ASTM: D6238: Oxygen Demand in Water

Summary
Analytes
Revision
Data and Sites

Official Method Name

Standard Test Method for Total Oxygen Demand in Water

Current Revision

Current edition approved March 10, 1998. Published March 1999.

Media

WATER

Instrumentation

Total Oxygen Demand Instrument

Method Subcategory

Inorganic

Method Source

ASTM

Citation

Annual Book of ASTM Standards, Section 11, Water and Environmental Technology, Volume 11.02, Water (I)

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 900^oC 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 Concentration 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.

Quality Control 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 4^oC.
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 .

Maximum Holding Time

Relative Cost

$51 to $200

Sample Preparation Methods

This method has 1 analyte associated with it.

Analyte	Detection Level	Bias	Precision	Pct False Positive	Pct False Negative	Spiking Level
Total Oxygen Demand(N-55629) TOD Total Oxygen Demand	100.000 mg/L	100% Rec (ML)	5.00			525.00 mg/L

Precision Descriptor Notes:	An interlaboratory study was conducted to determine the precision and bias for the determination of Total Oxygen Demand (TOD) in water. Standards prepared in reagent water were sent to 8 laboratories for evaluation. One laboratory did not participate as their instrument was awaiting repairs (Lab 4). A second lab failed to get required recoveries (? 10 % on the practice sample and also failed the ranking test (Lab3). The data from the other six laboratories was used for this evaluation. Eight samples (four pairs) were analyzed at each laboratory for Total Oxygen Demand (TOD). The study samples included six samples made from potassium acid phthalate (KHP?NIST 84j) and two samples were made from glacial acetic acid (GFS Chemicals). In addition, three practice samples were sent with concentrations noted on the labels to ensure operator familiarity with the method.
Detection Level Note:	None.

Precision Descriptor Notes:

An interlaboratory study was conducted to determine the precision and bias for the determination of Total Oxygen Demand (TOD) in water. Standards prepared in reagent water were sent to 8 laboratories for evaluation. One laboratory did not participate as their instrument was awaiting repairs (Lab 4). A second lab failed to get required recoveries (? 10 % on the practice sample and also failed the ranking test (Lab3). The data from the other six laboratories was used for this evaluation. Eight samples (four pairs) were analyzed at each laboratory for Total Oxygen Demand (TOD). The study samples included six samples made from potassium acid phthalate (KHP?NIST 84j) and two samples were made from glacial acetic acid (GFS Chemicals). In addition, three practice samples were sent with concentrations noted on the labels to ensure operator familiarity with the method.

Detection Level Note:

None.

Revision	PDF/Link
Current edition approved March 10, 1998. Published March 1999.

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ASTM: D6238: Oxygen Demand in Water

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