Chemical Oxygen Demand Test Calculation

COD Equation:

\[ COD = \frac{(blank - sample) \times normality \times 8 \times 1000}{volume} \]

Blank Titrant Volume:

Sample Titrant Volume:

Normality of Titrant:

Sample Volume:

Chemical Oxygen Demand (COD):

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1. What is Chemical Oxygen Demand (COD)?

Chemical Oxygen Demand (COD) is a measure of the amount of oxygen required to chemically oxidize organic matter in water. It's an important water quality parameter that indicates the degree of organic pollution in water samples.

2. How Does the Calculator Work?

The calculator uses the standard COD equation:

\[ COD = \frac{(blank - sample) \times normality \times 8 \times 1000}{volume} \]

Where:

\( blank \) — Volume of titrant used for blank (mL)
\( sample \) — Volume of titrant used for sample (mL)
\( normality \) — Normality of the titrant (N)
\( volume \) — Volume of water sample (mL)
8 — Equivalent weight of oxygen (g/eq)
1000 — Conversion factor (mg/g)

Explanation: The equation calculates the oxygen equivalent of the organic matter that can be chemically oxidized in the water sample.

3. Importance of COD Testing

Details: COD testing is crucial for assessing water quality, monitoring wastewater treatment efficiency, and ensuring compliance with environmental regulations. It provides a faster alternative to BOD (Biochemical Oxygen Demand) testing.

4. Using the Calculator

Tips: Enter all titration volumes in mL, ensure the normality is correct for your titrant (typically 0.25N or 0.1N for COD tests), and use the actual sample volume used in the test. All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What's the difference between COD and BOD?
A: COD measures all chemically oxidizable material while BOD measures only biologically oxidizable material. COD values are typically higher and results are available much faster (hours vs. days).

Q2: What are typical COD values for different water types?
A: Drinking water: 1-10 mg/L, Surface water: 10-50 mg/L, Raw sewage: 250-1000 mg/L, Industrial wastewater: up to 10,000+ mg/L.

Q3: Why is a blank titration needed?
A: The blank accounts for any oxidizable material in the reagents themselves, ensuring only the sample's oxygen demand is measured.

Q4: What are common interferences in COD testing?
A: Chlorides can interfere, which is why mercury sulfate is often added to the digestion solution to complex chlorides.

Q5: How should samples be preserved before COD testing?
A: Samples should be cooled to 4°C and tested within 7 days. Acidification with sulfuric acid to pH < 2 helps preserve samples.