ASTM D1293-18

Standard Test Methods for pH of Water

SIGNIFICANCE AND USE
5.1 The pH of water is a critical parameter affecting the solubility of trace minerals, the ability of the water to form scale or to cause metallic corrosion, and the suitability of the water to sustain living organisms. It is a defined scale, based on a system of buffer solutions2 with assigned values. In pure water at 25°C, pH 7.0 is the neutral point, but this varies with temperature and the ionic strength of the sample.5 Pure water in equilibrium with air has a pH of about 5.5, and most natural uncontaminated waters range between pH 6 and pH 9.
SCOPE
1.1 These test methods cover the determination of pH by electrometric measurement using the glass electrode as the sensor. Two test methods are given as follows:    
Sections  
Test Method A—Precise Laboratory Measurement  
8 to 15  
Test Method B—Routine or Continuous Measurement  
16 to 24  
1.2 Test Method A covers the precise measurement of pH in water utilizing at least two of seven standard reference buffer solutions for instrument standardization.  
1.3 Test Method B covers the routine measurement of pH in water and is especially useful for continuous monitoring. Two buffers are used to standardize the instrument under controlled parameters, but the conditions are somewhat less restrictive than those in Test Method A. For on-line measurement, also see Test Method D6569 which provides more detail.  
1.4 Both test methods are based on the pH scale established by NIST (formerly NBS) Standard Reference Materials.2  
1.5 Neither test method is considered to be adequate for measurement of pH in water whose conductivity is less than about 5 μS/cm. Refer to Test Methods D5128 and D5464.  
1.6 Precision and bias data were obtained using buffer solutions only. It is the user's responsibility to assure the validity of these test methods for untested types of water.  
1.7 The values stated in SI units are to be regarded as standard. The values given in parentheses are mathematical conversions to inch-pound units that are provided for information only and are not considered standard.  
1.8 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.  
1.9 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: 14-Jan-2018
• ICS: 13.060.50 - Examination of water for chemical substances
• Technical Committee: D19 - Water
• Drafting Committee: D19.03 - Sampling Water and Water-Formed Deposits, Analysis of Water for Power Generation and Process Use, On-Line Water Analysis, and Surveillance of Water

Relations

• Refers: ASTM E70-24 - Standard Test Method for pH of Aqueous Solutions With the Glass Electrode
• Effective Date: 01-Jan-2024
• Refers: ASTM D1129-13(2020)e2 - Standard Terminology Relating to Water
• Effective Date: 01-May-2020
• Refers: ASTM D1066-18 - Standard Practice for Sampling Steam
• Effective Date: 01-Aug-2018
• Refers: ASTM D1066-18e1 - Standard Practice for Sampling Steam
• Effective Date: 01-Aug-2018
• Refers: ASTM E70-07(2015) - Standard Test Method for pH of Aqueous Solutions With the Glass Electrode
• Effective Date: 01-Jun-2015
• Refers: ASTM D2777-12 - Standard Practice for Determination of Precision and Bias of Applicable Test Methods of Committee D19 on Water
• Effective Date: 15-Jun-2012
• Refers: ASTM D1066-11 - Standard Practice for Sampling Steam
• Effective Date: 15-Jun-2011
• Refers: ASTM D3370-10 - Standard Practices for Sampling Water from Closed Conduits
• Effective Date: 01-Dec-2010
• Refers: ASTM D1129-10 - Standard Terminology Relating to Water
• Effective Date: 01-Mar-2010
• Refers: ASTM D5128-09 - Standard Test Method for On-Line pH Measurement of Water of Low Conductivity
• Effective Date: 01-Oct-2009
• Refers: ASTM D6569-05(2009) - Standard Test Method for On-Line Measurement of pH¹
• Effective Date: 01-May-2009
• Refers: ASTM D3370-08 - Standard Practices for Sampling Water from Closed Conduits
• Effective Date: 01-Oct-2008
• Refers: ASTM D2777-08 - Standard Practice for Determination of Precision and Bias of Applicable Test Methods of Committee D19 on Water
• Effective Date: 15-Jan-2008
• Refers: ASTM D3370-07 - Standard Practices for Sampling Water from Closed Conduits
• Effective Date: 01-Dec-2007
• Refers: ASTM D5464-07 - Standard Test Method for pH Measurement of Water of Low Conductivity
• Effective Date: 01-Aug-2007

Overview

ASTM D1293-18 is an internationally recognized standard developed by ASTM International, establishing test methods for the accurate measurement of the pH of water using electrometric equipment, notably the glass electrode method. Monitoring water pH is an essential part of water quality assessment, since pH influences trace mineral solubility, scale formation, corrosion, and the capacity of water to sustain biological processes. The standard outlines two primary test methods: Test Method A for precise laboratory measurement and Test Method B for routine or continuous monitoring. Both methods are aligned with pH scales established by NIST Standard Reference Materials, ensuring consistent and reliable results across different applications.

Key Topics

• Importance of Water pH Measurement: pH is a critical indicator in water chemistry, affecting mineral solubility, corrosion potential, and biological viability.
• Test Method A - Precise Laboratory Measurement:
  • Utilizes at least two reference buffer solutions for calibration.
  • Requires stringent laboratory conditions for maximum accuracy.
  • Designed for applications demanding high precision, such as research and quality assurance.
• Test Method B - Routine/Continuous Measurement:
  • Suited for regular monitoring, including process waters and on-line measurement.
  • Involves less restrictive calibration using two buffers.
  • Compatible with continuous pH monitoring in industrial or field settings.
• Instrumentation and Calibration:
  • Use of reliable laboratory or process-grade pH meters.
  • Mandatory regular calibration with standard buffer solutions.
  • Glass electrodes and reference electrodes must be in optimal condition.
• Applicable Range and Limitations:
  • Accurate for water samples with conductivity above 5 μS/cm.
  • For low conductivity water, alternative ASTM methods should be referenced.
• Reporting Standards:
  • Results should include temperature and be reported to the nearest 0.01 pH unit (for pH 1-12).

Applications

ASTM D1293-18 is widely applied in:

• Drinking Water and Environmental Monitoring: Verifying that water is within safe pH ranges (often pH 6–9 for natural waters).
• Industrial and Power Generation Facilities: Monitoring process water to minimize scale, corrosion, or unfavorable chemical reactions.
• Wastewater Treatment and Compliance: Ensuring effluent meets regulatory pH standards before discharge.
• Laboratory Research and Quality Assurance: Providing reference measurements for traceability and consistency in water chemistry research.

The standard is suitable for both laboratory-grade pH determination (where exact, high-fidelity results are needed) and for more routine or continuous field measurements that inform process control or environmental compliance programmes.

Related Standards

When measuring pH of water, it is beneficial to consult additional ASTM standards that are referenced in or complement D1293-18, including:

• ASTM D5128: Test Method for On-Line pH Measurement of Water of Low Conductivity.
• ASTM D5464: Test Method for pH Measurement of Water of Low Conductivity.
• ASTM D6569: Test Method for On-Line Measurement of pH.
• ASTM D1066: Practice for Sampling Steam.
• ASTM D1067: Test Methods for Acidity or Alkalinity of Water.
• ASTM D1129: Terminology Relating to Water.
• ASTM E70: Test Method for pH of Aqueous Solutions with the Glass Electrode.

Consulting these related standards can help address specific measurement challenges (such as low-conductivity samples) and align with best practices across various water testing scenarios.

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Keywords: ASTM D1293-18, pH of water, water pH measurement, electrometric method, laboratory pH, routine pH testing, water quality, glass electrode method, buffer solutions, ASTM standards, water analysis.

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: D1293 − 18
Standard Test Methods for
pH of Water
This standard is issued under the fixed designation D1293; 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.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use.
1.1 These test methods cover the determination of pH by
1.9 This international standard was developed in accor-
electrometric measurement using the glass electrode as the
dance with internationally recognized principles on standard-
sensor. Two test methods are given as follows:
ization established in the Decision on Principles for the
Sections
Development of International Standards, Guides and Recom-
Test Method A—Precise Laboratory Measurement 8 to 15
Test Method B—Routine or Continuous Measurement 16 to 24 mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
1.2 Test MethodAcovers the precise measurement of pH in
water utilizing at least two of seven standard reference buffer
2. Referenced Documents
solutions for instrument standardization.
2.1 ASTM Standards:
1.3 Test Method B covers the routine measurement of pH in
D1066 Practice for Sampling Steam
water and is especially useful for continuous monitoring. Two
D1067 Test Methods for Acidity or Alkalinity of Water
buffers are used to standardize the instrument under controlled
D1129 Terminology Relating to Water
parameters, but the conditions are somewhat less restrictive
D1193 Specification for Reagent Water
thanthoseinTestMethodA.Foron-linemeasurement,alsosee
D2777 Practice for Determination of Precision and Bias of
Test Method D6569 which provides more detail.
Applicable Test Methods of Committee D19 on Water
1.4 Both test methods are based on the pH scale established
D3370 Practices for Sampling Water from Closed Conduits
by NIST (formerly NBS) Standard Reference Materials.
D5128 Test Method for On-Line pH Measurement of Water
1.5 Neither test method is considered to be adequate for
of Low Conductivity
measurement of pH in water whose conductivity is less than
D5464 Test Method for pH Measurement of Water of Low
about 5 µS/cm. Refer to Test Methods D5128 and D5464.
Conductivity
D6569 Test Method for On-Line Measurement of pH
1.6 Precision and bias data were obtained using buffer
E70 Test Method for pH of Aqueous Solutions With the
solutions only. It is the user’s responsibility to assure the
Glass Electrode
validity of these test methods for untested types of water.
1.7 The values stated in SI units are to be regarded as
3. Terminology
standard. The values given in parentheses are mathematical
3.1 Definitions:
conversions to inch-pound units that are provided for informa-
3.1.1 For definitions of terms used in this standard, refer to
tion only and are not considered standard.
Terminology D1129.
1.8 This standard does not purport to address all of the
3.2 Definitions of Terms Specific to This Standard:
safety concerns, if any, associated with its use. It is the
3.2.1 pH, n—the negative logarithm of the hydrogen ion
responsibility of the user of this standard to establish appro-
activity in an aqueous solution or the logarithm of the
reciprocal of the hydrogen ion activity.
3.2.1.1 Discussion—The pH of an aqueous solution is de-
These test methods are under the jurisdiction of ASTM Committee D19 on
rived from E, the electromotive force (emf) of the cell:
Water and are the direct responsibility of Subcommittee D19.03 on Sampling Water
and Water-Formed Deposits, Analysis of Water for Power Generation and Process
Use, On-Line Water Analysis, and Surveillance of Water.
Current edition approved Jan. 15, 2018. Published January 2018. Originally
approved in 1953. Last previous edition approved in 2011 as D1293 – 11. DOI: For referenced ASTM standards, visit the ASTM website, www.astm.org, or
10.1520/D1293-18. contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Wu, and Koch, “Standard Reference Materials: Standardization of pH Standards volume information, refer to the standard’s Document Summary page on
Measurements,” NBS Special Publications No. 260-53, 1988. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D1293 − 18
TABLE 1 Slope Factor at Various Temperatures
temperature and the ionic strength of the sample. Pure water
Temperature,° C Slope, millivolts in equilibrium with air has a pH of about 5.5, and most natural
0 54.20 uncontaminated waters range between pH 6 and pH 9.
5 55.19
10 56.18
6. Purity of Reagents
15 57.17
20 58.17
6.1 Reagent grade chemicals shall be used in all tests,
25 59.16
except as specifically noted for preparation of reference buffer
30 60.15
35 61.14
solutions. Unless otherwise indicated, it is intended that all
40 62.13
reagents shall conform to the specifications of the Committee
45 63.13
on Analytical Reagents of the American Chemical Society,
50 64.12
55 65.11 where such specifications are available. Other grades may be
60 66.10
used, provided it is first ascertained that the reagent is of
65 67.09
sufficiently high purity to permit its use without lessening the
70 68.09
75 69.08 accuracy of the determination.
80 70.07
6.2 Purity of Water—Unless otherwise indicated, references
85 71.06
90 72.05
towaterthatisusedforreagentpreparation,rinsing,ordilution
95 73.05
shall be understood to mean reagent water conforming to
Specification D1193, Type I.
7. Sampling
glass electrode | solution || reference electrode
7.1 Collect samples in accordance with Practice D1066,or
(where the double vertical line represents a liquid junc-
Practices D3370, whichever is applicable.
tion) when the electrodes are immersed in the solution in the
diagrammed position, and E is the electromotive force ob- TEST METHOD A—PRECISE LABORATORY
s
MEASUREMENT OF pH
tained when the electrodes are immersed in a reference buf-
fer solution.
8. Scope
With the assigned pH of the reference buffer designated as
pH , and E and E expressed in volts is the following:
s s
8.1 This test method covers the precise measurement of pH
in water under strictly controlled laboratory conditions.
E 2 E F
~ !
s
pH 5 pH 1
s
2.3026 RT
9. Interferences
where:
9.1 The glass electrode reliably measures pH in nearly all
F = Faraday constant,
aqueous solutions and in general is not subject to solution
R = gas constant, and
interference from color, turbidity, colloidal matter, oxidants, or
T = absolute temperature, t(°C) + 273.15.
reductants.
The reciprocal of F/2.3026 RT is known as the slope of the
9.2 The reference electrode may be subject to interferences
electrode, and is the expected difference in observed voltage
and should be chosen to conform to all requirements of
for two measurements one pH unit apart.Values of the slope at
Sections 10 and 12. Refer also to Appendix X1.3.
various temperatures are given in Table 1.
9.3 The true pH of an aqueous solution or extract is affected
4. Summary of Test Method
by the temperature. The electromotive force between the glass
and the reference electrode is a function of temperature as well
4.1 ThepHmeterandassociatedelectrodesarestandardized
as pH. The temperature effect can be compensated automati-
against at least two reference buffer solutions that closely
cally in many instruments or can be manually compensated in
bracket the anticipated sample pH.The sample measurement is
most other instruments. The temperature compensation cor-
made under strictly controlled conditions and prescribed tech-
rects for the effect of changes in electrode slope with tempera-
niques.
ture but does not correct for temperature effects on the
5. Significance and Use chemical system being monitored. It does not adjust the
5.1 The pH of water is a critical parameter affecting the
solubility of trace minerals, the ability of the water to form
The relative acidity or alkalinity measured by pH should not be confused with
scale or to cause metallic corrosion, and the suitability of the
total alkalinity or total acidity (for example,Test Methods D1067).Thus, 0.1 M HCl
watertosustainlivingorganisms.Itisadefinedscale,basedon
and 0.1 M acetic acid have the same total acidity, but the HCl solution will be more
acidic (approximately pH 1 versus pH 3).
a system of buffer solutions with assigned values. In pure
Reagent Chemicals, American Chemical Society Specifications, American
water at 25°C, pH 7.0 is the neutral point, but this varies with
Chemical Society, Washington, DC. For suggestions on the testing of reagents not
listed by the American Chemical Society, see Analar Standards for Laboratory
Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia
Bates, R. G., Determination of pH: Theory and Practice, 2nd ed., J. Wiley and and National Formulary, U.S. Pharmaceutical Convention, Inc. (USPC), Rockville,
Sons, New York, 1973, p. 29. MD.
D1293 − 18
measured pH to a common temperature; therefore, the tem- The reference electrode and junction shall perform satisfacto-
perature should be reported for each pH measurement. Tem- rily as required in the standardizing procedure described in
perature effects are discussed further in Appendix X1.2. 12.1through12.6.Adiscussionofreferenceelectrodesisgiven
in Appendix X1.3.
9.4 The pH response of the glass electrode/reference elec-
trode pair is imperfect at both ends of the pH scale. The 10.4 Temperature Compensator—The thermocompensator
indicatedpHvalueofhighlyalkalinesolutionsmaybetoolow, is a temperature-sensitive resistance element immersed in the
by as much as 1 pH, depending on electrode composition and watersamplewiththeelectrodes.Thethermocompensatormay
sample conditions. See X1.5.1. The indicated pH value of be incorporated into the pH electrode or may be a separate
strong aqueous solutions of salts and strong acids having a pH probe. The thermocompensator automatically corrects for the
less than 1, will often be higher than the true pH value. change in slope of the glass electrode (with change of
Interferences can be minimized by the selection of the proper temperature) but does not correct for actual changes in sample
glass and reference electrodes for measurements in highly pH with temperature. The automatic thermocompensator is not
alkaline or acidic solutions. required if the water temperature is essentially constant and the
analyst chooses to use the manual temperature compensation
9.5 A few substances sometimes dispersed in water appear
feature of the pH meter.
to poison the glass electrode. A discussion of this subject is
given in Appendix X1.4.
11. Reagents
10. Apparatus 11.1 Reference Buffer Solutions—The approximate pH val-
ues of the reference buffer solutions measured at several
10.1 Laboratory pH Meter—Almost all commercially avail-
temperatures are listed in Table 2. If traceability to NIST
able meters are of the digital type and will have either manual
standard reference materials (SRMs) is required, follow ex-
or automatic calibration, and either manual or automatic
actly the NIST certificate drying, preparation, and storage
temperature (slope) correction. All four types are permissible.
instructions for the given renewal of the respective SRM
However, readability to 0.01 pH is essential (Section 14), and
buffer,andobtainthecertifiedvaluefromthecertificateforthat
the ability to read in millivolts is useful in troubleshooting.
SRM renewal at the applicable temperature. The current
10.2 Glass Electrode—The pH response of the glass elec-
renewal of each NIST SRM should be used. Buffer solution
trode shall conform to the requirements set forth in 12.1
compositions and preparations given in Sections 11.1.1
through 12.6. The glass electrode lead wire shall be shielded.
through 11.1.7 are for use with non-NIST materials. For NIST
New glass electrodes and those that have been stored dry shall
SRMs, follow the instructions included with the SRM.
be conditioned and maintained as recommended by the manu-
Table 3 identifies each buffer salt by its National Institute of
facturer.
Standards and Technology (NIST) number Dry the buffer salt
before use. The drying procedure will be according to the
10.3 Reference Electrode—This may be used as separate
“half cell,” or it may be purchased integral with the glass pH current renewal of the NIST certificate. Recommended drying
proceduresfornon-NISTreferencematerialsarealsoprovided,
electrode body, as a combination electrode. The internal
reference element may be calomel (mercury/mercurous in the last column of the table. Keep the five reference buffer
solutions with pH less than 9.5 in bottles of chemically
chloride), silver/silver chloride, or an iodide-iodine redox
resistant glass. Keep the calcium hydroxide solutions in a
couple. For best performance, the reference element should be
plastic bottle that is nonporous to air (that is, polypropylene or
the same type in both the reference electrode and inside the pH
high density polyethylene). Keep all the reference buffer
electrode. For all three types, the junction between the refer-
solutions well-stoppered and replace if a visible change is
ence filling solution and the sample may be either a flowing or
observed.
nonflowing junction. The flowing liquid junction-type unit
ensures that a fresh liquid junction is formed for each mea- 11.1.1 Borax Reference Buffer Solution (pH = 9.18 at
s
surement and shall be used for Test Method A determinations. 25°C)—When using the NIST SRM, prepare in accordance
If a saturated calomel electrode is used, some potassium with the current renewal of the certificate. For other reference
chloride crystals shall be contained in the saturated potassium materials, follow associated instructions or dissolve 3.80 g of
chloride solution. If the reference electrode is of the flowing sodium tetraborate decahydrate (Na B O ·10H O) in water
2 4 7 2
junction type, the design of the electrode shall permit a fresh and dilute to 1 L.
liquid junction to be formed between the reference electrode 11.1.2 Calcium Hydroxide Reference Buffer Solution(pH =
s
solution and the buffer standard or tested water for each 12.45 at 25°C)—When using the NIST SRM, prepare in
measurement and shall allow traces of solution to be washed accordancewiththecurrentrenewalofthecertificate.Forother
from the outer surfaces of the electrodes. To ensure the desired reference materials, follow associated instructions or prepare
slow outward flow of reference electrode solution, the solution pure calcium hydroxide (Ca(OH) ) from well-washed calcium
pressure inside the liquid junction should be kept somewhat in carbonate (CaCO ) of low-alkali grade by slowly heating the
excess of that outside the junction. In nonpressurized carbonate in a platinum dish at 1000°C and calcining for at
applications, this requirement can be met by maintaining the least 45 min at that temperature. After cooling in a dessicator,
inside solution level higher than the outside water level. If the add the calcined product slowly to water with stirring, heat the
reference electrode is of the nonflowing junction type, these resultantsuspensiontoboiling,cool,andfilterthroughafunnel
outward flow and pressurization considerations do not apply. having a fritted-glass disk of medium porosity. Collect the
D1293 − 18
A
TABLE 2 Approximate pH of Reference Buffer Solutions
s
Temperature, Tetroxalate Tartrate Phthalate Phosphate Borax Sodium Calcium Hydroxide
°C Solution Solution Solution Solution Solution Bicarbonate Solution
Sodium Carbonate
0 1.67 . 4.00 6.98 9.46 10.32 13.42
5 1.67 . 4.00 6.95 9.39 10.25 13.21
10 1.67 . 4.00 6.92 9.33 10.18 13.00
15 1.67 . 4.00 6.90 9.28 10.12 12.81
20 1.68 . 4.00 6.88 9.23 10.06 12.63
25 1.68 3.56 4.00 6.86 9.18 10.01 12.45
30 1.68 3.55 4.01 6.85 9.14 9.97 12.29
35 1.69 3.55 4.02 6.84 9.11 9.93 12.13
40 1.69 3.55 4.03 6.84 9.07 9.89 11.98
45 1.70 3.55 4.04 6.83 9.04 9.86 11.84
50 1.71 3.55 4.06 6.83 9.02 9.83 11.71
55 1.72 3.55 4.07 6.83 8.99 . 11.57
60 1.72 3.56 4.09 6.84 8.96 . 11.45
70 1.74 3.58 4.12 6.85 8.92 . .
80 1.77 3.61 4.16 6.86 8.89 . .
90 1.79 3.65 4.19 6.88 8.85 . .
95 1.81 3.67 4.21 6.89 8.83 . .
A
For a discussion of the manner in which these pH values were assigned, see Bates, R. G., “Revised Standard Values for pH Measurements from 0 to 95°C,” Journal
ofResearch, NBS, Vol 66A, 1962, p. 179. The reference values were obtained without a liquid junction, which has an uncertainty of ±0.005. Liquid junction electrode values
may have an uncertainty of ±0.012, with uncertainty± 0.03 for the tetroxalate and the Ca(OH) . More recent values have been published in pH Measurement by Helmuth
Galster, VCH Publishers, Inc., New York, 1991.
TABLE 3 National Institute of Standards and Technology (NIST) Materials for Reference Buffer Solutions
NIST Standard Reference NIST SRM
A B
Buffer Salt Drying Procedure
Material Designation Drying Procedure
187 Borax (sodium tetraborate decahydrate) see NIST material certificate Drying not necessary (this salt should not be oven-
dried)
186 disodium hydrogen phosphate see NIST material certificate 2 h in oven at 130°
186 potassium dihydrogen phosphate see NIST material certificate 2 h in oven at 130°C
185 potassium hydrogen phthalate see NIST material certificate 2 h in oven at 110°C
188 potassium hydrogen tartrate see NIST material certificate drying not necessary
189 potassium tetroxalate dihydrate see NIST material certificate should not be dried
191 sodium bicarbonate see NIST material certificate should not be dried
192 sodium carbonate see NIST material certificate 2 h in oven at 275°C
2193 calcium carbonate see NIST material certificate see NIST material certificate
A
The buffer salts listed can be purchased from the Standard Reference Materials Program, National Institute of Standards and Technology, Gaithersburg, MD 20899.
B
The drying procedures in this column do not apply to NIST SRM preparations, but these procedures may be used with materials from other sources where NIST
traceability is not required.
solid from the filter, dry it in an oven at 110°C, and crush it to with the current renewal of the certificate. For other reference
a uniform and fine granular state. Prepare a saturated calcium materials, follow associated instructions or dissolve 10.12 g of
hydroxide solution by vigorously shaking a considerable ex-
potassiumhydrogenphthalate(KHC H O )inwateranddilute
8 4 4
cess (about 3 g/L) of the fine granular product in water at 25°C
to1L.
in a stoppered plastic bottle (that is, polypropylene or high
11.1.5 Tartrate Reference Buffer Solution (pH = 3.56 at
s
density polyethylene) that is essentially nonporous to gases.
25°C)—When using the NIST SRM, prepare in accordance
Allow the gross excess of solid to settle and filter the solution
with the current renewal of the certificate. For other reference
with suction through a fritted-glass funnel of medium porosity.
materials,followassociatedinstructionsorshakevigorouslyan
The filtrate is the reference buffer solution. Contamination of
excess (about 75 g/L) of potassium hydrogen tartrate
the solution with atmospheric carbon dioxide renders it turbid
(KHC H O ) with 100 to 300 mL of water at 25°C in a
4 4 6
and indicates need for replacement.
glass-stoppered bottle. Filter, if necessary, to remove sus-
11.1.3 Phosphate Reference Buffer Solution (pH = 6.86 at
s
pended salt. Add a crystal of thymol (about 0.1 g) as a
25°C)—When using the NIST SRM, prepare in accordance
preservative.
with the current renewal of the certificate. For other reference
11.1.6 Tetroxalate Reference Buffer Solution (pH = 1.68 at
materials, follow associated instructions or dissolve 3.39 g of s
25°C)—When using the NIST SRM, prepare in accordance
potassium dihydrogen phosphate (KH PO ) and 3.53 g of
2 4
with the current renewal of the certificate. For other reference
anhydrous disodium hydrogen phosphate (Na HPO ) in water
2 4
materials, follow associated instructions or dissolve 12.61 g of
and dilute to 1 L.
potassium tetroxalate dihydrate (KHC O ·H C O ·2H O) in
11.1.4 Phthalate Reference Buffer Solution (pH = 4.00 at
2 4 2 2 4 2
s
25°C)—When using the NIST SRM, prepare in accordance water and dilute to 1 L.
D1293 − 18
11.1.7 Sodium Bicarbonate—Sodium Carbonate Reference 12.6 Wash the electrodes and the sample container three
Buffer Solution (pH = 10.01 at 25°C)—When using the NIST times with water. Place the second reference buffer solution in
s
SRM, prepare in accordance with the current renewal of the the sample container, stir and measure the pH. Set the
certificate. For other reference materials, follow associated temperature corrected value of the second reference buffer
instructions or dissolve 2.092 g of sodium bicarbonate solution according to the meter manufacturer’s instructions.
(NaHCO )and2.640gofsodiumcarbonate(Na CO )inwater Use additional portions of the second reference buffer solution,
3 2 3
and dilute to 1 L. as before, until two successive readings differ by not more than
60.02 pH unit. The assembly shall be judged to be operating
11.2 Other Buffer Solutions—A buffer solution other than
satisfactorily if the reading obtained for the second reference
that specified may be used as a working standard in the method
buffer solution agrees with its assigned pH value within 0.05
s
providing that in each case such a solution is first checked
(or less) pH units.
against the corresponding reference buffer solution, using the
12.7 Use additional portions of the second reference buffer
procedures of the method, and is found to differ by not more
solution, as before, until two successive readings differ by not
than 60.02 pH unit.
more than 60.02 pH unit from pH . Alternatively, follow the
s
11.3 Commercial Buffer Solutions—Commercially available
manufacturer’s instructions for calibration with the second
prepared buffer solutions are not acceptable for the standard-
buffer.
ization in Test Method A.
12.8 Interim Checks: The assembly shall be judged to be
operating satisfactorily if the reading obtained for the second
12. Standardization of Assembly
reference buffer solution agrees with its assigned pH value
s
12.1 Turn on the instrument, allow it to warm up
within 60.05 (or less) pH units.
thoroughly, and bring it to electrical balance in accordance
12.9 If only an occasional pH determination is made,
with the manufacturer’s instructions. Wash the glass and
standardize the assembly each time it is used. In a long series
reference electrodes and the sample container with three
of measurements, supplemental interim checks at regular
changes of water or by means of flowing stream from a wash
intervals are recommended. Inasmuch as commercially avail-
bottle. Form a fresh liquid junction if a sleeve-type reference
able pH assemblies exhibit different degrees of measurement
junction is used. Note the temperature of the water to be tested.
stability, conduct these checks at intervals of 30 min, unless it
If temperature compensation is to be manual, adjust the
is ascertained that less frequent checking is satisfactory to
temperature setting of the meter to correspond to the tempera-
ensure the performance described in 12.2 to 12.6.
ture of the water to be tested and allow time for all buffers,
solutions, and electrodes to equilibrate thermally.
13. Procedure
12.2 Select at least two reference buffer solutions, the pH
s
13.1 Standardize the assembly with at least two reference
values of which closely bracket the anticipated pH (refer to
buffer solutions as described in 12.2 to 12.6 and then wash the
Table 2). Warm or cool the reference solutions as necessary to
electrodes with three changes of water or by means of a
match within 2°C the temperature of the solution to be tested.
flowing stream from a wash bottle.
Fill the sample container with the first reference buffer solution
13.2 Placethewatersampleinacleanglassbeakerprovided
and immerse the electrodes. Stir the solution as described in
with a stirring bar or stirrer and either a thermometer (for
13.3.
meters with manual temperature compensation) or an ATC
12.3 Fill the sample container with the first reference buffer
probe(formeterswithautomatictemperaturecompensation)or
solution and immerse the electrodes. Stir the solution as
use a pH electrode with integral temperature sensor.
described in 13.3.
13.3 Stir during the period of pH measurement at a rate that
12.4 Set the pH value of the reference buffer solution at the
will prevent splashing and that will avoid loss or gain of acidic
s
temperature of the buffer, as read from Table 2 or interpolated or basic gases by interchange with the atmosphere. When
from the data therein, according to the manufacturer’s instruc-
necessary, stir briskly enough to intermix the phases of a
tions. nonhomogeneous water sample. Stop the stirrer during periods
of measurement if fluctuations in readings are observed. (See
12.5 Empty the sample container and repeat, using succes-
Appendix X1.3.4 and X1.4.3.)
sive portions of the reference buffer solution, Repeat with
successive portions of the reference buffer solution until two 13.4 Insert the electrodes and determine a preliminary pH
value (since this value may drift somewhat, it should be
successive instrument readings are obtained which differ from
the pH value of the buffer solution by no more than 60.02 pH c
...

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: D1293 − 12 D1293 − 18
Standard Test Methods for
pH of Water
This standard is issued under the fixed designation D1293; 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.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope
1.1 These test methods cover the determination of pH by electrometric measurement using the glass electrode as the sensor. Two
test methods are given as follows:
Sections
Test Method A—Precise Laboratory Measurement 8 to 15
Test Method B—Routine or Continuous Measurement 16 to 24
Sections
Test Method A—Precise Laboratory Measurement 8 to 15
Test Method B—Routine or Continuous Measurement 16 to 24
1.2 Test Method A covers the precise measurement of pH in water utilizing at least two of seven standard reference buffer
solutions for instrument standardization.
1.3 Test Method B covers the routine measurement of pH in water and is especially useful for continuous monitoring. Two
buffers are used to standardize the instrument under controlled parameters, but the conditions are somewhat less restrictive than
those in Test Method A. For on-line measurement, also see Test Method D6569 which provides more detail.
1.4 Both test methods are based on the pH scale established by NIST (formerly NBS) Standard Reference Materials.
1.5 Neither test method is considered to be adequate for measurement of pH in water whose conductivity is less than about 5
μS/cm. Refer to Test Methods D5128 and D5464.
1.6 Precision and bias data were obtained using buffer solutions only. It is the user’s responsibility to assure the validity of these
test methods for untested types of water.
1.7 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this The values
given in parentheses are mathematical conversions to inch-pound units that are provided for information only and are not
considered standard.
1.8 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 safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.9 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:
D1066 Practice for Sampling Steam
D1067 Test Methods for Acidity or Alkalinity of Water
D1129 Terminology Relating to Water
These test methods are under the jurisdiction of ASTM Committee D19 on Water and are the direct responsibility of Subcommittee D19.03 on Sampling Water and
Water-Formed Deposits, Analysis of Water for Power Generation and Process Use, On-Line Water Analysis, and Surveillance of Water.
Current edition approved Jan. 1, 2012Jan. 15, 2018. Published January 2012January 2018. Originally approved in 1953. Last previous edition approved in 20052011 as
D1293 – 99(2005).11. DOI: 10.1520/D1293-11.10.1520/D1293-18.
“Standard Reference Materials: Standardization of pH Measurements” Wu and Koch, NBS Special Publications No. 260-53, 1988.Wu, and Koch, “Standard Reference
Materials: Standardization of pH Measurements,” NBS Special Publications No. 260-53, 1988.
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
D1293 − 18
D1192 Guide for Equipment for Sampling Water and Steam in Closed Conduits (Withdrawn 2003)
D1193 Specification for Reagent Water
D2777 Practice for Determination of Precision and Bias of Applicable Test Methods of Committee D19 on Water
D3370 Practices for Sampling Water from Closed Conduits
D5128 Test Method for On-Line pH Measurement of Water of Low Conductivity
D5464 Test Method for pH Measurement of Water of Low Conductivity
D6569 Test Method for On-Line Measurement of pH
E70 Test Method for pH of Aqueous Solutions With the Glass Electrode
3. Terminology
3.1 Definitions—For definitions of terms used in these test methods, refer to Terminology D1129.
3.1 Definitions:
3.1.1 For definitions of terms used in this standard, refer to Terminology D1129.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 pH, n—the negative logarithm of the hydrogen ion activity in an aqueous solution or the logarithm of the reciprocal of
the hydrogen ion activity.
3.2.1.1 Discussion—
TABLE 1 Slope Factor at Various Temperatures
Temperature,° C Slope, millivolts
0 54.20
5 55.19
10 56.18
15 57.17
20 58.17
25 59.16
30 60.15
35 61.14
40 62.13
45 63.13
50 64.12
55 65.11
60 66.10
65 67.09
70 68.09
75 69.08
80 70.07
85 71.06
90 72.05
95 73.05
theThe pH of an aqueous solution is derived from E, the electromotive force (emf) of the cellcell:
glass electrode | solution || reference electrode
(where the double vertical line represents a liquid junction) when the electrodes are immersed in the solution in the dia-
grammed position, and E is the electromotive force obtained when the electrodes are immersed in a reference buffer solution.
s
With the assigned pH of the reference buffer designated as pH , and E and E expressed in volts is the following:
s s
~E 2 E !F
s
pH 5 pH 1
s
2.3026 RT
where:
F = Faraday constant,
R = gas constant, and
T = absolute temperature, t(°C) + 273.15.
The reciprocal of F/2.3026 RT is known as the slope of the electrode, and is the expected difference in observed voltage for two
measurements one pH unit apart. Values of the slope at various temperatures are given in Table 1.
Bates, R. G., Determination of pH: Theory and Practice, 2nd Ed., J. Wiley and Sons, New York, 1973, p. 29.Bates, R. G., Determination of pH: Theory and Practice,
2nd ed., J. Wiley and Sons, New York, 1973, p. 29.
D1293 − 18
4. Summary of Test Method
4.1 The pH meter and associated electrodes are standardized against at least two reference buffer solutions that closely bracket
the anticipated sample pH. The sample measurement is made under strictly controlled conditions and prescribed techniques.
5. Significance and Use
5.1 The pH of water is a critical parameter affecting the solubility of trace minerals, the ability of the water to form scale or
to cause metallic corrosion, and the suitability of the water to sustain living organisms. It is a defined scale, based on a system of
buffer solutions with assigned values. In pure water at 25°C, pH 7.0 is the neutral point, but this varies with temperature and the
ionic strength of the sample. Pure water in equilibrium with air has a pH of about 5.5, and most natural uncontaminated waters
range between pH 6 and pH 9.
6. Purity of Reagents
6.1 Reagent grade chemicals shall be used in all tests, except as specifically noted for preparation of reference buffer solutions.
Unless otherwise indicated, it is intended that all reagents shall conform to the specifications of the Committee on Analytical
Reagents of the American Chemical Society, where such specifications are available. Other grades may be used, provided it is
first ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination.
6.2 Purity of Water—Unless otherwise indicated, references to water that is used for reagent preparation, rinsing, or dilution
shall be understood to mean reagent water conforming to Specification D1193, Type I.
7. Sampling
7.1 Collect samples in accordance with Practice D1066, or Practices D3370, whichever is applicable.
TEST METHOD A—PRECISE LABORATORY MEASUREMENT OF pH
8. Scope
8.1 This test method covers the precise measurement of pH in water under strictly controlled laboratory conditions.
8. Scope
8.1 This test method covers the precise measurement of pH in water under strictly controlled laboratory conditions.
9. Interferences
9.1 The glass electrode reliably measures pH in nearly all aqueous solutions and in general is not subject to solution interference
from color, turbidity, colloidal matter, oxidants, or reductants.
9.2 The reference electrode may be subject to interferences and should be chosen to conform to all requirements of Sections
10 and 12. Refer also to Appendix X1.3.
9.3 The true pH of an aqueous solution or extract is affected by the temperature. The electromotive force between the glass and
the reference electrode is a function of temperature as well as pH. The temperature effect can be compensated automatically in
many instruments or can be manually compensated in most other instruments. The temperature compensation corrects for the effect
of changes in electrode slope with temperature but does not correct for temperature effects on the chemical system being
monitored. It does not adjust the measured pH to a common temperature; therefore, the temperature should be reported for each
pH measurement. Temperature effects are discussed further in Appendix X1.2.
9.4 The pH response of the glass electrode/reference electrode pair is imperfect at both ends of the pH scale. The indicated pH
value of highly alkaline solutions may be too low, by as much as 1 pH, depending on electrode composition and sample conditions.
See X1.5.1. The indicated pH value of strong aqueous solutions of salts and strong acids having a pH less than 1, will often be
higher than the true pH value. Interferences can be minimized by the selection of the proper glass and reference electrodes for
measurements in highly alkaline or acidic solutions.
9.5 A few substances sometimes dispersed in water appear to poison the glass electrode. A discussion of this subject is given
in Appendix X1.4.
The relative acidity or alkalinity measured by pH should not be confused with total alkalinity or total acidity (for example, Test Methods D1067). Thus, 0.1 M HCl and
0.1 M acetic acid have the same total acidity, but the HCl solution will be more acidic (approximately pH 1 versus pH 3.).3).
Reagent Chemicals, American Chemical Society Specifications, American Chemical Society, Washington, DC. For suggestions on the testing of reagents not listed by
the American Chemical Society, see Analar Standards for Laboratory Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia and National
Formulary, U.S. Pharmaceutical Convention, Inc. (USPC), Rockville, MD.
D1293 − 18
A
TABLE 2 Approximate pH of Reference Buffer Solutions
s
Temperature, Tetroxalate Tartrate Phthalate Phosphate Borax Sodium Calcium Hydroxide
°C Solution Solution Solution Solution Solution Bicarbonate Solution
Sodium Carbonate
0 1.67 . 4.00 6.98 9.46 10.32 13.42
5 1.67 . 4.00 6.95 9.39 10.25 13.21
10 1.67 . 4.00 6.92 9.33 10.18 13.00
15 1.67 . 4.00 6.90 9.28 10.12 12.81
20 1.68 . 4.00 6.88 9.23 10.06 12.63
25 1.68 3.56 4.00 6.86 9.18 10.01 12.45
30 1.68 3.55 4.01 6.85 9.14 9.97 12.29
35 1.69 3.55 4.02 6.84 9.11 9.93 12.13
40 1.69 3.55 4.03 6.84 9.07 9.89 11.98
45 1.70 3.55 4.04 6.83 9.04 9.86 11.84
50 1.71 3.55 4.06 6.83 9.02 9.83 11.71
55 1.72 3.55 4.07 6.83 8.99 . 11.57
60 1.72 3.56 4.09 6.84 8.96 . 11.45
70 1.74 3.58 4.12 6.85 8.92 . .
80 1.77 3.61 4.16 6.86 8.89 . .
90 1.79 3.65 4.19 6.88 8.85 . .
95 1.81 3.67 4.21 6.89 8.83 . .
A
For a discussion of the manner in which these pH values were assigned, see Bates, R. G., “Revised Standard Values for pH Measurements from 0 to 95°C,” Journal
of Research, NBS, Vol 66A, 1962, p. 179. The reference values were obtained without a liquid junction, which has an uncertainty of ±0.005. Liquid junction electrode values
may have an uncertainty of ±0.012, with uncertainty± 0.03 for the tetroxalate and the Ca(OH) . More recent values have been published in pH Measurement by Helmuth
Galster, VCH Publishers, Inc., New York, 1991.
10. Apparatus
10.1 Laboratory pH Meter—Almost all commercially available meters are of the digital type and will have either manual or
automatic calibration, and either manual or automatic temperature (slope) correction. All four types are permissible. However,
readability to 0.01 pH is essential (Section 14), and the ability to read in millivolts is useful in troubleshooting.
10.2 Glass Electrode—The pH response of the glass electrode shall conform to the requirements set forth in 12.1 through 12.6.
The glass electrode lead wire shall be shielded. New glass electrodes and those that have been stored dry shall be conditioned and
maintained as recommended by the manufacturer.
10.3 Reference Electrode—This may be used as separate “half cell,” or it may be purchased integral with the glass pH electrode
body, as a combination electrode. The internal reference element may be calomel (mercury/mercurous chloride), silver/silver
chloride, or an iodide-iodine redox couple. For best performance, the reference element should be the same type in both the
reference electrode and inside the pH electrode. For all three types, the junction between the reference filling solution and the
sample may be either a flowing or nonflowing junction. The flowing liquid junction-type unit ensures that a fresh liquid junction
is formed for each measurement and shall be used for Test Method A determinations. If a saturated calomel electrode is used, some
potassium chloride crystals shall be contained in the saturated potassium chloride solution. If the reference electrode is of the
flowing junction type, the design of the electrode shall permit a fresh liquid junction to be formed between the reference electrode
solution and the buffer standard or tested water for each measurement and shall allow traces of solution to be washed from the outer
surfaces of the electrodes. To ensure the desired slow outward flow of reference electrode solution, the solution pressure inside the
liquid junction should be kept somewhat in excess of that outside the junction. In nonpressurized applications, this requirement
can be met by maintaining the inside solution level higher than the outside water level. If the reference electrode is of the
nonflowing junction type, these outward flow and pressurization considerations do not apply. The reference electrode and junction
shall perform satisfactorily as required in the standardizing procedure described in 12.1 through 12.6. A discussion of reference
electrodes is given in Appendix X1.3.
10.4 Temperature Compensator—The thermocompensator is a temperature-sensitive resistance element immersed in the water
sample with the electrodes. The thermocompensator may be incorporated into the pH electrode or may be a separate probe. The
thermocompensator automatically corrects for the change in slope of the glass electrode (with change of temperature) but does not
correct for actual changes in sample pH with temperature. The automatic thermocompensator is not required if the water
temperature is essentially constant and the analyst chooses to use the manual temperature compensation feature of the pH meter.
11. Reagents
11.1 Reference Buffer Solutions —Solutions—The approximate pH values of the reference buffer solutions measured at several
temperatures are listed in Table 2. If traceability to NIST standard reference materials (SRMs) is required, follow exactly the NIST
certificate drying, preparation, and storage instructions for the given renewal of the respective SRM buffer, and obtain the certified
value from the certificate for that SRM renewal at the applicable temperature. The current renewal of each NIST SRM should be
D1293 − 18
TABLE 3 National Institute of Standards and Technology (NIST) Materials for Reference Buffer Solutions
NIST Standard Reference NIST SRM
A B
Buffer Salt Drying Procedure
Material Designation Drying Procedure
187 Borax (sodium tetraborate decahydrate) see NIST material certificate Drying not necessary (this salt
should not be oven-dried)
186 disodium hydrogen phosphate see NIST material certificate 2 h in oven at 130°
186 potassium dihydrogen phosphate see NIST material certificate 2 h in oven at 130°C
185 potassium hydrogen phthalate see NIST material certificate 2 h in oven at 110°C
188 potassium hydrogen tartrate see NIST material certificate drying not necessary
189 potassium tetroxalate dihydrate see NIST material certificate should not be dried
191 sodium bicarbonate see NIST material certificate should not be dried
192 sodium carbonate see NIST material certificate 2 h in oven at 275°C
2193 calcium carbonate see NIST material certificate see NIST material certificate
A
The buffer salts listed can be purchased from the Standard Reference Materials Program, National Institute of Standards and Technology, Gaithersburg, MD 20899.
B
he The drying procedures in this column do not apply to NIST SRM preparations, but these procedures may be used with materials from other sources where NIST
traceability is not required.
used. Buffer solution compositions and preparations given in Sections 11.1.111.1.1 through 11.1.711.1.7 are for use with non-NIST
materials. For NIST SRMs, follow the instructions included with the SRM.
Table 3 identifies each buffer salt by its National Institute of Standards and Technology (NIST) number Dry the buffer salt before
use. The drying procedure will be according to the current renewal of the NIST certificate. Recommended drying procedures for
non-NIST reference materials are also provided, in the last column of the table. Keep the five reference buffer solutions with pH
less than 9.5 in bottles of chemically resistant glass. Keep the calcium hydroxide solutions in a plastic bottle that is nonporous to
air (that is, polypropylene or high density polyethylene). Keep all the reference buffer solutions well-stoppered and replace if a
visible change is observed.
11.1.1 Borax Reference Buffer Solution (pH = 9.18 at 25°C)—When using the NIST SRM, prepare in accordance with the
s
current renewal of the certificate. For other reference materials, follow associated instructions or dissolve 3.80 g of sodium
tetraborate decahydrate (Na B O ·10H O) in water and dilute to 1 L.
2 4 7 2
11.1.2 Calcium Hydroxide Reference Buffer Solution(pH = 12.45 at 25°C)—When using the NIST SRM, prepare in accordance
s
with the current renewal of the certificate. For other reference materials, follow associated instructions or prepare pure calcium
hydroxide (Ca(OH) ) from well-washed calcium carbonate (CaCO ) of low-alkali grade by slowly heating the carbonate in a
2 3
platinum dish at 1000°C and calcining for at least 45 min at that temperature. After cooling in a dessicator, add the calcined product
slowly to water with stirring, heat the resultant suspension to boiling, cool, and filter through a funnel having a fritted-glass disk
of medium porosity. Collect the solid from the filter, dry it in an oven at 110°C, and crush it to a uniform and fine granular state.
Prepare a saturated calcium hydroxide solution by vigorously shaking a considerable excess (about 3 g/L) of the fine granular
product in water at 25°C in a stoppered plastic bottle (that is, polypropylene or high density polyethylene) that is essentially
nonporous to gases. Allow the gross excess of solid to settle and filter the solution with suction through a fritted-glass funnel of
medium porosity. The filtrate is the reference buffer solution. Contamination of the solution with atmospheric carbon dioxide
renders it turbid and indicates need for replacement.
11.1.3 Phosphate Reference Buffer Solution (pH = 6.86 at 25°C)—When using the NIST SRM, prepare in accordance with the
s
current renewal of the certificate. For other reference materials, follow associated instructions or dissolve 3.39 g of potassium
dihydrogen phosphate (KH PO ) and 3.53 g of anhydrous disodium hydrogen phosphate (Na HPO ) in water and dilute to 1 L.
2 4 2 4
11.1.4 Phthalate Reference Buffer Solution (pH = 4.00 at 25°C)—When using the NIST SRM, prepare in accordance with the
s
current renewal of the certificate. For other reference materials, follow associated instructions or dissolve 10.12 g of potassium
hydrogen phthalate (KHC H O ) in water and dilute to 1 L.
8 4 4
11.1.5 Tartrate Reference Buffer Solution (pH = 3.56 at 25°C)—When using the NIST SRM, prepare in accordance with the
s
current renewal of the certificate. For other reference materials, follow associated instructions or shake vigorously an excess (about
75 g/L) of potassium hydrogen tartrate (KHC H O ) with 100 to 300 mL of water at 25°C in a glass-stoppered bottle. Filter, if
4 4 6
necessary, to remove suspended salt. Add a crystal of thymol (about 0.1 g) as a preservative.
11.1.6 Tetroxalate Reference Buffer Solution (pH = 1.68 at 25°C)—When using the NIST SRM, prepare in accordance with the
s
current renewal of the certificate. For other reference materials, follow associated instructions or dissolve 12.61 g of potassium
tetroxalate dihydrate (KHC O ·H C O ·2H O) in water and dilute to 1 L.
2 4 2 2 4 2
11.1.7 Sodium Bicarbonate—Sodium Carbonate Reference Buffer Solution (pH = 10.01 at 25°C)—When using the NIST SRM,
s
prepare in accordance with the current renewal of the certificate. For other reference materials, follow associated instructions or
dissolve 2.092 g of sodium bicarbonate (NaHCO ) and 2.640 g of sodium carbonate (Na CO ) in water and dilute to 1 L.
3 2 3
11.2 Other Buffer Solutions—A buffer solution other than that specified may be used as a working standard in the method
providing that in each case such a solution is first checked against the corresponding reference buffer solution, using the procedures
of the method, and is found to differ by not more than 6 0.02 60.02 pH unit.
11.3 Commercial Buffer Solutions —Solutions—Commercially available prepared buffer solutions are not acceptable for the
standardization in Test Method A.
D1293 − 18
12. Standardization of Assembly
12.1 Turn on the instrument, allow it to warm up thoroughly, and bring it to electrical balance in accordance with the
manufacturer’s instructions. Wash the glass and reference electrodes and the sample container with three changes of water or by
means of flowing stream from a wash bottle. Form a fresh liquid junction if a sleeve-type reference junction is used. Note the
temperature of the water to be tested. If temperature compensation is to be manual, adjust the temperature setting of the meter to
correspond to the temperature of the water to be tested and allow time for all buffers, solutions, and electrodes to equilibrate
thermally.
12.2 Select at least two reference buffer solutions, the pH values of which closely bracket the anticipated pH (refer to Table
s
2). Warm or cool the reference solutions as necessary to match within 2°C the temperature of the solution to be tested. Fill the
sample container with the first reference buffer solution and immerse the electrodes. Stir the solution as described in 13.3.
12.3 Fill the sample container with the first reference buffer solution and immerse the electrodes. Stir the solution as described
in 13.3.
12.4 Set the pH value of the reference buffer solution at the temperature of the buffer, as read from Table 2 or interpolated from
s
the data therein, according to the manufacturer’s instructions.
12.5 Empty the sample container and repeat, using successive portions of the reference buffer solution, Repeat with successive
portions of the reference buffer solution until two successive instrument readings are obtained which differ from the pH value of
s
the buffer solution by no more than 6 0.02 60.02 pH unit. Alternatively, follow the manufacturer’s instructions for calibration with
the first buffer.
NOTE 1—If the temperature of the electrode differs appreciably from that of the solution to be tested, use several portions of solution and immerse the
electrodes deeply to assure that both the electrodes and the solution are at the desired temperature. To reduce the effects of thermal lag, keep the
temperature of electrodes, reference buffer solutions, and the wash as close to that of the water sample as possible.
12.6 Wash the electrodes and the sample container three times with water. Place the second reference buffer solution in the
sample container, stir and measure the pH. Set the temperature corrected value of the second reference buffer solution according
to the meter manufacturer’s instructions. Use additional portions of the second reference buffer solution, as before, until two
successive readings differ by not more than 6 0.02 60.02 pH unit. The assembly shall be judged to be operating satisfactorily if
the reading obtained for the second reference buffer solution agrees with its assigned pH value within 0.05 (or less) pH units.
s
12.7 Use additional portions of the second reference buffer solution, as before, until two successive readings differ by not more
than 6 0.02 60.02 pH unit from pH . Alternatively, follow the manufacturer’s instructions for calibration with the second buffer.
s
12.8 Interim Checks: The assembly shall be judged to be operating satisfactorily if the reading obtained for the second reference
buffer solution agrees with its assigned pH value within 6 0.05 60.05 (or less) pH units.
s
12.9 If only an occasional pH determination is made, standardize the assembly each time it is used. In a long series of
measurements, supplemental interim checks at regular intervals are recommended. Inasmuch as commercially available pH
assemblies exhibit different degrees of measurement stability, conduct these checks at intervals of 30 min, unless it is ascertained
that less frequent checking is satisfactory to ensure the performance described in 12.2 to 12.6.
13. Procedure
13.1 Standardize the assembly with at least two reference buffer solutions as described in 12.2 to 12.6 and then wash the
electrodes with three changes of water or by means of a flowing stream from a wash bottle.
13.2 Place the water sample in a clean glass beaker provided with a stirring bar or stirrer and either a thermometer (for meters
with manual temperature compensation) or an ATC probe (for meters with automatic temperature compensation) or use a pH
electrode with integral temperature sensor.
13.3 Stir during the period of pH measurement at a rate that will prevent splashing and that will avoid loss or gain of acidic
or basic gases by interchange with the atmosphere. When necessary, stir briskly enough to intermix the phases of a
nonhomogeneous water sample. Stop the stirrer during periods of measurement if fluctuations in readings are observed. (See
Appendix X1.3.4 and X1.4.3).)
13.4 Insert the electrodes and determine a preliminary pH value (since this value may drift somewhat, it should be considered
an estimated value). Measure
...