Can I solve for different variables?

Yes! This calculator supports reverse solving. You can solve for: Normality of FAS (N), Chemical oxygen demand (COD), FAS for blank (A), FAS for sample (B), Volume of sample.

Chemical oxygen demand

Solve chemical oxygen demand relationships quickly using core chemistry formulas.

Formula shown4 inputs definedUpdated Nov 2025By Automated Chemistry GeneratorFree, no sign-up

Quick Summary

Use this when you need:

Convert titration buret readings into COD concentrations for wastewater
Validate laboratory blanks by ensuring A and B differ appropriately
Estimate how sample dilution impacts COD detection limits

You’ll need:Chemical oxygen demand (COD), FAS for blank (A), FAS for sample (B), Volume of sample

You’ll get:Normality of FAS (N)

How this is calculated

Formula:

COD (mg/L) = (A - B) * N * 8000 / Vₙ

In plain language:

Standard dichromate COD titrations subtract sample FAS volume B from blank A, multiply by the FAS normality N, and scale by 8000 mg O₂ per equivalent divided by the sample volume in mL.

The formula is always visible so you can verify the math and explain your numbers to anyone who asks.

What these terms mean

Normality of FAS (N)

Normality of FAS (N) used in the calculation.

Chemical oxygen demand (COD)

Chemical oxygen demand (COD) used in the calculation.

FAS for blank (A)

FAS for blank (A) used in the calculation.

FAS for sample (B)

FAS for sample (B) used in the calculation.

Volume of sample

Volume of sample used in the calculation.

When to use this calculator

Convert titration buret readings into COD concentrations for wastewater
Validate laboratory blanks by ensuring A and B differ appropriately
Estimate how sample dilution impacts COD detection limits

Last updated: November 19, 2025

Chemical oxygen demand calculator explained

The dichromate COD method quantifies how much oxidizable material is in water by titrating excess dichromate with ferrous ammonium sulfate (FAS). This calculator uses the Standard Methods relationship $(A - B) N 8000 / V$ so you can enter buret readings for the blank {titrantblank}, sample {titrantsample}, FAS normality {normality}, and sample volume {volumesample} to get COD in mg/L instantly.

Use it for wastewater compliance reports, fermentation monitoring, or quick checks of industrial effluent strength when you need accurate numbers without building a custom spreadsheet.

How the conversion works

After digestion, the dichromate remaining in the blank (volume $A$ ) and sample (volume $B$ ) are titrated with FAS of normality $N$ . The COD in mg/L is

COD = \\frac{(A - B) \\times N \\times 8000}{V_s}

where $V_s$ is the sample volume in mL and 8000 is the milligrams of oxygen equivalent to one equivalent of dichromate per liter. Positive COD values require $A > B$ ; if $B$ exceeds $A$ , dilute the sample or check digestion quality.

Units and conversions

Quantity	Units	Notes
$A$ , $B$	mL FAS	Read directly from the buret.
$N$	eq/L	FAS normality; standard COD methods use 0.10 or 0.25 N.
$V_s$	mL sample	Volume of digested sample.
COD	mg/L O $_2$	Multiply by dilution factors if the sample was diluted before digestion.

Worked examples

Municipal wastewater
$A = 12.40\\ \\text{mL}$ , $B = 3.25\\ \\text{mL}$ , $N = 0.250\\ \\text{N}$ , $V_s = 50.0\\ \\text{mL}$ .

COD = \\frac{(12.40 - 3.25) \\times 0.250 \\times 8000}{50.0} = 3.66 \\times 10^{2}\\ \\text{mg}\\,\\text{L}^{-1}

Report 366 mg/L COD.

High-strength food process water
$A = 9.85\\ \\text{mL}$ , $B = 1.10\\ \\text{mL}$ , $N = 0.100\\ \\text{N}$ , $V_s = 25.0\\ \\text{mL}$ .

COD = \\frac{(9.85 - 1.10) \\times 0.100 \\times 8000}{25.0} = 2.80 \\times 10^{2}\\ \\text{mg}\\,\\text{L}^{-1}

If this exceeds permit limits, dilute before digestion or adjust the process.

Tips and pitfalls

Always run reagent blanks; if $A$ drifts, re-make digestion reagents.
Normalize FAS normality weekly using primary standard sodium oxalate.
Dilute samples with COD above about 900 mg/L so the dichromate remains in excess.
Keep digestion tubes capped tightly; evaporative loss changes $V_s$ and inflates COD.

References and further reading

Frequently Asked Questions

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Calculator info

Version:: v1.0.0
Last reviewed:: Nov 15, 2025
Sensitivity:: low
References:: 1

Last updated: November 19, 2025

Chemical oxygen demand calculator explained

Use it for wastewater compliance reports, fermentation monitoring, or quick checks of industrial effluent strength when you need accurate numbers without building a custom spreadsheet.

How the conversion works

After digestion, the dichromate remaining in the blank (volume $A$ ) and sample (volume $B$ ) are titrated with FAS of normality $N$ . The COD in mg/L is

COD = \\frac{(A - B) \\times N \\times 8000}{V_s}

Units and conversions

Quantity	Units	Notes
$A$ , $B$	mL FAS	Read directly from the buret.
$N$	eq/L	FAS normality; standard COD methods use 0.10 or 0.25 N.
$V_s$	mL sample	Volume of digested sample.
COD	mg/L O $_2$	Multiply by dilution factors if the sample was diluted before digestion.

Worked examples

Municipal wastewater
$A = 12.40\\ \\text{mL}$ , $B = 3.25\\ \\text{mL}$ , $N = 0.250\\ \\text{N}$ , $V_s = 50.0\\ \\text{mL}$ .

COD = \\frac{(12.40 - 3.25) \\times 0.250 \\times 8000}{50.0} = 3.66 \\times 10^{2}\\ \\text{mg}\\,\\text{L}^{-1}

Report 366 mg/L COD.

High-strength food process water
$A = 9.85\\ \\text{mL}$ , $B = 1.10\\ \\text{mL}$ , $N = 0.100\\ \\text{N}$ , $V_s = 25.0\\ \\text{mL}$ .

COD = \\frac{(9.85 - 1.10) \\times 0.100 \\times 8000}{25.0} = 2.80 \\times 10^{2}\\ \\text{mg}\\,\\text{L}^{-1}

If this exceeds permit limits, dilute before digestion or adjust the process.

Tips and pitfalls

Always run reagent blanks; if $A$ drifts, re-make digestion reagents.
Normalize FAS normality weekly using primary standard sodium oxalate.
Dilute samples with COD above about 900 mg/L so the dichromate remains in excess.
Keep digestion tubes capped tightly; evaporative loss changes $V_s$ and inflates COD.

Quick Summary

Calculator Tool

How this is calculated

What these terms mean

Normality of FAS (N)

Chemical oxygen demand (COD)

FAS for blank (A)

FAS for sample (B)

Volume of sample

When to use this calculator

Chemical oxygen demand calculator explained

How the conversion works

Units and conversions

Worked examples

Tips and pitfalls

References and further reading

Frequently Asked Questions

Related Calculators

Neutralization

Normality

pKa

Beer-Lambert law

Molarity

Mole

Was this calculator helpful?

Found an error?

Calculator info

Chemical oxygen demand calculator explained

How the conversion works

Units and conversions

Worked examples

Tips and pitfalls

References and further reading

Frequently Asked Questions

Related Calculators

Neutralization

Normality

pKa

Beer-Lambert law

Molarity

Mole

Was this calculator helpful?

Found an error?

Calculator info