Chemical Oxygen Demand (COD) quantifies the mass of oxygen, expressed in milligrams per liter, required to chemically oxidize the organic and oxidizable inorganic matter contained in a water sample. Because it yields a result in two to three hours—rather than the five days required for BOD—it is the principal control parameter used in wastewater treatment plants, industrial discharge permits, and environmental monitoring programs.
This calculator implements the dichromate reflux methodology codified in APHA Standard Methods 5220. It accepts raw laboratory data from either the titrimetric (FAS) or colorimetric (closed-reflux) determinations and returns a regulatory-grade COD value, along with derived estimates for BOD₅ and TOC.
Required Laboratory Data
To obtain a valid result, the analyst must provide the following readings recorded during digestion and analysis:
- Blank Titration Volume (A) — millilitres of Ferrous Ammonium Sulfate (FAS) consumed by the reagent blank.
- Sample Titration Volume (B) — millilitres of FAS consumed by the digested sample.
- FAS Normality (M) — the exact molarity of the titrant, standardised daily against K₂Cr₂O₇.
- Sample Volume (V) — the volume of raw sample subjected to reflux, typically 50 mL.
- Absorbance Readings — for the colorimetric route, the sample and blank absorbance at 600 nm (high range) or 420 nm (low range).
- Calibration Slope & Intercept — coefficients of the standard curve built from potassium hydrogen phthalate (KHP).
- Dilution Factor (DF) — applied when the raw sample concentration exceeds the linear working range.
Theoretical Foundation & Formulas
The reaction underpinning the assay is the oxidation of organic carbon by hexavalent chromium in a boiling 50% sulfuric acid matrix, catalysed by silver sulfate. The dichromate ion is reduced to the chromic ion, producing a stoichiometric colour shift from orange to green.
$$C_nH_aO_bN_c + dCr_2O_7^{2-} + (8d+c)H^+ \rightarrow nCO_2 + \tfrac{a+8d-3c}{2}H_2O + cNH_4^+ + 2dCr^{3+}$$
Titrimetric Calculation (Method 5220 B/C)
Unreacted dichromate is back-titrated with standardised FAS using ferroin as the redox indicator. The endpoint is a sharp transition from blue-green to reddish-brown. COD is derived by difference between blank and sample:
$$COD\ \left(\tfrac{mg}{L}\right) = \frac{(A - B) \times M \times 8000}{V} \times DF$$
The constant 8000 is the product of the equivalent weight of oxygen (8 g eq⁻¹) and the millilitre-to-litre conversion (1000). It converts electron-equivalents consumed by the sample into an oxygen-equivalent mass.
Colorimetric Calculation (Method 5220 D)
Here the Cr³⁺ product (green, λₘₐₓ ≈ 600 nm) or residual Cr₂O₇²⁻ (yellow, λₘₐₓ ≈ 420 nm) is measured spectrophotometrically. Concentration is interpolated from a KHP calibration curve:
$$COD\ \left(\tfrac{mg}{L}\right) = \frac{(Abs_{sample} - Abs_{blank} - b)}{m} \times DF$$
Where $m$ is the curve slope and $b$ is the y-intercept. One mole of dichromate possesses the oxidation capacity of 1.5 moles of molecular O₂, which is why KHP exhibits a theoretical COD of 1.175 g O₂ per gram of phthalate.
Technical Reference Data
The working range, reagent chemistry, and expected precision differ by method. The following table summarises the decision matrix used in accredited environmental laboratories.
| Method | Standard | Sample Volume | Working Range | Wavelength | Key Interferent |
|---|---|---|---|---|---|
| Open Reflux, Titrimetric | 5220 B | 50 mL | 50 – 800 mg/L | N/A | Cl⁻ > 2000 mg/L |
| Closed Reflux, Titrimetric | 5220 C | 2.5 mL | 40 – 400 mg/L | N/A | Volatile organics |
| Closed Reflux, Colorimetric (Low) | 5220 D | 2.5 mL | 5 – 90 mg/L | 420 nm | Nitrite, Fe²⁺ |
| Closed Reflux, Colorimetric (High) | 5220 D | 2.5 mL | 100 – 900 mg/L | 600 nm | Turbidity |
Water Quality Classification Thresholds
Interpretation of the numerical result is guided by generally accepted ecological benchmarks:
- < 20 mg/L — Unpolluted surface water, consistent with potable source reservoirs.
- 20 – 100 mg/L — Moderately impacted; typical of treated municipal effluent.
- 100 – 500 mg/L — Heavily polluted; characteristic of raw domestic wastewater.
- > 500 mg/L — Severely contaminated; common in untreated industrial discharges (food processing, textiles, pulp).
Engineering Analysis & Real-World Application
The most critical check an analyst must perform is verifying that the sample titration volume (B) is less than the blank (A). A reversal indicates either a reagent error or a sample so dilute that it falls below the method detection limit. This calculator defensively returns zero when $B \geq A$ to prevent negative COD values from propagating into compliance reports.
The BOD/COD ratio is a powerful diagnostic of biodegradability. Ratios above 0.5 indicate readily biodegradable wastewater suitable for conventional activated sludge treatment. Ratios below 0.3 suggest recalcitrant compounds, often requiring advanced oxidation or membrane processes prior to biological polishing.
Chloride interference is the single most common source of positive bias. At concentrations above 2000 mg/L, mercuric sulfate complexation becomes insufficient, and the oxidation of Cl⁻ to Cl₂ generates a spurious oxygen demand. For saline effluents, alternative protocols based on the argentometric adjustment or marine-specific methods must be substituted.
Dilution strategy also governs accuracy. A sample diluted tenfold propagates all analytical uncertainty by the same factor; therefore, dilutions should be chosen so that the final titration consumes between 20% and 80% of the blank volume, placing the measurement within the optimal region of the response curve.
Frequently Asked Questions
This typically reflects the presence of reduced inorganic species that also consume dichromate. Ferrous iron, sulfide, sulfite, and nitrite each contribute to the measured value. For example, 1 mg/L of nitrite nitrogen contributes approximately 1.14 mg/L to the apparent COD. When inorganic demand is significant, sulfamic acid can be added to eliminate the nitrite contribution prior to digestion.
Only after a site-specific correlation has been established over at least twenty paired measurements. The BOD/COD ratio varies considerably across industries: domestic sewage typically exhibits ratios of 0.4 to 0.6, while petrochemical effluent may fall below 0.1 due to high fractions of aromatic and halogenated compounds. A single assumed ratio applied to a new waste stream is a frequent cause of undersized biological reactors and permit excursions.
The open-reflux technique uses a larger sample volume (50 mL), which improves the representativeness of heterogeneous wastewaters containing suspended solids. The closed-reflux variants use only 2 to 5 mL and can under-report COD when particulates are not homogenised. Conversely, closed reflux achieves better oxidation of volatile organics because they remain in the sealed vial rather than escaping through the condenser. Duplicate analyses should agree within ±5%; larger deviations indicate matrix-specific bias requiring method validation.
Professional Conclusion
Chemical Oxygen Demand remains the fastest and most reproducible index of organic pollution load available to the environmental practitioner. Manual computation of the 5220 formulas is error-prone, particularly when dilution factors, molarity corrections, and calibration intercepts must be combined under time pressure. This calculator applies the APHA stoichiometry deterministically, flags out-of-range inputs, and contextualises the output against both regulatory limits and established water-quality classifications—delivering defensible results suitable for permit reporting, process control, and academic research.