Standard Methods: 5220C: COD by Closed Reflux, Titration
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Official Method Name
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5220 C. Closed Reflux, Titrimetric Method |
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Current Revision
| Standard Methods 18th, 19th, 20th ed. |
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Media
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WATER |
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Instrumentation
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Not Applicable |
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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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Most types of organic matter are oxidized by a boiling mixture of chromic and sulfuric acids. A sample is refluxed in strongly acid solution with a known excess of potassium dichromate (K2Cr2O7). After digestion, the remaining unreduced K2Cr2O7 is titrated with ferrous ammonium sulfate to determine the amount of K2Cr2O7 consumed and the oxidizable organic matter is calculated in terms of oxygen equivalent. |
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Scope and Application
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This parameter is generally of interest in wastewaters and natural waters. The closed reflux method (5220 C) is economical in the use of metallic salt reagents, but requires homogenization of samples containing suspended solids to obtain reproducible results. |
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Applicable Concentration Range
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40 to 400 mg/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. Bromide, iodide, and any other reagent that inactivates the silver ion can interfere similarly. The difficulties caused by the presence of the chloride can be overcome largely, though not completely, by complexing with mercuric sulfate (HgSO4) before the refluxing procedure. Halide interferences may be removed by precipitation with silver ion and filtration before digestion. This approach may introduce substantial errors due to the occlusion and carrydown of COD matter from heterogenous samples. Nitrite exerts a COD of 1.1 mg O2/mg NO2- -N. Because concentrations of NO2- in waters rarely exceed 1 or 2 mg NO2- -N/L, the interference is considered insignificant and usually is ignored. To eliminate a significant interference due to NO2-, add 10 mg sulfamic acid for each mg NO2- -N present in the sample volume used; add the same amount of sulfamic acid to the reflux vessel containing the distilled water blank. Reduced inorganic species such as ferrous iron, sulfide, manganous manganese, etc., are oxidized quantitatively under the test conditions. |
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Quality Control Requirements
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See Section 5020 Quality Assurance/Quality Control |
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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 < or = 2 using conc H2SO4. Preferably acidify any sample that cannot be analyzed the same day it is collected. Blend samples containing settleable solids with a homogenizer to permit representative sampling. Make preliminary dilutions for wastes containing a high COD to reduce the error inherent in measuring small sample volumes. |
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Maximum Holding Time
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28 days (regulatory) |
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Relative Cost
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$51 to $200 |
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Sample Preparation Methods
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