ASTM E70-07(2015)
(Test Method)Standard Test Method for pH of Aqueous Solutions With the Glass Electrode
Standard Test Method for pH of Aqueous Solutions With the Glass Electrode
SIGNIFICANCE AND USE
4.1 pH is, within the limits described in 1.1, an accurate measurement of the hydrogen ion concentration and thus is widely used for the characterization of aqueous solutions.
4.2 pH measurement is one of the main process control variables in the chemical industry and has a prominent place in pollution control.
SCOPE
1.1 This test method specifies the apparatus and procedures for the electrometric measurement of pH values of aqueous solutions with the glass electrode. It does not deal with the manner in which the solutions are prepared. pH measurements of good precision can be made in aqueous solutions containing high concentrations of electrolytes or water-soluble organic compounds, or both. It should be understood, however, that pH measurements in such solutions are only a semiquantitative indication of hydrogen ion concentration or activity. The measured pH will yield an accurate result for these quantities only when the composition of the medium matches approximately that of the standard reference solutions. In general, this test method will not give an accurate measure of hydrogen ion activity unless the pH lies between 2 and 12 and the concentration of neither electrolytes nor nonelectrolytes exceeds 0.1 mol/L (M).
1.2 The values stated in SI units are to be regarded as standard. The values in parentheses are for information only.
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.
General Information
Relations
Buy Standard
Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: E70 − 07 (Reapproved 2015)
Standard Test Method for
pH of Aqueous Solutions With the Glass Electrode
ThisstandardisissuedunderthefixeddesignationE70;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginal
adoptionor,inthecaseofrevision,theyearoflastrevision.Anumberinparenthesesindicatestheyearoflastreapproval.Asuperscript
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* 2. Referenced Documents
1.1 This test method specifies the apparatus and procedures 2.1 ASTM Standards:
for the electrometric measurement of pH values of aqueous D1193Specification for Reagent Water
solutions with the glass electrode. It does not deal with the E180Practice for Determining the Precision of ASTM
manner in which the solutions are prepared. pH measurements Methods for Analysis and Testing of Industrial and Spe-
ofgoodprecisioncanbemadeinaqueoussolutionscontaining cialty Chemicals (Withdrawn 2009)
high concentrations of electrolytes or water-soluble organic E691Practice for Conducting an Interlaboratory Study to
compounds,orboth.Itshouldbeunderstood,however,thatpH Determine the Precision of a Test Method
measurements in such solutions are only a semiquantitative
3. Terminology
indication of hydrogen ion concentration or activity. The
measured pH will yield an accurate result for these quantities 3.1 Definitions:
only when the composition of the medium matches approxi- 3.1.1 pH—defined formally as the negative logarithm to the
mately that of the standard reference solutions. In general, this base 10 of the conventional hydrogen ion activity. See Appen-
test method will not give an accurate measure of hydrogen ion dix X1.
activity unless the pH lies between 2 and 12 and the concen-
3.2 Definitions of Terms Specific to This Standard:
tration of neither electrolytes nor nonelectrolytes exceeds 0.1
3.2.1 For the purpose of this test method, the term “meter”
mol/L (M).
shall apply to the instrument used for the measurement of
1.2 The values stated in SI units are to be regarded as potential (either in millivolts or in terms of pH units), the term
standard. The values in parentheses are for information only.
“electrodes” to the glass electrode and the reference electrode,
and the term “assembly” to the combination of the meter and
1.3 This standard does not purport to address all of the
the electrodes. The performance of the meter shall be differ-
safety concerns, if any, associated with its use. It is the
entiated from that of the electrodes.
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter-
4. Significance and Use
mine the applicability of regulatory limitations prior to use.
4.1 pH is, within the limits described in 1.1, an accurate
1.4 This international standard was developed in accor-
measurement of the hydrogen ion concentration and thus is
dance with internationally recognized principles on standard-
widely used for the characterization of aqueous solutions.
ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
4.2 pH measurement is one of the main process control
mendations issued by the World Trade Organization Technical
variablesinthechemicalindustryandhasaprominentplacein
Barriers to Trade (TBT) Committee.
pollution control.
1 2
This test method is under the jurisdiction of ASTM Committee D16 on For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Aromatic, Industrial, Specialty and Related Chemicals and is the direct responsi- contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
bility of Subcommittee D16.04 on Instrumental Analysis. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved June 1, 2015. Published June 2015. Originally the ASTM website.
approved in 1952. Last previous edition approved in 2007 as E70–07. DOI: The last approved version of this historical standard is referenced on
10.1520/E0070-07R15. www.astm.org.
*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
E70 − 07 (2015)
5. Apparatus 5.2.4 If the assembly is in intermittent use, the ends of the
electrodes shall be immersed in distilled water between mea-
5.1 pH meters—Many excellent pH meters are available
surements. The high-alkalinity type of glass electrode shall be
from commercial sources. To some extent, the choice of meter
storedintheboraxbuffersolution.Forprolongedstorage,glass
will depend on the desired precision of measurement. The
electrodes may be allowed to become dry, and reference
meter may operate on a null-detection principle or may utilize
electrodes shall be capped to prevent undue evaporation.
digital readout or a direct deflection meter with a large scale.
Power may be supplied by batteries or a-c operation may be
NOTE2—Newglasselectrodesandthosethathavebeenstoreddryshall
be conditioned as recommended by the manufacturer. Requirements for
provided. The maximum grid current drawn from the glass
−12
the physical dimensions and shape of the electrodes and the composition
electrode during measurement shall not exceed 2×10 A.
of the internal reference solution are not considered part of this test
Automatic or manual adjustment shall allow for changes in
method.
F/(RT ln 10) when the temperature of the assembly is altered.
For referee work, or in case of dispute, meters capable of
6. Reagents and Materials
discriminatingchangesofpHto0.01unit(0.6mV)orlessshall
6.1 The pH(S) of six recommended standard solutions at
be used.
several temperatures is listed in Table 1. The buffer solutions
5.2 Reference Electrodes and Glass Electrodes:
shall be prepared from highly purified materials sold specifi-
5.2.1 Thesaturatedcalomelelectrodeandthe3.5mol/L(M)
cally as pH standards (Note 3). Potassium hydrogen phthalate
calomel electrode are suitable as reference electrodes in pH
and the two phosphate salts shall be dried at 110°C for 1 h
assemblies (Note 1). If the saturated electrode is used, a few
before use, but borax and sodium bicarbonate shall not be
crystals of solid potassium chloride shall be present in the
heated above room temperature. Potassium dihydrogen citrate
chamber surrounding the electrode element at each tempera-
shall be dried for1hat 80°C, and sodium carbonate shall be
ture. The design of the electrode shall permit a fresh liquid
ignited for1hat 270°C before use. The standard solutions
junction between the solution of potassium chloride and the
shall be prepared as described in 6.4 – 6.9. They shall be
bufferortestsolutiontobeformedforeachtestandshallallow
preserved in bottles of chemically resistant glass or polyethyl-
traces of solution to be readily removed by washing.
ene and shall be replaced at an age of six weeks, or earlier if a
visible change should occur in the solution.
NOTE 1—Other reference electrodes of constant potential may be used,
provided no difficulty is experienced in standardizing the assembly as
NOTE 3—Six of the buffer salts can be obtained in the form of standard
described in Section 8.
reference materials from the National Bureau of Standards. These mate-
5.2.2 Thesilver-silverchlorideelectrodealsoisusedwidely rials are numbered as follows:
as a reference electrode. Buffer Salt SRM No.
Potassium hydrogen phthalate 185
5.2.3 Commercial glass electrodes are designed for certain
Potassium dihydrogen phosphate 186I
specific ranges of pH and temperature; consequently, the pH
Disodium hydrogen phosphate 186II
and temperature of the test solutions shall be considered in Borax 187
Sodium bicarbonate 191
selecting the glass electrode for use. The pH response shall
Sodium carbonate 192
conformwiththerequirementssetforthinSection7.Theleads
shall be shielded from the effects of body capacitance. The pH(S) values may vary slightly from one lot to another;
A,B
TABLE 1 pH(S) of Standard Solutions
Temperature, °C A B C D E F
0 3.863 4.003 6.984 7.534 9.464 10.317
10 3.820 3.998 6.923 7.472 9.332 10.179
20 3.788 4.002 6.881 7.429 9.225 10.062
25 3.776 4.008 6.865 7.413 9.180 10.012
30 3.766 4.015 6.853 7.400 9.139 9.966
35 3.759 4.024 6.844 7.389 9.102 9.925
40 3.753 4.035 6.838 7.380 9.068 9.889
50 3.749 4.060 6.833 7.367 9.011 9.828
60 . 4.091 6.836 . 8.962 .
70 . 4.126 6.845 . 8.921 .
80 . 4.164 6.859 . 8.885 .
90 . 4.205 6.877 . 8.850 .
A
The compositions of the standard solutions are:
−1
A—KH citrate, m = 0.05 mol kg
−1
B—KH phthalate, m = 0.05 mol kg
−1 −1
C—KH PO , m = 0.025 mol kg ;Na HPO , m = 0.025 mol kg
2 4 2 4
−1 −1
D—KH PO , m = 0.008695 mol kg ;Na HPO , m = 0.03043 mol kg
2 4 2 4
−1
E—Na B O , m = 0.01 mol kg
2 4 7
−1 −1
F—NaHCO , m = 0.025 mol kg ;Na CO , m = 0.025 mol kg
3 2 3
where m denotes molality.
B
For a discussion of the manner in which these pH(S) values were assigned, see Chapter 4 of the book by Bates, R. G., Determination of pH, Theory and Practice, John
Wiley and Sons, Second edition, New York, 1973.
E70 − 07 (2015)
consequently, the values given on the SRM certificate should be used in usualpHmeasurement,thestabilityofthemeter,theaccuracyofthescale
preference to those given in Table 2, if slight differences exist. reading, and the pH response of the glass electrode over the range of the
measurements are verified by checking the assembly with a series of
6.2 Commercialstandardbuffersareavailable.Forthemost
standard buffer solutions.
exactmeasurements,thevalueofthecommercialbuffershould
be verified using one of the recommended standard buffers in
7.1 Assembly—Theassemblyshallbejudgedtobeperform-
Table 1.
ing satisfactorily if it furnishes, within acceptable limits of
accuracy, the correct pH values for the standard buffer solu-
6.3 Distilled Water—The conductivity of the distilled water
−6 −1
tions listed in Table 2. When the electrodes are immersed in a
shall not exceed 2×10 s·cm . For the preparation of the
buffer solution, the measured potential difference shall be
citrate, phthalate, and phosphate solutions, the water need not
substantially constant, and the cause of any instability shall be
be freed of dissolved carbon dioxide. The water used for the
borax standard and the carbonate standard shall be boiled for determined.
15 min or purged with air free of carbon dioxide and shall be
7.2 Meter—The meter shall be brought to electrical balance
protected with a soda-lime tube or equivalent (Note 4) while
in accordance with the manufacturer’s instructions. The per-
coolingandinstorage.ThepHofthecarbondioxide-freewater
formance shall then be tested by applying a known variable
shall be between 6.6 and 7.5 at 25°C. The temperature of the
potential through a resistance of approximately 200 MΩ to the
water used to prepare the standards shall be within 2°C of
terminals of the meter, the high-resistance lead being con-
25°C. The amounts of the buffer salts given in 5.3 through 5.8
nectedtotheterminalcorrespondingtotheglasselectrode.The
areweightsinairnearsealeveldeterminedwithbrassweights.
source of potential may be a precision-type potentiometer with
NOTE 4—The water used for preparing the standard buffer solutions
a range of 1100 mV or more and a limit of error not greater
shall be Types I or II reagent water in accordance with Specification
than0.1mV.The200-MΩresistorshallbeproperlyshieldedto
D1193.Precautionsshallbetakentopreventcontaminationofthedistilled
water with traces of the material used for protection against carbon
avoid capacity pickup. Commencing with a value of zero, the
dioxide.
applied potential shall be increased in increments of 100 mV,
6.4 Citrate, Standard Solution A (molality = 0.05 mol/kg;
and the readings of the dial of the meter at balance shall be
pH(S) = 3.776 at 25°C)—Dissolve 11.41 g of potassium dihy-
noted. The process shall be extended to cover the entire range
drogen citrate in distilled water and dilute to 1 L.
ofthemeter.Innocaseshallthedifferencebetweentheapplied
voltage and that indicated by the meter differ by more than 1
6.5 Phthalate, Standard Solution B (molality = 0.05 mol/kg;
pH(S) = 4.008 at 25°C)—Dissolve 10.12 g of potassium hy- mV per increment of applied voltage.
drogen phthalate in distilled water and dilute to 1 L.
NOTE6—Ifthecumulativeerrorattheendofthescaleexceeds 63mV,
6.6 Phosphate, Standard Equimolal Solution C (molality of
a calibration curve for the meter shall be constructed and corrections
each phosphate salt = 0.025 mol⁄kg; pH(S) = 6.865 at applied to each measurement of electromotive force or pH. Differences of
electromotive force (volts) are converted to corresponding differences of
25°C)—Dissolve 3.388 g of potassium dihydrogen phosphate
pH by multiplying by F/(RTln 10) (Table X1.1). Inasmuch as the meter is
and 3.533 g of disodium hydrogen phosphate in distilled water
madetoreadcorrectlyatthepHofthestandard,thecalibrationcorrection
and dilute to 1 L.
to be applied to a pH measurement is the difference between the scale
6.7 Phosphate, Standard Solution D (1 + 3) (molality of
corrections at the pH of the standard and that of the unknown, with due
KH PO = 0.008695 mol/kg, molality of Na HPO = 0.03043 regard for sign.
2 4 2 4
mol/kg); pH(S) = 7.413 at 25°C)—Dissolve 1.179 g of potas-
7.3 Glass Electrodes—The difference of potential between
sium dihydrogen phosphate and 4.302 g of disodium hydrogen
the glass electrode and the standard hydrogen gas electrode
phosphate in distilled water and dilute to 1 L.
shall be measured when both electrodes are immersed in the
6.8 Borax, Standard Solution E (molality = 0.01 mol/kg;
same portion of various buffer solutions over the pH range in
pH(S) = 9.180 at 25°C)—Dissolve 3.80 g of sodium tetrabo-
which the glass electrode is to be used. For these comparisons
rate decahydrate (borax) in distilled water and dilute to 1 L.
the cell shall be placed in a water bath thermostatically
6.9 Carbonate, Standard Solution F (molality of each car-
controlledto 60.1°Cnear25°C.Thesolutionsusedforthistest
bonate salt = 0.025 mol⁄kg; pH(S) = 10.012 at 25°C)—
shallbethoselistedinSection6.ThestandardsofpH9.18and
Dissolve2.092gofsodiumbicarbonateand2.640gofsodium
below (at 25°C) shall be used to test electrodes of the
carbonate in distilled water and dilute to 1 L.
general-purpose type. The borax and carbonate standards shall
be used to test the high-alkalinity type of electrode. These
7. Performance Tests of Meter and Electrodes
buffer solutions shall be supplemented by a 0.1 mol/kg (M)
NOTE 5—Except for measurements of the high
...
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: E70 − 07 E70 − 07 (Reapproved 2015)
Standard Test Method for
pH of Aqueous Solutions With the Glass Electrode
This standard is issued under the fixed designation E70; 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 This test method specifies the apparatus and procedures for the electrometric measurement of pH values of aqueous
solutions with the glass electrode. It does not deal with the manner in which the solutions are prepared. pH measurements of good
precision can be made in aqueous solutions containing high concentrations of electrolytes or water-soluble organic compounds, or
both. It should be understood, however, that pH measurements in such solutions are only a semiquantitative indication of hydrogen
ion concentration or activity. The measured pH will yield an accurate result for these quantities only when the composition of the
medium matches approximately that of the standard reference solutions. In general, this test method will not give an accurate
measure of hydrogen ion activity unless the pH lies between 2 and 12 and the concentration of neither electrolytes nor
nonelectrolytes exceeds 0.1 mol/L (M).
1.2 The values stated in SI units are to be regarded as standard. The values in parentheses are for information only.
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.
2. Referenced Documents
2.1 ASTM Standards:
D1193 Specification for Reagent Water
E180 Practice for Determining the Precision of ASTM Methods for Analysis and Testing of Industrial and Specialty Chemicals
(Withdrawn 2009)
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
3. Terminology
3.1 Definitions:
3.1.1 pH—defined formally as the negative logarithm to the base 10 of the conventional hydrogen ion activity. See Appendix
X1.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 For the purpose of this test method, the term “meter” shall apply to the instrument used for the measurement of potential
(either in millivolts or in terms of pH units), the term “electrodes” to the glass electrode and the reference electrode, and the term
“assembly” to the combination of the meter and the electrodes. The performance of the meter shall be differentiated from that of
the electrodes.
4. Significance and Use
4.1 pH is, within the limits described in 1.1, an accurate measurement of the hydrogen ion concentration and thus is widely used
for the characterization of aqueous solutions.
This test method is under the jurisdiction of ASTM Committee E15 on Industrial and Specialty Chemicals and is the direct responsibility of Subcommittee E15.01 on
General Standards.
Current edition approved April 1, 2007June 1, 2015. Published May 2007June 2015. Originally approved in 1952. Last previous edition approved in 20022007 as E70 – 97
(2002).E70 – 07. DOI: 10.1520/E0070-07.10.1520/E0070-07R15.
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 the ASTM website.
The last approved version of this historical standard is referenced on www.astm.org.
*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
E70 − 07 (2015)
4.2 pH measurement is one of the main process control variables in the chemical industry and has a prominent place in pollution
control.
5. Apparatus
5.1 pH meters—Many excellent pH meters are available from commercial sources. To some extent, the choice of meter will
depend on the desired precision of measurement. The meter may operate on a null-detection principle or may utilize digital readout
or a direct deflection meter with a large scale. Power may be supplied by batteries or a-c operation may be provided. The maximum
−12
grid current drawn from the glass electrode during measurement shall not exceed 2 × 10 A. Automatic or manual adjustment
shall allow for changes in F/(RT ln 10) when the temperature of the assembly is altered. For referee work, or in case of dispute,
meters capable of discriminating changes of pH to 0.01 unit (0.6 mV) or less shall be used.
5.2 Reference Electrodes and Glass Electrodes:
5.2.1 The saturated calomel electrode and the 3.5 mol/L (M) calomel electrode are suitable as reference electrodes in pH
assemblies (Note 1). If the saturated electrode is used, a few crystals of solid potassium chloride shall be present in the chamber
surrounding the electrode element at each temperature. The design of the electrode shall permit a fresh liquid junction between
the solution of potassium chloride and the buffer or test solution to be formed for each test and shall allow traces of solution to
be readily removed by washing.
NOTE 1—Other reference electrodes of constant potential may be used, provided no difficulty is experienced in standardizing the assembly as described
in Section 88.
5.2.2 The silver-silver chloride electrode also is used widely as a reference electrode.
5.2.3 Commercial glass electrodes are designed for certain specific ranges of pH and temperature; consequently, the pH and
temperature of the test solutions shall be considered in selecting the glass electrode for use. The pH response shall conform with
the requirements set forth in Section 77. The leads shall be shielded from the effects of body capacitance.
5.2.4 If the assembly is in intermittent use, the ends of the electrodes shall be immersed in distilled water between
measurements. The high-alkalinity type of glass electrode shall be stored in the borax buffer solution. For prolonged storage, glass
electrodes may be allowed to become dry, and reference electrodes shall be capped to prevent undue evaporation.
NOTE 2—New glass electrodes and those that have been stored dry shall be conditioned as recommended by the manufacturer. Requirements for the
physical dimensions and shape of the electrodes and the composition of the internal reference solution are not considered part of this test method.
6. Reagents and Materials
6.1 The pH(S) of six recommended standard solutions at several temperatures is listed in Table 1. The buffer solutions shall be
prepared from highly purified materials sold specifically as pH standards (Note 3). Potassium hydrogen phthalate and the two
phosphate salts shall be dried at 110°C for 1 h before use, but borax and sodium bicarbonate shall not be heated above room
temperature. Potassium dihydrogen citrate shall be dried for 1 h at 80°C, and sodium carbonate shall be ignited for 1 h at 270°C
before use. The standard solutions shall be prepared as described in 6.4 – 6.9. They shall be preserved in bottles of chemically
resistant glass or polyethylene and shall be replaced at an age of 6six weeks, or earlier if a visible change should occur in the
solution.
NOTE 3—Six of the buffer salts can be obtained in the form of standard reference materials from the National Bureau of Standards. These materials
are numbered as follows:
Buffer Salt SRM No.
Potassium hydrogen phthalate 185
Potassium dihydrogen phosphate 186I
Disodium hydrogen phosphate 186II
Borax 187
Sodium bicarbonate 191
Sodium carbonate 192
Buffer Salt SRM No.
Potassium hydrogen phthalate 185
Potassium dihydrogen phosphate 186I
Disodium hydrogen phosphate 186II
Borax 187
Sodium bicarbonate 191
Sodium carbonate 192
The pH(S) values may vary slightly from one lot to another; consequently, the values given on the SRM certificate should be used in preference to
those given in Table 2, if slight differences exist.
6.2 Commercial standard buffers are available. For the most exact measurements, the value of the commercial buffer should be
verified using one of the recommended standard buffers in Table 1.
−6 −1
6.3 Distilled Water—The conductivity of the distilled water shall not exceed 2 × 10 s · cm . For the preparation of the citrate,
phthalate, and phosphate solutions, the water need not be freed of dissolved carbon dioxide. The water used for the borax standard
and the carbonate standard shall be boiled for 15 min or purged with air free of carbon dioxide and shall be protected with a
E70 − 07 (2015)
A,B
TABLE 1 pH(S) of Standard Solutions
Temperature, °C A B C D E F
0 3.863 4.003 6.984 7.534 9.464 10.317
10 3.820 3.998 6.923 7.472 9.332 10.179
20 3.788 4.002 6.881 7.429 9.225 10.062
25 3.776 4.008 6.865 7.413 9.180 10.012
30 3.766 4.015 6.853 7.400 9.139 9.966
35 3.759 4.024 6.844 7.389 9.102 9.925
40 3.753 4.035 6.838 7.380 9.068 9.889
50 3.749 4.060 6.833 7.367 9.011 9.828
60 . 4.091 6.836 . 8.962 .
70 . 4.126 6.845 . 8.921 .
80 . 4.164 6.859 . 8.885 .
90 . 4.205 6.877 . 8.850 .
A,B
TABLE 1 pH(S) of Standard Solutions
Temperature, °C A B C D E F
0 3.863 4.003 6.984 7.534 9.464 10.317
10 3.820 3.998 6.923 7.472 9.332 10.179
20 3.788 4.002 6.881 7.429 9.225 10.062
25 3.776 4.008 6.865 7.413 9.180 10.012
30 3.766 4.015 6.853 7.400 9.139 9.966
35 3.759 4.024 6.844 7.389 9.102 9.925
40 3.753 4.035 6.838 7.380 9.068 9.889
50 3.749 4.060 6.833 7.367 9.011 9.828
60 . 4.091 6.836 . 8.962 .
70 . 4.126 6.845 . 8.921 .
80 . 4.164 6.859 . 8.885 .
90 . 4.205 6.877 . 8.850 .
A
The compositions of the standard solutions are:
−1
A—KH citrate, m = 0.05 mol kg
−1
B—KH phthalate, m = 0.05 mol kg
−1 −1
C—KH PO , m = 0.025 mol kg ; Na HPO , m = 0.025 mol kg
2 4 2 4
−1 −1
D—KH PO , m = 0.008695 mol kg ; Na HPO , m = 0.03043 mol kg
2 4 2 4
−1
E—Na B O , m = 0.01 mol kg
2 4 7
−1 −1
F—NaHCO , m = 0.025 mol kg ; Na CO , m = 0.025 mol kg
3 2 3
where m denotes molality.
B
For a discussion of the manner in which these pH(S) values were assigned, see Chapter 4 of the book by Bates, R. G., Determination of pH, Theory and Practice, John
Wiley and Sons, Second edition, New York, 1973.
TABLE 2 Bias of pH Measurements
Nominal pH Hydrogen Electrode Glass Electrode Difference
3.7 3.715 3.73 + 0.015
6.5 6.519 6.53 + 0.011
8.2 8.174 8.18 + 0.006
8.4 8.478 8.45 – 0.028
TABLE 2 Bias of pH Measurements
Nominal pH Hydrogen Electrode Glass Electrode Difference
3.7 3.715 3.73 + 0.015
6.5 6.519 6.53 + 0.011
8.2 8.174 8.18 + 0.006
8.4 8.478 8.45 – 0.028
soda-lime tube or equivalent (Note 4) while cooling and in storage. The pH of the carbon dioxide-free water shall be between 6.6
and 7.5 at 25°C. The temperature of the water used to prepare the standards shall be within 2°C of 25°C. The amounts of the buffer
salts given in 5.3 through 5.8 are weights in air near sea level determined with brass weights.
NOTE 4—The water used for preparing the standard buffer solutions shall be Types I or II reagent water in accordance with Specification D1193.
Precautions shall be taken to prevent contamination of the distilled water with traces of the material used for protection against carbon dioxide.
6.4 Citrate, Standard Solution A (molality = 0.05 mol/kg; pH(S) = 3.776 at 25°C)—Dissolve 11.41 g of potassium dihydrogen
citrate in distilled water and dilute to 1 L.
6.5 Phthalate, Standard Solution B (molality = 0.05 mol/kg; pH(S) = 4.008 at 25°C)—Dissolve 10.12 g of potassium hydrogen
phthalate in distilled water and dilute to 1 L.
6.6 Phosphate, Standard Equimolal Solution C (molality of each phosphate salt = 0.025 mol/kg; pH(S) = 6.865 at 25°C)—
Dissolve 3.388 g of potassium dihydrogen phosphate and 3.533 g of disodium hydrogen phosphate in distilled water and dilute
to 1 L.
E70 − 07 (2015)
6.7 Phosphate, Standard Solution D (1 + 3) (molality of KH PO = 0.008695 mol/kg,mol ⁄kg, molality of Na HPO = 0.03043
2 4 2 4
mol/kg); pH(S) = 7.413 at 25°C)—Dissolve 1.179 g of potassium dihydrogen phosphate and 4.302 g of disodium hydrogen
phosphate in distilled water and dilute to 1 L.
6.8 Borax, Standard Solution E (molality = 0.01 mol/kg; pH(S) = 9.180 at 25°C)—Dissolve 3.80 g of sodium tetraborate
decahydrate (borax) in distilled water and dilute to 1 L.
6.9 Carbonate, Standard Solution F (molality of each carbonate salt = 0.025 mol/kg; pH(S) = 10.012 at 25°C)—Dissolve 2.092
g of sodium bicarbonate and 2.640 g of sodium carbonate in distilled water and dilute to 1 L.
7. Performance Tests of Meter and Electrodes
NOTE 5—Except for measurements of the highest precision, it will usually be unnecessary to perform the tests described in this section. In the usual
pH measurement, the stability of the meter, the accuracy of the scale reading, and the pH response of the glass electrode over the range of the
measurements are verified by checking the assembly with a series of standard buffer solutions.
7.1 Assembly—The assembly shall be judged to be performing satisfactorily if it furnishes, within acceptable limits of accuracy,
the correct pH values for the standard buffer solutions listed in Table 2. When the electrodes are immersed in a buffer solution,
the measured potential difference shall be substantially constant, and the cause of any instability shall be determined.
7.2 Meter—The meter shall be brought to electrical balance in accordance with the manufacturer’s instructions. The
performance shall then be tested by applying a known variable potential through a resistance of approximately 200 MΩ to the
terminals of the meter, the high-resistance lead being connected to the terminal corresponding to the glass electrode. The source
of potential may be a precision-type potentiometer with a range of 1100 mV or more and a limit of error not greater than 0.1 mV.
The 200-MΩ resistor shall be properly shielded to avoid capacity pickup. Commencing with a value of zero, the applied potential
shall be increased in increments of 100 mV, and the readings of the dial of the meter at balance shall be noted. The process shall
be extended to cover the entire range of
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.