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Blank Titrant Volume (mL) FAS volume consumed by the blank

Sample Titrant Volume (mL)

FAS Normality (N)

Sample Volume (mL)

Dilution Factor

Absorbance (AU)

Calibration Slope (mg/L per AU)

Calibration Intercept (mg/L)

Dilution Factor

Presets:

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COD — mg/L

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About

Chemical Oxygen Demand (COD) quantifies the oxygen equivalent of organic matter oxidizable by a strong chemical oxidant. Standard Method 5220 (APHA) prescribes potassium dichromate (K₂Cr₂O₇) in sulfuric acid at 150 °C for 2 hours. The unreacted oxidant is back-titrated with ferrous ammonium sulfate (FAS). Errors in blank correction, normality standardization, or dilution factor propagate directly into discharge compliance reports. Facilities exceeding permitted COD limits face regulatory fines and potential shutdown orders.

This calculator implements both titrimetric and colorimetric paths. The titrimetric mode uses the volumetric back-titration formula with FAS normality. The colorimetric mode applies Beer-Lambert absorbance against a user-supplied calibration curve. Pro tip: always run a potassium hydrogen phthalate (KHP) recovery check. Theoretical COD of KHP is 1.176 mg O₂/mg KHP. Recovery outside 96 - 104% indicates reagent degradation or incomplete digestion. Note: the dichromate method does not oxidize certain refractory compounds (pyridine, some volatile organics) and overestimates COD when chloride exceeds 2000 mg/L unless mercuric sulfate is added.

Formulas

The titrimetric COD formula per APHA Standard Method 5220 C:

COD = (B − S) × N × 8000V × D

Where B = volume of FAS used for blank titration (mL), S = volume of FAS used for sample titration (mL), N = normality of FAS (eq/L), 8000 = milliequivalent weight of oxygen (8) × 1000 mL/L, V = volume of sample (mL), and D = dilution factor (dimensionless, 1 if undiluted).

For the colorimetric method (Beer-Lambert), the COD concentration is derived from the absorbance reading against a calibration curve:

COD = (m × A + b) × D

Where A = measured absorbance at 600 nm (high range) or 420 nm (low range), m = calibration slope (mg/L per absorbance unit), b = calibration intercept (mg/L), and D = dilution factor.

FAS normality standardization against primary standard dichromate:

N_FAS = V_{K₂Cr₂O₇} × 0.25V_FAS

Where 0.25 N is the standard normality of the potassium dichromate solution, V_{K₂Cr₂O₇} = volume of dichromate pipetted (mL), and V_FAS = volume of FAS consumed at equivalence point (mL).

Reference Data

Sample Type	Typical COD Range mg/L	BOD/COD Ratio	Recommended Method	Digestion Time	Notes
Domestic Wastewater (Raw)	250 - 800	0.4 - 0.8	Closed Reflux Titrimetric	2 hr	Standard municipal influent
Domestic Wastewater (Treated)	30 - 150	0.1 - 0.3	Closed Reflux Colorimetric	2 hr	Secondary effluent
Pharmaceutical Wastewater	5000 - 50000	0.2 - 0.5	Open Reflux (diluted)	2 hr	High chloride interference possible
Brewery Wastewater	2000 - 6000	0.6 - 0.7	Closed Reflux Titrimetric	2 hr	Highly biodegradable
Dairy Industry	1500 - 5000	0.5 - 0.7	Closed Reflux Titrimetric	2 hr	Fat/oil may coat probe
Textile Dyeing	1000 - 10000	0.15 - 0.35	Open Reflux (diluted)	2 hr	Refractory dyes skew ratio
Pulp & Paper Mill	1000 - 4000	0.3 - 0.5	Open Reflux	2 hr	Lignin raises COD
Surface Water (Clean River)	5 - 30	0.3 - 0.6	Closed Reflux Colorimetric	2 hr	Low range; use micro method
Landfill Leachate (Young)	10000 - 60000	0.5 - 0.8	Open Reflux (high dilution)	2 hr	Volatile fatty acids dominant
Landfill Leachate (Mature)	500 - 5000	0.05 - 0.2	Closed Reflux Titrimetric	2 hr	Humic substances; low biodegradability
Slaughterhouse	3000 - 10000	0.5 - 0.65	Open Reflux (diluted)	2 hr	Blood proteins; high N interference
Olive Oil Mill	40000 - 200000	0.3 - 0.5	Open Reflux (extreme dilution)	2 hr	Polyphenols; dark color
Stormwater Runoff	20 - 200	0.3 - 0.5	Closed Reflux Colorimetric	2 hr	Variable; first flush highest
Drinking Water Source	< 10	-	Permanganate Index	10 min	KMnO₄ method (ISO 8467)
Sugar Mill Effluent	5000 - 15000	0.5 - 0.7	Open Reflux (diluted)	2 hr	High sucrose; easy oxidation

Frequently Asked Questions

Chloride ions are oxidized by dichromate, producing falsely elevated COD values. Each mg/L of Cl⁻ contributes approximately 0.226 mg/L apparent COD. APHA 5220 prescribes adding mercuric sulfate (HgSO₄) at a ratio of 10:1 (HgSO₄:Cl⁻) to complex chloride. Maximum tolerable chloride with HgSO₄ is approximately 2000 mg/L. Above that, increase the HgSO₄ dose or use the argentometric pre-precipitation method.

Potassium hydrogen phthalate (KHP) has a theoretical COD of 1.176 mg O₂/mg. Recovery outside 96-104% typically indicates: (1) degraded or improperly standardized FAS - re-standardize against fresh 0.25 N K₂Cr₂O₇; (2) insufficient digestion temperature - verify the reactor maintains 150 °C ± 2 °C; (3) expired digestion reagent - silver sulfate catalyst degrades over months; or (4) volumetric glassware error - use Class A pipettes and burets.

The permanganate index (ISO 8467) uses KMnO₄ at 100 °C for 10 minutes. It only oxidizes the most labile organic fraction, yielding values roughly 25-50% of the dichromate COD. It is specified for drinking water source monitoring where COD values are below 10 mg/L and for regulatory compliance in EU Drinking Water Directive reporting. It should not be used for wastewater or discharge compliance, as it underestimates total organic load.

The dilution factor D multiplies the final result, so any measurement error in B or S is also multiplied by D. For a 10× dilution, a 0.1 mL titration error on a 20 mL sample produces (0.1 × 0.25 × 8000 / 20) × 10 = 100 mg/L error instead of 10 mg/L without dilution. Minimize dilution factor by selecting appropriate sample volumes. For high-COD samples (> 900 mg/L with 0.25 N FAS and 20 mL sample), serial dilutions with duplicate analysis are recommended.

BOD₅ measures biologically degradable oxygen demand over 5 days; COD measures total chemically oxidizable matter. The BOD/COD ratio indicates biodegradability: values above 0.5 suggest the waste is amenable to biological treatment; values below 0.3 indicate refractory compounds dominate, requiring advanced oxidation or physical-chemical treatment. The ratio is not constant - it varies with wastewater composition and should be established through parallel testing, not assumed from literature.

Yes. Volatile organics (acetone, chloroform, low-MW alcohols) can evaporate during open reflux digestion before being oxidized. The closed reflux method (sealed vials) retains volatiles and is preferred when volatile organic content is significant. Even with closed reflux, extremely volatile compounds with boiling points below 60 °C may partially escape during sample handling. Pre-cool samples to 4 °C and minimize headspace in collection bottles.