ASTM D5128-90(2005)
(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 [mu]S/cm (see Tables 1 and 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.
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.
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Standards Content (Sample)
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Designation: D 5128 – 90 (Reapproved 2005)
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.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
TABLE 1 Calculated Conductivity and pH Values at 25°C of Low
1. Scope
A
Concentrations of NaOH in Pure Water
1.1 This test method covers the precise on-line determina-
NOTE 1—This table tabulates the theoretical conductivity and pH
tion of pH in water samples of conductivity lower than 100
values of low levels of NaOH in pure water as calculated from available
µS/cm (see Table 1 and Table 2) over the pH range of 3 to 11
thermodynamic data.
(seeFig.1),underfieldoperatingconditions,utilizingasealed,
NOTE 2—ToillustratethehighsensitivityofthesamplepHattheselow
non-refillable, reference electrode. pH measurements of water
concentrations to contaminants, the last column lists errors that would
of low conductivity are problematical for conventional pH
result if the sample were contaminated with an additional 1 mg/Lthrough
electrodes, methods, and related measurement apparatus.
sample or equipment handling errors.
1.2 Thistestmethodincludestheproceduresandequipment
Sample Sample D pH Error from Addi-
Sample
Concentration, Conductivity, tional 1 mg/L NaOH
required for the continuous pH measurement of low conduc-
pH
mg/L µS/cm Contaminate
tivity water sample streams including the requirements for the
0.001 0.055 7.05 D 2.35
control of sample stream pressure, flow rate, and temperature.
0.010 0.082 7.45 D 1.95
1.3 This standard does not purport to address all of the
0.100 0.625 8.40 D 1.03
safety concerns, if any, associated with its use. It is the
1.0 6.229 9.40 D 0.30
8.0 49.830 10.30 D 0.05
responsibility of the user of this standard to establish appro-
A
Data courtesy of Ref (13). This data developed from algorithms originally
priate safety and health practices and determine the applica-
published in Ref (14).
bility of regulatory limitations prior to use.
2. Referenced Documents
3. Terminology
2.1 ASTM Standards: 3.1 Definitions of Terms Specific to This Standard:
D1129 Terminology Relating to Water
3.1.1 liquid junction potential—a dc potential that appears
D1193 Specification for Reagent Water at the point of contact between the reference electrode’s salt
D1293 Test Methods for pH of Water
bridge and the sample solution. Ideally this potential is near
D2777 Practice for Determination of Precision and Bias of
Applicable Methods of Committee D19 on Water
TABLE 2 Calculated Conductivity and pH Values at 25°C of Low
A
D3864 Guide for Continual On-Line Monitoring Systems Concentrations of HCl in Pure Water
for Water Analysis
NOTE 1—This table tabulates the theoretical conductivity and pH
D4453 Practice for Handling of Ultra-Pure Water Samples
values of low levels of HCl in pure water as calculated from available
2.2 ASTM Proposal: thermodynamic data
NOTE 2—ToillustratethehighsensitivityofthesamplepHattheselow
P228 ProposedTestMethodsforpHMeasurementofWater
concentrations to contaminants, the last column lists errors that would
of Low Conductivity
result if the sample were contaminated with an additional 1 mg/Lthrough
sample or equipment handling errors.
Sample Sample D pH Error from Addi-
This test method is under the jurisdiction ofASTM Committee D19 on Water Sample
Concentration, Conductivity, tional 1 mg/L HCl Con-
pH
and is the direct responsibility of Subcommittee D19.03 on Sampling of Water and
mg/L µS/cm taminate
Water-Formed Deposits,Analysis of Water for Power Generation and Process Use,
0.001 0.060 6.94 D2.38
On-Line Water Analysis, and Surveillance of Water.
0.010 0.134 6.51 D 1.95
Current edition approved Jan. 1, 2005. Published January 2005.
0.100 1.166 5.56 D 1.03
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
1.0 11.645 4.56 D 0.30
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
8.0 93.163 3.66 D 0.05
Standards volume information, refer to the standard’s Document Summary page on
A
the ASTM website. Data courtesy of Ref (13). This data developed from algorithms originally
Withdrawn. published in Ref (14).
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D 5128 – 90 (2005)
diffusion contact to the sample, resists significant dilution for
periods up to several months on continuous operation.
4.2 Thistestmethoddescribestheapparatusandprocedures
to be used for the continuous on-line pH measurement of low
conductivity water sample streams. The type of pH sensor
assembly and pH instrument interface module are described in
detail. The requirements for sample stream manifolds for the
conditioning of sample pressure and flow rate are defined, and
arrangements for this associated equipment are illustrated.
Guidelines for the proper installation and calibration of the pH
sensor and associated sample manifold are discussed along
with the precautions that must be considered concerning
sample contamination and representative sampling for calibra-
tion purposes.
4.3 The apparatus and procedures described in this test
method are intended to be used with most state-of-the-art,
process-grade,pHanalyzer/transmitterinstrumentscurrentlyin
use or available from the major manufacturers of such instru-
mentation.
5. Significance and Use
5.1 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
quicklyandprecisely.Continuouson-linepHmeasurementsin
water samples of low conductivity are more difficult (4, 5).
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
FIG. 1 Restrictions Imposed by the Conductivity pH Relationship
low conductivity waters cause liquid junction potential shifts
(see3.1.1)resultinginpHmeasurementerrors.Theaggressive
zero, and is stable. However, in low conductivity water it
nature and the high electrical resistance of pure and ultra-pure,
becomeslargerbyanunknownamount,andisazerooffset.As
low conductivity waters may degrade the pH measurement
long as it remains stable its effect can be minimized by “grab
electrodes resulting in unstable and drifting pH output signals.
sample” calibration (1).
5.2 It is essential to make on-line pH measurements of low
3.1.2 streaming potential—thestaticelectricalchargethatis
conductivity water as accurately as possible to determine the
induced by the movement of a low ionic strength solution
propercontrolofpHadjustmentchemicals,theeffectivenessof
having a high electrical resistivity or low electrical conductiv-
demineralizer equipment, the event and nature of impurity
ity (such as pure water), across relatively non-conductive
contamination of the water, and information pertaining to the
surfaces such as the pH measurement electrode’s glass mem-
overall status of the pure water system.
brane or other non-conductive wetted materials found in
flowing sample streams.
6. Interferences
3.2 Definitions: Definitions—For definitions of other terms
6.1 Samplesystemsforhighpurity,lowconductivitywaters
used in this test method, refer to Terminology D1129 and
are especially sensitive to contamination from atmospheric
Practice D3864.
gases (especially carbon dioxide, see Appendix X1 and Table
3) from collection of “crud” (insoluble deposits of iron oxide
4. Summary of Test Method
4.1 pH is measured by a pair of electrodes contained in an
TABLE 3 Calculated pH and Conductivity Values at 25°C of
all stainless steel flow cell. The pH measurement half cell is
A
Water Solutions Containing Only Ammonia and Carbon Dioxide
constructed of a glass membrane suitable for continuous
pH Shift
serviceinlowconductivitywater.ManymodernpHelectrodes
Carbon Dioxide Carbon Dioxide
Caused by
0 mg/L 0.2 mg/L
are available that perform well in this service. However, the
Ammonia 0.2 mg/L
mg/L CO
bulb impedance should be kept low to minimize the effects of
Contamination
µS/cm pH µS/cm pH
“streaming potential” (see 3.1.2). The reference half cell is
of Sample
sealed (requiring no electrolyte replenishment) and is con-
0 0.056 7.00 0.508 5.89 D 1.11 pH
structed in such a manner that the salt bridge, while making
0.12 1.462 8.73 1.006 8.18 D 0.55 pH
0.51 4.308 9.20 4.014 9.09 D 0.11 pH
0.85 6.036 9.34 5.788 9.26 D 0.08 pH
1.19 7.467 9.44 7.246 9.38 D 0.06 pH
The boldface numbers given in parentheses refer to a list of references at the
A
end of this standard. Data extracted from Ref (15).
D 5128 – 90 (2005)
A
TABLE 4 pH versus Specific Conductivity At 25°C
and other by-products of metallic corrosion that are present
throughout the system) in sample lines, from exposure to high
NOTE 1—This table tabulates the theoretical pH and specific conduc-
ionicstrengthcalibrationbuffers,fromincorrectsamplesystem
tivity values of low levels of ammonium hydroxide in reagent water as
installation techniques, and from excessive KCl leakage from calculated from available thermodynamic data.
the pH reference half-cell. Refer to Practice D4453 and Refs
Ammonium Hy- Specific
Ammonia,
droxide, mg/L pH Conductivity,
(2) and (3).
mg/L NH
NH OH µS/cm
6.2 Streaming potentials that are developed in flowing, low
0.10 0.21 8.65 1.24
conductivity water sample streams, and which are dynamic in
0.15 0.31 8.79 1.72
nature,willaddtothepotential(millivolt)generatedbythepH
0.20 0.41 8.89 2.15
+ −
0.25 0.51 8.96 2.54
glass measurement half cell in proportion to the H and OH
0.30 0.62 9.02 2.91
activities. This resultant pH error appears as a noisy and
0.35 0.72 9.07 3.25
drifting pH signal from the pH sensor. These effects are
0.40 0.82 9.11 3.57
minimized by using a conductive flow cell and, in some cases, 0.45 0.93 9.15 3.88
0.50 1.03 9.18 4.17
a symmetrical combination measurement/reference electrode
1.00 2.06 9.38 6.58
(6).
1.50 3.09 9.49 8.47
6.3 Liquid junction potentials, that are most evident in low 2.00 4.11 9.56 10.08
A
conductivitywaters,shiftthepotentialofthepHreferencehalf
Data courtesy of Ref (13). This data developed from algorithms originally
published in Ref (14).
cell resulting in both short and long-term pH measurement
errors. The instability of liquid junction potentials depends
upon reference half-cell design, electrical conductivity of the
system should be properly grounded. Provisions for the nec-
sample water, time, and sampling conditions such as flow rate
essary shielding to eliminate noise pick-up and for minimizing
and pressure. Exposure of the pH sensor electrodes to pH
air entrapment and “crud” accumulation shall be furnished in
calibration buffer solutions, that have a higher ionic strength
the flow cell and sensor assembly design. The use of plastics
than the pure water sample stream, causes significant instabil-
suchasTFEandPVDFandotherwettedmaterialsthatwillnot
ity in liquid junction potentials resulting in pH measurement
leach any contaminates into the sample may be incorporated
errors. This pH measurement error is caused by the shifting of
into the sensor assembly where required.
the pH electrodes from one ionic strength solution to another.
NOTE 1—Thetemperatureresponseofthemeasurementelectrodesmay
6.3.1 Liquidjunctionpotentialsmustbestablesoastomake
affect the accuracy and repeatability of the measurement. Electrodes that
reliable calibration of the system possible. Reference elec-
quickly equilibrate to each other and the sample temperature must be
trodesthathavebeenexposedtothemuchhigherionicstrength
selected for this service. Refer to Practice D1293, X1.2 and Ref (2).
of buffer solutions will require considerable rinse time to
NOTE 2—ContinuousexposureofthepHelectrodetolowionicstrength
establish a stable liquid junction potential in high purity water.
solutions may result in the degradation of the glass membrane portion of
somepHelectrodes(11).Electrodessuitableforcontinuousserviceinlow
TodeterminethepHelectrode’ssuitabilityinlowconductivity
conductivity water should be included in the pH sensor assembly.
water, a comparative low conductivity water sample calibra-
NOTE 3—Changes in liquid junction potentials (1) with time and
tion, or on-line calibration with low conductivity standards
eventual degradation of the reference half cell caused by diffusion of low
similartothesamplesbeingaddressedshouldbeperformed,as
ionic strength sample water into the high ionic strength electrolyte of the
described in 9.5.
half cell, must be avoided in order to effect an accurate and stable pH
6.3.2 The severity of the error resulting from a liquid
measurement. A sealed reference half cell (requiring no electrolyte
junction potential shift when the ionic strength of the sample
replenishment) that is constructed in such a manner that the salt bridge,
while making diffusion contact to the sample, resists significant dilution
changes, for example, measuring 1.0 mg/L ammonia
for periods up to several months on continuous operation in low
(pH=9.38 and conductivity=6.58 µS/cm) followed by mea-
conductivity water measurements, must be included in the pH sensor
suring 0.1 mg/L ammonia (pH=8.65 and conductivity=1.24
assembly. A trace amount of KCl will diffuse with time into the sample.
µS/cm) is not known and is a deficiency in the state-of-the-art.
7.2 A sample stream manifold constructed of all stainless
See Table 4.
steel, PTFE, and glass wetted components as shown in Fig. 2
6.4 Temperature stability of the flowing sample stream and
shall be used immediately upstream of the pH sensor. The
pH correlation to the desired 25°C reference temperature has a
manifold will provide proper sample stream pressure and flow
direct effect which is more significant in low conductivity
rate control secondary to primary sample cooling and pressure
waterontheaccuracyofthepHmeasurement(5,6,7,8,9,10).
regulation.Thismanifoldshallalsoprovidegrabsampleoutlet
AdiscussionofthetemperatureeffectsonpHmeasurementsis
for proper calibration of the pH sensor. This manifold shall be
presented in Appendix X2.
constructed in such a manner that when a grab sample is being
6.5 The flow rate to the pH electrodes and related apparatus
takenforcalibrationpurposes,neitherthesampleflowratenor
must be controlled in order to obtain repeatable results. A
pressure shall be permitted to vary at the on-line pH sensor
discussion of the flow sensitivity is presented inAppendix X3.
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