Water for COD - Chemical Oxygen Demand

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Impact of Water

Water is used in COD tests for blank preparation, sample dilutions and glassware rinsing. The following water contaminants may have effects on the COD test:
  • Organics
    Since the COD test measures the oxygen demand of organic compounds in a sample of water, it is important that no outside organic material be accidentally added to the sample to be measured, as it may give falsely high results.
  • Bacteria
    It is best to minimize their levels in the dilution water, as they may release organics. 
  • Ions
    Oxidizable inorganic materials may interfere with the determination of COD. They may be oxidized by dichromate and give erroneously high COD results: chlorides are often the most serious source of interference. Nitrites, sulfides and disulfides, sulfites, thiosulfates and ferrous ions may also interfere with the COD test. 
Distilled water has been used to prepare dilution water; however, chlorine may co-distil with water and interfere with the COD test. Therefore, distilled water would require an additional sodium thiosulfate treatment. Distillation from alkaline permanganate was sometimes recommended, but this purification procedure is quite cumbersome as well.

Deionized water, purified only by ion exchange resins, may contain organics and microorganisms, thereby causing high COD results. Since the level of organic contamination of deionized water may vary with time, it is not recommended for this test.

Water purified with a combination of technologies, such as reverse-osmosis, ion exchange and/or electrodeionization and activated carbon is low in organics and oxidizable inorganic substances. It is therefore best fitted for the COD test.

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