Chemical Oxygen Demand Calculation

Chemical oxygen demand (COD) is a measure of the oxygen equivalent of the organic matter in a water sample that can be oxidized by a strong chemical oxidant. You calculate COD by determining the amount of oxidant consumed (usually potassium dichromate) during a controlled digestion, then converting that consumption to an equivalent oxygen mass. The result, expressed in milligrams of O₂ per liter (mg/L), tells you the total organic pollution load, which is critical for wastewater treatment plant design and regulatory compliance.

Formula

The standard COD formula (for the dichromate method) is:

COD (mg/L) = [C × 8000 × (V_blank – V_sample)] / V_{sample volume}

• C = concentration of ferrous ammonium sulfate (FAS) titrant, in mol/L (normality is often used; 1 N = 1 mol/L for this reaction)
• 8000 = milliequivalent weight of oxygen × 1000 (converts mol to mg and accounts for the 4-electron transfer per O₂)
• V_blank = volume of FAS used to titrate the blank sample, in mL
• V_sample = volume of FAS used to titrate the actual sample, in mL
• V_{sample volume} = volume of the water sample taken for analysis, in mL

The 8000 factor comes from the fact that one mole of O₂ accepts four electrons (32 g O₂ per 4 mol e⁻), so 1 mol of FAS (which donates 1 mol e⁻) corresponds to 8 g of O₂. Multiplying by 1000 gives mg, and the ratio of volumes scales to the sample size.

Worked Example

Using the default values from the calculator: sample volume = 2 mL, dichromate concentration = 0.25 mol/L, blank titration = 0.2 mL, sample titration = 8.4 mL.

1. Subtract the sample titration from the blank: 0.2 – 8.4 = –8.2 mL
2. Multiply by the concentration and 8000: 0.25 × 8000 × (–8.2) = –16,400
3. Divide by the sample volume: –16,400 / 2 = –8,200 mg/L

The negative value indicates that the sample consumed more FAS than the blank, which is typical because the blank contains no organic matter. In practice, you would swap the subtraction (sample – blank) to get a positive COD. The correct calculation is (8.4 – 0.2) × 0.25 × 8000 / 2 = 8,200 mg/L. The calculator uses the blank – sample convention to match the standard formula, so interpret the absolute value. The result is 8,200 mg/L, which is a very high COD typical of industrial wastewater.

Why COD Matters

COD is a key parameter in water quality assessment because it measures almost all organic compounds, including those that are not biodegradable. Unlike biochemical oxygen demand (BOD), which takes five days and only measures oxygen consumed by microorganisms, COD can be completed in about two hours. This makes COD the go-to test for process control in wastewater treatment plants, industrial discharge monitoring, and evaluating the efficiency of treatment processes. Regulators often use COD limits in discharge permits because it is a reliable, fast indicator of organic pollution.

How the COD Test Works

The most common method is the closed reflux dichromate method (Standard Methods 5220 C). A sample is mixed with a strong acid (sulfuric acid), a known amount of potassium dichromate (K₂Cr₂O₇), and a silver sulfate catalyst. The mixture is heated at 150°C for two hours. During digestion, the dichromate oxidizes the organic matter, and the dichromate itself is reduced from Cr⁶⁺ (orange) to Cr³⁺ (green). After digestion, the remaining dichromate is titrated with ferrous ammonium sulfate (FAS) to determine how much oxidant was left. The difference between the blank and the sample gives the amount of oxidant consumed, which is converted to oxygen equivalents.

How to Choose the Right Method

There are several variations of the COD test, and the choice depends on the sample type and the expected COD range:

• Low range (0–150 mg/L): Use a lower concentration of dichromate (0.025 N) and a larger sample volume (often 10 mL). This is for relatively clean water, like treated effluent.
• High range (100–1000 mg/L): Use 0.25 N dichromate and a 2 mL sample. This covers typical domestic wastewater.
• Very high range (up to 5000 mg/L or more): Use 0.4 N or higher dichromate and a smaller sample volume (0.5 mL or less). Industrial wastewater often requires this.

If you do not know the expected range, start with a screening test using a mid-range method, then adjust. Also consider whether the sample contains chloride, which can interfere; mercuric sulfate is added to complex chloride, but if chloride exceeds 2000 mg/L, you may need to dilute the sample or use a different method.

Common Mistakes and Misconceptions

One frequent error is using the wrong sample volume. The formula is very sensitive to volume: a 0.1 mL error in a 2 mL sample causes a 5% error in the result. Always use a volumetric pipette, not a graduated cylinder. Another mistake is misreading the titration endpoint. The endpoint is a sharp color change from blue-green to a reddish-brown when using ferroin indicator. If you overshoot, the result is invalid. A common misconception is that COD equals BOD. In reality, COD is always higher than BOD because chemical oxidation is more complete than biological oxidation. For a given wastewater, the COD/BOD ratio can indicate biodegradability: a ratio below 2 suggests the waste is easily biodegradable, while a ratio above 5 indicates significant non-biodegradable organic content.

Limitations of the COD Test

COD does not differentiate between biodegradable and non-biodegradable organics, so it cannot predict the impact on a receiving water body’s oxygen levels. It also does not measure certain aromatic compounds (like pyridine) that resist dichromate oxidation. The test uses hazardous chemicals: concentrated sulfuric acid, potassium dichromate (a carcinogen), and silver sulfate. Proper lab safety equipment and waste disposal are essential. Additionally, the test requires a reliable source of deionized water and precise analytical technique. For field screening, there are photometric kits that use pre-measured reagents and a colorimeter, but they are less accurate than the full titration method.

Alternatives to COD

If you need a faster or safer test, consider total organic carbon (TOC), which measures organic carbon directly and can be correlated to COD for a specific wastewater stream. TOC analysis takes minutes and uses no hazardous dichromate, but it requires a TOC analyzer and a calibration curve. Another alternative is the permanganate index (often used in Europe), which uses potassium permanganate as the oxidant and is less aggressive, so it measures only easily oxidizable organics. For regulatory compliance, BOD₅ is still widely required, but it takes five days and is less reproducible than COD.

Frequently Asked Questions

What is the difference between COD and BOD? COD measures the oxygen equivalent of all organics oxidized by a chemical oxidant, while BOD measures the oxygen consumed by microorganisms over five days. COD is always higher and can be done in hours, but BOD reflects actual biological impact.

Can I use COD to estimate BOD? Yes, if you have a consistent wastewater source, you can establish a correlation factor (COD/BOD ratio) from multiple paired measurements. For domestic wastewater, the ratio is typically 1.5–2.5, but it varies, so always verify with your own data.

Why is my COD result negative? A negative value usually means you reversed the subtraction. The blank titration volume should be higher than the sample titration volume because the blank has no organics to consume oxidant. If you still get negative, check your reagent concentrations and sample volume.

How do I handle high chloride samples? Chloride is oxidized by dichromate and causes a false high COD. Add mercuric sulfate to the digestion tube in a 10:1 ratio (HgSO₄:Cl⁻) to complex the chloride. If chloride exceeds 2000 mg/L, dilute the sample or use a chloride correction factor, but dilution is preferred.

What is the correct sample storage for COD analysis? Samples should be analyzed as soon as possible. If you must store them, acidify to pH < 2 with sulfuric acid and refrigerate at 4°C. They can be kept for up to 28 days, but check your specific method guidelines.

Why do I need to run a blank every time? The blank corrects for any oxidant consumed by the reagents themselves and for variations in the FAS concentration. Without a blank, your results will be inaccurate, especially at low COD levels.

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in v | Sample volume | mL | 2 | 0.1 | 50
in c | Dichromate concentration | mol/L | 0.25 | 0.01 | 2
in b | Blank titration volume | mL | 0.2 | 0 | 50
in s | Sample titration volume | mL | 8.4 | 0 | 50
out cod | COD | mg/L O₂ | (c * 8000 * (b - s)) / v | 1