Chemical Oxygen Demand (COD) Determination

Chemical oxygen demand is a measure of the oxygen required for the chemical oxidation of organic waste matter in water. Thus, COD assesses the quality of water and wastewater. The widely used method for chemical oxygen demand analysis is the wet chemical method, which involves high-temperature digestion in the presence of a strong oxidant (e.g. potassium dichromate, potassium iodate, potassium permanganate) under acidic conditions, where potassium dichromate is used as a common oxidant for organic matter in water samples. Silver sulfate is present as the catalyst, and mercuric sulfate serves to complex out any interfering chlorides. After the digestion, the extent of oxidation is measured by indirectly measuring the electrons consumed in the reduction of Cr⁶⁺ to Cr³⁺. This can be done by titration or spectrophotometry. BOC Sciences has accumulated many years of experience in wet chemical analysis. Our chemists are able to provide fast and simple determination of chemical oxygen demand in liquids and solids. Whether you want to determine COD in ultrapure water, drinking water, surface water, seawater, wastewater, soil eluate, soils, sediments or solid waste, we always have the perfect solution for your application.

Why Measure COD?

COD is used as a general indicator of water quality and is an important component of water quality management. In addition, COD is often used to estimate BOD (Biochemical Oxygen Demand) because of the strong correlation between COD and BOD, however COD is a much faster and more accurate test.

Advantages of COD

• COD test only requires 2-3 hours, while the Biochemical Oxygen Demand (BOD) test requires 5 days
• COD can be used to measure all organic contaminants, including those that are not biodegradable
• Unlike for the BOD test, toxic compounds (such as heavy metals and cyanides) in the samples to be analyzed do not have an effect on the oxidants used in the COD test. Therefore, the COD test can be used to measure the strength of wastes that are too toxic for the BOD test

Figure 1. Contribution to chemical oxygen demand (COD) by constituents of cabbage wash water. (Teng, Z.; et al. 2018)

COD Determination Workflow

• Firstly, we collect your sample in the appropriate size container (unpreserved or pre-preserved ), and properly label enough prepared COD vials for all samples and standards to be digested
• Then, a known excess of the oxidant (the reagent contains Cr(VI) or sulfuric acid) is added to the sample, and the sample is digested at 150°C for two hours. We also homogenize your sample by mixing or shaking and aliquot 2 mL of sample or standard into each vial
• After two hours remove the vials from the reactor and allow them to cool to room temperature. Once the oxidation is complete (converted to Cr(III) by the organic contaminants of the sample during the thermal pretreatment), the amount of oxidant remaining in the solution is estimated by titration using an indicator solution( such as using a ferroin indicator)
• Finally, the amount of Cr(VI) consumed is proportional to the amount of COD and can be directly detected by photometry (Set your spectrophotometer to the appropriate wavelength and read the absorbance)

Figure 2. Chemical oxygen demand measurement with dichromate oxidizing reagent. (Do Thi Kim Hue.; et al. 2017)

Quantitative Analysis of COD

• Titration method

EPA 410.3 (contains instructions for overcoming chlorides in excess of 2000 mg/L)

ASTM D1252-06(12) (a)

U.S. Geological Survey I-3560-85

• Photometric Method

EPA 410.4 (does not contain provisions for low concentration measurements)

SM5220 D-2011

ASTM D1252-06(12) (b)

U.S. Geological Survey I-3561-85