Chemical Oxygen Demand Calculator

How to use this chemical oxygen demand calculator

What this calculator does

This calculator estimates chemical oxygen demand, or COD, from a ferrous ammonium sulfate titration. COD expresses the oxygen-equivalent amount needed to oxidize susceptible material in a water or wastewater sample, reported here in milligrams per liter. It is useful for checking laboratory arithmetic, reviewing a worksheet, comparing samples, and documenting a titration result. It does not identify individual pollutants, certify drinking-water safety, replace blanks and quality-control samples, or determine whether a result complies with a particular permit.

When to use it

Use it when you have a blank titration reading, a sample titration reading, the standardized normality of FAS, and the original sample volume. Typical uses include checking a manual COD calculation after a dichromate reflux test, training students on the titrimetric equation, reviewing laboratory data before transcription, and comparing how a changed blank, sample endpoint, or titrant normality affects the reported oxygen demand. The U.S. EPA titrimetric COD method describes the underlying oxidation and back-titration approach.

How to calculate

1. The calculator opens with a complete demonstration set: 300 mL for the blank, 129 mL for the sample, FAS normality 3 N, and a 45,000 mL sample volume. Its result and a validated example Excel workbook are immediately available.
2. Replace each demonstration value with your laboratory readings. Use plain decimal notation with a period as the decimal separator; commas, scientific notation, unit text, and negative values are rejected.
3. Read the COD result in mg/L and review the calculation breakdown. The result updates as soon as all required fields are valid.
4. Select Download Excel to create a current-state OOXML workbook containing the inputs, formula steps, result, and notes.
5. Select Reset to clear the demonstration and all calculated output. Download Excel is then disabled until you enter another complete valid set.

Input guide

FAS for blank (A) is a required positive titration volume in milliliters. Enter a plain number such as 300. It should normally be greater than or equal to the sample reading; lowering A while other fields stay fixed lowers COD. A common mistake is swapping A and B, which creates a negative result and signals that the test or transcription should be reviewed.

FAS for sample (B) is the required positive FAS volume used for the digested sample, also in milliliters. A realistic example is 129. Increasing B reduces the A – B difference and therefore reduces COD. Do not enter burette start and end readings separately unless you have already converted them to the actual titrant volume used.

Normality of FAS (N) is a required positive decimal representing equivalents per liter. The demonstration uses 3 N. COD changes in direct proportion to normality, so a 1% change in N produces a 1% change in the calculated COD when all other values are unchanged. Use the standardized value for the batch rather than a nominal label when accuracy matters.

Volume of sample is the required original sample volume in milliliters and must be greater than zero. The example uses 45,000 mL to reproduce the demonstration calculation. A larger sample volume lowers the reported mg/L result because the same oxygen-equivalent demand is distributed over more sample. Do not enter liters without converting them to milliliters first.

Output guide

Chemical oxygen demand (COD) is the primary estimate in mg/L. It is driven by all four inputs. A value of zero occurs when A equals B; a negative mathematical result is treated as invalid because the sample titration should not exceed the blank under this model. Higher values indicate more oxidizable material under the test conditions, but they are not a direct inventory of specific contaminants.

FAS difference (A – B) shows the titrant-volume difference in mL and is the first arithmetic checkpoint. Oxygen-equivalent numerator shows (A – B) × N × 8000 before division by sample volume. The summary pills repeat the current COD, FAS difference, and validity state. The breakdown table lists each operation and value so the result can be audited line by line.

Worked example

With A = 300 mL and B = 129 mL, the difference is 171 mL. Multiplying by 3 N gives 513. Multiplying 513 by 8000 gives 4,104,000. Dividing by the 45,000 mL sample volume gives exactly 91.2 mg/L, displayed as 91.20 mg/L. The same typed values and unrounded result are written into the startup workbook.