ASTM G51-95(2000)
(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
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 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 2000)
Standard Test Method for
Measuring pH of Soil for Use in Corrosion Testing
ThisstandardisissuedunderthefixeddesignationG 51;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginal
adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscript
epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 4.2.1 Use a saturated calomel reference electrode or its
equivalent in the pH determination. A few crystals of solid
1.1 This test method covers a procedure for determining the
potassium chloride should always be present within the cham-
pH of a soil in corrosion testing. The principle use of the test
ber surrounding the calomel to assure that the solution is
is to supplement soil resistivity measurements and thereby
saturated under the conditions of use. The design of the
identify conditions under which the corrosion of metals in soil
electrode must permit the formation of a fresh liquid junction
may be accentuated (see G 57 – 78 (1984)).
between the solution of potassium chloride and the buffer or
1.2 This standard does not purport to address all of the
test soil for each test and allow traces of soil to be readily
safety concerns, if any, associated with its use. It is the
removed by washing.
responsibility of the user of this standard to establish appro-
4.2.2 A glass electrode of rugged construction is required.
priate safety and health practices and determine the applica-
The performance of the glass electrode is satisfactory if it
bility of regulatory limitations prior to use.
furnishes the correct pH value (60.1 pH unit) for standard
2. Referenced Documents
buffered solutions.
4.2.3 A combination electrode consisting of a saturated
2.1 ASTM Standards:
calomel reference electrode and a glass electrode (4.2.1 and
E 177 Practice for Use of the Terms Precision and Bias in
4.2.2) combined as a single electrode is acceptable. However,
ASTM Test Methods
the requirements outlined above are equally applicable to the
E 691 Practice for Conducting an Interlaboratory Study to
electrodes used in this combination unit.
Determine the Precision of a Test Method
4.3 Subsurface Probe—When pH measurements below the
G 57 Method for Field Measurement of Soil Resistivity
surface of the soil are required, it is necessary to use a probe of
Using the Wenner Four-Electrode Method
suitable length which will allow measurements to be made at
3. Significance and Use
the depth of interest.This probe consists of a glass electrode or
a combination electrode in a rubber housing at the end of a
3.1 Information on pH of soil is used as an aid in evaluating
plastic tube. One type of probe is illustrated in Fig. 1.
the corrosivity of a soil environment. Some metals are more
4.4 Soil Thermometer—Some pH electrodes have tempera-
sensitive to the pH of their environment than others, and
ture compensation built in as part of the pH electrode, but most
information on the stability of a metal as a function of pH and
do not (see manufacturers’ specifications). A thermometer of
potential is available in the literature.
rugged construction is required for soil use, and a stainless
4. Apparatus
steel sheathed thermometer is preferred. Metal sheathed ther-
mometers come in different lengths, and a length appropriate
4.1 pH Meters—A portable, battery-powered pH meter is
for the depth of interest should be chosen.
necessary for field measurements. Most instruments can also
function as a high-impedance voltmeter. An LCD display is
5. Reagents and Materials
preferred for its readability in a bright, outdoor environment.
5.1 During the calibration procedure for the pH meter,
4.2 Calomel and Glass Electrodes:
standard buffered solutions of known pH are necessary. These
solutions, or tablets to make up these solutions, can be
This test method is under the jurisdiction of ASTM Committee G-1 on
purchased from chemical supply companies or pH equipment
Corrosion of Metals, and is the direct responsibility of Subcommittee G01.10 on
manufacturers.
Corrosion in Soils.
Current edition approved Jan. 15, 1995. Published March 1995. Originally
6. Sampling
published as G 51 – 77. Last previous edition G 51 – 92.
Annual Book of ASTM Standards, Vol 14.02.
6.1 By the nature of the measurement, pH is determined for
Annual Book of ASTM Standards, Vol 03.02.
a small volume of soil at each reading, and it is important that
Pourbaix, M., “Atlas of Electrochemical Equilibria in Aqueous Solutions,”
at least three measurements at different locations be made and
Pergamon Press, 1966.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
G51
FIG. 1 Subsurface probe
a simple average calculated. The regions of interest, surface, 7.2.2 Immerse the electrodes in the known standard solution
subsurface, or both, where applicable, must be sampled. and calibrate the meter in accordance with the manufacturer’s
instructions.
7. Calibration and Standardization
8. Procedure
7.1 Test for Linearity—Prior to field use, or periodically
when used extensively in the field, test the apparatus for
8.1 Soil Preparation for pH Determination:
linearity of response. This procedure is as follows:
8.1.1 Ideally the pH measurement should be made in the
7.1.1 Turn on the instrument, allow to warm up thoroughly,
field with the glass electrode contacting the soil at the specific
and bring to electrical balance in accordance with the manu-
depth of interest. If the surface soil pH is desired, then the soil
facturer’sinstructions.Beforeuse,cleanandrinsetheglassand
can be broken up if compacted so as to accept the electrodes.
calomel electrodes in distilled water.
If a subsurface pH is desired, then a boring or an excavation
7.1.2 At least two standard buffered pH solutions that span
must be done so that the electrode can be placed in the soil at
the soil pH to be measured are required. From practical
the desired depth.The subsurface probe (Fig. 1) is ideal for use
experience standard solutions of pH 4, 7, and 8 are recom-
with a boring tool such as hand auger.After boring through the
mended. For the test, the temperature of these solutions shall
soil to the depth of interest, carefully lower the probe into the
not differ from each other by more than 5°C. A laboratory
cavity without further preparation of the soil.
thermometer can be used for these measurements.
8.1.2 Soil samples can be brought to the surface with a
7.1.3 Adjust the temperature compensating dial on the pH
boring tool or a post-hole digger, and the measurement made in
meter to the standard solution temperature.
the field on the soil thus obtained. This technique is less
7.1.4 Immerse the electrodes in a small volume of the first
desirable than the prob
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