ASTM G51-95(2012)
(Test Method)Standard Test Method for Measuring pH of Soil for Use in Corrosion Testing
Standard Test Method for Measuring pH of Soil for Use in Corrosion Testing
SIGNIFICANCE AND USE
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.
SCOPE
1.1 This test method covers a procedure for determining the pH of a soil in corrosion testing. 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(1984)).
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 and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: G51 − 95 (Reapproved 2012)
Standard Test Method for
Measuring pH of Soil for Use in Corrosion Testing
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.Anumber in parentheses indicates the year of last reapproval.Asuperscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope information on the stability of a metal as a function of pH and
potential is available in the literature.
1.1 This test method covers a procedure for determining the
pH of a soil in corrosion testing. The principle use of the test
4. Apparatus
is to supplement soil resistivity measurements and thereby
4.1 pH Meters—A portable, battery-powered pH meter is
identify conditions under which the corrosion of metals in soil
necessary for field measurements. Most instruments can also
may be accentuated (see G57–78(1984)).
function as a high-impedance voltmeter. An LCD display is
1.2 The values stated in SI units are to be regarded as
preferred for its readability in a bright, outdoor environment.
standard. No other units of measurement are included in this
4.2 Calomel and Glass Electrodes:
standard.
4.2.1 Use a saturated calomel reference electrode or its
1.3 This standard does not purport to address all of the
equivalent in the pH determination. A few crystals of solid
safety concerns, if any, associated with its use. It is the
potassium chloride should always be present within the cham-
responsibility of the user of this standard to establish appro-
ber surrounding the calomel to assure that the solution is
priate safety and health practices and determine the applica-
saturated under the conditions of use. The design of the
bility of regulatory limitations prior to use.
electrode must permit the formation of a fresh liquid junction
between the solution of potassium chloride and the buffer or
2. Referenced Documents
test soil for each test and allow traces of soil to be readily
2.1 ASTM Standards:
removed by washing.
E177 Practice for Use of the Terms Precision and Bias in
4.2.2 A glass electrode of rugged construction is required.
ASTM Test Methods
The performance of the glass electrode is satisfactory if it
E691 Practice for Conducting an Interlaboratory Study to
furnishes the correct pH value (60.1 pH unit) for standard
Determine the Precision of a Test Method
buffered solutions.
G57 Test Method for Field Measurement of Soil Resistivity
4.2.3 A combination electrode consisting of a saturated
Using the Wenner Four-Electrode Method
calomel reference electrode and a glass electrode (4.2.1 and
4.2.2) combined as a single electrode is acceptable. However,
3. Significance and Use
the requirements outlined above are equally applicable to the
3.1 Information on pH of soil is used as an aid in evaluating electrodes used in this combination unit.
the corrosivity of a soil environment. Some metals are more
4.3 Subsurface Probe—When pH measurements below the
sensitive to the pH of their environment than others, and
surface of the soil are required, it is necessary to use a probe of
suitable length which will allow measurements to be made at
This test method is under the jurisdiction of ASTM Committee G01 on the depth of interest. This probe consists of a glass electrode or
Corrosion of Metals and is the direct responsibility of Subcommittee G01.10 on
a combination electrode in a rubber housing at the end of a
Corrosion in Soils.
plastic tube. One type of probe is illustrated in Fig. 1.
Current edition approved May 1, 2012. Published June 2012. Originally
approved in 1977. Last previous edition approved in 2005 as G51–95(2005). DOI:
4.4 Soil Thermometer—Some pH electrodes have tempera-
10.1520/G0051-95R12.
ture compensation built in as part of the pH electrode, but most
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on Pourbaix, M., Atlas of Electrochemical Equilibria in Aqueous Solutions,
the ASTM website. Pergamon Press, 1966.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G51 − 95 (2012)
FIG. 1 Subsurface Probe
do not (see manufacturer’s specifications). A thermometer of 7.1.5 Removetheelectrodesfromthefirststandardsolution,
rugged construction is required for soil use, and a stainless and rinse in distilled water. Immerse the electrodes in the
steel sheathed thermometer is preferred. Metal sheathed ther- second known standard solution and read the pH value. Judge
mometers come in different lengths, and a length appropriate the system to be operating satisfactorily if the reading obtained
for the depth of interest should be chosen. for the second standard agrees within 60.1 unit of the assigned
pH.
5. Reagents and Materials
7.2 Calibration of pH Meter—Calibrate the pH meter im-
5.1 During the calibration procedure for the pH meter,
mediately before use. If a series of measurements are to be
standard buffered solutions of known pH are necessary. These
made, repeat the calibration procedure at intervals of about 30
solutions, or tablets to make up these solutions, can be
min. Perform the pH meter calibration as follows:
purchased from chemical supply companies or pH equipment
7.2.1 Use a standard pH solution in the range of the pH of
manufacturers.
the soil to be tested, if such information is known beforehand.
Otherwise, begin with a standard solution having a pH of 7.
6. Sampling
Stabilize the temperature of the solution so that it matches the
6.1 By the nature of the measurement, pH is determined for
temperature of the soil to within 10°C.
a small volume of soil at each reading, and it is important that
7.2.2 Immerse the electrodes in the known standard solution
at least three measurements at different locations be made and
and calibrate the meter in accordance with the manufacturer’s
a simple average calculated. The regions of interest, surface,
instructions.
subsurface, or both, where applicable, must be sampled.
8. Procedure
7. Calibration and Standardization
8.1 Soil Preparation for pH Determination:
7.1 Test for Linearity—Prior to field use, or periodically
8.1.1 Ideally the pH measurement should be made in the
when used extensively in the field, test the apparatus for
field with the glass electrode contacting the soil at the specific
linearity of response. This procedure is as follows:
depth of interest. If the surface soil pH is desired, then the soil
7.1.1 Turn on the instrument, allow to warm up thoroughly,
can be broken up if compacted so as to accept the electrodes.
and bring to electrical balance in accordance with the manu-
If a subsurface pH is desired, then a boring or an excavation
facturer’sinstructions.Beforeuse,cleanandrinsetheglassand
must be done so that the electrode can be placed in the soil at
calomel electrodes in distilled water.
the desired depth.The subsurface probe (Fig. 1) is ideal for use
7.1.2 At least two s
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