Designation: D4972 − 13

Standard Test Method for

pH of Soils1
This standard is issued under the fixed designation D4972; 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* D1193 Specification for Reagent Water

1.1 This test method covers the measurement of the pH of D3740 Practice for Minimum Requirements for Agencies
soils for uses other than for corrosion testing. Such measure- Engaged in Testing and/or Inspection of Soil and Rock as
ments are used in the agricultural, environmental, and natural Used in Engineering Design and Construction
resources fields. This measurement determines the degree of D6026 Practice for Using Significant Digits in Geotechnical
acidity or alkalinity in soil materials suspended in water and a Data
0.01 M calcium chloride solution. Measurements in both G51 Test Method for Measuring pH of Soil for Use in
liquids are necessary to fully define the soil’s pH. This variable Corrosion Testing
is useful in determining the solubility of soil minerals and the
mobility of ions in the soil and assessing the viability of the 3. Terminology
soil-plant environment. A more detailed discussion of the 3.1 Definitions:
usefulness of this parameter is not warranted here; however, it 3.1.1 For common definitions of terms used in this standard,
can be found in many discussions of the subject. A few such refer to Terminology D653.
discussions are given as Refs (1-6)2 at the end of the text.
4. Summary of Test Method
1.2 The values stated in SI units are to be regarded as
standard. No other units of measurement are included in this 4.1 Measurement of the pH of soils in both suspensions of
standard. water and a calcium chloride solution are made with either a
potentiometer using a pH sensitive electrode system (Method
1.3 All measured and calculated values shall conform to the
A), or pH sensitive paper (Method B). The potentiometer is
guidelines for significant digits and rounding established in
calibrated with buffer solutions of known pH. The pH sensitive
Practice D6026.
paper is a less accurate measurement and should only be used
1.4 This standard does not purport to address all of the for a rough estimate of the soil pH. The electrode must be used
safety concerns, if any, associated with its use. It is the for this measurement unless the pH sensitive paper is specified.
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica- NOTE 1—The quality of the result produced by this standard is
dependent on the competence of the personnel performing it, and the
bility of regulatory limitations prior to use. suitability of the equipment and facilities used. Agencies that meet the
criteria of Practice D3740 are generally considered capable of competent
2. Referenced Documents and objective testing/sampling/inspection and the like. Users of this
2.1 ASTM Standards:3 standard are cautioned that compliance with Practice D3740 does not in
itself assure reliable results. Reliable results depend on many factors;
C670 Practice for Preparing Precision and Bias Statements Practice D3740 provides a means of evaluating some of those factors.
for Test Methods for Construction Materials
D653 Terminology Relating to Soil, Rock, and Contained 5. Significance and Use
Fluids
5.1 The pH of the soil is a useful variable in determining the
solubility of soil minerals and the mobility of ions in the soil
1
This test method is under the jurisdiction of ASTM Committee D18 on Soil and and assessing the viability of the soil-plant environment.
Rock and is the direct responsibility of Subcommittee D18.22 on Soil as a Medium
for Plant Growth. 5.2 pH measurements are made in both water and a calcium
Current edition approved Nov. 1, 2013. Published November 2013. Originally chloride solution because the calcium displaces some of the
approved in 1989. Last previous edition approved in 2007 as D4972 – 01 (2007).
DOI: 10.1520/D4972-13. exchangeable aluminum. The low ionic strength counters the
2
The boldface numbers in parentheses refer to a list of references at the end of dilution effect on the exchange equilibrium by setting the salt
this standard.
3
concentration of the solution closer to that expected in the soil
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
solution. The pH values obtained in the solution of calcium
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 chloride are slightly lower than those measured in water due to
the ASTM website. the release of more aluminum ions which then hydrolyses.

*A Summary of Changes section appears at the end of this standard

Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States

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 D4972 − 13
Therefore, both measurements are required to fully define the 8.3 Acid Potassium Phthalate Buffer Solution (0.05 M)—
character of the soil’s pH. Dissolve 10.21 g (dried 1 hour at 105 6 5°C) of potassium
5.3 For the purpose of this test method the test soil must be phthalate in water and dilute to 1 L. The pH of this solution
sieved through a 2-mm (No. 10) sieve. Measurements on soils should be 4.0 at 20°C. Protect the solution against evaporation
or soil fractions having particle sizes larger than 2 mm by this and against contamination with molds. Replace the solution
test method may be invalid. If soil or soil fractions with when mold is noticed. The effect of temperature is as follows:
particles larger than 2 mm are used, it must be stated in the °C pH
5 to 37 4.0
report since the results may be significantly different.
This illustrates that the pH of the solution does not change
5.4 All water used for this test method must be ASTM Type over the range in temperature from 5 to 37°C.
III or better. Type III water is defined by Specification D1193.
It is prepared by distillation, ion exchange, reverse osmosis, or 8.4 Calcium Chloride Stock Solution (1.0 M)—Dissolve
a combination thereof. 147 g of CaCl2 * 2H2O in water in a 1-L volumetric flask, cool,
dilute to volume with water, and mix.
6. Interferences 8.5 Calcium Chloride Solution (0.01 M)—Dilute 20.0 mL of
6.1 This test method as measured by a pH probe has stock 1.0 M CaCl2 solution to 2 L with water. The pH of this
possible interferences due to a suspension effect or sedimen- solution should be between 5 and 7.
tation potential. Users interested in a detailed discussion of the 8.6 Phosphate Buffer Solution (0.025 M)—Dissolve 3.40 g
mechanism of this effect can find it in Refs (5) and (6). of KH2PO4 and 3.55 g of KH2HPO4 in water and dilute to 1 L.
6.2 This effect is the main reason Test Method G51 can not Dry salts 2 h at 130°C before use. The pH of this solution
be used for general measurement of pH outside of that for should be 6.9 at 20°C. The effect of temperature is as follows:
corrosion analysis. Test Method G51 measures pH (an aqueous °C pH
0 7.0
parameter) without adding any aqueous phase to the soil. This 10 6.9
results in excessive soil particle-pH probe contact that overes- 20 6.9
timates the activity of the hydrogen ions in solution and is 30 6.8
40 6.8
therefore unacceptable for general soil analysis.
8.7 Carbonate Buffer Solution (0.025 M)—Dissolve 2.10 g
6.3 The suspension effect can be mitigated by careful
of NaHCO3 and 2.65 g of Na2CO3 in water and dilute to 1 L.
attention to 10.1.
Dry salts 2 hours at 130°C before use. The pH of this solution
7. Apparatus should be 10.1 at 20°C. The effect of temperature is as follows:
°C pH
7.1 Method A, pH Meter—Potentiometer equipped with an 0 10.3
electrode system. Follow the manufacturer’s instructions for 10 10.2
20 10.1
the pH meter used. A silver/silver chloride electrode system or 30 10.0
similar is also acceptable. 40 9.9
7.2 Method B, pH Paper—pH paper sensitive to a pH range
9. Calibration of pH Meter
from 1 to 12, with resolution to the nearest 0.2 pH unit.
9.1 Calibrate the pH meter using the buffer solutions in
8. Reagents Section 8 or other NIST traceable purchased buffers. Select
8.1 Purity of Reagents—Reagent grade chemicals should be buffers so that the expected pH value of the tested soil is
used in all tests. Unless otherwise indicated, it is intended that bracketed. Adjustment of the pH meter should follow the
all reagents should conform to the specifications of the manufacturer’s direction.
Committee on Analytical Reagents of the American Chemical 10. Procedure
Society, where such specifications are available.4 Other grades
may be used, provided it is first ascertained that the reagent is 10.1 When making measurements with the pH electrode,
of sufficient purity to permit its use without lessening the place the electrode into the partially settled suspension to
accuracy of the determination. mitigate the suspension effect.
8.2 Purity of Water—All water used for this test method 10.2 For both methods, begin with an air dried soil that has
must be ASTM Type III or better. Type III water is defined by been sieved through a 2-mm (No. 10) sieve to remove the
Specification D1193. It is prepared by distillation, ion coarser soil fraction. Air drying the soil is necessary to
exchange, reverse osmosis, or a combination thereof. accomplish sieving and to control the amount of water present
at the time of measurement.
NOTE 2—Some laboratories have reported that 10 ml of water and 10 g
4
Reagent Chemicals, American Chemical Society Specifications , American of soil is insufficient material to fully submerge the pH electrode on their
Chemical Society, Washington, DC. For suggestions on the testing of reagents not equipment. In these cases, a larger sample such as 40 ml of water and 40
listed by the American Chemical Society, see Analar Standards for Laboratory g of soil is permitted
Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia
and National Formulary, U.S. Pharmaceutical Convention, Inc. (USPC), Rockville, 10.3 pH in Distilled Water—For both methods, weigh out
MD. approximately 10 g of air dried soil. Place the soil into a glass

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 D4972 − 13
container and add approximately 10 mL of water. Mix thor- 12.1.1 Within-Laboratory Precision—The within laboratory
oughly and let stand for 1 h. standard deviations for Method A are 0.031 (pH units) for the
10.4 Method A—Read pH on pH meter. water mixture and 0.1395 (pH units) for the calcium chloride
mixture. Therefore, results of two properly conducted tests in
10.5 Method B—Read pH on pH paper. the same or different laboratories should not differ by more
10.6 pH in 0.01 M Calcium Chloride Solution—For both than 0.0655 (pH units) for the water mixture and 0.389 pH units
methods weigh out approximately 10 g of air dried soil. Place for the calcium chloride mixture. The within-laboratory stan-
the soil into a glass container and add approximately 10 mL of dard deviations for Method B are 0.189 (pH units) for the water
the 0.01 M CaCl2 solution. Mix thoroughly and let stand for 1 mixture and 0.2125 (pH units) for the calcium chloride mixture.
h. Therefore, results of two properly conducted tests in the same
10.7 Method A—Read pH on pH meter. or different laboratories should not differ by more than 0.535
(pH units) for the water mixture and 0.60 pH units for the
10.8 Method B—Read pH on pH paper. calcium chloride mixture.
10.9 The mixture should be at approximately room tempera- 12.1.1.1 The precision of Method A presented was deter-
ture (15 to 25°C) at the time of pH measurement. mined by the National Technical Center of the United States
Department of Agriculture. In their evaluation they used 174
11. Report:Test Data Sheet(s)/Form(s) replicates for the water mixture and 32 replicates in testing the
11.1 Record as a minimum the following general informa- calcium chloride mixture.
tion: 12.1.1.2 The precision of Method B presented was deter-
11.1.1 Sample/specimen identifying information, such as mined by the United States Army Environmental Hygiene
Project No., Boring No., Sample No., Depth, etc. Agency. In their evaluation they used 25 replicates in testing
11.1.2 Any special selection and preparation process, such each mixture.
as removal of gravel or other materials. 12.1.2 Between-Laboratory Precision—The between-
11.1.3 Technician name, method used and date. laboratory standard deviation has not been determined for
11.2 Record as a minimum the following test information: either method. Subcommittee D18.22 is actively seeking data
11.2.1 Report the pH of the soil to the first decimal place to evaluate the between laboratory precision of this test
11.2.2 Specify which of the pH measurements is in water method.
and which is in the calcium chloride solution. 12.2 Bias—There is no accepted reference value for this test
11.2.3 Specify whether the determinations were made with method; therefore, bias cannot be determined.
Method A or Method B.
11.2.4 Record if size fractions other than sieved through the 13. Keywords
2 mm (No. 10) sieve are used, it must be stated in the report
since the results may be significantly different. 13.1 acidity; alkalinity; pH; reaction; soil

12. Precision and Bias

12.1 Precision: 5
These data satisfy the 1S and D2S requirements outlined in Practice C670.

REFERENCES

(1) Garrels, R. M., and Christ, C. L., Solutions, Minerals, and Equilibria, (4) Davies, J. T., and Rideal, E. K., Interfacial Phenomena, Academic,
Freeman Cooper, San Francisco, CA, 1965. New York, NY, 1963.
(2) Greenland, D. J., and Hayes, M. H. B., The Chemistry of Soil (5) Hunter, R. J., Zeta Potential in Colloid Science, Academic, New York,
Processes, Wiley, New York, NY, 1981. NY, 1981.
(3) Sposito, G., The Thermodynamics of Soil Solutions, Clarendon, (6) Perrin, D. D., and Dempsey, B., Buffers for pH and Metal Ion Control,
Oxford, United Kindom, 1981. Chapman and Hall, London, United Kindom, 1974.

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 D4972 − 13
SUMMARY OF CHANGES

Committee D18 has identified the location of selected changes to this standard since the last issue (D4972 – 01
(2007)) that may impact the use of this standard. (Approved Nov. 1, 2013.)

(1) Revised Sections 1, 8, 9, 11 (2) Added Note 1 and Note 2.

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