Chemicals Used in the COD Test Method
The COD (Chemical Oxygen Demand) method is used to measure the amount of organic and Inorganic pollutants in water. It provides an estimate of the water quality and its pollution level by determining how much oxygen is required to chemically oxidize the organic substances present.
In the COD test method, various chemicals are added, each serving a specific purpose to ensure accurate measurement of the Chemical Oxygen Demand. Here is a breakdown of the key chemicals and their roles:
01. Potassium Dichromate (K₂Cr₂O₇):
- Purpose: It acts as a strong oxidizing agent to oxidize organic substances in the water sample. During the reaction, potassium dichromate is reduced, and organic matter is oxidized into carbon dioxide and water.
- Reaction:The dichromate is reduced to chromium (III) ions (Cr³⁺) during the oxidation process.
02. Sulfuric Acid (H₂SO₄):
- Purpose: Provides an acidic environment necessary for the oxidation reaction to occur efficiently. It also helps maintain the correct pH for the potassium dichromate to act as an effective oxidizer.
- Role in COD: The acid helps break down organic molecules and promotes the complete oxidation of organic matter.
03. Silver Sulfate (Ag₂SO₄):
- Purpose:It acts as a catalyst in the COD test. It enhances the oxidation of certain organic compounds, especially those that are difficult to oxidize, like certain fats, oils, and alcohols.
- Function: Silver sulfate speeds up the reaction between potassium dichromate and the organic matter.
04. Mercuric Sulfate (HgSO₄):
- Purpose: It helps to eliminate interference from chloride ions (Cl⁻), which can otherwise react with the potassium dichromate and give falsely high COD readings.
- Role in COD: Mercuric sulfate binds with chloride ions to form mercuric chloride, preventing chloride interference in the oxidation reaction.
05. Ferrous Ammonium Sulfate (FAS, Fe(NH₄)₂(SO₄)₂):
- Purpose:Used in the titration process to determine how much potassium dichromate was consumed during the oxidation of the organic matter.
- Role in COD: The remaining unreacted dichromate after oxidation is titrated with ferrous ammonium sulfate, and the amount of FAS used helps calculate the COD.
06. Indicator (Ferroin):
- Purpose: Acts as an indicator in the titration process. It changes color to signal the endpoint of the titration when all remaining dichromate has been reduced.
- Color Change:Typically changes from blue-green to reddish-brown, indicating the completion of the titration.
These chemicals work together to ensure that the COD test provides a reliable measure of the organic pollution in a water sample.