ASTM D1498-14(2022)e1

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
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Designation: D1498 − 14 (Reapproved 2022)
Standard Test Method for
Oxidation-Reduction Potential of Water
This standard is issued under the fixed designation D1498; 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.
ε NOTE—An editorial update was made to Section 1 in November 2022.
1. Scope D3370 Practices for Sampling Water from Flowing Process
Streams
1.1 This test method covers the apparatus and procedure for
theelectrometricmeasurementofoxidation-reductionpotential
3. Terminology
(ORP) in water. It does not deal with the manner in which the
3.1 Definitions—For definitions of terms used in this test
solutions are prepared, the theoretical interpretation of the
method, refer to Terminology D1129.
oxidation-reduction potential, or the establishment of a stan-
dard oxidation-reduction potential for any given system. The
3.2 Definitions of Terms Specific to This Standard:
test method described has been designed for the routine and
3.2.1 oxidation-reduction potential, n—the electromotive
process measurement of oxidation-reduction potential.
force, E , developed between a noble metal electrode and a
m
standard reference electrode.
1.2 The values stated in SI units are to be regarded as
3.2.1.1 Discussion—This oxidation-reduction potential
standard. No other units of measurement are included in this
(ORP) is related to the solution composition by:
standard.
RT
1.3 This standard does not purport to address all of the
o
E 5 E 12.3 log A /A
@ #
m ox red
nF
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
where:
priate safety, health, and environmental practices and deter-
E = ORP,
m
mine the applicability of regulatory limitations prior to use.
o
E = constant that depends on the choice of refer-
1.4 This international standard was developed in accor-
ence electrodes,
dance with internationally recognized principles on standard-
F = Faraday constant,
ization established in the Decision on Principles for the
R = gas constant,
Development of International Standards, Guides and Recom-
T = absolute temperature, °C + 273.15,
mendations issued by the World Trade Organization Technical
n = number of electrons involved in process
Barriers to Trade (TBT) Committee.
reaction, and
A and A = activities of the reactants in the process.
ox red
2. Referenced Documents
4. Summary of Test Method
2.1 ASTM Standards:
D1129 Terminology Relating to Water 4.1 This is a test method designed to measure the ORP
D1193 Specification for Reagent Water which is defined as the electromotive force between a noble
D2777 Practice for Determination of Precision and Bias of metal electrode and a reference electrode when immersed in a
Applicable Test Methods of Committee D19 on Water solution. The test method describes the equipment available to
make the measurement, the standardization of the equipment
and the procedure to measure ORP. The ORP electrodes are
inert and measure the ratio of the activities of the oxidized to
This test method is under the jurisdiction of ASTM Committee D19 on Water
the reduced species present.
and is the direct responsibility of Subcommittee D19.03 on Sampling Water and
Water-Formed Deposits, Analysis of Water for Power Generation and Process Use,
On-Line Water Analysis, and Surveillance of Water.
5. Significance and Use
Current edition approved Nov. 1, 2022. Published December 2022. Originally
5.1 Various applications include monitoring the
approved in 1957. Last previous edition approved in 2014 as D1498 – 14. DOI:
10.1520/D1498-14R22E01.
chlorination/dechlorination process of water, recognition of
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
oxidants/reductants present in wastewater, monitoring the
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
cyclechemistryinpowerplants,andcontrollingtheprocessing
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. of cyanide and chrome waste in metal plating baths.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
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D1498 − 14 (2022)
5.2 ThemeasurementofORPhasbeenfoundtobeusefulin considered include, for example, the types of signals which the
the evaluation of soils, for evaluating treatment design data at analyzer can produce to drive external devices, and the span
sites contaminated with certain chemicals, and in evaluating
ranges available.
solid wastes.
7.1.2 ForremoteORPmeasurementsthepotentialgenerated
can be transmitted to an external indicating meter. Special
6. Interferences
shielded cable is required to transmit the signal.
6.1 The ORP electrodes reliably measure ORP in nearly all
7.2 Reference Electrode—A calomel, silver-silver chloride,
aqueous solutions and in general are not subject to solution
or other reference electrode of constant potential shall be used.
interference from color, turbidity, colloidal matter, and sus-
If a saturated calomel electrode is used, some potassium
pended matter.
chloride crystals shall be contained in the saturated potassium
chloride solution. If the reference electrode is of the flowing
6.2 The ORP of an aqueous solution is sensitive to change
junction type, a slow outward flow of the reference-electrode
in temperature of the solution, but temperature correction is
solution is desired.To achieve this, the solution pressure inside
rarely done due to its minimal effect and complex reactions.
the liquid junction should be somewhat in excess of that
Temperature corrections are usually applied only when it is
outside the junction. In nonpressurized applications this re-
desired to relate the ORP to the activity of an ion in the
quirement can be met by maintaining the inside solution level
solutions.
higherthantheoutsidesolutionlevel.Ifthereferenceelectrode
6.3 The ORP of an aqueous solution is almost always
is of the nonflowing junction type, these outward flow and
sensitive to pH variations even to reactions that do not appear
pressurization considerations shall not apply. The reference
toinvolvehydrogenorhydroxylions.TheORPgenerallytends
electrode and junction shall perform satisfactorily as required
to increase with an increase in hydrogen ions and to decrease
in the procedure for checking sensitivity described in 11.2.
with an increase in hydroxyl ions during such reactions.
7.3 Oxidation-Reduction Electrode—A noble metal is used
6.4 Reproducible oxidation-reduction potentials cannot be
in the construction of oxidation-reduction electrodes.The most
obtained for chemical systems that are not reversible. Most
common metals employed are platinum and gold; silver is
naturalandgroundwatersdonotcontainreversiblesystems,or
rarely used. It is important to select a metal that is not attacked
maycontainsystemsthatareshiftedbythepresenceofair.The
by the test solution. The construction of the electrode shall be
measurement of end point potential in oxidation-reduction
such that only the noble metal comes in contact with the test
titration is sometimes of this type.
solution. The area of the noble metal in contact with the test
6.5 If the metallic portion of the ORP electrode is sponge-
solution should be approximately 1 cm .
like, materials absorbed from solutions may not be washed
7.4 ElectrodeAssembly—Aconventionalelectrodeholderor
away, even by repeated rinsings. In such cases, the electrode
support can be employed for laboratory measurements. Many
may exhibit a memory effect, particularly if it is desired to
different styles of electrode holders are suitable for various
detect a relatively low concentration of a particular species
process applications such as measurements in an open tank,
immediately after a measurement has been made in a relatively
process pipe line, pressure vessel, or a high pressure sample
concentrated solution. A brightly polished metal electrode
line.
surface is required for accurate measurements.
6.6 The ORP resulting from interactions among several
8. Reagents and Materials
chemical systems present in mixed solutions may not be
8.1 Purity of Reagents—Reagent grade chemicals shall be
assignable to any single chemical.
used in all tests. Unless otherwise indicated, it is intended that
all reagents shall conform to the specifications of the Commit-
7. Apparatus
teeonAnalyticalReagentsoftheAmericanChemicalSociety.
7.1 Meter—Most laboratory pH meters can be used for
Other grades may be used, provided it is first ascertained that
measurements of ORP by substitution of an appropriate set of
the reagent is of sufficiently high purity to permit its use
electrodes and meter scale. Readability to 1 mV is adequate.
without lessening the accuracy of the determination.
The choice will depend on the accuracy desired in the
8.2 Purity of Water—References to water that is used for
determination.
reagent preparation, rinsing, or dilution shall be understood to
7.1.1 Most process pH meters can be used for measurement
mean water that conforms to the quantitative specifications for
of ORP by substitution of an appropriate set of electrodes and
type I or II reagent water of Specification D1193.
meter scale. These instruments are generally much more
rugged than those which are used for very accurate measure-
ments in the laboratory. Usually, these more rugged instru-
ments produce results that are somewhat less accurate and
ACS Reagent Chemicals, Specifications and Procedures for Reagents and
Standard-Grade Reference Materials, American Chemical Society, Washington,
precise than those obtained from laboratory instruments. The
DC. For suggestions on the testing of reagents not listed by theAmerican Chemical
choice of process ORP analyzer is generally based on how
Society, see Analar Standards for Laboratory Chemicals, BDH Ltd., Poole, Dorset,
closely the characteristics of the analyzer match the require-
U.K., and the United States Pharmacopeia and National Formulary, U.S. Pharma-
ments of the application. Typical factors which may be copeial Convention, Inc. (USPC), Rockville, MD.
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D1498 − 14 (2022)
TABLE 1 National Institute of Standards and Technology (NIST) TABLE 2 Potential of the Platinum Electrode for Several
Materials for Reference Buffer Solutions Reference Electrodes at 25 °C in Ferrous-Ferric Reference
Solution
NIST Standard
Sample Buffer Salt Drying Procedure
Reference Electrode Potential EMF,
A
Designation
mV
186-II-e disodium hydrogen phosphate 2 h in oven at 130 °C
Hg, Hg Cl , satd KCl + 430
2 2
186-I-e potassium dihydrogen phosphate 2 h in oven at 130 °C
Ag, AgCl, 1.00 M KCl + 439
185-g potassium hydrogen phthalate drying not necessary
Ag, AgCl, 4.00 M KCl + 475
Ag, AgCl, satd KCl + 476
A
The buffer salts listed can be purchased from the Office of Standard Reference
Pt, H (p=1), H (a=1) + 675
Materials, National Institute of Standards and Technology (NIST), Gaithersburg,
MD 20899.
nominalmillivoltredoxreadingsforthequinhydronereference
8.3 Aqua Regia—Mix 1 volume of concentrated nitric acid
solutions at temperatures of 20 °C, 25 °C, and 30 °C.
(HNO , sp gr 1.42) with 3 volumes of concentrated hydrochlo-
8.10 Redox Standard Solution; Iodide/Triiodide—Dissolve
ric acid (HCl, sp gr 1.18). It is recommended that only enough
664.04 g of potassium iodide (KI), 1.751 g of resublimed I ,
solution be prepared for immediate requirements.
12.616 g of boric acid (H BO ), and 20 ml of 1 M potassium
3 3
8.4 Buffer Standard Salts—Table 1 lists the buffer salts
hydroxide(KOH)inwateranddiluteto1 L.Mixsolution.This
available from the National Institute of Standards and Tech- solution is stable at least one year. Solution can be stored in a
nology specifically for the preparation of standard buffer
closed glass or plastic container. Table 4 provides the potential
solutions. The NIST includes numbers and drying procedures. of the platinum electrode for various reference electrodes at
8.4.1 Phthalate Reference Buffer Solution (pH = 4.00 at
various temperatures in the standard Iodide/Triiodide solution.
s
25 °C)—Dissolve 10.12 g of potassium hydrogen phthalate
9. Sampling
(KHC H O ) in water and dilute to 1 L.
8 4 4
8.4.2 Phosphate Reference Buffer Solution (pH = 6.86 at
9.1 Collect the samples in accordance with Practices
s
25 °C)—Dissolve 3.39 g of potassium dihydrogen phosphate
D3370.
(KH PO ) and 3.53 g of anhydrous disodium hydrogen phos-
2 4
10. Preparation
phate (Na HPO ) in water and dilute to 1 L.
2 4
10.1 Electrode Treatment—Condition and maintain ORP
8.5 Chromic Acid Cleaning Solution—Dissolve about5gof
electrodes as recommended by the manufacturer. If the assem-
potassium dichromate (K Cr O ) in 500 mL of concentrated
2 2 7
bly is in intermittent use, the immersible ends of the electrode
sulfuric acid (H SO , sp gr 1.84).
2 4
should be kept in water between measurements. Cover the
8.6 Detergent—Use any commercially available “low-suds”
junctions and fill-holes of reference electrodes to reduce
liquid or solid detergent.
evaporation during prolonged storage.
8.7 Nitric Acid(1+1)—Mix equal volumes of concentrated
10.2 ORP Electrode Cleaning—It is desirable to clean the
nitric acid (HNO , sp gr 1.42) and water.
electrode daily. Remove foreign matter by a preliminary
8.8 Redox Standard Solution; Ferrous-Ferric Reference
treatment with a detergent or mild abrasive, such as toothpaste.
Solution —Dissolve 39.21 g of ferrous ammonium sulfate
If this is insufficient, use 1 + 1 nitric acid. Rinse the electrode
(Fe(NH ) -(SO ) ·6H O), 48.22 g of ferric ammonium sulfate
4 2 4 2 2 in water several times. An alternative cleaning procedure is to
(FeNH (SO ) ·12H O) and 56.2 mL of sulfuric acid (H SO ,
4 4 2 2 2 4 immerse the electrode in chromic acid cleaning mixture at
sp gr 1.84) in water and dilute to 1 L.
...