ASTM D6569-05(2009)
(Test Method)Standard Test Method for On-Line Measurement of pH1
Standard Test Method for On-Line Measurement of pH<sup>1</sup>
SIGNIFICANCE AND USE
pH is a measure of the hydrogen ion activity in water. It is a major parameter affecting the corrosivity and scaling properties of water, biological life in water and many applications of chemical process control. It is therefore important in water purification, use and waste treatment before release to the environment.
On-line pH measurement is preferred over laboratory measurement to obtain real time, continuous values for automatic control and monitoring purposes.
SCOPE
1.1 This test method covers the continuous determination of pH of water by electrometric measurement using the glass, the antimony or the ion-selective field-effect transistor (ISFET) electrode as the sensor.
1.2 This test method does not cover measurement of samples with less than 100 μS/cm conductivity. Refer to Test Method D 5128.
1.3 This test method does not cover laboratory or grab sample measurement of pH. Refer to Test Method D 1293.
1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.5 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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Designation: D6569 − 05(Reapproved 2009)
Standard Test Method for
On-Line Measurement of pH
This standard is issued under the fixed designation D6569; 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 D3864 Guide for Continual On-Line Monitoring Systems
for Water Analysis
1.1 This test method covers the continuous determination of
D5128 Test Method for On-Line pH Measurement of Water
pH of water by electrometric measurement using the glass, the
of Low Conductivity
antimony or the ion-selective field-effect transistor (ISFET)
electrode as the sensor.
3. Terminology
1.2 This test method does not cover measurement of
3.1 Definitions—For definitions of terms used in this test
samples with less than 100 µS/cm conductivity. Refer to Test
method, refer to Terminology D1129, Test Method D1293 and
Method D5128.
Guide D3864.
1.3 This test method does not cover laboratory or grab
3.2 Definitions of Terms Specific to This Standard:
sample measurement of pH. Refer to Test Method D1293.
3.2.1 liquid junction potential—the dc potential which ap-
pears at the point of contact between the reference electrode’s
1.4 The values stated in SI units are to be regarded as
salt bridge and the sample solution. Ideally this potential is
standard. No other units of measurement are included in this
near zero and is stable. However, in samples with extreme pH
standard.
it becomes larger by an unknown amount and is a zero offset.
1.5 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
4. Summary of Test Method
responsibility of the user of this standard to establish appro-
4.1 pH is measured as a voltage between measuring elec-
priate safety and health practices and determine the applica-
trode and reference electrode elements. The sensor assembly
bility of regulatory limitations prior to use.
typically includes a temperature compensator to compensate
for the varying output of the measuring electrode due to
2. Referenced Documents
temperature.
2.1 ASTM Standards:
4.2 The sensor signals are processed with an industrial pH
D1129 Terminology Relating to Water
analyzer/transmitter.
D1193 Specification for Reagent Water
4.3 The equipment is calibrated with standard pH buffer
D1293 Test Methods for pH of Water
solutionsencompassingorincloseproximitytotheanticipated
D2777 Practice for Determination of Precision and Bias of
pH measurement range.
Applicable Test Methods of Committee D19 on Water
D3370 Practices for Sampling Water from Closed Conduits
5. Significance and Use
5.1 pH is a measure of the hydrogen ion activity in water. It
is a major parameter affecting the corrosivity and scaling
This test method is under the jurisdiction of ASTM Committee D19 on Water
properties of water, biological life in water and many applica-
and is the direct responsibility of Subcommittee D19.03 for Sampling of Water and
Water-Formed Deposits, Surveillance of Water, and Flow Measurement of Water.
tions of chemical process control. It is therefore important in
Current edition approved May 1, 2009. Published June 2009. Originally
water purification, use and waste treatment before release to
approved in 2000. Last previous edition approved in 2005 as D6569 – 05. DOI:
the environment.
10.1520/D6569-05R09.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
5.2 On-line pH measurement is preferred over laboratory
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
measurement to obtain real time, continuous values for auto-
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. matic control and monitoring purposes.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6569 − 05 (2009)
6. Interferences samples being measured and make the correction on all
measurements. The pH to be reported is referenced to 25°C
6.1 Pressure and temperature variations may force process
unless another temperature is specified. Some process instru-
sample into the liquid junction of non-flowing junction refer-
ments have built-in solution temperature compensation which
ence electrodes and cause changes in the junction potential.
allows entry of a user-defined linear temperature coefficient
Estimates of 0.2 to 0.5 pH errors from this source have been
into instrument memory for on-line correction of this effect.
cited. (1)
The temperature of the solution measured for pH should be
6.2 Liquid junction potentials at the reference electrode can
monitored and recorded since this information may be critical
varydependingonthecompositionofthesample.Strongacids,
to understanding the base state of the solution.
bases and extremely high and low ionic strength samples
NOTE1—Forregulatorymonitoring,correctionforsolutiontemperature
develop liquid junction potentials different from typical cali-
effects should not be done without consulting the governing authority.
brating buffer solutions.(2) Where these conditions exist, the
6.6 A small temperature influence can occur due to differ-
most stable junction potential is obtained using a flowing
encesinthecompositionofmeasuringandreferencehalf-cells.
junction reference electrode—one that requires refilling with
This is not compensated by any instrumentation. For this
electrolyte solution. However, providing positive flow of
reason it is advisable to calibrate as near the measuring
electrolyte through the reference junction places limitations on
temperature as possible.
the sample pressure that can be tolerated. Follow manufactur-
ers’ recommendations.
6.7 Coating of the measuring electrode may produce a slow
or erroneous response since the sensing surface is in contact
6.3 pH reference electrodes must not be allowed to dry.
with the coating layer rather than the bulk sample. Flat surface
Electrolyte salts can crystallize in the liquid junction and
electrodes and high sample flow velocity have been found to
produce a high liquid junction impedance. Subsequent pH
provide some self-cleaning effects. Cleaning may be accom-
measurements could be noisy, drifting or off-scale. When pH
plished manually using solvents, acids, detergents, etc. Clean-
sensors are not in use, they should be typically stored wet per
ing may be automated by a number of approaches. See
manufacturers’ instructions.
Appendix X1.
6.4 There are several temperature effects on pH measure-
6.8 Abrasion of measuring electrode surfaces from particles
ment. The pH electrode signal is described by the Nernst
in the sample can shorten sensor life. Where abrasive particles
equation with its output proportional to the absolute tempera-
are present, the flow velocity past the electrode surface should
ture times the pH deviation from the isopotential point—
be controlled low enough to minimize abrasion and provide
usually 7 pH for glass electrodes. Compensation for this effect
satisfactory electrode life yet high enough to prevent particles
may be accomplished automatically with a temperature sensor
from accumulating into a coating as in 6.7.
integral to the combination pH probe and an algorithm in the
instrument. Alternatively, some instruments may be set manu-
6.9 High pH conditions can produce an alkaline error as the
ally for a fixed temperature when a temperature signal is not
glass pH sensor responds to sodium or other small cations in
available. Errors caused by deviations from the manual setting
addition to hydrogen. This type of error is greater at higher
may be calculated from the following (for a conventional glass
temperatures.The result is always a negative error in the range
electrode system with 7 pH isopotential point).
of 0 to -1 pH depending on the pH, temperature, sodium
concentration and sensor glass formulation. Some manufactur-
~pH27! 3 ~T2Tf!
Glass Electrode pH error5 (1)
ers have characterized the alkaline or sodium error sufficiently
Tf1273
to closely estimate those errors. Some process ISFET elec-
where:
trodes do not experience these errors.
pH = uncorrected process pH
6.10 While fluorides in the sample do not interfere with the
T = process temperature (°C)
measurement, if present at pH below 5, they attack silica,
Tf = temperature setting of fixed compensation (°C)
greatly shortening the life of glass and ISFET electrodes.
Other types of electrodes, (antimony, ISFET) have different
6.11 Antimonyelectrodemeasurementsaresubjecttomajor
isopotential points and therefore different corrections. Consult
interferences from oxidizing or reducing species, non-linearity,
the manufacturer.
irregular temperature characteristics and the physical condition
6.5 Solution temperature effects may be caused by changes
of the electrode surface. However, the antimony electrode can
in the sample, such as ionization of constituents, off-gassing,
withstand hydrofluoric acid which other electrodes cannot and
and precipitation, which occur with changes in temperature.
this application is its primary use. The typical useful range of
These are generally small for many samples over moderate
the antimony electrode is 3-9 pH. Performance is very
temperature ranges. In waste streams with variation in compo-
application-dependent and should be carefully evaluated.
sition, such effects are usually not predictable. However, for
6.12 Electrical noise induced on the pH sensor-to-
samples with uniform or predictable composition with tem-
instrument cable can cause erratic and offset readings. Route
perature changes > 5°C, one may determine the effect for the
pH signal cables separately from AC power and switching
circuit wiring.
6.13 Electrical insulation leakage in electrode connectors
The boldface numbers given in parentheses refer to a list of references at the
end of this standard. and cable or cracking of a glass electrode membrane can cause
D6569 − 05 (2009)
the high impedance pH signal to be attenuated or completely repeatable response as given in Test Method D1293. It shall
lost. This results in a dead response where the measurement have pH, temperature and pressure ratings suitable for the
system will not give response away from the calibration point.
process conditions. It shall be conditioned in the process
Keep pH signal cables and connectors clean and dry. Pream-
sample for at least 30 minutes or as recommended by the
plifiers are normally located close to pH sensors to minimize
manufacturer before accurate readings can be taken.
the distance high impedance signals must be transported—a
7.2.2 ISFET measuring—The ISFET measuring electrode
help in minimizing noise interference in 6.12 as well.
along with its unique measuring circuit shall give response
6.14 Ground loop interference can occur if the pH measur- equivalent to a glass electrode measuring system. (ISFET
ing circuit is not galvanically isolated from earth ground, electrodes typically require an adapter circuit to be compatible
except for the electrodes themselves. Such interference can with glass electrode measuring instruments.)
give an offset or off-scale reading when measuring in a
7.2.3 Antimony measuring—The antimony measuring elec-
grounded process installation but will give satisfactory re-
trode shall be pure polished antimony metal that has been
sponse in grab samples or calibration solutions that are not
conditioned by soaking in water to produce an oxide layer,
grounded. Sources of ground loops include improper wiring of
according to manufacturer’s instructions.
sensor cables, lack of isolation of analog or digital output
7.2.4 Non-flowing Liquid Junction Reference
signals from the measuring circuit, or a leaking sensor body
7.2.4.1 Thenon-flowingreferenceelectrodeshallcontainan
which allows electrical contact of the sample to a part of the
electrode half-cell similar to the glass measuring electrode, if
measuring circuit beyond the external electrode surfaces.
used, to cancel the temperature effects of the half-cells. It shall
Remove output wiring, check sensor wiring and observe
contain sufficient electrolyte with gelling agent or other means
readings to locate the cause of grounding problems.
to restrict its loss and give acceptable life in the application.
6.15 Measurements on samples with conductivity less than
Despite the name “non-flowing,” the electrolyte is consumable
100 µS/cm are vulnerable to streaming potentials, large junc-
as a trace amount of it diffuses through the junction into the
tion potentials and other difficulties and are beyond the scope
sample. The only opening of the electrode is its interface with
of this method. Use Test Method D5128.
the process through its liquid junction—a small passage of
7. Apparatus
porous ceramic, polymer, wood, fiber, ground glass surfaces or
other material that allows electrical continuity with the sample
7.1 Process instrument
while limiting loss of electrolyte. Some non-flowing reference
7.1.1 The measuring system shall use a high impedance
electrodes are refillable.
preamplifier, preferably located near the electrode but may be
contained within the instrument, capable of measuring the high 7.2.4.2 For fouling processes containing sulfides, or other
impedancepHsensorvoltage.Whenlocatedneartheelectrode, speciesthatcouldreactwiththeelectrolyte,asecondordouble
the preamplifier shall be sealed against moisture intrusion. A liquid junction shall be provided as a barrier to contamination
glass pH electrode measuring circuit must have at least 10 or dilution of the inner electrolyte. A long path between the
Megohm input impedance to preserve the signal. Some mea-
liquid junction and the inner half-cell is also helpful. Some
suring circuits use a differential input and solution ground
electrode systems use another pH glass membrane within the
which can tolerate a higher reference junction impedance and
reference electrode in place of a second junction. In that case,
reduce liquid junction potential errors.
the intermediate electrolyte is a concentrated pH buffer which
7.1.2 The instrument shall provide indication, alarms, re-
holds the reference potential constant.
lays, isolated analog outputs and digital outputs as needed for
7.2.4.3 For oil, grease or suspended solids-bearing samples,
the application. Where output signal isolation from the mea-
the liquid junction should have a relatively large surface area,
surement circuit is not provided within the instrument, the 2
typically greater than 15 mm , to reduce the chances of
signal must pass through an externa
...
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.
An American National Standard Designation: D 6569 – 05 (Reapproved 2009)
Designation:D6569–00
Standard Test Method for
On-Line Measurement of pH
This standard is issued under the fixed designation D 6569; 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
1.1 This test method covers the continuous determination of pH of water by electrometric measurement using the glass, the
antimony or the ion-selective field-effect transistor (ISFET) electrode as the sensor.
1.2 This test method does not cover measurement of samples with less than 100 µS/cm conductivity. Refer to Test Method
D 5128.
1.3 This test method does not cover laboratory or grab sample measurement of pH. Refer to Test Method D 1293.
1.4
1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.5 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:
D 1129 Terminology Relating to Water
D 1193 Specification for Reagent Water
D 1293 Test Methods for pH of Water
D 2777 Practice for Determination of Precision and Bias of Applicable Test Methods of Committee D-19D19 on Water
D 3370 Standard Practices for Sampling Water from Closed Conduits
D 3864 Guide for Continual On-Line Monitoring Systems for Water Analysis
D 5128 Test Method for On-Line pH Measurement of Water of Low Conductivity
3. Terminology
3.1 Definitions—For definitions of terms used in this test method, refer to Terminology D 1129, Test Method D 1293 and
PracticeGuide D 3864.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 liquid junction potential—the dc potential which 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 samples with extreme pH it becomes
larger by an unknown amount and is a zero offset.
4. Summary of Test Method
4.1 pH is measured as a voltage between measuring electrode and reference electrode elements. The sensor assembly typically
includes a temperature compensator to compensate for the varying output of the measuring electrode due to temperature.
4.2 The sensor signals are processed with an industrial pH analyzer/transmitter.
4.3 The equipment is calibrated with standard pH buffer solutions encompassing or in close proximity to the anticipated pH
measurement range.
5. Significance and Use
5.1 pH is a measure of the hydrogen ion activity in water. It is a major parameter affecting the corrosivity and scaling properties
of water, biological life in water and many applications of chemical process control. It is therefore important in water purification,
This test method is under the jurisdiction ofASTM Committee D-19 D19 on Water and is the direct responsibility of Subcommittee D19.03 for Sampling of Water and
Water-Formed Deposits, Surveillance of Water, and Flow Measurement of Water.
Current edition approved June 10, 2000. Published September 2000.
Current edition approved May 1, 2009. Published June 2009. Originally approved in 2000. Last previous edition approved in 2005 as D 6569 – 05.
For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
, Vol 11.01.volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D 6569 – 05 (2009)
use and waste treatment before release to the environment.
5.2 On-line pH measurement is preferred over laboratory measurement to obtain real time, continuous values for automatic
control and monitoring purposes.
6. Interferences
6.1 Pressure and temperature variations may force process sample into the liquid junction of non-flowing junction reference
electrodes and cause changes in the junction potential. Estimates of 0.2 to 0.5 pH errors from this source have been cited. (1)
6.2 Liquid junction potentials at the reference electrode can vary depending on the composition of the sample. Strong acids,
basesandextremelyhighandlowionicstrengthsamplesdevelopliquidjunctionpotentialsdifferentfromtypicalcalibratingbuffer
solutions.(2) Where these conditions exist, the most stable junction potential is obtained using a flowing junction reference
electrode—one that requires refilling with electrolyte solution. However, providing positive flow of electrolyte through the
reference junction places limitations on the sample pressure that can be tolerated. Follow manufacturers’ recommendations.
6.3 pHreferenceelectrodesmustnotbeallowedtodry.Electrolytesaltscancrystallizeintheliquidjunctionandproduceahigh
liquidjunctionimpedance.SubsequentpHmeasurementscouldbenoisy,driftingoroff-scale.WhenpHsensorsarenotinuse,they
should be typically stored wet per manufacturers’ instructions.
6.4 There are several temperature effects on pH measurement.The pH electrode signal is described by the Nernst equation with
its output proportional to the absolute temperature times the pH deviation from the isopotential point—usually 7 pH for glass
electrodes. Compensation for this effect may be accomplished automatically with a temperature sensor integral to the combination
pH probe and an algorithm in the instrument. Alternatively, some instruments may be set manually for a fixed temperature when
a temperature signal is not available. Errors caused by deviations from the manual setting may be calculated from the following
(for a conventional glass electrode system with 7 pH isopotential point).
~pH–7! 3 ~T – Tf!
Glass Electrode pH error 5 (1)
Tf 1 273
where:
pH = uncorrected process pH
T = process temperature (°C)
Tf = temperature setting of fixed compensation (°C)
Other types of electrodes, (antimony, ISFET) have different isopotential points and therefore different corrections. Consult the
manufacturer.
6.5 Solution temperature effects may be caused by changes in the sample, such as ionization of constituents, off-gassing, and
precipitation, which occur with changes in temperature. These are generally small for many samples over moderate temperature
ranges.Inwastestreamswithvariationincomposition,sucheffectsareusuallynotpredictable.However,forsampleswithuniform
or predictable composition with temperature changes > 5°C, one may determine the effect for the samples being measured and
make the correction on all measurements. The pH to be reported is referenced to 25°C unless another temperature is specified.
Someprocessinstrumentshavebuilt-insolutiontemperaturecompensationwhichallowsentryofauser-definedlineartemperature
coefficient into instrument memory for on-line correction of this effect. The temperature of the solution measured for pH should
be monitored and recorded since this information may be critical to understanding the base state of the solution.
NOTE 1—For regulatory monitoring, correction for solution temperature effects should not be done without consulting the governing authority.
6.6 A small temperature influence can occur due to differences in the composition of measuring and reference half-cells. This
is not compensated by any instrumentation. For this reason it is advisable to calibrate as near the measuring temperature as
possible.
6.7 Coating of the measuring electrode may produce a slow or erroneous response since the sensing surface is in contact with
the coating layer rather than the bulk sample. Flat surface electrodes and high sample flow velocity have been found to provide
some self-cleaning effects. Cleaning may be accomplished manually using solvents, acids, detergents, etc. Cleaning may be
automated by a number of approaches. See Appendix X1.
6.8 Abrasion of measuring electrode surfaces from particles in the sample can shorten sensor life. Where abrasive particles are
present,theflowvelocitypasttheelectrodesurfaceshouldbecontrolledlowenoughtominimizeabrasionandprovidesatisfactory
electrode life yet high enough to prevent particles from accumulating into a coating as in 6.7.
6.9 High pH conditions can produce an alkaline error as the glass pH sensor responds to sodium or other small cations in
addition to hydrogen. This type of error is greater at higher temperatures. The result is always a negative error in the range of 0
to -1 pH depending on the pH, temperature, sodium concentration and sensor glass formulation. Some manufacturers have
characterized the alkaline or sodium error sufficiently to closely estimate those errors. Some process ISFET electrodes do not
experience these errors.
6.10 While fluorides in the sample do not interfere with the measurement, if present at pH below 5, they attack silica, greatly
shortening the life of glass and ISFET electrodes.
The boldface numbers given in parentheses refer to a list of references at the end of this standard.
D 6569 – 05 (2009)
6.11 Antimony electrode measurements are subject to major interferences from oxidizing or reducing species, non-linearity,
irregular temperature characteristics and the physical condition of the electrode surface. However, the antimony electrode can
withstand hydrofluoric acid which other electrodes cannot and this application is its primary use. The typical useful range of the
antimony electrode is 3-9 pH. Performance is very application-dependent and should be carefully evaluated.
6.12 Electrical noise induced on the pH sensor-to-instrument cable can cause erratic and offset readings. Route pH signal cables
separately from AC power and switching circuit wiring.
6.13 Electrical insulation leakage in electrode connectors and cable or cracking of a glass electrode membrane can cause the
high impedance pH signal to be attenuated or completely lost. This results in a dead response where the measurement system will
not give response away from the calibration point. Keep pH signal cables and connectors clean and dry. Preamplifiers are normally
located close to pH sensors to minimize the distance high impedance signals must be transported—a help in minimizing noise
interference in 6.12 as well.
6.14 Ground loop interference can occur if the pH measuring circuit is not galvanically isolated from earth ground, except for
the electrodes themselves. Such interference can give an offset or off-scale reading when measuring in a grounded process
installation but will give satisfactory response in grab samples or calibration solutions that are not grounded. Sources of ground
loops include improper wiring of sensor cables, lack of isolation of analog or digital output signals from the measuring circuit, or
a leaking sensor body which allows electrical contact of the sample to a part of the measuring circuit beyond the external electrode
surfaces. Remove output wiring, check sensor wiring and observe readings to locate the cause of grounding problems.
6.15 Measurements on samples with conductivity less than 100 µS/cm are vulnerable to streaming potentials, large junction
potentials and other difficulties and are beyond the scope of this method. Use Test Method D 5128.
7. Apparatus
7.1 Process instrument
7.1.1 Themeasuringsystemshalluseahighimpedancepreamplifier,preferablylocatedneartheelectrodebutmaybecontained
within the instrument, capable of measuring the high impedance pH sensor voltage. When located near the electrode, the
preamplifiershallbesealedagainstmoistureintrusion.AglasspHelectrodemeasuringcircuitmusthaveatleast10 Megohminput
impedance to preserve the signal. Some measuring circuits use a differential input and solution ground which can tolerate a higher
reference junction impedance and reduce liquid junction potential errors.
7.1.2 The instrument shall provide indication, alarms, relays, isolated analog outputs and digital outputs as needed for the
application.Whereoutputsignalisolationfromthemeasurementcircuitisnotprovidedwithintheinstrument,thesignalmustpass
through an external signal isolator before connection to a grounded computer, data acquisition or control system.This will prevent
ground loop errors in the measurement as described in 6.14.
7.1.3 Some instruments provide as a part of their measuring circuit, sensor diagnostics which check the impedance of the glass
electrode, reference electrode or both to assure their integrity.
7.2 Process electrodes—Although measuring and reference electrodes and the temperature compensator are described
individually below, they may also be constructed into a single probe housing, frequently called a combination electrode. The
differenttypesofmeasuringelectrodesandreferenceelectrodesbelowareoptions:onlyonemeasuringelectrodeandonereference
electrode are used for measurement.
7.2.1 Glass measuring—The pH glass measuring electrode is by far the most common type of pH sensor. It shall have a
repeatable response as given in Test Method D 1293. It shall have pH, temperature and pressure ratings suitable for the process
conditions. It shall be conditioned in the process sample for at least 30 minutes or as recommended by the manufacturer before
accurate readings can be taken.
7.2.2 ISFETmeasuring—TheISFETmeasuringelectrodealongwithitsuniquemeasuringcircuitshallgiveresponseequivalent
to a glass electrode measuring system. (ISFET electrodes typically require an adapter circuit to be compatible with glass electrode
measuring instruments.)
7.2.3 Antimony measuring—Theantimonymeasuringelectrodeshallbepurepolishedantimonymetalthathasbeenconditioned
by soaking in water to produce an oxide layer, according to manufacturer’s instructions.
7.2.4 Non-flowing Liquid Junction Reference
7.2.4.1 Thenon-flowingreferenceelectrodeshallcontainanelectrodehalf-cellsimilartotheglassmeasuringelectrode,ifused,
to cancel the temperature effects of the half-cells. It shall contain sufficient electrolyte with gelling agent or other means to restrict
itslossandgiveacceptablelifeintheapplication.Despitethename“non-flowing,”theelectrolyteisconsumableasatraceamount
of it diffuses through t
...
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