Chemical Oxygen Demand COD Analyzer

Chemical Oxygen Demand is carried out according to two different methods and makes it possible to determine the organic matter content of water by chemical means. The first technique targets homogeneous liquids where a Spectroquant kit is used. The second method concerns solids and will also be used if more precision is required compared to the kit.

For a kit COD, the sample is added to the tubes containing a potassium dichromate solution K2Cr2O7 in the presence of concentrated sulfuric acid H2SO4. After heating in a thermoreactor (2h at 148°C), the absorbance of the tube (Cr2O72- is orange and the Cr3+ product is green) is measured in a specific spectrophotometer which provides the result in mg_DCO/L.

For a COD of solids or more precise, a conventional method is used based on the Belgian standard "NBN T 91-201 - Water analysis".

The materials which can be oxidized under the conditions of the test are oxidized in an acid medium (concentrated sulfuric acid H2SO4) by an excess of potassium dichromate solution K2Cr2O7 in the presence of mercury sulphate which forms a complex with the chlorides (allowing them to be trapped and avoiding interference) and the catalyst, silver sulphate.

A reflux heating step is carried out for 30 minutes (15 minutes for liquids). The excess potassium dichromate is titrated with a solution of iron and ammonium sulphate (Mohr salt) in the presence of an indicator, iron orthophenanthroline (ferroin).

The oxidation is more extensive than that obtained with potassium permanganate (see NBN T91-202) although all the organic materials are not completely oxidized.

Mineral reducers (ferrous iron, sulphides, sulphites, nitrites ...) being also oxidized, can interfere.

The result is expressed in milligrams of oxygen consumed per liter (mg_DCO/L) for liquids or in grams of oxygen consumed per grams of material (g_DCO/g) for solids.