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Chemical Oxygen Demand Calculator

Ready to calculate
COD = (A−B)·N·8000/V.
EPA 410.4 / SM 5220-D.
Water-quality bands.
100% Free.
No Data Stored.

How it Works

01Digest Sample with Dichromate

Reflux sample with K₂Cr₂O₇ in conc. H₂SO₄ (Ag₂SO₄ catalyst, HgSO₄ for Cl⁻) at 150 °C × 2 h.

02Run a Blank in Parallel

Reagent water replaces sample; same digestion. Records the dichromate available before reduction by sample.

03Back-titrate with FAS

Titrate excess dichromate with standardized Ferrous Ammonium Sulfate; ferroin indicator → red endpoint.

04Apply COD = (A−B)·N·8000/V

(A − B) is the dichromate reduced by sample; multiply by N and 8000, divide by sample volume → mg O₂/L.

What is a Chemical Oxygen Demand (COD) Calculator?

Chemical Oxygen Demand (COD) is the most widely-reported water-quality parameter in regulated wastewater laboratories — a single number, in mg of O₂ per liter, that summarizes the total oxidizable load (organic plus reduced inorganic) in a water sample. It is the regulatory currency for industrial discharge permits worldwide, the operational set-point for activated-sludge plants, and the diagnostic that separates "treatable" from "industrial-strength" effluent. The standard method (US EPA 410.4, APHA Standard Methods 5220-D, ISO 6060) uses hot acidic dichromate digestion followed by ferroin-indicated back-titration with Ferrous Ammonium Sulfate (FAS), and the result is computed by a single defining identity: COD (mg O₂/L) = (A − B) × N × 8000 / V_sample.

In that formula A is the FAS titrant volume for the reagent blank (mL), B is the FAS volume for the digested sample (mL), N is the standardized FAS normality (eq/L), V_sample is the sample volume (mL), and 8000 = 8 g O₂/equivalent × 1000 mL/L (the equivalent weight of oxygen is 32/4 = 8 g/eq, since O₂ accepts 4 electrons per molecule). The calculator accepts FAS volumes and sample volume in mL / L / µL, outputs COD in mg/L (numerically equal to ppm in water), g/L, and oz/gal (US), and applies an automatic water-quality classification — drinking-water grade (< 10 mg/L), treated effluent (10-100), moderately polluted (100-500), raw municipal wastewater (500-1500), industrial wastewater (1500-50,000), and extreme (> 50,000 mg/L beyond direct method range).

Smart warnings catch the four most common laboratory mistakes: (1) B ≥ A, which is impossible because the sample must consume MORE dichromate than the reagent blank; (2) (A − B) below 0.5 mL, where the small titrimetric difference inflates relative error past 5%; (3) COD outside the 5-700 mg/L reliable range of the standard open-reflux method; (4) off-spec FAS normality (standard methods use 0.025 N for low range, 0.10 N for mid, 0.25 N for high). Designed for environmental laboratories running hundreds of COD analyses per week, wastewater treatment plant operators monitoring influent/effluent loads, environmental-engineering students learning the dichromate method, and regulatory inspectors verifying compliance reports — runs entirely in your browser, no account, no data stored.

Pro Tip: Pair this with our Molarity Calculator for solution preparation, our Dilution Factor Calculator for high-COD sample dilution, or our Normality Calculator for FAS standardization.

How to Use the COD Calculator?

Run the Open / Closed-Reflux Digestion: Add 2.5 mL of sample (or volume per method) + 1.5 mL standardized 0.0167 M K₂Cr₂O₇ + 3.5 mL conc. H₂SO₄/Ag₂SO₄ catalyst reagent into a sealed reflux vial. Add HgSO₄ if Cl⁻ > 50 mg/L (10:1 Hg:Cl mass ratio). Heat at 150 °C for 2 h. Cool to room T. Run a BLANK in parallel using reagent water in place of sample.
Back-titrate Both Vials with Standardized FAS: Transfer digested mixture to an Erlenmeyer flask; add 2-3 drops of ferroin indicator (turns blue-green initially). Titrate with FAS until the color changes sharply from blue-green to red-brown — the endpoint is the first stable reddish hue. Record A (FAS volume for blank) and B (FAS volume for sample) to ±0.05 mL.
Verify B < A: The sample reduces some of the dichromate, leaving less to be back-titrated; therefore B must always be SMALLER than A. If B ≥ A, the sample contained reducing agents that did not survive digestion, the blank was contaminated, or there's a transcription error — re-run.
Enter the 4 Inputs: A (mL), B (mL), N (FAS normality in eq/L), and V_sample (mL). Common FAS normalities: 0.025 N for low-range COD (< 100 mg/L), 0.10 N for mid-range (100-700), 0.25 N for high-range (250-15,000). For higher COD, dilute the sample BEFORE digestion.
Apply COD = (A − B) × N × 8000 / V: The 8000 factor combines (8 g O₂/equivalent) × (1000 mL/L) so that the result has units of mg/L when A, B, V are in mL and N is in eq/L. Algebraic check: (mL · eq/L · 8000) / mL = 8000 eq/L → ÷ 1000 mL/L = 8 g/eq × eq → mg/L scaling.
Read COD with Water-Quality Classification: The calculator automatically labels the result by EPA / Standard Methods convention — drinking-water grade (< 10 mg/L, indicates very clean water; use TOC for quantitation), treated effluent (10-100, within US POTW discharge limit 125 mg/L), moderately polluted (100-500), raw municipal wastewater (500-1500), industrial wastewater (1500-50,000), extreme (> 50,000, dilute and re-test).
Cross-Check with QC Standard: Theoretical COD of glucose is 1.067 mg O₂ per mg glucose; KHP (potassium hydrogen phthalate) is 1.176 mg O₂ per mg. Run a 500 mg/L KHP standard alongside the sample; recovery should be 97-103% for the analysis to be acceptable.
Report According to Permit: Most NPDES permits require COD in mg/L (= ppm). Some industrial permits use lb COD/day (multiply mg/L by flow in gal/day × 8.345 × 10⁻⁶). The calculator provides both mg/L and oz/gal-US for cargo-style reporting.

How is Chemical Oxygen Demand calculated?

The COD dichromate back-titration is the single most-cited analytical method in environmental chemistry — it has been the regulatory standard for over 60 years (since EPA Method 410 was promulgated in the 1970s). The math is straightforward; the chemistry is exact stoichiometry; the value of the result depends entirely on careful technique.

References: US EPA Method 410.4 (1993); APHA Standard Methods for the Examination of Water and Wastewater 5220-D (24th ed., 2017); ISO 6060:1989.

Core Formula

COD (mg O₂/L) = (A − B) × N × 8000 / V_sample

A and B in mL; N in eq/L (FAS normality); V_sample in mL; 8000 = 8 g O₂/equivalent × 1000 mL/L. The result is mg of O₂ equivalent per liter of original sample.

Where Does the 8000 Factor Come From?

  • Oxygen equivalent weight: O₂ accepts 4 electrons (½O₂ + 2e⁻ + 2H⁺ → H₂O). Equivalent weight = molecular weight / electrons gained = 32 / 4 = 8 g/eq.
  • Unit conversion: g/L → mg/L is × 1000.
  • Combined: 8 g/eq × 1000 mg/g = 8000 mg/eq; the formula gives mg O₂ per liter of original sample.

The Underlying Chemistry

In the digestion vial, dichromate oxidizes organic matter:

(organic) + Cr₂O₇²⁻ + H⁺ → CO₂ + H₂O + Cr³⁺ (catalyzed by Ag₂SO₄)

Excess dichromate is back-titrated with Fe²⁺ (FAS):

Cr₂O₇²⁻ + 6 Fe²⁺ + 14 H⁺ → 2 Cr³⁺ + 6 Fe³⁺ + 7 H₂O (ferroin endpoint blue-green → red)

Worked Example — Municipal Wastewater Influent

Sample volume V = 2.5 mL. Blank: A = 9.80 mL of 0.10 N FAS. Sample: B = 4.50 mL of 0.10 N FAS.

  • (A − B) = 9.80 − 4.50 = 5.30 mL.
  • COD = 5.30 × 0.10 × 8000 / 2.5 = 4240 / 2.5 = 1696 mg O₂/L.
  • Classification: Raw municipal wastewater — high end of typical 500-1500 mg/L range, suggests strong wastewater (concentrated sewage or industrial mix).

Worked Example — Treated Effluent

After secondary biological treatment. V = 5.0 mL. A = 4.95 mL, B = 4.65 mL of 0.025 N FAS.

  • (A − B) = 0.30 mL.
  • COD = 0.30 × 0.025 × 8000 / 5.0 = 60 / 5.0 = 12 mg O₂/L.
  • Classification: Treated effluent / drinking-water grade. Within US EPA secondary treatment standard (typically < 125 mg/L for municipal POTW discharge).

Worked Example — Industrial Discharge

Pulp-mill bleach effluent. V = 0.50 mL (pre-diluted 10×). A = 9.95 mL, B = 1.20 mL of 0.25 N FAS.

  • (A − B) = 8.75 mL.
  • COD (diluted) = 8.75 × 0.25 × 8000 / 0.50 = 17,500 / 0.50 = 35,000 mg O₂/L.
  • Multiply by dilution factor 10: COD (original) = 350,000 mg O₂/L. Far beyond direct-method range — typical of pulp-mill liquor; requires substantial pre-treatment before discharge.

Typical COD Values You Should Know (mg O₂/L)

  • Pure deionized water: < 5 (limit of detection).
  • Drinking water (post-treatment): 1-10.
  • River water (clean): 5-30.
  • Treated municipal effluent: 30-100.
  • Raw municipal sewage: 250-1000.
  • Concentrated raw sewage / septic tank: 1000-3000.
  • Food processing wastewater: 1000-10,000 (dairy 2000-4000; brewery 1500-4000).
  • Pulp & paper effluent: 5000-50,000 (black liquor 100,000+).
  • Petrochemical effluent: 1000-50,000.
  • Landfill leachate (young): 10,000-50,000; (mature): 1000-5000.
  • Glucose 1 mg/mL standard (theoretical): 1067 mg O₂/L.
  • KHP 1 mg/mL standard (theoretical): 1176 mg O₂/L.

COD vs BOD₅ vs TOC — Three Different Measurements

  • COD: CHEMICALLY oxidizable matter via dichromate. Includes organics + reduced inorganics (sulfide, ferrous, nitrite). 2-h analysis. mg O₂/L.
  • BOD₅: BIOLOGICALLY oxidizable matter via 5-day microbial respiration test. Includes only biodegradable organics. 5-day analysis. mg O₂/L.
  • TOC: Total Organic Carbon by combustion / NDIR detection. Direct organic carbon mass. ~10-min analysis. mg C/L.
  • Typical relationships (municipal wastewater): BOD₅ / COD ≈ 0.4-0.6; COD / TOC ≈ 3 (by mass). High COD/BOD₅ ratio (> 3) signals non-biodegradable or toxic organics.
Real-World Example

Worked Example — Compute COD for Mid-Range Wastewater

Scenario. A municipal wastewater plant operator samples raw influent. Sample volume V = 2.0 mL. Standardized FAS at 0.100 N. Blank titration: A = 10.10 mL FAS to ferroin endpoint. Sample titration: B = 6.40 mL FAS.

Step 1 — Compute the titration difference.

  • (A − B) = 10.10 − 6.40 = 3.70 mL.
  • This is the volume of FAS NOT consumed by sample-side dichromate — equivalent to the dichromate that the sample reduced.

Step 2 — Apply the COD formula.

  • COD = (A − B) × N × 8000 / V_sample
  • COD = 3.70 × 0.100 × 8000 / 2.0
  • COD = 2960 / 2.0 = 1480 mg O₂/L.

Step 3 — Classification.

  • 1480 mg O₂/L falls in the 500-1500 mg/L band → Raw municipal wastewater (high end).
  • This is at the upper limit of typical municipal influent — likely indicates concentrated industrial discharge mixed in (food processing, brewery, pharmaceutical) or wet-weather first-flush.

Step 4 — QC Cross-Check (run a KHP standard alongside).

  • 500 mg/L KHP standard. Theoretical COD = 500 × 1.176 = 588 mg O₂/L.
  • Suppose KHP titration gives (A − B) = 1.47 mL with same N, V: COD = 1.47 × 0.10 × 8000 / 2.0 = 588 mg/L.
  • Recovery = 588 / 588 = 100% — analytical method is in control.

Step 5 — Reporting.

  • Report: COD = 1480 mg O₂/L (or 1.48 g/L) for sample collected on [date], analyzed by EPA Method 410.4 / SM 5220-D.
  • For loading reporting at 10 MGD flow: 1480 mg/L × 10 × 10⁶ gal/day × 8.345 × 10⁻⁶ lb/(mg·L⁻¹·gal) = 123,500 lb COD/day.

Who Should Use the COD Calculator?

1
Daily monitoring of influent and effluent COD — track removal efficiency (typical 80-95% for activated sludge), detect upsets, optimize aeration energy and chemical doses, and document compliance with discharge permits.
2
EPA Method 410.4 is the regulatory reference for self-monitoring under NPDES permits. Required by US POTW pretreatment programs, EU Urban Waste Water Directive, and most national environmental agencies.
3
Dairy, brewery, distillery, slaughterhouse, and food-canning effluents have very high COD (1000-50,000 mg/L). On-site COD monitoring drives anaerobic-digestion design, biogas yield prediction, and sludge management.
4
Black liquor, bleach effluent, oil refinery wastewater all run COD > 10,000 mg/L; on-site digesters, MBRs, and ozonation systems are sized from measured COD load. Required for environmental permitting.
5
River, lake, and estuary monitoring under Clean Water Act § 305(b) and TMDL programs. Detect non-point pollution, agricultural runoff, urban stormwater impact.
6
Methane potential ≈ 0.35 m³ CH₄ per kg COD removed (theoretical max). Design AD reactor volume from feedstock COD load; track removal efficiency and biogas yield.
7
COD is a textbook environmental-chemistry concept — understanding the dichromate back-titration, the 8000 factor, and the COD/BOD/TOC relationships is fundamental for water-quality coursework.

Technical Reference

Standard Method Equivalents. US EPA Method 410.4 (Determination of Chemical Oxygen Demand by Semi-Automated Colorimetry, 1993); APHA Standard Methods for the Examination of Water and Wastewater 5220-B (Open Reflux), 5220-C (Closed Reflux Titrimetric), 5220-D (Closed Reflux Colorimetric); ISO 6060:1989 (Water Quality — Determination of Chemical Oxygen Demand); HACH Method 8000 (Reactor Digestion). All methods rely on the same chemistry; differences are in digestion vessel (open vs sealed), endpoint detection (titration vs colorimetric absorbance at 600 nm for Cr³⁺ or 420 nm for residual Cr₂O₇²⁻), and pre-dosed reagent vials vs lab-prepared reagents.

Reagents and Concentrations. (1) Standard dichromate digestion solution: 0.0167 M K₂Cr₂O₇ in 50% H₂SO₄ with HgSO₄ for chloride control and Ag₂SO₄ catalyst. Typical: 1.022 g K₂Cr₂O₇ + 167 mL conc. H₂SO₄ + 33.3 g HgSO₄ per liter. (2) Sulfuric acid reagent: 5.5 g Ag₂SO₄ per kg conc. H₂SO₄ (~10.5 g/L). (3) FAS titrant (Ferrous Ammonium Sulfate, Fe(NH₄)₂(SO₄)₂·6H₂O, MW 392.14): 0.025 N (9.8 g/L), 0.10 N (39.2 g/L), or 0.25 N (98.0 g/L) — standardize daily against the dichromate digestion solution. (4) Ferroin indicator: 1,10-phenanthroline ferrous sulfate, 0.025 M.

Range and Method Detection Limit. Open-reflux titrimetric (5220-B): MDL ~5 mg/L, working range 50-700 mg/L. Closed-reflux titrimetric (5220-C): MDL ~5 mg/L, working range 50-700 mg/L (or 5-50 with low-range protocol). Closed-reflux colorimetric (5220-D / HACH 8000): three vial ranges — ultra-low (0.7-40 mg/L), low (3-150 mg/L), high (20-1500 mg/L), with high-high vials extending to 15,000 mg/L. Beyond 700 mg/L by direct titration, dilute the sample 1:5, 1:10, or 1:20 with reagent water and re-test.

Chloride Interference and Correction. Chloride is the most significant interference; Cr₂O₇²⁻ oxidizes Cl⁻ to Cl₂ during digestion, registering as false COD. Each mg/L Cl⁻ contributes ~0.225 mg/L apparent COD if unmasked. Standard correction: add HgSO₄ at 10:1 Hg:Cl mass ratio, which forms HgCl₂ (negligible Cr₂O₇²⁻ reactivity). Effective up to ~2000 mg/L Cl⁻; for higher chloride, dilute sample, use silver-removal pretreatment, or use the modified Pitwell method. Modern HACH chloride-free vials use a different masking chemistry to eliminate Hg waste.

Other Interferences. Nitrite (NO₂⁻): oxidizes to NO₃⁻ contributing 1.14 mg COD per mg N. Add 10 mg sulfamic acid per mg NO₂⁻-N to mask. Volatile organics: partially escape during open-reflux digestion; closed-reflux retains them quantitatively. Reduced inorganics (S²⁻, Fe²⁺, Mn²⁺, NH₃ at high concentrations) contribute to COD legitimately — distinguishing organic vs inorganic COD requires separate TOC analysis. Pyridine and aromatic-N compounds are not fully oxidized; this is one of the documented limitations of the dichromate method.

Quality Control Standards. Glucose (C₆H₁₂O₆, MW 180.16): theoretical COD = 6 × 32 / 180.16 = 1.067 mg O₂ per mg glucose. A 200 mg/L glucose standard gives theoretical 213 mg/L COD. Recovery 95-105% expected for in-control analysis. Potassium hydrogen phthalate (KHP, KC₈H₅O₄, MW 204.22): theoretical COD = 7.5 × 32 / 204.22 = 1.176 mg O₂ per mg KHP. KHP is the preferred standard because it is non-hygroscopic, stable, and certified pure. A 425 mg/L KHP gives 500 mg/L COD theoretical. Run one QC standard per batch of 10-20 samples; reject batch if recovery falls outside 90-110%.

COD vs BOD₅ Relationship. COD and BOD₅ measure different fractions of the same waste. BOD₅/COD ratio is a biodegradability index: > 0.6 = highly biodegradable (typical fresh municipal sewage); 0.3-0.6 = treatable (typical municipal mixed); < 0.3 = non-biodegradable (industrial, presence of toxics, refractory organics). For most municipal influent BOD₅ ≈ 0.45 × COD; for municipal effluent after biological treatment BOD₅ ≈ 0.1 × COD (the easily-biodegradable fraction has been removed, leaving recalcitrant compounds).

Theoretical Oxygen Demand (ThOD) Calculation. For a single compound C_a H_b N_c O_d S_e with stoichiometric oxidation to CO₂, H₂O, NH₃, SO₄²⁻: ThOD = 32 × (a + b/4 − c/2 − d/2 + 1.5e) / MW. Examples: methanol CH₃OH ThOD = 1.50 mg O₂/mg; ethanol ThOD = 2.09; glucose 1.067; sucrose 1.122; cellulose 1.185; phenol 2.38. Measured COD typically achieves 85-95% of ThOD because some compounds (pyridines, aromatic amines, straight-chain hydrocarbons over C₆) are not fully oxidized in the 2-hour digestion. The COD/ThOD ratio is a useful sanity check for known-composition test solutions.

Conclusion

Chemical Oxygen Demand is the workhorse of every environmental laboratory — a single mg O₂/L value summarizes the entire oxidizable load of a water sample, drives discharge-permit compliance, and tracks treatment-plant performance day to day. The math is one formula: COD = (A − B) × N × 8000 / V, where A and B are the FAS titration volumes for blank and sample, N is the FAS normality, V is the sample volume, and 8000 = 8 g O₂/equivalent × 1000 mL/L. The chemistry is exact dichromate-iron stoichiometry. The accuracy depends entirely on technique: a fresh blank, careful endpoint detection, mercuric sulfate to mask chloride interference, and a glucose or KHP QC standard to verify the analysis is in control.

Two pitfalls dominate real-world COD failures: (1) Chloride interference inflates apparent COD by 200-500% in saline samples — always add HgSO₄ at 10:1 Hg:Cl mass ratio (or use a chloride-free alternative method). (2) Sample matrix effects — high turbidity, color, or volatiles partially escape the open-reflux digestion; closed-reflux sealed vials (HACH-style) eliminate this and are the modern standard. For routine lab work, prefer a closed-reflux pre-dosed-vial system with a colorimetric finish (HACH DR series or equivalent) when budget allows; the manual back-titration remains the regulatory reference. Use this calculator alongside your lab notebook to verify computed values against your titration data, classify results by EPA water-quality bands, and catch transcription errors before they reach a regulatory report.

Frequently Asked Questions

What is the Chemical Oxygen Demand Calculator?
It implements the EPA 410.4 / Standard Methods 5220 dichromate back-titration: COD (mg O₂/L) = (A − B) × N × 8000 / V_sample. A is FAS volume for blank, B is FAS for sample, N is FAS normality (eq/L), V_sample is sample volume (mL); 8000 = 8 g O₂/equivalent × 1000 mL/L. The calculator outputs COD in mg/L (= ppm), g/L, oz/gal-US, plus an automatic water-quality classification (drinking-water grade < 10, treated 10-100, raw 500-1500, industrial 1500-50000 mg/L) and smart warnings for the four most common laboratory mistakes.

Pro Tip: Pair this with our Normality Calculator for FAS standardization.

What is the formula for COD?
COD (mg O₂/L) = (A − B) × N × 8000 / V_sample. A and B in mL (FAS for blank and sample), N in eq/L (FAS normality), V_sample in mL, 8000 = 8 g O₂/equivalent × 1000 mL/L. Algebraic check: (mL · eq/L · mg/eq) / mL = mg/L. The 8 g/eq comes from O₂ accepting 4 electrons (½O₂ + 2e⁻ + 2H⁺ → H₂O); equivalent weight = 32/4 = 8 g/eq.
Why does the formula multiply by 8000?
8000 = 8 × 1000. The 8 is the equivalent weight of oxygen in grams per equivalent (since O₂ has molecular weight 32 g/mol and accepts 4 electrons per molecule, the equivalent weight is 32/4 = 8 g/eq). The 1000 converts the result from g/L to mg/L. Combined factor: each milliliter of FAS at 0.10 N reduces 0.0001 eq of dichromate, which is equivalent to 0.0001 × 8 = 0.0008 g of O₂ = 0.8 mg O₂. Per mL of sample: 0.8 / V_sample × 1000 = 800 / V_sample mg/L per mL of FAS at 0.10 N — combining with N gives the (A−B)·N·8000/V form.
Why must B (FAS for sample) be less than A (FAS for blank)?
Algebraic necessity. The blank contains the FULL initial dichromate (no sample to consume it), so all the dichromate must be back-titrated by FAS — large A. The sample reduces some of the dichromate to Cr³⁺, leaving LESS dichromate for FAS to reduce — small B. The difference (A − B) directly equals the dichromate consumed by the sample, which is what we want. If B ≥ A you have a method failure: reagent contamination, transcription error, blank degraded, sample carryover, or the sample contained REDUCING agents that survived digestion (rare but possible for resistant aromatic compounds). Re-run the analysis.
What FAS normality should I use?
Three standard normalities cover the typical range. 0.025 N FAS for low-range COD < 100 mg/L (drinking water, treated effluent — small dichromate dose, sensitive titration). 0.10 N FAS for mid-range 50-700 mg/L (most municipal samples — standard choice). 0.25 N FAS for high-range 250-15,000 mg/L (industrial wastewater, raw process streams). For COD > 700 mg/L by direct method, dilute the sample 1:5, 1:10, or 1:20 with reagent water before digestion and use 0.10 N FAS. Always standardize FAS daily — the Fe²⁺ slowly oxidizes to Fe³⁺ on storage.
What sample volume should I use?
Standard EPA / Standard Methods sample volume is 2.5 mL for closed-reflux methods (HACH-style sealed vials), or up to 50 mL for open-reflux scaled-up methods. Choose volume based on expected COD: small volume (2.5 mL) for high COD (1000+ mg/L); larger volume (10-50 mL) for low COD < 100 mg/L to ensure (A − B) is > 0.5 mL for accurate quantitation. If (A − B) is too small, the relative error in the small titration difference dominates the total uncertainty (a 0.05 mL endpoint precision on a 0.3 mL difference is 17% RSD).
How does COD relate to BOD₅ and TOC?
Three different measurements. COD = chemically oxidizable matter (organics + reduced inorganics) via 2-h dichromate digestion; mg O₂/L. BOD₅ = biologically oxidizable matter via 5-day microbial respiration; mg O₂/L; only includes biodegradable organics. TOC = total organic carbon by combustion; mg C/L; direct organic carbon mass. Typical relationships: for fresh municipal sewage BOD₅/COD ≈ 0.45-0.55; COD/TOC ≈ 3 (by mass). BOD₅/COD < 0.3 indicates non-biodegradable / refractory organics or toxic samples. BOD₅/COD > 0.6 indicates highly biodegradable, treatable organic load.
How do I correct for chloride interference?
Cl⁻ is the most common interference — it gets oxidized to Cl₂ by dichromate during digestion, registering as false COD (~0.23 mg COD per mg Cl⁻). Standard correction: add HgSO₄ at 10:1 Hg:Cl mass ratio to form HgCl₂ (which is non-reactive with Cr₂O₇²⁻). Effective up to ~2000 mg/L Cl⁻ (a typical seawater contains 19,000 mg/L Cl⁻ — too high; dilute first). Pre-dosed HACH vials include the correct HgSO₄ amount for typical sample chloride. For chloride-free / mercury-free analysis, use the modified silver-mediated method or chloride-removal column pretreatment.
What is a typical COD value for raw sewage?
Raw municipal sewage: 250-1000 mg O₂/L (median ~500). Treated effluent (secondary): 30-100 mg/L. Concentrated raw or septic-tank: 1000-3000 mg/L. Food-processing wastewater: 1000-10,000 mg/L (dairy 2-4×10³, brewery 1.5-4×10³). Pulp & paper effluent: 5,000-50,000 mg/L. Black liquor / spent kraft cooking liquor: 100,000+ mg/L. Drinking water: < 10 mg/L (often below detection by standard COD; use TOC). The calculator automatically classifies your result by these EPA / Standard Methods bands.
Why use COD instead of BOD₅?
COD is faster (2 h vs 5 days), more reproducible (~3% RSD vs ~15% for BOD₅), and broader in scope (catches non-biodegradable + reduced inorganics that BOD₅ misses). BOD₅ is the historical regulatory parameter (the original Royal Commission on Sewage Disposal report 1898 established BOD₅ as the gold standard), still widely required by NPDES permits and CWA. Modern operational use: COD for daily monitoring, process control, real-time decisions; BOD₅ for permit-compliance reports and biodegradability assessment. Many regulated facilities establish a site-specific BOD₅/COD correlation, then use rapid COD for hourly process control and BOD₅ for monthly compliance.
What is the theoretical COD of glucose / KHP for QC?
Glucose (C₆H₁₂O₆, MW 180.16): ThOD = 6 × 32 / 180.16 = 1.067 mg O₂ per mg glucose. A 200 mg/L glucose standard has theoretical COD = 213 mg O₂/L. Potassium hydrogen phthalate (KHP, MW 204.22): ThOD = 7.5 × 32 / 204.22 = 1.176 mg O₂ per mg KHP. A 425 mg/L KHP gives theoretical COD = 500 mg/L. KHP is the preferred QC standard because it is non-hygroscopic, NIST-traceable certified, stable indefinitely, and reaches near-100% recovery (95-105%) when the analytical method is in control. Run one QC standard per batch of 10-20 samples; reject the batch and re-run if recovery falls outside 90-110%.

Author Spotlight

The ToolsACE Team - ToolsACE.io Team

The ToolsACE Team

Our ToolsACE chemistry team built this calculator to handle the standard <strong>Chemical Oxygen Demand (COD)</strong> back-titration math used in every regulated wastewater laboratory worldwide. The defining identity is <strong>COD (mg O₂/L) = (A − B) × N × 8000 / V_sample</strong>, where A is the volume of Ferrous Ammonium Sulfate (FAS) titrant for the reagent BLANK, B is the FAS volume for the digested SAMPLE, N is the FAS normality (eq/L), V_sample is the sample volume (mL), and 8000 = 8 g O₂/equivalent × 1000 mL/L. The calculator accepts FAS volumes and sample volume in mL / L / µL; outputs COD in mg/L (= ppm), g/L, and oz/gal-US, plus an automatic water-quality classification (drinking-water grade &lt; 10, treated effluent 10-100, moderately polluted 100-500, raw municipal 500-1500, industrial 1500-50000, extreme &gt; 50000 mg/L). Smart warnings detect common errors: B ≥ A (impossible — sample must consume more dichromate than blank), (A−B) too small for accurate quantitation, COD beyond the standard method's reliable 5-700 mg/L range, and unusual FAS normality.

EPA Method 410.4 (Determination of Chemical Oxygen Demand)Standard Methods for the Examination of Water and Wastewater (SM 5220)ISO 6060:1989 (Determination of COD)

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COD measures chemically oxidizable matter (organic + reduced inorganic) via hot acidic dichromate, distinct from BOD₅ (biologically oxidizable, 5-day) and TOC (total organic carbon mg C/L). Typical BOD₅/COD ratio: 0.4-0.6 for municipal wastewater. Chloride interferes — add HgSO₄ at 10:1 Hg:Cl mass ratio (effective up to 2000 mg/L Cl⁻); for higher Cl⁻, dilute first. Method ranges: low (5-150 mg/L, 0.025 N FAS), medium (50-700, 0.10 N), high (250-15,000, 0.25 N). Always run a glucose (1.067 mg O₂/mg) or KHP (1.176 mg O₂/mg) QC standard with each batch; reject batch if recovery is outside 90-110%. References: US EPA Method 410.4 (1993); APHA Standard Methods 5220 (24th ed., 2017); ISO 6060:1989; HACH Method 8000.