ASTM D6238-98
(Test Method)Standard Test Method for Total Oxygen Demand in Water
Standard Test Method for Total Oxygen Demand in Water
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)
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Designation: D 6238 – 98
Standard Test Method for
Total Oxygen Demand in Water
This standard is issued under the fixed designation D 6238; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope streams. Its application for monitoring secondary sewage
effluents is not established. Its use for the monitoring of natural
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.
water and wastewater including brackish waters and brines (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 (see 4.1), the measurement range is a function of the
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
D 888 Test Methods for Dissolved Oxygen in Water
flow rate, oxygen detector temperature, and reference sensor
D 1129 Terminology Relating to Water
voltage. Combined, these variables limit the minimum recom-
D 1192 Specification for Equipment for Sampling Water
mended range to 2 to 100 mg/L.
and Steam
1.1.3 The upper measurement range is limited by the
D 1193 Specification for Reagent Water
maximum oxygen concentration in the carrier gas (100 %).
D 2777 Practice for Determination of Precision and Bias of
With the recommended conditions of carrier gas concentration
Applicable Test Methods of Committee D-19 on Water
being two to four times the maximum oxygen demand, this
D 3370 Practices for Sampling Water
limits the maximum possible oxygen demand to between
D 3856 Practice for Evaluating Laboratories Engaged in
250 000 to 500 000 mg/L. However, as a practical application
Sampling and Analysis of Water and Waste Water
to water analysis, this test method will consider a maximum
D 5789 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
D 5847 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 D 1129.
Annex A2.
3.2 Definitions of Terms Specific to This Standard:
1.3 This test method is particularly useful for measuring
3.2.1 total oxygen demand (TOD)—the amount of oxygen
oxygen demand in certain industrial effluents and process
required to convert the elements in compounds to their most
stable oxidized forms.
This test method is under the jurisdiction of ASTM Committee D-19 on Water
and is the direct responsibility of Subcommittee D19.06 on Methods for Analysis for
Organic Substances in Water. Annual Book of ASTM Standards, Vol 11.01.
Current edition approved March 10, 1998. Published March 1999. Annual Book of ASTM Standards, Vol 11.02.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
D 6238
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
achieved by continuous analysis of the concentration of oxygen 6.2 Sulfuric acid will normally decompose under sample
in a combustion process gas effluent. The decrease in oxygen combustion conditions as follows:
resulting from introduction of the sample into the combustion
900°C 1
H SO H O 1 SO 1 O (1)
zone is a measure of oxygen demand.
2 4 2 2 2
Catalyst 2
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.
4.3 The TOD for the sample is obtained by comparing the
6.3 Nitrate salts decompose under sample combustion con-
peak height to a calibration curve of peak heights for TOD
ditions as follows:
standard solutions. The TOD for the standard solution is based
900°C 1
on experimentally observed reactions in which carbon is
2 NaNO Na O 1 2NO 1 1 O (2)
3 2 2
Catalyst 2
converted to carbon dioxide, hydrogen to water, combined
nitrogen including ammonia to nitric oxide, and elemental or The resulting generation of oxygen reduces the oxygen
organic sulfur to sulfur dioxide. Sample injection is achieved demand.
by means of an automatic valve, that provides unattended
6.4 Heavy metal ions have been reported to accumulate in
multiple sampling in the laboratory or on-stream monitoring.
the system resulting in a significant loss of sensitivity. The
4.4 For monitoring applications, pretreatment of the sample
history of the combustion column appears to be a major factor
may be required. However, no single instruction can be written
contributing to interferences of this nature. Similarly, high
since pretreatment steps will be a function of the specific
concentrations of dissolved inorganic salts will tend to build up
characteristics of the sample stream.
and coat the catalyst as indicated by a loss of sensitivity. To
correct the problem, replace the combustion tube and refrac-
5. Significance and Use
tory packing material and clean the catalyst in accordance with
5.1 The measurement of oxygen demand parameters is
the manufacturer’s recommendations. The effects of these
critical to the control of process wastewaters. Biochemical
problems can be minimized by dilution of the sample.
oxygen demand (BOD) and chemical oxygen demand (COD)
6.5 Some brackish waters and natural brines may exhibit
analyzers have long time cycles and in the case of COD
base line drift. In such cases, continue to inject samples until a
analyzers use corrosive reagents with the inherent problem of
stable response is observed.
disposal. Total oxygen demand analysis is faster, approxi-
mately 3 min, and uses no liquid reagents in its analysis.
7. Apparatus
5.2 TOD can be correlated to both COD and BOD, provid-
ing effective on-line control.
7.1 Total Oxygen Demand Instrument—(See Fig. 1), includ-
5.3 TOD offers several features which make it a more
ing a pure nitrogen source, an oxygen permeation system,
attractive measurement than carbon monitoring using Total
sample injection valve, catalyst-combustion zone, gas flow
Carbon (TC) or Total Organic Carbon (TOC) analyzers. TOD
controls, oxygen sensor and display or recorder, as detailed in
is unaffected by the presence of inorganic carbon. TOD
Annex A2.
analysis will also indicate noncarbonaceous materials that
7.2 Homogenizing Apparatus—A high speed blender, or a
consume or contribute oxygen. For example, the oxygen
mechanical or ultrasonic homogenizer is satisfactory for ho-
demand of ammonia, sulfite and sulfides will be reflected in the
mogenizing immiscible liquid samples and suspended solids
TOD measurement. Also, since the actual measurement is
(see Annex A1).
oxygen consumption, TOD reflects the oxidation state of the
chemical compound (that is, urea and formic acid have the
8. Reagents and Materials
same number of carbon atoms, yet urea has five times the
oxygen demand of formic acid).
8.1 Purity of Reagents—Reagent grade chemicals shall be
used in all tests. Unless otherwise indicated, it is intended that
6. Interferences
all reagents shall conform to the specifications of the Commit-
6.1 The dissolved oxygen concentrations will contribute a
tee on Analytical Reagents of the American Chemical Society,
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
The sole source of supply of the apparatus known to the committee at this time
samples contain low DO concentrations then compensation
is Ionics, Inc., P.O. Box 9131, 65 Grove Street, Watertown, MA 02272. If you are
may be necessary. Measure the dissolved oxygen (DO) in both
aware of alternative suppliers, please provide this information to ASTM Headquar-
solutions in accordance with Test Method D 888. Adjust the
ters. Your comments will receive careful consideration at a meeting of the
TOD result as follows: If DO of the sample is less than in the responsible technical committee that you may attend.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
D 6238
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-
to water shall be understood to mean reagent water conforming rioration. Refrigeration extends the shelf-life.
to Specification D 1193, 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
ppm is recommended. Other pure inert gases, such as helium or 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-
carrier gas by means of the permeation system in the apparatus. 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. D 1192 and Practices D 3370.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
D 6238
9.2 Because of the possibility of oxidation or bacterial 11.4 Prepare a standard curve by plotting mg/L TOD 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 After servicing the analyzer, or replacing carrier gas, 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
gas flow rate, permeation tube lengths etc.) according to
12. Procedure
manufacturer’s specifications for the desired oxygen demand
range. Set the furnace temperature to the specified temperature
12.1 Laboratory Analysis:
...
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