previousBiochemical Oxygen Demand (BOD) Total Organic Carbon (TOC)next
Chemical Oxygen Demand (COD)

The equivalent amount of oxygen required to oxidise organic matter present in a water sample by means of a strong chemical oxidising agent is called chemical oxygen demand (COD). COD is also a sum parameter and is used to measure the content of organic matter of wastewater. The COD values include the oxygen demand created by biodegradable as well as non-biodegradable substances. As a result, COD values are greater than BOD. In comparison with BOD5, COD measurement has an advantage in that it requires a short digestion period of about 3 hours rather than incubation of 5 days period required for BOD5 measurement. For many types of wastes, it is possible to correlate COD with BOD. Once the correlation has been established, COD measurements can be used to good advantage for treatment-plant control and operation.

In the COD method, not the product (CO2) formed by oxidation of the organic wastewater constituents is measured, but the consumption of the oxidant (calculated as oxygen O2). Thus, an exact amount of oxidant has to be used for the oxidation of the organics in a given volume of a wastewater sample, and the excessive oxidant which is not consumed for complete oxidation of organics must be quantified. Complete oxidation postulates that the only oxidation product formed is CO2 and not any organic intermediate with high carbon oxidation numbers. Although COD is given as mg of consumed oxygen per litre of wastewater, the oxidant used in the analytical procedure is not oxygen, but potassium dichromate (K2Cr2O7) in concentrated sulfuric acid. The substance K2Cr2O7 is a powerful oxidant in an acid milieu (H2Cr2O7 is formed) at elevated temperature (148°C). Oxidation is the abstraction of electrons from a substance that is oxidized. As one molecule K2Cr2O7 can accept 1.5 times more electrons than the molecule O2, this is considerated by calculation.

The analytical standard procedure prescribes to place 50 ml of the wastewater sample in a 500-ml-refluxing flask, to add 1 g of HgSO4 (caution: toxic!), 5 ml of a mixture of Ag2SO4 (which serves as a catalyst for the oxidation of the organics) in concentrated sulfuric acid (caution: corrosive!), to add subsequently 25 ml of a solution of the oxidant K2Cr2O7 (caution: powerful carcinogen!) in concentrated sulfuric acid and to heat the mixture under reflux after vigorous mixing for two hours. The K2Cr2O7 that has not been consumed for oxidation is then quantified by titration with an aqueous solution of Fe(NH4)2SO4 with known concentration. The residual K2Cr2O7 oxidizes the Fe2+ of the titrant Fe(NH4)2SO4 to give Fe3+. When all the residual K2Cr2O7 is consumed (reduced by Fe2+), the indicator ferroin, which has to be added to the wastewater/K2Cr2O7 mixture prior to titration, turns from blue-green to reddish brown. At this end-point of titration, the volume of the Fe(NH4)2SO4 is read from the burette and the residual amount of the oxidant K2Cr2O7 after oxidizing the organic constituents in the wastewater sample can be calculated. By this, the consumption of K2Cr2O7 during oxidation - and its oxygen equivalent - are calculated, giving the COD.

Besides problems with working safety (use of the carcinogenic K2Cr2O7), there are also some analytical problems, because K2Cr2O7 does not only oxidize organic, but also some inorganic molecules or ions. Chloride, which is a normal constituent of wastewaters, is oxidized by K2Cr2O7 forming Cl2 gas. In order to prevent oxidation of chloride, it is masked by the addition of HgSO4. Chloride bound to Hg2+ is not oxidized by K2Cr2O7. However, the addition of mercury sulfate to all samples for COD determinations generates a large amount of toxic waste in laboratories for wastewater analyses (some laboratories purify this liquid waste stream by ion exchange giving the ion exchange regenerates to recycling companies for mercury and also for silver and chromium recovery). If the chloride concentration in the wastewater sample exceeds 1 g/l, the chloride has to be removed prior to COD analysis from the sample by heating it after addition of sulfuric acid and removing the formed hydrochloric acid from the gas phase by absorption to alkaline materials. But there are other substances which can cause troubles with the COD analysis: If the wastewater contains e.g. bromide, iodide, sulfite, Fe2+, Co+ or hydrogen peroxide, these reducing agents will also be oxidized by K2Cr2O7. This K2Cr2O7 consumption, however, is not caused by organics leading to misinterpretations about the content of organics of the wastewater. Another problem is that not every organic substance is completely oxidized under the conditions of COD analysis. Many nitrogen-containing heterocycles (e.g. pyridine) consume significantly less K2Cr2O7 than theoretically assumed.

There are also cuvette tests for COD analyses available from several companies. These cuvette tests offer the advantage, that smaller volumes of reagents (and also of wastewater samples) are used limiting the toxic waste of laboratories

previousBiochemical Oxygen Demand (BOD) Total Organic Carbon (TOC)next