Chemical Oxygen Demand Testing

In environmental analytical chemistry, the chemical oxygen demand (COD) is used to measure the amount of reducing substance that needs to be oxidized by chemical reactions in a measured solution. It is generally expressed in mass of oxygen consumed over volume of solution (mg/L). COD reflects the extent of the reducing substance pollution, and it is also one of the comprehensive indexes of the relative content of organics in water. It is an important organic pollution parameter in research of river pollution and industrial wastewater，because similar to BOD, it provides an index to assess the discharged wastewater effect on the receiving environment. Higher COD level means a greater amount of oxidizable organic material in the sample, which will reduce dissolved oxygen (DO) levels. A reduction in DO can lead to anaerobic conditions, which is deleterious to higher aquatic life forms. The COD test is often used as an alternate to BOD due to shorter length of testing time. The higher value of COD, the more serious the water pollution. Many governments have imposed strict regulations regarding the maximum COD allowed in waste water. For example, a maximum oxygen demand between 200 and 1000 mg/L must be reached before waste water or industrial water can be returned to the environment in Switzerland. What is more, it is also an index for drinking water quality.

The Environmental Analytical Testing Division at Alfa Chemistry is fully accredited at local and international level. It can provide a broad range of potable and non-potable water testing, including tap-water, drinking water, groundwater, surface water, seawater, wastewater, process water, and industrial cooling water. We can ensure the accuracy of the measurement results in a relatively short period of time using advanced instruments. Here are some methods to measure the COD.

The potassium dichromate titrimetric method of COD test

Fig. 1. The automatic potentiometric titrator

Potassium dichromate titrimetric method is a traditional method for Chemical Oxygen Demand analysis. The method uses a strong oxidizing chemical, potassium dichromate Cr₂O₇^2-, to oxidize the organic matter in solution to carbon dioxide and water under acidic conditions. In general, in order to ensure the complete oxidation of the organic material, the oxidant was added in excess. By determining the quantity of excess oxidant using a reducing agent such as ferrous ammonium sulfate, the COD value can be calculated. The automatic potentiometric titrator (Fig.1.) can be applied to detect COD in water to provide simple, fast and accurate results.

The colorimetric method of COD test

Fig. 2. The spectrophotometer

Similar to the titrimetric method, another way to determine the content of excess potassium dichromate after oxidation is offered by AB. Currently, potassium dichromate is a hexavalent chromium salt that is bright orange in color. Once dichromate oxidizes the organic material of sample, the hexavalent chromium (Cr₆⁺) will be converted to (Cr₃⁺), which is a dull green color. Therefore, a photometer or spectrophotometer can be used to measure the content of trivalent chromium (Cr₃⁺) in a sample after oxidation or the content of excess hexavalent chromium (Cr₆⁺) at an absorption wavelength of 600 nm. This method is more convenient compared to the titrimetric method since no reagents such as ferrous ammonium sulfate need to be premixed.

Feel free to contact us at any time for your COD testings. Our professional team will be glad to listen to your concerns and look forward to cooperating with you.

Reference

• L F Liao, X G Liu, F X Qiu and X L Xiao. Analytical Chemistry [M]. Wuhan: Huazhong University of Science and Technology Press, 2015: 325 (in Chinese).