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**:
- The primary purpose of measuring COD is to assess the water quality, particularly in wastewater treatment. It indicates how much oxygen is needed to break down pollutants in the water.

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