NEMI Method Summary

Method Summary Information

Method Number: D6238 Media: WATER
Revision: Current edition approved March 10, 1998. Published March 1999.
Method Source: ASTM International
Subcategory: INORGANIC Analytes in this method
Official Name: Standard Test Method for Total Oxygen Demand in Water
Descriptive Name Oxygen Demand in Water
Source Info: ASTM International
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Citation: ASTM Volume 11.02
 
Brief Method
Summary:
 The total oxygen demand (TOD) measurement is achieved by continuous analysis of the concentration of oxygen in a combustion process gas effluent. The decrease in oxygen resulting from introduction of the sample into the combustion zone is a measure of oxygen demand.
The oxidizable components in a liquid sample introduced into a carrier gas stream containing a fixed amount of oxygen flowing through a 900oC combustion tube are converted to their stable oxides. The momentary reduction in the oxygen concentration in the carrier gas is detected by an oxygen sensor and indicated on a digital display or recorded. The TOD for the sample is obtained by comparing the peak height to a calibration curve of peak heights for TOD standard solutions. The TOD for the standard solution is based on experimentally observed reactions in which carbon is converted to carbon dioxide, hydrogen to water, combined nitrogen including ammonia to nitric oxide, and elemental or organic sulfur to sulfur dioxide. Sample injection is achieved by means of an automatic valve, that provides unattended multiple sampling in the laboratory or on-stream monitoring.
Scope And
Application:
 This test method covers the determination of total oxygen demand in the range from 100 to 100 000 mg/L, in water and wastewater including brackish waters and brines. Larger concentrations, or samples with high suspended solids, or both, may be determined by suitable dilution of the sample.
Applicable
Conc Range:
 100-100,000 mg/L
Interferences: The dissolved oxygen concentrations will contribute a maximum error of 8 ppm. This error is only significant on ranges below 0 to 100 ppm when samples have no dissolved oxygen (DO) content. When operating in this range and samples contain low DO concentrations then compensation may be necessary.
Sulfuric acid will normally decompose under sample combustion conditions. The oxygen release will result in a reduction in the TOD reading. However, alkali metal sulfates (that is, sodium and potassium salts) do not decompose under the combustion conditions. If sulfates are present in the samples, adjust to pH 11 with NaOH prior to analysis.
Nitrate salts decompose under sample combustion conditions. The resulting generation of oxygen reduces the oxygen demand.
Heavy metal ions have been reported to accumulate in the system resulting in a significant loss of sensitivity. The history of the combustion column appears to be a major factor contributing to interferences of this nature. Similarly, high concentrations of dissolved inorganic salts will tend to build up and coat the catalyst as indicated by a loss of sensitivity.
Some brackish waters and natural brines may exhibit base line drift. In such cases, continue to inject samples until a stable response is observed.
QC Requirements: Minimum quality control requirements are initial demonstration of proficiency, plus calibration verification, analysis of method blanks, quality control samples, and recovery spikes and, duplicate samples.
Sample Handling: Collect the sample in accordance with Specification D 1192 and Practices D 3370.
Because of the possibility of oxidation or bacterial decomposition of some components of aqueous samples, the time lapse between collection of samples and analysis must be kept to a minimum. After collection, keep the samples at approximately 4oC.
Sample preservation may also be accomplished by the addition of NaOH to a pH of 12 or higher, or HCl to a pH of 2 or lower. Do not use sulfuric acid or nitric acid to preserve the sample .
Max Holding Time:
 
Relative Cost: $51 to $200
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