For the COD test of sewage, organic matter is oxidized by Potassium dichromate (K 2Cr 2C 7) in the presence
Sulphuric Acid
Understanding the COD Test for Sewage
The Chemical Oxygen Demand (COD) test is a crucial environmental analysis method used to measure the total amount of oxygen required to chemically oxidize the organic and inorganic matter present in a water sample, such as sewage. It provides an indicator of the pollution load in the water.
In the standard COD test, a strong oxidizing agent is used to break down the substances in the water sample. The most common oxidizing agent employed is Potassium dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$).
Role of Potassium Dichromate in COD Test
Potassium dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$) acts as the primary oxidizing agent. It is a very strong oxidant, capable of oxidizing a wide range of organic compounds. The amount of dichromate consumed during the oxidation process is measured, which is then used to calculate the COD value of the sample.
The Importance of Acidic Medium in COD Test
For the oxidation reaction by Potassium dichromate to proceed effectively and completely, the reaction must be carried out in a strongly acidic medium. This acidic environment is essential for the dichromate ion ($\text{Cr}_2\text{O}_7^{2-}$) to function as a powerful oxidizing agent. The acidic conditions help to drive the reaction forward and ensure complete oxidation of the organic matter.
Identifying the Correct Acid for COD Test
The question asks for the acid used in the presence of Potassium dichromate for the COD test. Let's consider the common acids used or potentially considered in such tests:
- Sulphuric Acid ($\text{H}_2\text{SO}_4$): This is the standard acid used in the COD test. Concentrated sulphuric acid provides the necessary acidic medium and also acts as a catalyst, raising the temperature during mixing, which helps in the digestion process.
- Nitric Acid ($\text{HNO}_3$): While a strong acid and oxidizing agent, nitric acid is generally not used in the standard COD test with dichromate. It can itself react with organic matter, leading to inaccurate COD values.
- Hydrochloric Acid ($\text{HCl}$): Hydrochloric acid cannot be used in significant amounts in the COD test because the chloride ions ($\text{Cl}^-$) present can be oxidized by Potassium dichromate to chlorine ($\text{Cl}_2$). This reaction consumes dichromate, leading to an overestimation of the COD value. Mercuric sulfate is often added in the standard COD test to complex with chloride ions and minimize this interference.
- Acetic Acid ($\text{CH}_3\text{COOH}$): Acetic acid is a weak acid and does not provide the strong acidic environment required for the complete oxidation of organic matter by Potassium dichromate in the COD test.
Based on the standard methodology for the Chemical Oxygen Demand test using Potassium dichromate, concentrated Sulphuric acid is added to provide the necessary acidic conditions and facilitate the oxidation of organic matter.
| Acid | Suitability for COD Test | Reason |
|---|---|---|
| Sulphuric Acid ($\text{H}_2\text{SO}_4$) | Suitable (Standard) | Provides strong acidic medium, aids digestion. |
| Nitric Acid ($\text{HNO}_3$) | Not Suitable | Interferes as it's an oxidizing agent itself. |
| Hydrochloric Acid ($\text{HCl}$) | Not Suitable (Interferes) | Chloride ions are oxidized by dichromate. |
| Acetic Acid ($\text{CH}_3\text{COOH}$) | Not Suitable | Too weak an acid. |
Therefore, for the COD test of sewage, organic matter is oxidized by Potassium dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$) in the presence of Sulphuric Acid.
Revision Table: Key Components of COD Test
| Component | Role in COD Test |
|---|---|
| Potassium Dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$) | Primary Oxidizing Agent |
| Sulphuric Acid ($\text{H}_2\text{SO}_4$) | Provides Strong Acidic Medium; Aids Digestion |
| Silver Sulfate ($\text{Ag}_2\text{SO}_4$) | Catalyst for Oxidation of Aliphatic Compounds (commonly added) |
| Mercuric Sulfate ($\text{HgSO}_4$) | Masks Chloride Ion Interference (commonly added) |
Additional Information on COD Test Procedure
The COD test typically involves adding a known amount of Potassium dichromate and concentrated Sulphuric acid (often containing Silver sulfate as a catalyst and Mercuric sulfate to complex chlorides) to a sewage sample. The mixture is then heated under reflux for a specific period (usually 2 hours) to ensure complete oxidation. After digestion, the remaining unreacted dichromate is titrated with a standard solution of Ferrous ammonium sulfate ($\text{FAS}$). An indicator, such as Ferroin, is used to detect the endpoint. The amount of dichromate consumed is calculated, and from this, the COD of the sample is determined using appropriate stoichiometry and sample volume.
The COD value is usually expressed in milligrams of oxygen per liter (mg/L) or parts per million (ppm).
Important Questions from Quality and Characteristics of Sewage
Bosco has developed the following equation:
K = K’ + \(\frac{\nu}{H}\)η
Here, v = average velocity of the river stream, H = average depth of the river, and η = bed activity coefficient of the river.
In this equation:
High COD to BOD ratio of an organic pollutant represents
Consider the following statements:
1. Ammonia nitrogen is a measure of nitrogen present as ammonium hydroxide and ammonium salts. It will progressively decrease as sewage gets treated.
2. Organic nitrogen is the total nitrogenous matter in sewages excepting that present as ammonia nitrogen, nitrites and nitrates. It becomes ammonia in anaerobic decomposition and nitrites or nitrates in aerobic decomposition.
Which of the above statements is/are correct?A rapid test to indicate the intensity of pollution of water is-
Imhoff cone is used to measure-
