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Organic compounds are found in large quantities in wastewater. The quantity of these compounds can be found out by determining the wastewater COD. This is also a vital measurement in determining the water quality. In this article I discuss one of the reactions that laboratories use for determining the cod in wastewater.

In a solution with a strength of 50 % of sulphuric acid (H2SO4), potassium dichromate (K2Cr2O7) is used to oxidize the organic substances found in the sample of wastewater taken for testing. For this oxidation, the reaction is conducted at a reflux temperature. Since there can be an excess of the dichromate in the solution, it is usually titrated. The titration requires ferrous sulphate with a ferroin indicator. The potassium dichromate imparts an orange hue to the solution while the formation of potassium bisulfate (KHSO4) imparts a creamy color to the solution. The ferroin indicator is an important element in the determination of wastewater cod as the change in color to reddish-brown from bluish green indicates the exhaustion of the dichromates during the reaction.

Here’s the reaction:

K2Cr2O7 + 6FeSO + 8H2SO -> 2KHSO4 + Cr2(SO)3 + 3Fe2(SO) + 7H2O

The by products of this reaction are chromium sulfur trioxide, and iron sulfur monoxide.

In this reaction used for determining wastewater cod, silver sulfate is used as a catalyst. If there are chloride ions in the solution, then they react with mercuric sulfate to form mercurous chloride (Hg2Cl2). The formation of this mercurous chloride prevents the interference of other substances in the solution. The ferroin indicator used in the reaction is a composition of o-phenanthroline represented as (Fe(C12H8N2))2^+ and ferrous iron.

This is a start of a cyclical process. As a second stage of this process, the dichromate in the solution undergoes a reaction with the iron from the ferroin indicator that causes the o-phenanthroline to be released back to its original state. The result is that the excess iron left in the solution is free to react and this reaction forms the ferroin indicator. This completes the cyclical process. This will go on till there is no excess iron left in the solution.

The above reaction is one of the examples used in the measure of cod in wastewater. The cod has to be checked to be able to treat wastewater properly.

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