ASTM D5128-14(2022)
(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
5.1 Continuous pH measurements in low conductivity samples are sometimes required in pure water treatment using multiple pass reverse osmosis processes. Membrane rejection efficiency for several contaminants depends on pH measurement and control between passes where the conductivity is low.
5.2 Continuous pH measurement is used to monitor power plant cycle chemistry where small amounts of ammonia or amines or both are added to minimize corrosion by high temperature pure water and steam.
5.3 Conventional pH measurements are 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 3.2.1) resulting in pH measurement errors. Special precautions are required.
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. pH measurements of water of low conductivity are problematic for conventional pH electrodes, methods, and related measurement apparatus.
FIG. 1 Restrictions Imposed by the Conductivity pH Relationship
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 D5464.
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.4 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.
1.5 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
Relations
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: D5128 − 14 (Reapproved 2022)
Standard Test Method for
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. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope D1193 Specification for Reagent Water
D1293 Test Methods for pH of Water
1.1 This test method covers the precise on-line determina-
D2777 Practice for Determination of Precision and Bias of
tion of pH in water samples of conductivity lower than
Applicable Test Methods of Committee D19 on Water
100 µS⁄cm (see Table 1 and Table 2) over the pH range of 3 to
D3864 Guide for On-Line Monitoring Systems for Water
11 (see Fig. 1), under field operating conditions. pH measure-
Analysis
ments of water of low conductivity are problematic for
D4453 Practice for Handling of High Purity Water Samples
conventionalpHelectrodes,methods,andrelatedmeasurement
D5464 Test Method for pH Measurement of Water of Low
apparatus.
Conductivity
1.2 This test method includes the procedures and equipment
required for the continuous pH measurement of low conduc-
3. Terminology
tivity water sample streams including the requirements for the
3.1 Definitions—For definitions of other terms used in this
control of sample stream pressure, flow rate, and temperature.
test method, refer to Terminology D1129 and Practice D3864.
For off-line pH measurements in low conductivity samples,
refer to Test Method D5464. 3.2 Definitions of Terms Specific to This Standard:
3.2.1 liquid junction potential, n—a dc potential that ap-
1.3 The values stated in SI units are to be regarded as
pears at the point of contact between the reference electrode’s
standard. No other units of measurement are included in this
salt bridge (sometimes called diaphragm) and the sample
standard.
solution.
1.4 This standard does not purport to address all of the
3.2.1.1 Discussion—Ideally this potential is near zero, and
safety concerns, if any, associated with its use. It is the
is stable. However, in low conductivity water it becomes larger
responsibility of the user of this standard to establish appro-
by an unknown amount, and is a zero offset. As long as it
priate safety, health, and environmental practices and deter-
remains stable its effect can be minimized by “grab sample”
mine the applicability of regulatory limitations prior to use. 3
calibration (3).
1.5 This international standard was developed in accor-
3.2.2 streaming potential, n—the static electrical charge that
dance with internationally recognized principles on standard-
is induced by the movement of a low ionic strength solution
ization established in the Decision on Principles for the
having a high electrical resistivity or low electrical conductiv-
Development of International Standards, Guides and Recom-
ity (such as pure water), across relatively non-conductive
mendations issued by the World Trade Organization Technical
surfaces such as the pH electrode or other non-conductive
Barriers to Trade (TBT) Committee.
wetted materials found in flowing sample streams.
2. Referenced Documents
4. Summary of Test Method
2.1 ASTM Standards:
4.1 pH is measured by electrodes contained in an all
D1129 Terminology Relating to Water
stainless steel flow cell. The pH measurement half-cell is
constructed of a glass membrane suitable for continuous
1 service in low conductivity water. The reference half-cell is
This test method is under the jurisdiction of ASTM Committee D19 on Water
and is the direct responsibility of Subcommittee D19.03 on Sampling Water and constructed in such a manner that the salt bridge uses either a
Water-Formed Deposits, Analysis of Water for Power Generation and Process Use,
flowing liquid electrolyte, or a pressurized gel electrolyte that
On-Line Water Analysis, and Surveillance of Water.
resists significant dilution for periods up to several months of
Current edition approved Nov. 1, 2022. Published December 2022. Originally
continuous operation.
approved in 1990. Last previous edition approved in 2014 as D5128 – 14. DOI:
10.1520/D5128-14R22.
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 boldface numbers given in parentheses refer to a list of references at the
the ASTM website. end of this standard.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D5128 − 14 (2022)
TABLE 1 Calculated Conductivity and pH Values at 25 °C of Low
A
Concentrations of NaOH in Pure Water
NOTE1—ThistabletabulatesthetheoreticalconductivityandpHvalues
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/Lthrough
sample or equipment handling errors.
Sample Sample ∆ pH Error from Addi-
Sample
Concentration, Conductivity, tional 1 mg/L NaOH
pH
mg/L µS/cm Contaminate
0.001 0.055 7.05 ∆ 2.35
0.010 0.082 7.45 ∆ 1.95
0.100 0.625 8.40 ∆ 1.03
1.0 6.229 9.40 ∆ 0.30
8.0 49.830 10.30 ∆ 0.05
A B
Data courtesy of Ref (1). This data developed from algorithms originally
published in Ref (2).
B
The boldface numbers in parentheses refer to a list of references at the end of
this standard.
TABLE 2 Calculated Conductivity and pH Values at 25 °C of Low
A
Concentrations of HCl in Pure Water
NOTE1—ThistabletabulatesthetheoreticalconductivityandpHvalues
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/Lthrough
sample or equipment handling errors.
FIG. 1 Restrictions Imposed by the Conductivity pH Relationship
Sample Sample ∆ pH Error from Addi-
Sample
Concentration, Conductivity, tional 1 mg/L HCl Con-
pH
mg/L µS/cm taminate
0.001 0.060 6.94 ∆2.38
5.3 Conventional pH measurements are made in solutions
0.010 0.134 6.51 ∆ 1.95
that contain relatively large amounts of acid, base, or dissolved
0.100 1.166 5.56 ∆ 1.03
1.0 11.645 4.56 ∆ 0.30
salts. Under these conditions, pH determinations may be made
8.0 93.163 3.66 ∆ 0.05
quickly and precisely. Continuous on-line pH measurements in
A
Data courtesy of Ref (1). This data developed from algorithms originally
water samples of low conductivity are more difficult (4, 5).
published in Ref (2).
These low ionic strength solutions are susceptible to contami-
nation from the atmosphere, sample stream hardware, and the
pH electrodes. Variations in the constituent concentration of
4.2 This test method describes the apparatus and procedures
low conductivity waters cause liquid junction potential shifts
to be used for the continuous on-line pH measurement of low
(see 3.2.1) resulting in pH measurement errors. Special pre-
conductivity water sample streams. The requirements for
cautions are required.
conditioning sample pressure and flow rate are defined, and
arrangements for this associated equipment are illustrated.
6. Interferences
4.3 The apparatus and procedures described in this test
6.1 Sample systems for high purity, low conductivity waters
method are intended to be used with process-grade, pH
are especially sensitive to contamination from atmospheric
analyzer/transmitter instruments.
gases (especially carbon dioxide, see Appendix X1 and Table
3), and to accumulation of power plant corrosion particles in
5. Significance and Use
samplelinesthatcanabsorbordesorbcontaminants.Excessive
5.1 Continuous pH measurements in low conductivity KCl electrolyte leakage from the pH reference half-cell can
samples are sometimes required in pure water treatment using also affect the sample. Refer to Practice D4453 and Refs (6)
multiple pass reverse osmosis processes. Membrane rejection and (7).
efficiency for several contaminants depends on pH measure-
6.2 Streaming potentials that are developed by flowing, low
mentandcontrolbetweenpasseswheretheconductivityislow.
conductivitywateracrossnon-conductivesurfacesaredynamic
5.2 Continuous pH measurement is used to monitor power in nature and will add to the potential (millivolt) generated by
plant cycle chemistry where small amounts of ammonia or the pH sensor. This resultant pH error appears as a noisy,
amines or both are added to minimize corrosion by high flow-sensitive and drifting pH signal. These effects are mini-
temperature pure water and steam. mized by using a conductive flow cell, low sample flowrates
D5128 − 14 (2022)
A
TABLE 3 Calculated pH and Conductivity Values at 25 °C of TABLE 4 pH versus Specific Conductivity At 25 °C
A
Water Solutions Containing Only Ammonia and Carbon Dioxide
NOTE 1—This table tabulates the theoretical pH and specific conduc-
pH Shift
tivity values of low levels of ammonium hydroxide in reagent water as
Carbon Dioxide Carbon Dioxide
Caused by
calculated from available thermodynamic data.
0 mg/L 0.2 mg/L
Ammonia 0.2 mg/L
mg/L CO Ammonium Hy- Specific
Ammonia,
droxide, mg/L pH Conductivity,
Contamination
µS/cm pH µS/cm pH mg/L NH
of Sample NH OH µS/cm
0.10 0.21 8.65 1.24
0 0.056 7.00 0.508 5.89 ∆ 1.11 pH
0.12 1.462 8.73 1.006 8.18 ∆ 0.55 pH 0.15 0.31 8.79 1.72
0.20 0.41 8.89 2.15
0.51 4.308 9.20 4.014 9.09 ∆ 0.11 pH
0.85 6.036 9.34 5.788 9.26 ∆ 0.08 pH 0.25 0.51 8.96 2.54
0.30 0.62 9.02 2.91
1.19 7.467 9.44 7.246 9.38 ∆ 0.06 pH
0.35 0.72 9.07 3.25
A
Data extracted from Ref (8).
0.40 0.82 9.11 3.57
0.45 0.93 9.15 3.88
0.50 1.03 9.18 4.17
1.00 2.06 9.38 6.58
1.50 3.09 9.49 8.47
and, in some cases, a symmetrical combination measurement/
2.00 4.11 9.56 10.08
reference electrode (9).
A
Data courtesy of Ref (1). This data developed from algorithms originally
published in Ref (2).
6.3 Reference electrode liquid junction potentials are sig-
nificantinlowconductivitywatersandshiftthepotentialofthe
pHreferencehalf-cell,resultinginbothshortandlong-termpH
measurement errors. The instability of liquid junction poten-
6.4.2 In addition, pH measurements in low conductivity
tials depends on reference half-cell design, electrical conduc-
water in the power industry must compensate for the change of
tivity of the sample water, time, and sampling conditions such
the dissociation of water and ammonia or amines with tem-
as flow rate and pressure. Generally, reference electrodes with
perature to report pH at 25 °C. This is typically set into the
refillable liquid electrolyte flowing junctions provide more
instrument by the user with a solution temperature coefficient
stable junction potential than non-refillable, sealed reference
in units of pH per °C. Most process pH instrumentation for use
electrodes. Exposure of the pH electrodes to pH calibration
in power plants has a setting for solution temperature compen-
buffer solutions, that have a higher ionic strength than the pure
sation which the user must enter to activate this compensation.
water sample stream, causes significant change in the liquid
Laboratory instrumentation typically does not have this capa-
junction potential of sealed reference electrodes from what it is
bility (5, 9, 10, 11, 12, 13).
in a low conductivity sample, resulting in pH measurement
6.4.3 Further discussion of the temperature effects on pH
errors that appear immediately after calibration in buffer
measurements is presented in Annex A1.
solutions.
6.5 The flow rate to the pH electrodes and related apparatus
6.3.1 Liquid junction potentials must be stable to make a
must be controlled in order to obtain repeatable results. A
reliable calibration of the system. Sealed reference electrodes
discussion of the flow sensitivity is presented in Annex A2.
that have been exposed to the much higher ionic strength of
buffer solutions require considerable rinse time to establish a
7. Apparatus
stable liquid junction potential in high purity water. To deter-
mine the pH electrode’s suitability in low conductivity water, 7.1 A complete high purity water pH sensor assembly is
a comparative low conductivity water sample calibration, or required. The pH flow cell and connecting tubing should be
on-line calibration with low conductivity standards similar to constructedofstainlesssteelandbeearthgrounded.Thesensor
thesamplesbeingaddressedshouldbeperformed,asdescribed assembly design shall provide shielding to prevent noise
pick-up and shall minimize air entrapment and corrosion
in 9.5.
6.3.2 The severity of the error resulting from a liquid particle accumulation. Sample discharge shall be near the top
junction potential shift when the ionic strength of the sample of the flow cell to purge any air bubbles rapidly and shall go
changes, for example, measuring 1.0 mg/L ammonia downwardtoanopendraintopreventanybackpressureonthe
(pH = 9.38andconductivity = 6.58S/cm)followedbymeasur- electrode(s).
ing 0.1 mg/L ammonia (pH = 8.65 and conductiv- 7.1.1 A single probe combination measuring and reference
ity = 1.24 S⁄cm) is not known and is a deficiency in the electrodewithintegraltemperaturecompensatorenablesuseof
state-of-the-art. Table 4 and Fig. X1.1 provide a correlation averysmallvolumeflowcellwhichcreatesahighsampleflow
between pH and specific conductivity of dilute ammonia. velocity that prevents power plant sample corrosion particles
from accumulating.
6.4 Temperature compensation of pH in low conductivity
7.1.2 Where separate measuring, reference and temperature
water is more significant and more involved than in conven-
compensator probes are used, a larger volume flow cell is
tional measurements.
necessary and the design should enable convenient periodic
6.4.1 All pH measurements must compensate for the chang-
cleanout of accumulated corrosion particles.
ing output of the electrode with temperature. This effect is
representedbyachangingslopewithunitsofmillivoltsperpH. 7.2 Electrodes should have half-cells that quickly equili-
This slope is proportional to absolute temperature according to brate to each other and the sample temperature. Refer to Ref
the Nernst equation. (6).
D5128 − 14 (2022)
7.3 Elect
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