The Chemical Oxygen Demand (COD) test is a vital analytical method used to measure the amount of oxygen required to chemically oxidize organic and inorganic matter present in a water sample. This test is crucial for assessing water quality and the pollution load in wastewater, such as domestic sewage.

Chemical Oxygen Demand (COD) Test Principle

The fundamental principle of the Chemical Oxygen Demand (COD) test involves the use of a strong chemical oxidizing agent, typically potassium dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$), under acidic conditions (sulphuric acid) and at high temperatures. This agent oxidizes most of the organic and some inorganic substances in the sample to carbon dioxide and water. The amount of oxygen equivalent to the consumed dichromate is then calculated. Since not all the added dichromate is consumed by the sample's oxidizable matter, the unreacted portion must be determined.

Reagents in Chemical Oxygen Demand Digestion

In the COD test, several specific reagents play crucial roles during the digestion process:

• Potassium Dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$): This is the primary oxidizing agent. A known excess amount of potassium dichromate is added to the domestic sewage sample. It reacts with and oxidizes the organic and oxidizable inorganic compounds present. The reaction can be generalized as:

  $\text{Organic Matter} + \text{Cr}_2\text{O}_7^{2-} + \text{H}^+ \xrightarrow{\text{Heat}} \text{CO}_2 + \text{H}_2\text{O} + \text{Cr}^{3+}$

• Sulphuric Acid ($\text{H}_2\text{SO}_4$): Concentrated sulphuric acid provides the highly acidic medium necessary for the efficient oxidation of organic matter by potassium dichromate. It also helps in maintaining the stability of the dichromate and acts as a solvent.
• Silver Sulphate ($\text{Ag}_2\text{SO}_4$): Silver sulphate serves as a catalyst. It aids in the oxidation of certain types of organic compounds, such as straight-chain aliphatic compounds and some aromatic compounds, which are otherwise resistant to oxidation by dichromate. Without the silver sulphate catalyst, the oxidation might be incomplete.
• Mercuric Sulphate ($\text{HgSO}_4$): This reagent is added to complex with chloride ions ($\text{Cl}^-$) that might be present in the domestic sewage sample. Chloride ions can also be oxidized by potassium dichromate, leading to an overestimation of the COD value. Mercuric sulphate prevents this interference by forming stable mercury-chloride complexes.

  $\text{Hg}^{2+} + 2\text{Cl}^- \rightarrow \text{HgCl}_2 \text{ (complex)}$

Titration with Ferrous Ammonium Sulphate (FAS)

After the digestion process is complete, the sample contains the unreacted (remaining) potassium dichromate, which did not oxidize any substances in the domestic sewage. To determine this unreacted amount, a back-titration is performed using a standard solution of ferrous ammonium sulphate (FAS), often called Mohr's salt ($\text{Fe(NH}_4)_2(\text{SO}_4)_2 \cdot 6\text{H}_2\text{O}$).

During the titration, the ferrous ions ($\text{Fe}^{2+}$) from the FAS solution react with the unreacted dichromate ions ($\text{Cr}_2\text{O}_7^{2-}$). The reaction is as follows:

$\text{Cr}_2\text{O}_7^{2-} \text{ (unreacted)} + 6\text{Fe}^{2+} + 14\text{H}^+ \rightarrow 2\text{Cr}^{3+} + 6\text{Fe}^{3+} + 7\text{H}_2\text{O}$

The endpoint of the titration is usually indicated by a color change using an indicator (like ferroin indicator), from blue-green to reddish-brown, marking the point where all the unreacted potassium dichromate has been consumed by the FAS.

By knowing the initial amount of potassium dichromate added and the amount that remained unreacted (determined by FAS titration), the amount of potassium dichromate that was consumed by the oxidizable matter in the domestic sewage sample can be calculated. This consumed amount is directly related to the Chemical Oxygen Demand.

Therefore, after the digestion of the domestic sewage sample, the subsequent titration with standard ferrous ammonium sulphate (FAS) is performed to determine the unreacted amount of potassium dichromate.