, Worked Once
Strong samples are diluted into the method's range, and the bookkeeping deserves one explicit pass because it is the commonest clerical error in COD reporting. A food-process effluent is diluted tenfold (10 mL made to 100 mL) and the diluted aliquot is digested: blank 24.0 mL, sample titre 9.6 mL, FAS 0.25 N, 20 mL aliquot. The diluted sample's COD = (24.0 − 9.6) × 0.25 × 8000 / 20 = 14.4 × 0.25 × 8000 / 20 = 1,440 mg/L. That figure describes the diluted liquid; the effluent itself is tenfold stronger, 14,400 mg/L, and the report that forgets the final multiplication understates a discharge by an order of magnitude — the kind of error permits are revoked over. State the dilution on the worksheet, multiply at the end, and say in the report which of the two numbers is being quoted.
Related Tools
More from the Chemistry Calculators hub: the Enzyme Activity Calculator for the other great absorbance-and-titration bench routine, the live Concentration Calculator for preparing the standards this method consumes, and the Molar Ratio Calculator for the stoichiometry underneath the titration.
Frequently Asked Questions
Why does the sample titre come second in the subtraction?
Because the sample's load consumes dichromate before the titration begins: less oxidant survives, so less FAS is needed to finish it. The blank (nothing consumed) always titrates highest. A sample titre above the blank is therefore a physical impossibility in this method — the calculator flags it, because in practice it means swapped labels, a contaminated blank, or a misread burette.
COD or BOD — which should I report?
Both answer different questions and permits usually name the one they want. COD is fast (hours), near-total, and reproducible; BOD₅ is slow (five days), partial, and biologically truthful. Report COD for process control and total load, BOD for biodegradability and receiving-water impact — and the ratio of the two when characterising a new waste stream.
What COD values are normal?
Unpolluted surface water: under ~20 mg/L. Raw municipal sewage: roughly 250–1,000 mg/L. Biologically treated effluent: commonly 30–100 mg/L against consents often set at 75–125. Industrial streams vary by orders of magnitude — food processing in the low thousands, some chemical and pharmaceutical wastes in the tens of thousands, diluted before analysis.
How does the KHP standard check work?
Potassium hydrogen phthalate has a calculable theoretical oxygen demand — a solution of 425 mg/L KHP corresponds to 500 mg/L COD. Running it as a sample should return 500 within the method's precision; laboratories typically accept recovery within about ±5–10%. It validates digestion, titration and arithmetic (including this formula) in one aliquot.
Can I use the vial (photometric) method's numbers here?
The chemistry is the same dichromate oxidation; the readout differs (absorbance of the chromium colour change against a calibration, instead of a titration). The titration formula on this page belongs to the reflux-titration variant; photometric results are read from the instrument's calibration curve, though they must agree when both are done properly.
Why is chloride such a problem?
Hot acid dichromate oxidises chloride to chlorine, registering it as phantom COD — roughly 0.23 mg of false COD per mg of chloride unmasked. Mercuric sulfate complexes the chloride out of harm's way up to a design limit; beyond it (seawater-strength samples), dilution or chloride corrections are mandatory before the formula's answer can be trusted.