FULL FORM OF COD IN CHEMISTRY

INTRODUCTION

COD – “Chemical Oxygen Demand”

A chemical oxygen demand measuring parameter is used to estimate how much oxygen is needed to oxidize the organic and inorganic materials in a water sample. It is measured using particular chemical assays and frequently used as a gauge for the degree of organic contamination in water bodies.

Water contamination can be caused by organic materials such hydrocarbons, proteins, lipids, and carbohydrates. These organic substances are biodegraded by bacteria when they get into water bodies. Microbes use organic material as a food supply throughout this process. The amount and type of organic molecules in the water directly affect the oxygen consumption rate.

COD Testing Process

1. COD testing uses a chemical oxidation procedure to simulate the organic matter's natural decay. In the COD analysis, potassium dichromate (K2Cr2O7) is frequently utilized as an oxidizing agent. Dichromate combines with organic chemicals in the water sample, enabling them to oxidize in the presence of a potent acid.
2. The consequence is the reduction of the orange-coloured dichromate ions (Cr2O7-2) to the green chromium ions (Cr3+). Dichromate ions are consumed in proportion to the amount of organic matter present, making it possible to calculate COD.
3. Before performing a COD test, a portion of the water sample must first be taken and filtered to get rid of any suspended solids. A known volume of acidified potassium dichromate solution is added to the filtered sample following filtering. The oxidation reactions are promoted by the acidic environment that the acidified medium provides.
4.  The combination is heated for a predetermined amount of time, usually 2 hours, at a predetermined temperature, often between 150°C and 160°C. The oxidation process accelerated by the increased temperature ensures the complete decomposition of organic components.
5. Organic molecules in the water sample are broken down during digestion into carbon dioxide (CO2) and water (H2O). As they interact with the organic substance, the dichromate ions reduce while acting as an electron acceptor. An appropriate indicator or a spectrophotometer is then used to measure the change in dichromate ion concentration.

Equations involved in calculation of cod

Several formulae are used in the Chemical Oxygen Demand (COD) analysis to compute and determine the COD value. Potassium dichromate (K2Cr2O7) oxidizing organic molecules in the presence of acid is the main reaction in COD measurement. This reaction's general equation can be written as:

CnHmOo + Cr2O7^2- + H+ -> CO2 + H2O + Cr^3+

• Here, the organic component undergoing oxidation is symbolized by CnHmOo. H+ stands in for the acid in the solution, and the dichromate ion (Cr2O7-2) serves as the oxidizing agent. Carbon dioxide (CO2), water (H2O), as well as reduced chromium ions (Cr3+) are the products of the process.

Two widely used techniques—the titrimetric approach and the spectrophotometric method—are used to assess the concentration of dichromate ions that remain after the reaction.

1. Titrimetric Method

In this approach, excess dichromate ions are titrated using a reducing agent, commonly ferrous ammonium sulphate (FAS) or ferrous sulphate (FeSO4). The titrimetric approach uses the formula.

Cr2O7--> + 6Fe+ + 14H+ -> 2Cr+-> + 6Fe+-> + 7H2O

(reaction between dichromate ions & ferrous ions).

6Fe^2+ + 6Fe^3+ -> 12Fe^2+

(Redox reaction between ferrous ions and ferric ions)

The quantity of reducing agent needed to finish the titration defines the concentration of remaining dichromate ions, which is used to compute the COD value.

2. Spectrophotometric Method

The spectrophotometric approach calculates the remaining concentration of dichromate ions using the sample's absorbance at a certain wavelength. The following equation represents the interaction between the organic component and the dichromate ions:

CnHmOo + Cr2O7^2- + 8H+ -> nCO2 + mH2O + 2Cr^3+

The intensity of the solution's color changes in direct proportion to the change in dichromate ion concentration. A spectrophotometer can be used to measure this at a specific wavelength, and the obtained absorbance is then used to calculate the COD value using a calibration curve.

• It's vital to remember that the actual COD analysis equations may change based on the method and reagents used in the lab. Various reducing agents, acid concentrations, and reaction circumstances can impact the stoichiometry of the reactions.
• To achieve accurate and dependable COD measurements, it is essential to adhere to the standardized procedures and guidelines offered by recognized organizations, such as the American Public Health Association (APHA) or the International Organization for Standardization (ISO).

CONCLUSION

The Chemical Oxygen Demand (COD) parameter measures the amount of oxygen needed to oxidize  organic and inorganic compounds in water. Scientists can determine pollution sources, gauge the extent of organic contamination, and develop effective remediation plans through COD analysis. COD testing uses chemical oxidation reactions & subsequent measurement techniques to reveal important information about the water's purity and environmental health.