ASTM D1498-93
(Practice)Standard Practice for Oxidation-Reduction Potential of Water
Standard Practice for Oxidation-Reduction Potential of Water
SCOPE
1.1 This practice covers the apparatus and procedure for the electrometric measurement of oxidation-reduction potential (ORP) in water. It does not deal with the manner in which the solutions are prepared, the theoretical interpretation of the oxidation-reduction potential, or the establishment of a standard oxidation-reduction potential for any given system. The practice described has been designed for the routine and process measurement of oxidation-reduction potential.
1.2 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.
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Designation: D 1498 – 93
AMERICAN SOCIETY FOR TESTING AND MATERIALS
100 Barr Harbor Dr., West Conshohocken, PA 19428
Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
Standard Practice for
1
Oxidation-Reduction Potential of Water
This standard is issued under the fixed designation D 1498; 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
n 5 number of electrons involved in process
reaction, and
1.1 This practice covers the apparatus and procedure for the
A and A 5 activities of the reactants in the process.
ox red
electrometric measurement of oxidation-reduction potential
3.2 For definitions of other items used in this practice, refer
(ORP) in water. It does not deal with the manner in which the
to Terminology D 1129.
solutions are prepared, the theoretical interpretation of the
oxidation-reduction potential, or the establishment of a stan-
4. Summary of Practices
dard oxidation-reduction potential for any given system. The
4.1 This is a practice designed to measure the ORP which is
practice described has been designed for the routine and
defined as the electromotive force between a noble metal
process measurement of oxidation-reduction potential.
electrode and a reference electrode when immersed in a
1.2 This standard does not purport to address all of the
solution. The practice describes the electronic equipment
safety problems, if any, associated with its use. It is the
available to make the measurement and describes how to
responsibility of the user of this standard to establish appro-
determine the sensitivity of the electrodes as well as the
priate safety and health practices and determine the applica-
calibration of equipment to solutions having a known potential.
bility of regulatory limitations prior to use.
The ORP electrodes are inert and measure the ratio of the
activities of the oxidized to the reduced species in the process
2. Referenced Documents
reactions.
2.1 ASTM Standards:
2
5. Interferences
D 1129 Terminology Relating to Water
2
D 1193 Specification for Reagent Water
5.1 The ORP electrodes reliably measured ORP in nearly all
D 3370 Practices for Sampling Water from Closed Con-
aqueous solutions and in general are not subject to solution
2
duits
interference from color, turbidity, colloidal matter, and
suspended matter.
3. Terminology
5.2 The ORP of an aqueous solution is sensitive to change
in temperature of the solution, but temperature correction is
3.1 Definitions of Terms Specific to This Standard:
rarely done due to its minimal effect and complex reactions.
3.1.1 oxidation-reduction potential—the electromotive
Temperature corrections are usually applied only when it is
force, Em, developed between a noble metal electrode and a
desired to relate the ORP to the activity of an ion in the
standard reference electrode. This oxidation-reduction poten-
solutions.
tial (ORP) is related to the solution composition by:
5.3 The ORP of an aqueous solution is almost always
RT
o
sensitive to pH variations even to reactions that do not appear
E 5 E 1 2.3 log A / A
m ox red
nF
to involve hydrogen or hydroxyl ions. The ORP generally tends
to increase with an increase in hydrogen ions and to decrease
where:
with an increase in hydroxyl ions during such reactions.
E 5 ORP,
m
o
5.4 Reproducible oxidation-reduction potentials cannot be
E 5 constant that depends on the choice of
obtained for chemical systems that are not reversible. Most
reference electrodes.
natural and ground waters do not contain reversible systems, or
F 5 Faraday constant,
R 5 gas constant, may contain systems that are shifted by the presence of air. The
T 5 absolute temperature, °C + 273.15,
measurement of end point potential in oxidation-reduction
titration is sometimes of this type.
5.5 If the metallic portion of the ORP electrode is sponge-
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This practice is under the jurisdiction of ASTM Committee D-19 on Water and like, materials absorbed from solutions may not be washed
is the direct responsibility of Subcommittee D19.11 on Water for Power Generation
away, even by repeated rinsings. In such cases, the electrode
and Process Use.
may exhibit a memory effect, particularly if it is desired to
Current edition approved Sept. 15, 1993. Published December 1993. Originally
2
detect a relatively low concentration of a particular species
published as D 1498 – 57 T. Last previous edition D 1498 – 76 (1981)e .
2
Annual Book of ASTM Standards, Vol 11.01. immediately after a measurement has been made in a relatively
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D 1498
concentrated solution. A bright
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