ASTM D1293-99(2005)
(Test Method)Standard Test Methods for pH of Water
Standard Test Methods for pH of Water
SIGNIFICANCE AND USE
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.6 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: SectionsTest Method A-Precise Laboratory Measurement 8 to 15Test 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.
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 S/cm. Refer to Test Methods D 5128 and D 5464.
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.
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.
1.7 This test method covers the precise measurement of pH in water under strictly controlled laboratory conditions.
1.8 This test method is used for the routine measurement of pH in the laboratory and the measurement of pH under various process conditions.
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NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation:D1293–99 (Reapproved 2005)
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 Department of Defense.
1. Scope priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use.
1.1 These test methods cover the determination of pH by
electrometric measurement using the glass electrode as the
2. Referenced Documents
sensor. Two test methods are given as follows:
2.1 ASTM Standards:
Sections
D1066 Practice for Sampling Steam
Test Method A—Precise Laboratory Measurement 8 to 15
Test Method B—Routine or Continuous Measurement 16 to 24
D1067 Test Methods for Acidity or Alkalinity of Water
D1129 Terminology Relating to Water
1.2 Test MethodAcovers the precise measurement of pH in
D1192 Guide for Equipment for SamplingWater and Steam
water utilizing at least two of seven standard reference buffer
in Closed Conduits
solutions for instrument standardization.
D1193 Specification for Reagent Water
1.3 Test Method B covers the routine measurement of pH in
D2777 Practice for Determination of Precision and Bias of
water and is especially useful for continuous monitoring. Two
Applicable Test Methods of Committee D19 on Water
buffers are used to standardize the instrument under controlled
D3370 Practices for Sampling Water from Closed Conduits
parameters, but the conditions are somewhat less restrictive
D5128 Test Method for On-Line pH Measurement of Water
than those in Test Method A.
of Low Conductivity
1.4 Both test methods are based on the pH scale established
D5464 Test Method for pH Measurement of Water of Low
by NIST (formerly NBS) Standard Reference Materials.
Conductivity
1.5 Neither test method is considered to be adequate for
E70 Test Method for pH of Aqueous Solutions With the
measurement of pH in water whose conductivity is less than
Glass Electrode
about 5 µS/cm. Refer to Test Methods D5128 and D5464.
1.6 Precision and bias data were obtained using buffer
3. Terminology
solutions only. It is the user’s responsibility to assure the
3.1 Definitions—For definitions of terms used in these test
validity of these test methods for untested types of water.
methods, refer to Terminology D1129.
1.7 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 pH of an aqueous solution is derived from
responsibility of the user of this standard to establish appro-
E, the electromotive force (emf) of the cell
glass electrode | solution || reference electrode
These test methods are under the jurisdiction of ASTM Committee D19 on
Water and are the direct responsibility of Subcommittee D19.03 on Sampling of
Water and Water-Formed Deposits, Analysis of Water for Power Generation and For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Process Use, On-Line Water Analysis, and Surveillance of Water. contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Current edition approved Jan. 1, 2005. Published January 2005. Originally Standards volume information, refer to the standard’s Document Summary page on
approved in 1953. Last previous edition approved in 1999 as D1293 – 99. DOI: the ASTM website.
10.1520/D1293-99R05. Withdrawn.
2 5
“Standard Reference Materials: Standardization of pH Measurements” Wu and Withdrawn. The last approved version of this historical standard is referenced
Koch, NBS Special Publications No. 260-53, 1988. on www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D1293–99 (2005)
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
uncontaminated waters range between pH 6 and pH 9.
0 54.20
5 55.19
6. Purity of Reagents
10 56.18
15 57.17
6.1 Reagent grade chemicals shall be used in all tests,
20 58.17
except as specifically noted for preparation of reference buffer
25 59.16
solutions. Unless otherwise indicated, it is intended that all
30 60.15
35 61.14
reagents shall conform to the specifications of the Committee
40 62.13
on Analytical Reagents of the American Chemical Society,
45 63.13
where such specifications are available. Other grades may be
50 64.12
55 65.11
used, provided it is first ascertained that the reagent is of
60 66.10
sufficiently high purity to permit its use without lessening the
65 67.09
70 68.09 accuracy of the determination.
75 69.08
6.2 Purity of Water—Unless otherwise indicated, references
80 70.07
towatershallbeunderstoodtomeanreagentwaterconforming
85 71.06
to Specification D1193, Type I.
90 72.05
95 73.05
7. Sampling
7.1 Collect samples in accordance with Practice D1066,
(where the double vertical line represents a liquid junction)
Specification D1192, or Practices D3370, whichever is appli-
when the electrodes are immersed in the solution in the
cable.
diagrammed position, and E is the electromotive force ob-
s
tained when the electrodes are immersed in a reference buffer
TEST METHOD A—PRECISE LABORATORY
solution.
MEASUREMENT OF pH
With the assigned pH of the reference buffer designated as
pH , and E and E expressed in volts is the following:
s s
8. Scope
~E 2 E !F
s
8.1 This test method covers the precise measurement of pH
pH 5 pH 1
s
2.3026 RT
in water under strictly controlled laboratory conditions.
where:
9. Interferences
F = Faraday,
R = gas constant, and
9.1 The glass electrode reliably measures pH in nearly all
T = absolute temperature, t (°C) + 273.15.
aqueous solutions and in general is not subject to solution
The reciprocal of F/2.3026 RT is known as the slope of the
interference from color, turbidity, colloidal matter, oxidants, or
electrode, and is the expected difference in observed voltage
reductants.
for two measurements one pH unit apart.Values of the slope at
9.2 The reference electrode may be subject to interferences
various temperatures are given in Table 1.
and should be chosen to conform to all requirements of
Sections 10 and 12. Refer also to Appendix X1.3.
4. Summary of Test Method
9.3 The true pH of an aqueous solution or extract is affected
4.1 ThepHmeterandassociatedelectrodesarestandardized
by the temperature. The electromotive force between the glass
against two reference buffer solutions that closely bracket the
and the reference electrode is a function of temperature as well
anticipated sample pH. The sample measurement is made
as pH. The temperature effect can be compensated automati-
under strictly controlled conditions and prescribed techniques.
cally in many instruments or can be manually compensated in
most other instruments. The temperature compensation cor-
5. Significance and Use
rects for the effect of changes in electrode slope with tempera-
5.1 The pH of water is a critical parameter affecting the
ture but does not correct for temperature effects on the
solubility of trace minerals, the ability of the water to form
chemical system being monitored. It does not adjust the
scale or to cause metallic corrosion, and the suitability of the
watertosustainlivingorganisms.Itisadefinedscale,basedon
a system of buffer solutions with assigned values. In pure
The relative acidity or alkalinity measured by pH should not be confused with
water at 25°C, pH 7.0 is the neutral point, but this varies 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.).
6 8
Bates, R. G., Determination of pH: Theory and Practice, 2nd Ed., J. Wiley and Reagent Chemicals, American Chemical Society Specifications, American
Sons, New York, 1973, p. 29. 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.
D1293–99 (2005)
measured pH to a common temperature; therefore, the tem- rily as required in the standardizing procedure described in
perature should be reported for each pH measurement. Tem- 12.1through12.5.Adiscussionofreferenceelectrodesisgiven
perature effects are discussed further in Appendix X1.2. in Appendix X1.3.
10.4 Temperature Compensator—The thermocompensator
9.4 The pH response of the glass electrode/reference elec-
is a temperature-sensitive resistance element immersed in the
trode pair is imperfect at both ends of the pH scale. The
water sample with the electrodes. The thermocompensator
indicatedpHvalueofhighlyalkalinesolutionsmaybetoolow,
automatically corrects for the change in slope of the glass
by as much as 1 pH, depending on electrode composition and
electrode (with change of temperature) but does not correct for
sample conditions. See X1.5.1. The indicated pH value of
actual changes in sample pH with temperature. The automatic
strong aqueous solutions of salts and strong acids having a pH
thermocompensator is not required if the water temperature is
less than 1, will often be higher than the true pH value.
essentially constant and the analyst chooses to use the manual
Interferences can be minimized by the selection of the proper
temperature compensation feature of the pH meter.
glass and reference electrodes for measurements in highly
alkaline or acidic solutions.
11. Reagents
9.5 A few substances sometimes dispersed in water appear
11.1 Reference Buffer Solutions—The pH values of the
to poison the glass electrode. A discussion of this subject is
reference buffer solutions measured at several temperatures are
given in Appendix X1.4.
listed in Table 2. Table 3 identifies each buffer salt by its
NationalInstituteofStandardsandTechnology(NIST)number
10. Apparatus
and provides a recommended drying procedure prior to use.
10.1 LaboratorypHMeter—Almostallcommerciallyavail-
The current renewal of each NIST standard reference material
able meters are of the digital type and will have either manual
should be used. Keep the five reference buffer solutions with
or automatic calibration, and either manual or automatic
pH less than 9.5 in bottles of chemically resistant glass. Keep
temperature (slope) correction. All four types are permissible.
the calcium hydroxide solutions in a plastic bottle that is
However, readability to 0.01 pH is essential (Section 14), and
nonporous to air (that is, polypropylene or high density
the ability to read in millivolts is useful in troubleshooting.
polyethylene). Keep all the reference buffer solutions well-
10.2 Glass Electrode—The pH response of the glass elec-
stoppered and replace if a visible change is observed.
trode shall conform to the requirements set forth in 12.1
11.1.1 Borax Reference Buffer Solution (pH = 9.18 at
s
through 12.5. The glass electrode lead wire shall be shielded.
25°C)—Dissolve 3.80 g of sodium tetraborate decahydrate
New glass electrodes and those that have been stored dry shall
(Na B O ·10H O) in water and dilute to 1 L.
2 4 7 2
be conditioned and maintained as recommended by the manu-
11.1.2 Calcium Hydroxide Reference Buffer Solution (pH =
s
facturer.
12.45 at 25°C)—Prepare pure calcium hydroxide (Ca(OH) )
10.3 Reference Electrode—This may be used as separate from well-washed calcium carbonate (CaCO ) of low-alkali
“half cell,” or it may be purchased integral with the glass pH grade by slowly heating the carbonate in a platinum dish at
electrode body, as a combination electrode. The internal 1000°C and calcining for at least 45 min at that temperature.
reference element may be calomel (mercury/mercurous chlo- After cooling in a dessicator, add the calcined product slowly
ride), silver/silver chloride, or an iodide-iodine redox couple. to water with stirring, heat the resultant suspension to boiling,
For best performance, the reference element should be the cool, and filter through a funnel having a fritted-glass disk of
same type in both the reference electrode and inside the pH medium porosity. Collect the solid from the filter, dry it in an
electrode. For all three types, the junction between the refer- ovenat110°C,andcrushittoauniformandfinegranularstate.
ence filling solution and the sample may be either a flowing or Prepare a saturated calcium hydroxide solution by vigorously
nonflowing junction. The flowing liquid junction-type unit shakingaconsiderableexcess(about3g/L)ofthefinegranular
ensures that a fresh liquid junction is formed for each mea- product in water at 25°C in a stoppered plastic bottle (that is,
surement and shall be used for Test Method A determinations. polypropylene or high density polyethylene) that is essentially
If a saturated calomel electrode is used, some potassium nonporous to gases. Allow the gross excess of solid to settle
chloride crystals shall be contained in the saturated potassium
and filter the solution with suction through a fritted-glass
chloride solution. If the reference electrode is of the flowing funnel of medium porosity. The filtrate is the reference buffer
junction type, the design of the electrode shall permit a fresh solution. Contamination of the solution with atmospheric
liquid junction to be formed between the reference electrode carbon dioxide renders it turbid and indicates need for replace-
solution and the buffer standard or tested water for each ment.
measurement and shall allow traces of solution to be washed 11.1.3 Phosphate Reference Buffer Solution (pH = 6.86 at
s
from the outer surfaces of the electrodes. To ensure the desired 25°C)—Dissolve 3.39 g of potassium dihydrogen phosphate
slow outward flow of reference electrode solution, the solution (KH PO ) and 3.53 g of anhydrous disodium hydrogen phos-
2 4
pressure inside the liquid junction should be kept somewhat in phate (Na HPO ) in water and dilute to 1 L.
2 4
excess of that outside the junction. In nonpressurized applica- 11.1.4 Phthalate Reference Buffer Solution (pH = 4.00 at
s
tions, this requirement can be met by maintaining the inside 25°C)—Dissolve 10.12 g of potassium hydrogen phthalate
solution level higher than the outside water level. If the (KHC H O ) in
...
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