ASTM G51-23

Standard Test Method for Measuring pH of Soil for Use in Corrosion Evaluations

SIGNIFICANCE AND USE
4.1 Information on pH of soil is used as an aid in evaluating the corrosivity of a soil environment. Some metals are more sensitive to the pH of their environment than others, and information on the stability of a metal as a function of pH and potential is available in the literature.3
SCOPE
1.1 This test method covers a procedure for determining the pH of a soil in corrosion evaluations. The principle use of the test is to supplement soil resistivity measurements and thereby identify conditions under which the corrosion of metals in soil may be accentuated (see G57 – 78 (2012)).  
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.  
1.3 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.4 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.

General Information

• Status: Published
• Publication Date: 31-Oct-2023
• ICS: 13.080.10 - Chemical characteristics of soils
• Technical Committee: G01 - Corrosion of Metals
• Drafting Committee: G01.10 - Corrosion in Soils

Relations

• Replaces: ASTM G51-18 - Standard Test Method for Measuring pH of Soil for Use in Corrosion Testing
• Effective Date: 01-Nov-2023
• Referred By: ASTM E2277-14(2019) - Standard Guide for Design and Construction of Coal Ash Structural Fills
• Effective Date: 01-Nov-2023
• Referred By: ASTM D420-18 - Standard Guide for Site Characterization for Engineering Design and Construction Purposes
• Effective Date: 01-Nov-2023
• Referred By: ASTM G218-19 - Standard Guide for External Corrosion Protection of Ductile Iron Pipe Utilizing Polyethylene Encasement Supplemented by Cathodic Protection
• Effective Date: 01-Nov-2023
• Referred By: ASTM D4972-19 - Standard Test Methods for pH of Soils
• Effective Date: 01-Nov-2023
• Referred By: ASTM G162-23 - Standard Practice for Conducting and Evaluating Laboratory Corrosion Tests in Soils
• Effective Date: 01-Nov-2023
• Referred By: ASTM G158-98(2021) - Standard Guide for Three Methods of Assessing Buried Steel Tanks
• Effective Date: 01-Nov-2023
• Referred By: ASTM D7765-18a - Standard Practice for Use of Foundry Sand in Structural Fill and Embankments
• Effective Date: 01-Nov-2023
• Referred By: ASTM E2788/E2788M-18 - Standard Specification for Use of Expanded Shale, Clay and Slate (ESCS) as a Mineral Component in the Growing Media and the Drainage Layer for Vegetative (Green) Roof Systems
• Effective Date: 01-Nov-2023
• Referred By: ASTM D2976-22a - Standard Test Method for pH of Peat Materials
• Effective Date: 01-Nov-2023
• Referred By: ASTM G200-20 - Standard Test Method for Measurement of Oxidation-Reduction Potential (ORP) of Soil
• Effective Date: 01-Nov-2023

Overview

ASTM G51-23: Standard Test Method for Measuring pH of Soil for Use in Corrosion Evaluations is an internationally recognized ASTM standard that specifies procedures for determining soil pH in the context of corrosion studies. Soil pH is a critical factor influencing the corrosion rate of metals buried in soil environments. This standard outlines best practices for accurate, repeatable pH measurements to supplement soil resistivity data and help identify conditions that may accelerate underground corrosion, ensuring reliable corrosion assessment.

ASTM G51-23 is under the jurisdiction of ASTM Committee G01 on Corrosion of Metals and is aligned with globally recognized standardization principles, as stipulated by the World Trade Organization’s TBT Committee. The test method employs SI units exclusively and emphasizes the significance of appropriate safety, health, and environmental protocols during implementation.

Key Topics

• Measurement Procedure: The standard describes step-by-step procedures for measuring soil pH, highlighting the importance of calibration, equipment selection, and appropriate sampling techniques, whether in-situ, from disturbed samples, or laboratory analysis.
• Recommended Equipment:
  • Portable battery-powered pH meters with digital displays suitable for field use
  • Reference electrodes, such as silver/silver chloride or combination electrodes
  • Glass electrodes and subsurface probes for measurements at depth
  • Thermometers for temperature compensation during measurement
• Calibration and Quality Control: Regular calibration using standard pH buffer solutions (commonly pH 4, 7, and 10) ensures accurate readings. The standard also discusses linearity checks and the need for periodic recalibration, especially when conducting multiple measurements.
• Reporting Results: Results should include location details, soil description, sampling methods, soil temperature, individual and average pH readings, and information on calibration standards used, to allow for traceability and data integrity.
• Precision and Reproducibility: The method includes procedures for evaluating repeatability within a laboratory and reproducibility across laboratories, in accordance with referenced ASTM practices.

Applications

The methods described in ASTM G51-23 have broad practical applications across industries concerned with corrosion management of metals in soil environments, such as:

• Pipeline and Underground Utility Protection: Soil pH measurement informs risk assessment and design for cathodic protection systems.
• Construction and Civil Engineering: Foundational investigations for buildings, bridges, and other structures in contact with soil rely on corrosion evaluation informed by soil pH data.
• Environmental Assessment: Soil pH affects metal mobility and stability, important in remediation planning and monitoring.
• Agricultural Infrastructure: Evaluation of buried equipment or irrigation systems prone to corrosion.

By facilitating accurate soil pH assessment, the standard helps stakeholders optimize corrosion control strategies, extend asset life, and ensure regulatory compliance.

Related Standards

ASTM G51-23 is commonly used in conjunction with other standards to provide a comprehensive corrosion evaluation:

• ASTM G57 - Test Method for Measurement of Soil Resistivity Using the Wenner Four-Electrode Method: Complements soil pH analysis by assessing electrical resistivity, another major factor affecting corrosion rates in soils.
• ASTM D1193 - Specification for Reagent Water: Defines requirements for water purity when preparing calibration solutions.
• ASTM E177 and ASTM E691 - Provide guidance on precision, bias, and interlaboratory studies for test methods.
• ASTM G215 - Guide for Electrode Potential Measurement: Offers additional detail for electrode selection and use.
• NACE/ASTM G193 - Standard Terminology and Acronyms Relating to Corrosion: Ensures standardized terminology across related documents.

Practical Value

Using ASTM G51-23 to measure soil pH for corrosion evaluations provides:

• Accurate risk assessment for underground corrosion
• Standardized methods for infrastructure asset management
• Improved reliability and reproducibility of environmental data
• Support for regulatory compliance and industry best practices

By leveraging this standard, organizations can more effectively identify at-risk installations, design appropriate corrosion control measures, and document conditions for both operational excellence and regulatory reporting.

Standards Content (Sample)

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.
Designation: G51 − 23
Standard Test Method for
Measuring pH of Soil for Use in Corrosion Evaluations
This standard is issued under the fixed designation G51; 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 3. Terminology
1.1 This test method covers a procedure for determining the 3.1 Definitions:
pH of a soil in corrosion evaluations. The principle use of the
3.1.1 calibration solution, n—a commercially available so-
test is to supplement soil resistivity measurements and thereby
lution with a stable pH used for calibrating a pH measuring
identify conditions under which the corrosion of metals in soil
system (meter and probe).
may be accentuated (see G57 – 78 (2012)).
3.1.2 calomel electrode, n—an electrode that develops a
1.2 The values stated in SI units are to be regarded as
standard electric potential and is used to provide a reference
standard. No other units of measurement are included in this
voltage in the circuit for sensing electrodes. It is composed of
standard.
an amalgam of mercury and mercury (I) chloride.
1.3 This standard does not purport to address all of the
3.1.2.1 Discussion—Due to shipping restrictions on mer-
safety concerns, if any, associated with its use. It is the
cury containing materials, calomel electrodes have been re-
responsibility of the user of this standard to establish appro-
placed by silver/silver chloride electrodes in newer pH meters.
priate safety, health, and environmental practices and deter-
3.1.3 combination electrode, n—an electrochemical appara-
mine the applicability of regulatory limitations prior to use.
tus that incorporates an ion-selective electrode and a reference
1.4 This international standard was developed in accor-
electrode in a single assembly thereby avoiding the need for a
dance with internationally recognized principles on standard-
separate reference electrode.
ization established in the Decision on Principles for the
3.1.4 pH, n—the negative logarithm of the hydrogen ion
Development of International Standards, Guides and Recom-
+ +
activity written as pH = –log (a ) where a = Hydrogen ion
mendations issued by the World Trade Organization Technical
10 H H
activity = the molar concentration of hydrogen ions multiplied
Barriers to Trade (TBT) Committee.
by the mean ion-activity coefficient.
2. Referenced Documents
3.1.5 pH meter, n—an instrument with a display of pH units.
2.1 ASTM Standards: For field use, it is portable and battery-powered.
D1193 Specification for Reagent Water
3.1.6 purified water, n—water that meets Specification
E177 Practice for Use of the Terms Precision and Bias in
D1193, Type IV requirements.
ASTM Test Methods
3.1.7 silver/silver chloride electrode, n—an electrode that
E691 Practice for Conducting an Interlaboratory Study to
develops a standard electric potential and is used to provide a
Determine the Precision of a Test Method
reference voltage. The electrode functions as a redox electrode
G57 Test Method for Measurement of Soil Resistivity Using
and the equilibrium is between the silver metal (Ag) and its salt
the Wenner Four-Electrode Method
– Silver Chloride (AgCl).
NACE/ASTM G193 Terminology and Acronyms Relating to
Corrosion 3.1.7.1 Discussion—Refer to detailed description and dis-
G215 Guide for Electrode Potential Measurement
cussion in Guide G215.
3.2 The terminology used herein, if not specifically defined
otherwise, shall be in accordance with NACE/ASTM G193.
This test method is under the jurisdiction of ASTM Committee G01 on
Definitions provided herein and not given in NACE/ASTM
Corrosion of Metals and is the direct responsibility of Subcommittee G01.10 on
G193 are limited only to this standard.
Corrosion in Soils.
Current edition approved Nov. 1, 2023. Published November 2023. Originally
approved in 1977. Last previous edition approved in 2018 as G51 – 18. DOI:
4. Significance and Use
10.1520/G0051-23.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
4.1 Information on pH of soil is used as an aid in evaluating
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
the corrosivity of a soil environment. Some metals are more
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. sensitive to the pH of their environment than others, and
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G51 − 23
information on the stability of a metal as a function of pH and mometers come in different lengths, and a length appropriate
potential is available in the literature. for the depth of interest should be chosen.
5.5 Miscellaneous—Depending on how and where the soil
5. Apparatus
will be analyzed, one or more of the following may be needed:
5.1 pH Meter—A portable, battery-powered pH meter that
hand auger, post-hole diggers, sample bags, glass container,
reads or attaches the electrodes is necessary for field measure-
etc.
ments. An LCD (liquid-crystal display) is preferred for its
readability in a bright, outdoor environment. 6. Reagents and Materials
5.2 Electrodes: 6.1 During the calibration procedure for the pH meter,
5.2.1 Silver/Silver Chloride Electrode—A saturated silver/ standard buffered solutions of known pH are necessary. These
silver chloride reference electrode or its equivalent is used in solutions, or tablets to make up these solutions, can be
the pH determination. A few crystals of solid potassium purchased from chemical supply companies or pH equipment
chloride should always be present within the chamber sur- manufacturers. From practical experience, standard solutions
rounding the silver to assure that the solution is saturated under of pH 4, 7, and 10 are recommended. Reagent grade water such
the conditions of use. The design of the electrode must permit as those described in Specification D1193 may be required to
the formation of a fresh liquid junction between the solution of make solutions.
potassium chloride and the buffer or test soil for each test and
7. Sampling, Test Specimens, and Test Units
allow traces of soil to be readily removed by washing.
5.2.2 Glass Electrode—A glass electrode of rugged con-
7.1 By the nature of the measurement, pH is determined for
struction is required. The performance of the glass electrode is
a small volume of soil at each reading, and it is important that
satisfactory if it furnishes the correct pH value (60.1 pH unit)
at least three measurements at different adjacent locations be
for standard buffered solutions.
made and a simple average calculated.
5.2.3 Combination Electrode—A combination electrode
7.2 In-situ Soil Sampling—Ideally, the pH measurement
consisting of a reference electrode and a glass electrode (5.2.1
should be made in the field with the glass electrode contacting
and 5.2.2) combined as a single electrode is acceptable.
the soil at the specific depth of interest. If the surface soil pH
However, the requirements outlined above are equally appli-
is desired, then the soil can be broken up if compacted so as to
cable to the electrodes used in this combination unit.
accept the electrodes. If a subsurface pH is desired, then a
5.3 Subsurface Probe—When pH measurements below the
boring or an excavation must be done so that the electrode can
surface of the soil are required, it is necessary to use a probe of
be placed in the soil at the desired depth. The subsurface probe
suitable length which will allow measurements to be made at
(Fig. 1) is ideal for use with a boring tool such as hand auger.
the depth of interest. This probe consists of a glass electrode or
After boring through the soil to the depth of interest, carefully
a combination electrode in a rubber housing at the end of a
lower the probe into the cavity without further preparation of
plastic tube. One type of probe is illustrated in Fig. 1.
the soil.
5.4 Soil Thermometer—Some pH electrodes have tempera-
7.3 Disturbed Soil Sampling—Soil samples can be brought
ture compensation built in as part of the pH electrode, but most
to the surface with a boring tool or a post-hole digger, and the
do not (see manufacturer’s specifications). A thermometer of
measurement made in the field on the soil specimen thus
rugged construction is required for soil use, and a stainless
obtained. This technique is less desirable than the probe
steel sheat
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: G51 − 18 G51 − 23
Standard Test Method for
Measuring pH of Soil for Use in Corrosion
TestingEvaluations
This standard is issued under the fixed designation G51; 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
1.1 This test method covers a procedure for determining the pH of a soil in corrosion testing.evaluations. The principle use of the
test is to supplement soil resistivity measurements and thereby identify conditions under which the corrosion of metals in soil may
be accentuated (see G57 – 78 (2012)).
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3 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, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.4 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.
2. Referenced Documents
2.1 ASTM Standards:
D1193 Specification for Reagent Water
E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
G57 Test Method for Measurement of Soil Resistivity Using the Wenner Four-Electrode Method
NACE/ASTM G193 Terminology and Acronyms Relating to Corrosion
G215 Guide for Electrode Potential Measurement
3. Terminology
3.1 Definitions:
3.1.1 calibration solution, n—a commercially available solution with a stable pH used for calibrating a pH measuring system
(meter and probe).
3.1.2 calomel electrode, n—an electrode that develops a standard electric potential and is used to provide a reference voltage in
the circuit for sensing electrodes. It is composed of an amalgam of mercury and mercury (I) chloride.
This test method is under the jurisdiction of ASTM Committee G01 on Corrosion of Metals and is the direct responsibility of Subcommittee G01.10 on Corrosion in
Soils.
Current edition approved Nov. 1, 2018Nov. 1, 2023. Published December 2018November 2023. Originally approved in 1977. Last previous edition approved in 20122018
as G51 – 95G51 – 18.(2012). DOI: 10.1520/G0051-18.10.1520/G0051-23.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G51 − 23
3.1.2.1 Discussion—
Due to shipping restrictions on mercury containing materials, calomel electrodes have been replaced by silver/silver chloride
electrodes in newer pH meters.
3.1.3 combination electrode, n—an electrochemical apparatus that incorporates an ion-selective electrode and a reference
electrode in a single assembly thereby avoiding the need for a separate reference electrode.
+ +
3.1.4 pH, n—the negative logarithm of the hydrogen ion activity written as pH = –log (a ) where a = Hydrogen ion activity
10 H H
= the molar concentration of hydrogen ions multiplied by the mean ion-activity coefficient.
3.1.5 pH meter, n—an instrument with a display of pH units. For field use, it is portable and battery-powered.
3.1.6 purified water, n—water that meets Specification D1193, Type IV requirements.
3.1.7 silver/silver chloride electrode, n—an electrode that develops a standard electric potential and is used to provide a reference
voltage. The electrode functions as a redox electrode and the equilibrium is between the silver metal (Ag) and itits salt – Silver
Chloride (AgCl).
3.1.7.1 Discussion—
Refer to detailed description and discussion in Guide G215.
3.2 The terminology used herein, if not specifically defined otherwise, shall be in accordance with NACE/ASTM G193.
Definitions provided herein and not given in NACE/ASTM G193 are limited only to this standard.
4. Significance and Use
4.1 Information on pH of soil is used as an aid in evaluating the corrosivity of a soil environment. Some metals are more sensitive
to the pH of their environment than others, and information on the stability of a metal as a function of pH and potential is available
in the literature.
5. Apparatus
5.1 pH Meter—A portable, battery-powered pH meter that reads or attaches the electrodes is necessary for field measurements.
An LCD (liquid-crystal display) is preferred for its readability in a bright, outdoor environment.
5.2 Electrodes:
5.2.1 Silver/Silver Chloride Electrode—A saturated silver/silver chloride reference electrode or its equivalent is used in the pH
determination. A few crystals of solid potassium chloride should always be present within the chamber surrounding the silver to
assure that the solution is saturated under the conditions of use. The design of the electrode must permit the formation of a fresh
liquid junction between the solution of potassium chloride and the buffer or test soil for each test and allow traces of soil to be
readily removed by washing.
NOTE 1—A few crystals of solid potassium chloride should always be present within the chamber surrounding the silver to assure that the solution is
saturated under the conditions of use. The design of the electrode must permit the formation of a fresh liquid junction between the solution of potassium
chloride and the buffer or test soil for each test and allow traces of soil to be readily removed by washing.
5.2.2 Glass Electrode—A glass electrode of rugged construction is required. The performance of the glass electrode is satisfactory
if it furnishes the correct pH value (60.1 pH unit) for standard buffered solutions.
5.2.3 Combination Electrode—A combination electrode consisting of a reference electrode and a glass electrode (5.2.1 and 5.2.2)
combined as a single electrode is acceptable. However, the requirements outlined above are equally applicable to the electrodes
used in this combination unit.
5.3 Subsurface Probe—When pH measurements below the surface of the soil are required, it is necessary to use a probe of suitable
Pourbaix, M., Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, 1966.
G51 − 23
length which will allow measurements to be made at the depth of interest. This probe consists of a glass electrode or a combination
electrode in a rubber housing at the end of a plastic tube. One type of probe is illustrated in Fig. 1.
5.4 Soil Thermometer—Some pH electrodes have temperature compensation built in as part of the pH electrode, but most do not
(see manufacturer’s specifications). A thermometer of rugged construction is required for soil use, and a stainless steel sheathed
thermometer is preferred. Metal sheathed thermometers come in different lengths, and a length appropriate for the depth of interest
should be chosen.
5.5 Miscellaneous—Depending on how and where the soil will be analyzed, one or more of the following may be needed: hand
auger, post-hole diggers, sample bags, glass container, etc.
6. Reagents and Materials
6.1 During the calibration procedure for the pH meter, standard buffered solutions of known pH are necessary. These solutions,
or tablets to make up these solutions, can be purchased from chemical supply companies or pH equipment manufacturers. From
practical experience, standard solutions of pH 4, 7, and 10 are recommended. Reagent grade water such as those described in
Specification D1193 may be required to make solutions.
7. Sampling, Test Specimens, and Test Units
7.1 By the nature of the measurement, pH is determined for a small volume of soil at each reading, and it is important that at least
three measurements at different adjacent locations be made and a simple average calculated.
7.2 In-situ Soil Sampling—Ideally, the pH measurement should be made in the field with the glass electrode contacting the soil
at the specific depth of interest. If the surface soil pH is desired, then the soil can be broken up if compacted so as to accept the
electrodes. If a subsurface pH is desired, then a boring or an excavation must be done so that the electrode can be placed in the
soil at the desired depth. The subsurface probe (Fig. 1) is ideal for use with a boring tool such as hand auger. After boring through
the soil to the depth of interest, carefully lower the probe into the cavity without further preparation of the soil.
7.3 Disturbed Soil Sampling—Soil samples can be brought to the surface with a boring tool or a post-ho
...