Chemical Oxygen Demand Determination Procedure

Chemical Oxygen Demand Determination Procedure

Chemical Oxygen Demand (COD) is a critical measure for assessing water quality, particularly in wastewater management. This guide details the procedure for determining COD using potassium dichromate oxidation, following standards set by IS: 3025 (Part 58) and APHA methods. It includes essential information on sample handling, reagent preparation, and titration techniques. Ideal for environmental science students and professionals, this document provides insights into the significance of COD in evaluating organic pollutants in water. The methodology covers a range of applications, from industrial wastewater to surface water analysis.

Key Points

  • Explains the COD determination process using potassium dichromate.
  • Details sample handling and preservation techniques for accurate results.
  • Includes reagent preparation steps for reliable testing.
  • Covers titration methods and calculations for COD values.
  • newtopiccyclegrowin
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12.0 EXPERIMENT ON DETERMINATION OF
CHEMICAL OXYGEN DEMAND
Sl. No. Contents
Preamble
12.1 Aim
12.2 Introduction
12.2.1 Environmental Significance
12.3 Principle
12.4 Materials Required
12.4.1 Apparatus Required
12.4.2 Chemicals Required
12.5
Sample Handling and Preservation
12.5.1 Precautions
12.6 Procedure
12.6.1 Preparation of Reagents
12.6.2 Testing of Sample
12.7 Calculation
12.7.1 Table
12.7.2 Data Sheet
12.8 Interpretation of Results
12.9 Inference
12.10 Evaluation
12.0 EXPERIMENT ON DETERMINATION OF
CHEMICAL OXYGEN DEMAND
PREAMBLE:
“How to determine chemical oxygen demand in Water and Wastewater”.
Test procedure is in accordance to IS: 3025 (Part 58) - Reaffirmed 2006.
In addition to our Indian Standard, we also discuss in brief regarding the
procedure stated in
(1) APHA Standard Methods for the Examination of Water and Wastewater - 20
th
Edition. Method 5220 C.
(2) Methods for Chemical Analysis of Water and Wastes, EPA-600/4-79-020,
USEPA, Method 410.1.
12.1 AIM
To determine chemical oxygen demand in the given water sample with the
stipulations as per IS: 3025 (Part 58) - Reaffirmed 2006.
12.2 INTRODUCTION
Before performing this experiment, few questions may arise to the learners:
What is meant by chemical oxygen demand?
Why do we need to determine COD?
What are the methods available to measure COD?
Is it measured in water or wastewater?
Whether is it mandatory to determine COD as per our codal provision?
The chemical oxygen demand (COD) test is commonly used to indirectly
measure the amount of organic compounds in water. Most applications of COD
determine the amount of organic pollutants found in surface water (e.g. lakes and
rivers), making COD a useful measure of water quality. It is expressed in
milligrams per liter (mg/L), which indicates the mass of oxygen consumed per
liter of solution.
COD is the measurement of the amount of oxygen in water consumed for
chemical oxidation of pollutants.
COD determines the quantity of oxygen required to oxidize the organic matter in
water or waste water sample, under specific conditions of oxidizing agent,
temperature, and time.
This method covers the determination of COD in ground and surface waters,
domestic and industrial wastewaters. The applicable range is 3-900 mg/L.
12.2.1 ENVIRONMENTAL SIGNIFICANCE
COD values are particularly important in the surveys designed to determine and
control the losses to sewer systems.
The ratio of BOD to COD is useful to assess the amenability of waste for
biological treatment. Ratio of BOD to COD greater than or equal to 0.8 indicates
that wastewater highly polluted and amenable to the biological treatment.
It is useful to assess strength of wastes, which contain toxins and biologically
resistant organic substances.
COD can be related to TOC, however, does not account for oxidation state of the
organic matter.
BOD value is always lower than COD value. For domestic and some industrial
wastewater, COD value is about 2.5 times BOD value.
12.3 PRINCIPLE
The organic matter present in sample gets oxidized completely by potassium
dichromate (K
2
Cr
2
O
7
) in the presence of sulphuric acid (H
2
SO
4
), silver sulphate
(AgSO
4
) and mercury sulphate (HgSO
4
) to produce CO
2
and H
2
O. The sample is
refluxed with a known amount of potassium dichromate (K
2
Cr
2
O
7
) in the sulphuric
acid medium and the excess potassium dichromate (K
2
Cr
2
O
7
) is determined by
titration against ferrous ammonium sulphate, using ferroin as an indicator. The
dichromate consumed by the sample is equivalent to the amount of O
2
required
to oxidize the organic matter.
12.4 MATERIALS REQUIRED
12.4.1 APPARATUS REQUIRED
1. COD Digester
2. Burette & Burette stand
3. COD Vials with stand
4. 250 mL conical flask (Erlenmeyer Flask)
5. Pipettes
6. Pipette bulb
7. Tissue papers
8. Wash Bottle
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End of Document
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FAQs of Chemical Oxygen Demand Determination Procedure

What is Chemical Oxygen Demand and why is it important?
Chemical Oxygen Demand (COD) measures the amount of oxygen required to chemically oxidize organic matter in water. It is crucial for assessing water quality, particularly in wastewater, as it indicates the level of organic pollutants present. Higher COD values suggest greater pollution levels, which can impact aquatic life and water treatment processes. Understanding COD helps in evaluating the effectiveness of wastewater treatment and ensuring compliance with environmental regulations.
What are the key steps in the COD determination procedure?
The COD determination procedure involves several key steps: First, a water sample is mixed with potassium dichromate and sulfuric acid in a COD vial. The mixture is then heated in a COD digester at 150°C for two hours to facilitate oxidation. After cooling, the excess potassium dichromate is titrated with ferrous ammonium sulfate using ferroin as an indicator. The volume of ferrous ammonium sulfate used in the titration helps calculate the COD value of the sample.
How does the presence of chlorides affect COD measurements?
Chlorides can interfere with COD measurements as they are also oxidized by potassium dichromate, leading to inaccurate results. To mitigate this interference, mercuric sulfate is added to the digestion tubes. This complexing agent binds with chlorides, preventing them from reacting during the COD determination. By addressing this potential source of error, the procedure ensures more reliable and accurate COD results.
What is the significance of the BOD to COD ratio?
The ratio of Biochemical Oxygen Demand (BOD) to Chemical Oxygen Demand (COD) is significant in assessing the treatability of wastewater. A BOD/COD ratio of 0.8 or higher indicates that the wastewater is highly polluted and amenable to biological treatment. Conversely, a lower ratio suggests the presence of toxic or resistant organic compounds that may hinder biological treatment processes. This ratio helps environmental scientists and engineers evaluate the effectiveness of treatment methods.

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