Standard Methods: 5220D: Chemical Oxygen Demand
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Official Method Name
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Closed Reflux, Colorimetric Method |
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Current Revision
| Standard Methods Online |
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Media
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WATER |
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Instrumentation
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Spectroscopy (Colorimetry; Photometry) |
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Method Subcategory
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Organic |
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Method Source
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Citation
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Brief Method Summary
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When a sample is digested, the dichromate ion oxidizes COD material in the sample. This results in the change of chromium from the hexavalent (VI) state to the trivalent (III) state. Both of these chromium species are colored and absorb in the visible region of the spectrum. The dichromate ion absorbs strongly in the 400-nm region, where the chromic ion absorption is much less. The chromic ion absorbs strongly in the 600-nm region, where the dichromate has nearly zero absorption. |
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Scope and Application
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This method determines the COD of water samples. |
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Applicable Concentration Range
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> 50 mg O2/L |
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Interferences
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The most common interferent is the chloride ion. Chloride reacts with silver ion to precipitate silver chloride, and thus inhibits the catalytic activity of silver. |
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Quality Control Requirements
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Sample Handling
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Preferably collect samples in glass bottles. Test unstable samples without delay. If delay before analysis is unavoidable, preserve sample by acidification to pH less than or equal to 2 using conc sulfuric acid. Blend (homogenize) all samples containing suspended solids before analysis. |
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Maximum Holding Time
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28 days at 4oC |
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Relative Cost
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Less than $50 |
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Sample Preparation Methods
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