Chemical Oxygen Demand: COD Test & Standard Methods

Introduction - COD is a slightly different category of test. There is no one COD atom or molecule. You can’t hold COD in your hand and point to it. It is also not necessarily a specific method defined parameter. It is simply a definition. COD is the amount of oxygen consumed when all materials present in a sample are fully oxidized. Note the distinction between this and the other oxygen demand test (BOD). COD is when all materials are fully chemically oxidized; BOD is only that material which is oxidized by (occasionally uncooperative) bacteria over a period of 5 days. Any method that is capable of oxidizing compounds and allows the amount of oxidation to be measured could theoretically be used to determine COD. The dichromate ion (Cr2O72-) has proven itself to be best suited for general purpose COD and is required in all approved methods.

Approved Methods – There are only two major distinctions in approved methods for COD

Titrimetric –

• EPA 410.3 (contains instructions for overcoming chloride in excess of 2000 mg/L)
• SM5220 C
• ASTM D1252-95, 00(A)
• USGS I-3560-85

Photometric –

• EPA 410.4 (does not contain provisions for low-level determinations)
• SM5220 D
• ASTM D1252-95, 00(B)
• USGS I-3561-85

Method Summary – An aliquot of the sample is digested for two hours at 150°C in the presence of dichromate and sulfuric acid. The resulting solution is titrated to a colored endpoint with ferroin indicator or read on a spectrophotometer at an appropriate wavelength.

Click here for a more detailed Lab Matters PDF on Understanding Chemical Oxygen Demand (COD)