ASTM D6238-98(2011)
(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 standard. No other units of measurement are included in this 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 regu...
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Designation: D6238 − 98 (Reapproved 2011)
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 1.3 This test method is particularly useful for measuring
oxygen demand in certain industrial effluents and process
1.1 This test method covers the determination of total
streams. Its application for monitoring secondary sewage
oxygen demand in the range from 100 to 100 000 mg/L, in
effluents is not established. Its use for the monitoring of natural
waterandwastewaterincludingbrackishwatersandbrines(see
waters is greatly limited by the interferences defined in Section
6.5). Larger concentrations, or samples with high suspended
6.
solids, or both, may be determined by suitable dilution of the
sample. 1.4 In addition to laboratory analysis, this test method is
1.1.1 Since the analysis is based on the change in oxygen applicable to on-stream monitoring. Sample conditioning tech-
reading of the carrier gas compared to that when a sample is niques for solids pretreatment applications are noted in Annex
introduced(see4.1),themeasurementrangeisafunctionofthe A2.
amount of oxygen in the carrier gas. The higher the desired
1.5 The values stated in SI units are to be regarded as
concentration range, the more oxygen required in the carrier
standard. No other units of measurement are included in this
gas. Under recommended conditions, the carrier gas concen-
standard.
tration should be between two to four times the maximum
1.6 This standard does not purport to address all of the
desired oxygen demand.
safety concerns, if any, associated with its use. It is the
1.1.2 The lower measurement range is limited by the
responsibility of the user of this standard to establish appro-
stability of the baseline oxygen detector output. This signal is
priate safety and health practices and determine the applica-
a function of the permeation system temperature, carrier gas
bility of regulatory limitations prior to use.
flow rate, oxygen detector temperature, and reference sensor
voltage. Combined, these variables limit the minimum recom-
2. Referenced Documents
mended range to 2 to 100 mg/L.
2.1 ASTM Standards:
1.1.3 The upper measurement range is limited by the
D888 Test Methods for Dissolved Oxygen in Water
maximum oxygen concentration in the carrier gas (100 %).
D1129 Terminology Relating to Water
With the recommended conditions of carrier gas concentration
D1192 Guide for Equipment for Sampling Water and Steam
being two to four times the maximum oxygen demand, this
in Closed Conduits (Withdrawn 2003)
limits the maximum possible oxygen demand to between
D1193 Specification for Reagent Water
250 000 to 500 000 mg/L. However, as a practical application
D2777 Practice for Determination of Precision and Bias of
to water analysis, this test method will consider a maximum
Applicable Test Methods of Committee D19 on Water
range of 100 000 mg/L.
D3370 Practices for Sampling Water from Closed Conduits
1.2 This test method is applicable to all oxygen-demanding
D3856 Guide for Management Systems in Laboratories
substances under the conditions of the test contained in the
Engaged in Analysis of Water
sample that can be injected into the reaction zone. The injector
D5789 Practice for Writing Quality Control Specifications
opening limits the maximum size of particles that can be
for Standard Test Methods for Organic Constituents
injected. If oxygen-demanding substances that are water-
(Withdrawn 2002)
insoluble liquids or solids are present, a preliminary treatment
D5847 Practice for Writing Quality Control Specifications
may be desired. These pretreatment methods are described in
for Standard Test Methods for Water Analysis
Annex A2.
1 2
This test method is under the jurisdiction of ASTM Committee D19 on Water For referenced ASTM standards, visit the ASTM website, www.astm.org, or
andisthedirectresponsibilityofSubcommitteeD19.06onMethodsforAnalysisfor contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Organic Substances in Water. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved May 1, 2011. Published June 2011. Originally the ASTM website.
approved in 1998. Last previous edition approved in 2003 as D6238 – 98 (2003). The last approved version of this historical standard is referenced on
DOI: 10.1520/D6238-98R11. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6238 − 98 (2011)
3. Terminology same number of carbon atoms, yet urea has five times the
oxygen demand of formic acid).
3.1 Definitions:
3.1.1 For definitions of terms used in this test method, refer
6. Interferences
to Terminology D1129.
6.1 The dissolved oxygen concentrations will contribute a
3.2 Definitions of Terms Specific to This Standard:
maximum error of 8 ppm. This error is only significant on
3.2.1 total oxygen demand (TOD)—the amount of oxygen
ranges below 0 to 100 ppm when samples have no dissolved
required to convert the elements in compounds to their most
oxygen (DO) content. When operating in this range and
stable oxidized forms.
samples contain low DO concentrations then compensation
may be necessary. Measure the dissolved oxygen (DO) in both
4. Summary of Test Method
solutions in accordance with Test Method D888. Adjust the
4.1 The total oxygen demand (TOD) measurement is TOD result as follows: If DO of the sample is less than in the
standard, subtract DO variation. If DO of the sample is greater
achievedbycontinuousanalysisoftheconcentrationofoxygen
in a combustion process gas effluent. The decrease in oxygen than in the standard, add DO variation to the TOD result.
resulting from introduction of the sample into the combustion
6.2 Sulfuric acid will normally decompose under sample
zone is a measure of oxygen demand.
combustion conditions as follows:
4.2 The oxidizable components in a liquid sample intro-
900°C 1
H SO H O1SO 1 O (1)
duced into a carrier gas stream containing a fixed amount of 2 4 2 2 2
Catalyst 2
oxygen flowing through a 900°C combustion tube are con-
The oxygen release will result in a reduction in the TOD
verted to their stable oxides. The momentary reduction in the
reading. However, alkali metal sulfates (that is, sodium and
oxygen concentration in the carrier gas is detected by an
potassium salts) do not decompose under the combustion
oxygen sensor and indicated on a digital display or recorded.
conditions. If sulfates are present in the samples, adjust to pH
4.3 The TOD for the sample is obtained by comparing the
11 with NaOH prior to analysis.
peak height to a calibration curve of peak heights for TOD
6.3 Nitrate salts decompose under sample combustion con-
standard solutions. The TOD for the standard solution is based
ditions as follows:
on experimentally observed reactions in which carbon is
converted to carbon dioxide, hydrogen to water, combined 900°C 1
2 NaNO Na O12NO11 O (2)
3 2 2
nitrogen including ammonia to nitric oxide, and elemental or Catalyst 2
organic sulfur to sulfur dioxide. Sample injection is achieved
The resulting generation of oxygen reduces the oxygen
by means of an automatic valve, that provides unattended
demand.
multiple sampling in the laboratory or on-stream monitoring.
6.4 Heavy metal ions have been reported to accumulate in
4.4 For monitoring applications, pretreatment of the sample
the system resulting in a significant loss of sensitivity. The
may be required. However, no single instruction can be written
history of the combustion column appears to be a major factor
since pretreatment steps will be a function of the specific
contributing to interferences of this nature. Similarly, high
characteristics of the sample stream.
concentrationsofdissolvedinorganicsaltswilltendtobuildup
and coat the catalyst as indicated by a loss of sensitivity. To
5. Significance and Use
correct the problem, replace the combustion tube and refrac-
5.1 The measurement of oxygen demand parameters is tory packing material and clean the catalyst in accordance with
critical to the control of process wastewaters. Biochemical the manufacturer’s recommendations. The effects of these
problems can be minimized by dilution of the sample.
oxygen demand (BOD) and chemical oxygen demand (COD)
analyzers have long time cycles and in the case of COD
6.5 Some brackish waters and natural brines may exhibit
analyzers use corrosive reagents with the inherent problem of
base line drift. In such cases, continue to inject samples until a
disposal. Total oxygen demand analysis is faster, approxi-
stable response is observed.
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-
ing a pure nitrogen source, an oxygen permeation system,
5.3 TOD offers several features which make it a more
sample injection valve, catalyst-combustion zone, gas flow
attractive measurement than carbon monitoring using Total
controls, oxygen sensor and display or recorder, as detailed in
Carbon (TC) or Total Organic Carbon (TOC) analyzers. TOD
Annex A2.
is unaffected by the presence of inorganic carbon. TOD
analysis will also indicate noncarbonaceous materials that
consume or contribute oxygen. For example, the oxygen
The sole source of supply of the apparatus known to the committee at this time
demandofammonia,sulfiteandsulfideswillbereflectedinthe
is Ionics, Inc., P.O. Box 9131, 65 Grove Street, Watertown, MA 02272. If you are
TOD measurement. Also, since the actual measurement is
aware of alternative suppliers, please provide this information toASTM Headquar-
oxygen consumption, TOD reflects the oxidation state of the
ters. Your comments will receive careful consideration at a meeting of the
chemical compound (that is, urea and formic acid have the responsible technical committee that you may attend.
D6238 − 98 (2011)
FIG. 1 Flow Diagram for TOD Analyzer
7.2 Homogenizing Apparatus—A high speed blender, or a 8.2 Purity of Water—Unless otherwise indicated, references
mechanical or ultrasonic homogenizer is satisfactory for ho- to water shall be understood to mean reagent water conforming
mogenizing immiscible liquid samples and suspended solids to Specification D1193, Type II except that distillation is not
(see Annex A1). necessary.
8.3 Carrier Gas Supply—Prepurified nitrogen containing
8. Reagents and Materials
oxidizable or reducible gases in concentrations of less than 10
8.1 Purity of Reagents—Reagent grade chemicals shall be
ppmisrecommended.Otherpureinertgases,suchasheliumor
used in all tests. Unless otherwise indicated, it is intended that
argon, are acceptable. The required oxygen is added to the
all reagents shall conform to the specifications of the Commit-
carriergasbymeansofthepermeationsystemintheapparatus.
tee onAnalytical Reagents of theAmerican Chemical Society,
Alternatively, a bottled, fixed oxygen concentration carrier gas
where such specifications are available. Other grades may be
may be used in place of a permeation system.
used, provided it is first ascertained that the reagent is of
8.4 Total Oxygen Demand Calibration Standard Solutions:
sufficiently high purity to permit its use without lessening the
8.4.1 Potassium Acid Phthalate (KHP) Solution Stock—
accuracy of the determination.
(10 000 mg/L TOD) Dissolve 8.509 g of potassium acid
phthalate (KHP) in water in a volumetric flask and dilute 1L.
5 This solution is stable for several weeks at average room
Reagent Chemicals, American Chemical Society Specifications, American
Chemical Society, Washington, DC. For suggestions on the testing of reagents not temperature but is eventually subject to bacteriological dete-
listed by the American Chemical Society, see Analar Standards for Laboratory
rioration. Refrigeration extends the shelf-life.
Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia
8.4.2 AceticAcid Solution, Stock—(111 900mg/LTOD)For
and National Formulary, U.S. Pharmaceutical Convention, Inc. (USPC), Rockville,
MD. calibration standards above 10 000 mg/L, the use of acetic acid
D6238 − 98 (2011)
is recommended. Pipet 100 mL of glacial acetic acid toa1L 10 000 mg⁄L stock standard solution into separate 100 mL
volumetric flask containing approximately 500 mL of water. volumetric flasks and dilute to volume with reagent water.
Dilute to 1 L with water and mix thoroughly. This solution is
11.2 In operation, standard (or sample) is drawn from the
stable for several weeks at average room temperature but is
sample/standard inlet tubing to a sample injection valve which
eventually subject to bacteriological deterioration. Refrigera-
delivers a 20 to 100 µL sample into the combustion chamber.
tion extends the shelf-life.
Insert the sample/standard inlet tubing into the container of
8.4.3 Water solutions of other pure organic compounds may
standard.
be used as standards based on the compound’s theoretical
oxygen demand.
NOTE 1—Nominal consumption 10 mL/analysis.
8.4.4 Calibration Standards—Prepare by appropriate dilu-
11.2.1 Introduce the highest concentration standard solution
tion of the above stock solutions.
for the selected range into the analyzer and run three peaks.
Adjust peak height by means of the calibration dial so that this
9. Sampling
standard reads 100 % of full scale.
9.1 Collect the sample in accordance with Specification
11.3 Successively introduce each standard to the sample/
D1192 and Practices D3370.
standard inlet tube and automatically run three replicates of
9.2 Because of the possibility of oxidation or bacterial
each standard. The peak should automatically return to base-
decomposition of some components of aqueous samples, the
line between injections. Replicates must agree within 63%of
time lapse between collection of samples and analysis must be
full scale.
kept to a minimum. After collection, keep the samples at
11.4 Prepare a standard curve by plotting mg/LTOD versus
approximately 4°C.
peak height on rectangular coordinate paper.
9.3 Sample preservation may also be accomplished by the
addition of NaOH to a pH of 12 or higher, or HCl to a pH of 11.5 Use a single mid-range standard for checking calibra-
tion curve drift. If this result deviates more than 6 3 % then
2 or lowe
...
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