How to Measure Chemical Oxygen Demand
As mentioned before, COD measures organic matter by using a chemical oxidant. It’s critical that a strong enough oxidant is used to react with virtually all organic material in the sample. Historically, potassium permanganate filled this role, but it was found to be inconsistent in its ability to oxidize all the organic matter in a wide variety of waste samples.
Currently, most COD tests use potassium dichromate as the oxidant. Potassium dichromate is a hexavalent chromium salt that is bright orange in colour and is a very strong oxidant. Between 95-100% of organic material can be oxidized by dichromate. Once dichromate oxidizes a substance it’s converted to a trivalent form of chromium, which is a dull green colour.
Digestion is performed on the samples with a set amount of the oxidant, sulfuric acid, and heat (150°C). Metal salts are usually included to suppress any interferences and to catalyze the digestion. The digestion typically takes two hours to perform.
During the digestion, it’s necessary to have excess oxidant; this ensures complete oxidation of the sample. As a result, it’s important to determine the quantity of excess oxidant. The two most common methods for this are titration and colourimetry.
