ASTM D6238-98(2003)
(Test Method)Standard Test Method for Total Oxygen Demand in Water
Standard Test Method for Total Oxygen Demand in Water
SIGNIFICANCE AND USE
The measurement of oxygen demand parameters is critical to the control of process wastewaters. Biochemical oxygen demand (BOD) and chemical oxygen demand (COD) analyzers have long time cycles and in the case of COD analyzers use corrosive reagents with the inherent problem of disposal. Total oxygen demand analysis is faster, approximately 3 min, and uses no liquid reagents in its analysis.
TOD can be correlated to both COD and BOD, providing effective on-line control.
TOD offers several features which make it a more attractive measurement than carbon monitoring using Total Carbon (TC) or Total Organic Carbon (TOC) analyzers. TOD is unaffected by the presence of inorganic carbon. TOD analysis will also indicate noncarbonaceous materials that consume or contribute oxygen. For example, the oxygen demand of ammonia, sulfite and sulfides will be reflected in the TOD measurement. Also, since the actual measurement is oxygen consumption, TOD reflects the oxidation state of the chemical compound (that is, urea and formic acid have the same number of carbon atoms, yet urea has five times the oxygen demand of formic acid).
SCOPE
1.1 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 (see 6.5). Larger concentrations, or samples with high suspended solids, or both, may be determined by suitable dilution of the sample.
1.1.1 Since the analysis is based on the change in oxygen reading of the carrier gas compared to that when a sample is introduced (see 4.1), the measurement range is a function of the amount of oxygen in the carrier gas. The higher the desired concentration range, the more oxygen required in the carrier gas. Under recommended conditions, the carrier gas concentration should be between two to four times the maximum desired oxygen demand.
1.1.2 The lower measurement range is limited by the stability of the baseline oxygen detector output. This signal is a function of the permeation system temperature, carrier gas flow rate, oxygen detector temperature, and reference sensor voltage. Combined, these variables limit the minimum recommended range to 2 to 100 mg/L.
1.1.3 The upper measurement range is limited by the maximum oxygen concentration in the carrier gas (100%). With the recommended conditions of carrier gas concentration being two to four times the maximum oxygen demand, this limits the maximum possible oxygen demand to between 250 000 to 500 000 mg/L. However, as a practical application to water analysis, this test method will consider a maximum range of 100 000 mg/L.
1.2 This test method is applicable to all oxygen-demanding substances under the conditions of the test contained in the sample that can be injected into the reaction zone. The injector opening limits the maximum size of particles that can be injected. If oxygen-demanding substances that are water-insoluble liquids or solids are present, a preliminary treatment may be desired. These pretreatment methods are described in Annex A2.
1.3 This test method is particularly useful for measuring oxygen demand in certain industrial effluents and process streams. Its application for monitoring secondary sewage effluents is not established. Its use for the monitoring of natural waters is greatly limited by the interferences defined in Section 6.
1.4 In addition to laboratory analysis, this test method is applicable to on-stream monitoring. Sample conditioning techniques for solids pretreatment applications are noted in Annex A2.
1.5 The values stated in SI units are to be regarded as the standard.
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
General Information
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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation:D6238–98 (Reapproved 2003)
Standard Test Method for
Total Oxygen Demand in Water
This standard is issued under the fixed designation D6238; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope streams. Its application for monitoring secondary sewage
effluentsisnotestablished.Itsuseforthemonitoringofnatural
1.1 This test method covers the determination of total
waters is greatly limited by the interferences defined in Section
oxygen demand in the range from 100 to 100 000 mg/L, in
6.
waterandwastewaterincludingbrackishwatersandbrines(see
1.4 In addition to laboratory analysis, this test method is
6.5). Larger concentrations, or samples with high suspended
applicable to on-stream monitoring. Sample conditioning tech-
solids, or both, may be determined by suitable dilution of the
niques for solids pretreatment applications are noted in Annex
sample.
A2.
1.1.1 Since the analysis is based on the change in oxygen
1.5 The values stated in SI units are to be regarded as the
reading of the carrier gas compared to that when a sample is
standard.
introduced(see4.1),themeasurementrangeisafunctionofthe
1.6 This standard does not purport to address all of the
amount of oxygen in the carrier gas. The higher the desired
safety concerns, if any, associated with its use. It is the
concentration range, the more oxygen required in the carrier
responsibility of the user of this standard to establish appro-
gas. Under recommended conditions, the carrier gas concen-
priate safety and health practices and determine the applica-
tration should be between two to four times the maximum
bility of regulatory limitations prior to use.
desired oxygen demand.
1.1.2 The lower measurement range is limited by the
2. Referenced Documents
stability of the baseline oxygen detector output. This signal is
2.1 ASTM Standards:
a function of the permeation system temperature, carrier gas
D888 Test Methods for Dissolved Oxygen in Water
flow rate, oxygen detector temperature, and reference sensor
D1129 Terminology Relating to Water
voltage. Combined, these variables limit the minimum recom-
D1192 Guide for Equipment for SamplingWater and Steam
mended range to 2 to 100 mg/L.
in Closed Conduits
1.1.3 The upper measurement range is limited by the
D1193 Specification for Reagent Water
maximum oxygen concentration in the carrier gas (100 %).
D2777 Practice for Determination of Precision and Bias of
With the recommended conditions of carrier gas concentration
Applicable Test Methods of Committee D19 on Water
being two to four times the maximum oxygen demand, this
D3370 Practices for Sampling Water from Closed Conduits
limits the maximum possible oxygen demand to between
D3856 Guide for Good Laboratory Practices in Laborato-
250 000 to 500 000 mg/L. However, as a practical application
ries Engaged in Sampling and Analysis of Water
to water analysis, this test method will consider a maximum
D5789 Practice for Writing Quality Control Specifications
range of 100 000 mg/L.
for Standard Test Methods for Organic Constituents
1.2 This test method is applicable to all oxygen-demanding
D5847 Practice for Writing Quality Control Specifications
substances under the conditions of the test contained in the
for Standard Test Methods for Water Analysis
sample that can be injected into the reaction zone. The injector
opening limits the maximum size of particles that can be
3. Terminology
injected. If oxygen-demanding substances that are water-
3.1 Definitions:
insoluble liquids or solids are present, a preliminary treatment
3.1.1 For definitions of terms used in this test method, refer
may be desired. These pretreatment methods are described in
to Terminology D1129.
Annex A2.
1.3 This test method is particularly useful for measuring
oxygen demand in certain industrial effluents and process
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
This test method is under the jurisdiction of ASTM Committee D19 on Water Standards volume information, refer to the standard’s Document Summary page on
andisthedirectresponsibilityofSubcommitteeD19.06onMethodsforAnalysisfor the ASTM website.
Organic Substances in Water. Withdrawn.
Current edition approved March 10, 1998. Published March 1999. DOI: Withdrawn. The last approved version of this historical standard is referenced
10.1520/D6238-98R03. on www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D6238–98 (2003)
3.2 Definitions of Terms Specific to This Standard: oxygen (DO) content. When operating in this range and
3.2.1 total oxygen demand (TOD)—the amount of oxygen samples contain low DO concentrations then compensation
required to convert the elements in compounds to their most may be necessary. Measure the dissolved oxygen (DO) in both
stable oxidized forms. solutions in accordance with Test Method D888. Adjust the
TOD result as follows: If DO of the sample is less than in the
4. Summary of Test Method standard, subtract DO variation. If DO of the sample is greater
than in the standard, add DO variation to the TOD result.
4.1 The total oxygen demand (TOD) measurement is
6.2 Sulfuric acid will normally decompose under sample
achievedbycontinuousanalysisoftheconcentrationofoxygen
combustion conditions as follows:
in a combustion process gas effluent. The decrease in oxygen
resulting from introduction of the sample into the combustion 900°C 1
H SO H O 1 SO 1 O (1)
2 4 2 2 2
Catalyst 2
zone is a measure of oxygen demand.
4.2 The oxidizable components in a liquid sample intro-
The oxygen release will result in a reduction in the TOD
duced into a carrier gas stream containing a fixed amount of
reading. However, alkali metal sulfates (that is, sodium and
oxygen flowing through a 900°C combustion tube are con-
potassium salts) do not decompose under the combustion
verted to their stable oxides. The momentary reduction in the
conditions. If sulfates are present in the samples, adjust to pH
oxygen concentration in the carrier gas is detected by an
11 with NaOH prior to analysis.
oxygen sensor and indicated on a digital display or recorded.
6.3 Nitrate salts decompose under sample combustion con-
4.3 The TOD for the sample is obtained by comparing the
ditions as follows:
peak height to a calibration curve of peak heights for TOD
900°C 1
standard solutions. The TOD for the standard solution is based 2 NaNO Na O 12NO 1 1 O (2)
3 2 2
Catalyst 2
on experimentally observed reactions in which carbon is
The resulting generation of oxygen reduces the oxygen
converted to carbon dioxide, hydrogen to water, combined
demand.
nitrogen including ammonia to nitric oxide, and elemental or
6.4 Heavy metal ions have been reported to accumulate in
organic sulfur to sulfur dioxide. Sample injection is achieved
the system resulting in a significant loss of sensitivity. The
by means of an automatic valve, that provides unattended
history of the combustion column appears to be a major factor
multiple sampling in the laboratory or on-stream monitoring.
contributing to interferences of this nature. Similarly, high
4.4 For monitoring applications, pretreatment of the sample
concentrationsofdissolvedinorganicsaltswilltendtobuildup
may be required. However, no single instruction can be written
and coat the catalyst as indicated by a loss of sensitivity. To
since pretreatment steps will be a function of the specific
correct the problem, replace the combustion tube and refrac-
characteristics of the sample stream.
tory packing material and clean the catalyst in accordance with
the manufacturer’s recommendations. The effects of these
5. Significance and Use
problems can be minimized by dilution of the sample.
5.1 The measurement of oxygen demand parameters is
6.5 Some brackish waters and natural brines may exhibit
critical to the control of process wastewaters. Biochemical
base line drift. In such cases, continue to inject samples until a
oxygen demand (BOD) and chemical oxygen demand (COD)
stable response is observed.
analyzers have long time cycles and in the case of COD
analyzers use corrosive reagents with the inherent problem of
7. Apparatus
disposal. Total oxygen demand analysis is faster, approxi-
7.1 Total Oxygen Demand Instrument—(SeeFig.1),includ-
mately 3 min, and uses no liquid reagents in its analysis.
ing a pure nitrogen source, an oxygen permeation system,
5.2 TOD can be correlated to both COD and BOD, provid-
sample injection valve, catalyst-combustion zone, gas flow
ing effective on-line control.
controls, oxygen sensor and display or recorder, as detailed in
5.3 TOD offers several features which make it a more
Annex A2.
attractive measurement than carbon monitoring using Total
7.2 Homogenizing Apparatus—A high speed blender, or a
Carbon (TC) or Total Organic Carbon (TOC) analyzers. TOD
mechanical or ultrasonic homogenizer is satisfactory for ho-
is unaffected by the presence of inorganic carbon. TOD
mogenizing immiscible liquid samples and suspended solids
analysis will also indicate noncarbonaceous materials that
(see Annex A1).
consume or contribute oxygen. For example, the oxygen
demandofammonia,sulfiteandsulfideswillbereflectedinthe
8. Reagents and Materials
TOD measurement. Also, since the actual measurement is
8.1 Purity of Reagents—Reagent grade chemicals shall be
oxygen consumption, TOD reflects the oxidation state of the
used in all tests. Unless otherwise indicated, it is intended that
chemical compound (that is, urea and formic acid have the
all reagents shall conform to the specifications of the Commit-
same number of carbon atoms, yet urea has five times the
tee onAnalytical Reagents of theAmerican Chemical Society,
oxygen demand of formic acid).
6. Interferences
The sole source of supply of the apparatus known to the committee at this time
is Ionics, Inc., P.O. Box 9131, 65 Grove Street, Watertown, MA 02272. If you are
6.1 The dissolved oxygen concentrations will contribute a
aware of alternative suppliers, please provide this information toASTM Headquar-
maximum error of 8 ppm. This error is only significant on
ters. Your comments will receive careful consideration at a meeting of the
ranges below 0 to 100 ppm when samples have no dissolved responsible technical committee that you may attend.
D6238–98 (2003)
FIG. 1 Flow Diagram for TOD Analyzer
where such specifications are available. Other grades may be 8.4.1 Potassium Acid Phthalate (KHP) Solution Stock—
used, provided it is first ascertained that the reagent is of (10 000 mg/L TOD) Dissolve 8.509 g of potassium acid
sufficiently high purity to permit its use without lessening the phthalate (KHP) in water in a volumetric flask and dilute 1L.
accuracy of the determination. This solution is stable for several weeks at average room
8.2 Purity of Water—Unless otherwise indicated, references temperature but is eventually subject to bacteriological dete-
towatershallbeunderstoodtomeanreagentwaterconforming rioration. Refrigeration extends the shelf-life.
to Specification D1193, Type II except that distillation is not 8.4.2 Acetic Acid Solution, Stock—(111 900 mg/L TOD)
necessary. For calibration standards above 10 000 mg/L, the use of acetic
8.3 Carrier Gas Supply—Prepurified nitrogen containing acid is recommended. Pipet 100 mL of glacial acetic acid to a
oxidizable or reducible gases in concentrations of less than 10 1 L volumetric flask containing approximately 500 mL of
ppmisrecommended.Otherpureinertgases,suchasheliumor water. Dilute to 1 L with water and mix thoroughly. This
argon, are acceptable. The required oxygen is added to the solution is stable for several weeks at average room tempera-
carriergasbymeansofthepermeationsystemintheapparatus. ture but is eventually subject to bacteriological deterioration.
Alternatively, a bottled, fixed oxygen concentration carrier gas Refrigeration extends the shelf-life.
may be used in place of a permeation system. 8.4.3 Water solutions of other pure organic compounds may
8.4 Total Oxygen Demand Calibration Standard Solutions: be used as standards based on the compound’s theoretical
oxygen demand.
8.4.4 Calibration Standards—Prepare by appropriate dilu-
Reagent Chemicals, American Chemical Society Specifications, American
tion of the above stock solutions.
Chemical Society, Washington, DC. For suggestions on the testing of reagents not
listed by the American Chemical Society, see Analar Standards for Laboratory
9. Sampling
Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia
9.1 Collect the sample in accordance with Specification
and National Formulary, U.S. Pharmaceutical Convention, Inc. (USPC), Rockville,
MD. D1192 and Practices D3370.
D6238–98 (2003)
9.2 Because of the possibility of oxidation or bacterial 11.4 Prepare a standard curve by plotting mg/LTOD versus
decomposition of some components of aqueous samples, the peak height on rectangular coordinate paper.
time lapse between collection of samples and analysis must be 11.5 Use a single mid-range standard for checking calibra-
kept to a minimum. After collection, keep the samples at tion curve drift. If this result deviates more than 6 3 % then
approximately 4°C. re-adjust the analyzers’ calibration settings.
9.3 Sample preservation may also be accomplished by the 11.6 Afterservicingtheanalyzer,orreplacingcarriergas,or
addition of NaOH to a pH of 12 or higher, or HCl to a pH of replacing catalyst, perform a complete 5 point Calibration as
2 or lower. Do not use sulfuric acid or nitric acid to preserve described in 11.1-11.4.
the sample (see Section 6). 11.7 Modern instrumentation may use an integral computer
to automatically handle the data from the above step. Follow
10. Preparation of Apparatus
the manufacturer’s instructions for handling this data.
10.1 Provide required services and adjust variables (carrier
12. Procedure
gas flow rate, permeation tube lengths etc.) according to
manufacturer’s specifications for the desired oxygen demand
12.1 Laboratory Analysis:
range. Set the furnace temperature to the specified temperature
12.1.1 Dilute samples with reagent water, if required, to
setting. Allow approximately 1 h for the instrument to ap-
bring the homogeneous oxygen demand level within the
proach equilibrium.
selected operating range.
10.2 Monitoroxygensensoroutputstabilityasanindication
12.1.2 I
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