ASTM D5128-09
(Test Method)Standard Test Method for On-Line pH Measurement of Water of Low Conductivity
Standard Test Method for On-Line pH Measurement of Water of Low Conductivity
SIGNIFICANCE AND USE
pH measurements are typically made in solutions that contain relatively large amounts of acid, base, or dissolved salts. Under these conditions, pH determinations may be made quickly and precisely. Continuous on-line pH measurements in water samples of low conductivity are more difficult (4, 5). These low ionic strength solutions are susceptible to contamination from the atmosphere, sample stream hardware, and the pH electrodes. Variations in the constituent concentration of low conductivity waters cause liquid junction potential shifts (see 3.1.1) resulting in pH measurement errors. The aggressive nature and the high electrical resistance of pure and ultra-pure, low conductivity waters may degrade the pH measurement electrodes resulting in unstable and drifting pH output signals.
It is essential to make on-line pH measurements of low conductivity water as accurately as possible to determine the proper control of pH adjustment chemicals, the effectiveness of demineralizer equipment, the event and nature of impurity contamination of the water, and information pertaining to the overall status of the pure water system.
SCOPE
1.1 This test method covers the precise on-line determination of pH in water samples of conductivity lower than 100 μS/cm (see Table 1 and Table 2) over the pH range of 3 to 11 (see Fig. 1), under field operating conditions, utilizing a sealed, non-refillable, reference electrode. pH measurements of water of low conductivity are problematical for conventional pH electrodes, methods, and related measurement apparatus.
1.2 This test method includes the procedures and equipment required for the continuous pH measurement of low conductivity water sample streams including the requirements for the control of sample stream pressure, flow rate, and temperature. For off-line pH measurements in low conductivity samples, refer to Test Method D 5464.
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.
TABLE 1 Calculated Conductivity and pH Values at 25°C of Low Concentrations of NaOH in Pure Water A
Note 1—This table tabulates the theoretical conductivity and pH values of low levels of NaOH in pure water as calculated from available thermodynamic data.
Note 2—To illustrate the high sensitivity of the sample pH at these low concentrations to contaminants, the last column lists errors that would result if the sample were contaminated with an additional 1 mg/L through sample or equipment handling errors. Sample
Concentration,
mg/LSample
Conductivity,
μS/cmSample
pHΔ pH Error from Additional 1 mg/L NaOH Contaminate 0.001 0.055 7.05Δ 2.35 0.010 0.082 7.45 Δ 1.95 0.100 0.625 8.40 Δ 1.03 1.0 6.229 9.40 Δ 0.30 8.0 49.83010.30 Δ 0.05
A Data courtesy of Ref (13). This data developed from algorithms originally published in Ref (14).
TABLE 2 Calculated Conductivity and pH Values at 25°C of Low Concentrations of HCl in Pure Water A
Note 1—This table tabulates the theoretical conductivity and pH values of low levels of HCl in pure water as calculated from available thermodynamic data
Note 2—To illustrate the high sensitivity of the sample pH at these low concentrations to contaminants, the last column lists errors that would result if the sample were contaminated with an additional 1 mg/L through sample or equipment handling errors. Sample
Concentration,
mg/LSample
Conductivity,
μS/cmSample
pHΔ pH Error from Additional 1 mg/L HCl Contaminate 0.001 0.0606.94Δ2.38 0.010 0.134 6.51 Δ 1.95 0.100 1.166 5.56 Δ 1.03 1.0 11.645 4.56 Δ 0.30 8.0 93.163 3.66 Δ 0.05
A Data courtesy of Ref (13). This data developed from algorithms originally published in Ref (14).
FIG. 1 Restrictions Imposed by the Conductivity pH Relationship
General Information
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Designation: D5128 − 09
StandardTest Method for
1
On-Line pH Measurement of Water of Low Conductivity
This standard is issued under the fixed designation D5128; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 3. Terminology
1.1 This test method covers the precise on-line determina- 3.1 Definitions of Terms Specific to This Standard:
tion of pH in water samples of conductivity lower than 100 3.1.1 liquid junction potential—a dc potential that appears
µS/cm (see Table 1 and Table 2) over the pH range of 3 to 11 at the point of contact between the reference electrode’s salt
(seeFig.1),underfieldoperatingconditions,utilizingasealed, bridge and the sample solution. Ideally this potential is near
non-refillable, reference electrode. pH measurements of water zero, and is stable. However, in low conductivity water it
of low conductivity are problematical for conventional pH becomeslargerbyanunknownamount,andisazerooffset.As
electrodes, methods, and related measurement apparatus. long as it remains stable its effect can be minimized by “grab
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sample” calibration (1).
1.2 Thistestmethodincludestheproceduresandequipment
3.1.2 streaming potential—thestaticelectricalchargethatis
required for the continuous pH measurement of low conduc-
induced by the movement of a low ionic strength solution
tivity water sample streams including the requirements for the
having a high electrical resistivity or low electrical conductiv-
control of sample stream pressure, flow rate, and temperature.
ity (such as pure water), across relatively non-conductive
For off-line pH measurements in low conductivity samples,
surfaces such as the pH measurement electrode’s glass mem-
refer to Test Method D5464.
brane or other non-conductive wetted materials found in
1.3 This standard does not purport to address all of the
flowing sample streams.
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro- 3.2 Definitions: For definitions of other terms used in this
priate safety and health practices and determine the applica- test method, refer to Terminology D1129 and Practice D3864.
bility of regulatory limitations prior to use.
4. Summary of Test Method
2. Referenced Documents
4.1 pH is measured by a pair of electrodes contained in an
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all stainless steel flow cell. The pH measurement half cell is
2.1 ASTM Standards:
constructed of a glass membrane suitable for continuous
D1129Terminology Relating to Water
serviceinlowconductivitywater.ManymodernpHelectrodes
D1193Specification for Reagent Water
are available that perform well in this service. However, the
D1293Test Methods for pH of Water
bulb impedance should be kept low to minimize the effects of
D2777Practice for Determination of Precision and Bias of
“streaming potential” (see 3.1.2). The reference half cell is
Applicable Test Methods of Committee D19 on Water
sealed (requiring no electrolyte replenishment) and is con-
D3864Guide for On-Line Monitoring Systems for Water
structed in such a manner that the salt bridge, while making
Analysis
diffusion contact to the sample, resists significant dilution for
D4453Practice for Handling of High Purity Water Samples
periods up to several months on continuous operation.
D5464Test Method for pH Measurement of Water of Low
Conductivity
4.2 Thistestmethoddescribestheapparatusandprocedures
to be used for the continuous on-line pH measurement of low
conductivity water sample streams. The type of pH sensor
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This test method is under the jurisdiction ofASTM Committee D19 on Water
assembly and pH instrument interface module are described in
and is the direct responsibility of Subcommittee D19.03 on Sampling Water and
detail. The requirements for sample stream manifolds for the
Water-Formed Deposits,Analysis of Water for Power Generation and Process Use,
conditioning of sample pressure and flow rate are defined, and
On-Line Water Analysis, and Surveillance of Water.
arrangements for this associated equipment are illustrated.
Current edition approved Oct. 1, 2009. Published October 2009. Originally
approved in 1990. Last previous edition approved in 2005 as D512890 (2005).
Guidelines for the proper installation and calibration of the pH
DOI: 10.1520/D5128-09.
2
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
3
Standards volume information, refer to the standard’s Document Summary page on The boldface numbers given in parentheses refer to a list of references at the
the ASTM website. end of this stan
...
This document is not anASTM standard and is intended only to provide the user of anASTM 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:D5128–90(Reapproved2005) Designation: D 5128 – 09
Standard Test Method for
1
On-Line pH Measurement of Water of Low Conductivity
This standard is issued under the fixed designation D5128; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 Thistestmethodcoversthepreciseon-linedeterminationofpHinwatersamplesofconductivitylowerthan100µS/cm(see
Table 1 and Table 2) over the pH range of 3 to 11 (see Fig. 1), under field operating conditions, utilizing a sealed, non-refillable,
referenceelectrode.pHmeasurementsofwateroflowconductivityareproblematicalforconventionalpHelectrodes,methods,and
related measurement apparatus.
1.2 This test method includes the procedures and equipment required for the continuous pH measurement of low conductivity
watersamplestreamsincludingtherequirementsforthecontrolofsamplestreampressure,flowrate,andtemperature.Foroff-line
pH measurements in low conductivity samples, refer to Test Method D5464.
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
2.1 ASTM Standards:
D1129 Terminology Relating to Water
D1193 Specification for Reagent Water
D1293 Test Methods for pH of Water
D2777 Practice for Determination of Precision and Bias of Applicable Test Methods of Committee D19 on Water
D3864 Guide for Continual On-Line Monitoring Systems for Water Analysis
D4453 Practice for Handling of Ultra-Pure Water Samples
2.2 ASTM Proposal:
P228Proposed
D5464 Test MethodsMethod for pH Measurement of Water of Low Conductivity
3. Terminology
3.1 Definitions of Terms Specific to This Standard:
3.1.1 liquid junction potential—a dc potential that appears at the point of contact between the reference electrode’s salt bridge
and the sample solution. Ideally this potential is near zero, and is stable. However, in low conductivity water it becomes larger by
an unknown amount, and is a zero offset. As long as it remains stable its effect can be minimized by “grab sample” calibration
3
(1).
3.1.2 streaming potential—the static electrical charge that is induced by the movement of a low ionic strength solution having
a high electrical resistivity or low electrical conductivity (such as pure water), across relatively non-conductive surfaces such as
the pH measurement electrode’s glass membrane or other non-conductive wetted materials found in flowing sample streams.
3.2 Definitions: Definitions—ForFor definitions of other terms used in this test method, refer to Terminology D1129 and
Practice D3864.
4. Summary of Test Method
4.1 pH is measured by a pair of electrodes contained in an all stainless steel flow cell. The pH measurement half cell is
1
This test method is under the jurisdiction of ASTM Committee D19 on Water and is the direct responsibility of Subcommittee D19.03 on Sampling of Water and
Water-Formed Deposits, Analysis of Water for Power Generation and Process Use, On-Line Water Analysis, and Surveillance of Water.
CurrenteditionapprovedJan.1,2005.PublishedJanuary2005.onSamplingWaterandWater-FormedDeposits,AnalysisofWaterforPowerGenerationandProcessUse,
On-Line Water Analysis, and Surveillance of Water.
Current edition approved Oct. 1, 2009. Published October 2009. Originally approved in 1990. Last previous edition approved in 2005 as D512890 (2005).
2
ForreferencedASTMstandards,visittheASTMwebsite,www.astm.org,orcontactASTMCustomerServiceatservice@astm.org.For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
3
Withdrawn.
3
The boldface numbers given in parentheses refer to a list of references at the end of this standard.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
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D5128–09
TABLE 1 Calculated Conductivity and pH Values at 25°C of Low
A
Concentrations of NaOH in Pure Water
NOTE 1—This table tabulates the theoretical conductivity and pH
values of low levels of NaOH in pure water as calculated from available
thermodynamic data.
NOTE 2—Toillustratethehighsensitivityofth
...
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