What is COD ? Write chemical substance is generally used in measurement ?

Explanation

### Step-by-Step Solution:

1. **Definition of COD**:
- COD stands for Chemical Oxygen Demand. It is a measure of the amount of oxygen required to chemically oxidize organic and inorganic matter in water.

2. **Purpose of COD Measurement**:
- COD is used to quantify the amount of oxidizable pollutants present in surface water or wastewater. It helps in assessing the level of pollution in water bodies.

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