ASTM D6569-23

Standard Test Method for On-Line Measurement of pH

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, 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.
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 D5128.  
1.3 This test method does not cover laboratory or grab sample measurement of pH. Refer to Test Method D1293.  
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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.6 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

• Status: Published
• Publication Date: 31-Mar-2023
• ICS: 71.040.50 - Physicochemical methods of analysis
• Technical Committee: D19 - Water
• Drafting Committee: D19.03 - Sampling Water and Water-Formed Deposits, Analysis of Water for Power Generation and Process Use, On-Line Water Analysis, and Surveillance of Water

Relations

• Refers: ASTM D1129-13(2020)e2 - Standard Terminology Relating to Water
• Effective Date: 01-May-2020
• Refers: ASTM D2777-12 - Standard Practice for Determination of Precision and Bias of Applicable Test Methods of Committee D19 on Water
• Effective Date: 15-Jun-2012
• Refers: ASTM D1293-12 - Standard Test Methods for pH of Water
• Effective Date: 01-Jan-2012
• Refers: ASTM D3370-10 - Standard Practices for Sampling Water from Closed Conduits
• Effective Date: 01-Dec-2010
• Refers: ASTM D1129-10 - Standard Terminology Relating to Water
• Effective Date: 01-Mar-2010
• Refers: ASTM D5128-09 - Standard Test Method for On-Line pH Measurement of Water of Low Conductivity
• Effective Date: 01-Oct-2009
• Refers: ASTM D3370-08 - Standard Practices for Sampling Water from Closed Conduits
• Effective Date: 01-Oct-2008
• Refers: ASTM D2777-08 - Standard Practice for Determination of Precision and Bias of Applicable Test Methods of Committee D19 on Water
• Effective Date: 15-Jan-2008
• Refers: ASTM D3370-07 - Standard Practices for Sampling Water from Closed Conduits
• Effective Date: 01-Dec-2007
• Refers: ASTM D1129-06a - Standard Terminology Relating to Water
• Effective Date: 01-Sep-2006
• Refers: ASTM D1129-06ae1 - Standard Terminology Relating to Water
• Effective Date: 01-Sep-2006
• Refers: ASTM D2777-06 - Standard Practice for Determination of Precision and Bias of Applicable Test Methods of Committee D19 on Water
• Effective Date: 15-Aug-2006
• Refers: ASTM D3864-06 - Standard Guide for Continual On-Line Monitoring Systems for Water Analysis
• Effective Date: 01-Jul-2006
• Refers: ASTM D1193-06 - Standard Specification for Reagent Water
• Effective Date: 01-Mar-2006
• Refers: ASTM D1129-06 - Standard Terminology Relating to Water
• Effective Date: 15-Feb-2006

Overview

ASTM D6569-23: Standard Test Method for On-Line Measurement of pH provides a comprehensive procedure for the continuous, real-time determination of pH in water using electrometric methods. This ASTM standard is essential for applications where the monitoring and control of water quality is critical, including water treatment, industrial processes, and environmental protection.

On-line pH measurement, as opposed to laboratory grab sample methods, delivers immediate and continuous data, enabling automated process control and improved system responsiveness. The standard focuses on the use of glass, antimony, or ion-selective field-effect transistor (ISFET) electrodes for on-line pH sensing. The method ensures accurate pH monitoring in water samples with conductivity equal to or exceeding 100 μS/cm, supporting compliance with industrial and regulatory requirements.

Key Topics

• Purpose of On-Line pH Measurement

  • pH affects water corrosivity, scaling, biological activity, and efficacy of chemical processes.
  • Continuous measurement is crucial for real-time control and environmental compliance.

• Measurement Technology

  • Specifies use of electrometric sensors: glass, antimony, or ISFET electrodes.
  • Focuses exclusively on on-line measurement, not laboratory or grab sample testing.

• System Requirements

  • High input impedance preamplifiers to capture accurate high-impedance pH signals.
  • Sensor calibration with NIST-traceable pH buffer solutions.
  • Temperature compensation, either integrated or manual, is essential for reliable results.

• Installation and Maintenance

  • Electrodes must be correctly mounted (submersion, insertion, flow-through, or retractable) to ensure accuracy and longevity.
  • Regular calibration and cleaning procedures are critical to maintaining measurement precision and sensor health.

• Limitations

  • Not applicable for water samples with conductivity below 100 μS/cm.
  • Excludes laboratory and grab sample pH methods.
  • Users must address any safety, health, or environmental risks.

Applications

Implementing ASTM D6569-23 facilitates precise process monitoring and control in:

• Water Treatment Plants: Ensures proper pH levels during purification, wastewater treatment, and before environmental discharge.
• Industrial Process Control: Provides continuous feedback for chemical dosing, minimizing risks related to corrosivity and scaling.
• Environmental Monitoring: Helps maintain compliance with regulatory bodies by delivering real-time water quality data.
• Power Generation: Critical for boiler and cooling water systems where pH influences scaling, efficiency, and equipment life.
• Pharmaceutical and Food Industries: Supports quality assurance by controlling pH in process water.

Related Standards

• ASTM D5128: Test Method for On-Line pH Measurement of Water of Low Conductivity (for samples with less than 100 μS/cm).
• ASTM D1293: Test Methods for pH of Water (laboratory and grab sample methods).
• ASTM D1129: Terminology Relating to Water.
• ASTM D3370: Practices for Sampling Water from Flowing Process Streams.
• ASTM D3864: Guide for On-Line Monitoring Systems for Water Analysis.
• ASTM D1193: Specification for Reagent Water.

These standards together support a comprehensive approach to accurate pH measurement, monitoring, and control in water systems.

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Keywords: on-line pH measurement, ASTM D6569-23, process pH monitoring, electrometric pH sensor, water quality, continuous pH measurement, glass electrode, ISFET electrode, antimony electrode, water treatment standards, industrial process control.

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: D6569 − 23
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 D3370 Practices for Sampling Water from Flowing Process
Streams
1.1 This test method covers the continuous determination of
D3864 Guide for On-Line Monitoring Systems for Water
pH of water by electrometric measurement using the glass, the
Analysis
antimony or the ion-selective field-effect transistor (ISFET)
D5128 Test Method for On-Line pH Measurement of Water
electrode as the sensor.
of Low Conductivity
1.2 This test method does not cover measurement of
samples with less than 100 μS/cm conductivity. Refer to Test
3. Terminology
Method D5128.
3.1 Definitions—For definitions of terms used in this test
1.3 This test method does not cover laboratory or grab
method, refer to Terminology D1129, Test Method D1293 and
sample measurement of pH. Refer to Test Method D1293.
Guide D3864.
1.4 The values stated in SI units are to be regarded as
3.2 Definitions of Terms Specific to This Standard:
standard. No other units of measurement are included in this
3.2.1 liquid junction potential, n—the dc potential which
standard.
appears at the point of contact between the reference elec-
1.5 This standard does not purport to address all of the
trode’s salt bridge and the sample solution.
safety concerns, if any, associated with its use. It is the
3.2.1.1 Discussion—Ideally this potential is near zero and is
responsibility of the user of this standard to establish appro-
stable. However, in samples with extreme pH it becomes larger
priate safety, health, and environmental practices and deter-
by an unknown amount and is a zero offset.
mine the applicability of regulatory limitations prior to use.
1.6 This international standard was developed in accor-
4. Summary of Test Method
dance with internationally recognized principles on standard-
4.1 pH is measured as a voltage between measuring elec-
ization established in the Decision on Principles for the
trode and reference electrode elements. The sensor assembly
Development of International Standards, Guides and Recom-
typically includes a temperature compensator to compensate
mendations issued by the World Trade Organization Technical
for the varying output of the measuring electrode due to
Barriers to Trade (TBT) Committee.
temperature.
2. Referenced Documents
4.2 The sensor signals are processed with an industrial pH
2.1 ASTM Standards: analyzer/transmitter.
D1129 Terminology Relating to Water
4.3 The equipment is calibrated with standard pH buffer
D1193 Specification for Reagent Water
solutions encompassing or in close proximity to the anticipated
D1293 Test Methods for pH of Water
pH measurement range.
D2777 Practice for Determination of Precision and Bias of
Applicable Test Methods of Committee D19 on Water
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 April 1, 2023. Published April 2023. Originally
water purification, use and waste treatment before release to
approved in 2000. Last previous edition approved in 2014 as D6569 – 14 which was
the environment.
withdrawn January 2023 and reinstated in April 2023. DOI: 10.1520/D6569-23.
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 [e-mail]. 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 − 23
6. Interferences the 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
vary depending on the composition of the sample. Strong acids,
to understanding the base state of the solution.
bases and extremely high and low ionic strength samples
NOTE 1—For regulatory monitoring, correction for solution temperature
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
ences in the composition of measuring and reference half-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-
~pH 2 7! × ~T 2 Tf!
Glass Electrode pH error 5 (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), and
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 Antimony electrode measurements are subject to major
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
application-dependent and should be carefully evaluated.
composition, such effects are usually not predictable. However,
6.12 Electrical noise induced on the pH sensor-to-
for samples with uniform or predictable composition with
instrument cable can cause erratic and offset readings. Route
temperature changes >5 °C, one may determine the effect for
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 − 23
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 The non-flowing reference electrode shall contain an
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
impedance pH sensor voltage. When located near the electrode, species that could react with the electrolyte, a second or double
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,
holds the reference potential constant.
relays, 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 external signal isolator before
becoming completely blocked.
connection to a grounded computer, data acquisition or control
7.2.5 Flowing Junction Reference:
system. This will prevent ground loop errors in the measure-
7.2.5.1 The flowing junction reference electrode shall con-
ment as described in 6.14.
tain an electrode half-cell similar to the glass measuring
7.1.3 Some instruments provide as a part of their measuring
electrode, if used, to cancel the temperature effects of the
circuit, sensor diagnostics which check the impedance of the
half-cells. It shall have a reservoir of electrolyte solution that is
glass electrode, reference electrode or both to assure their
continuously forced through the liquid junction by gravity head
integrity.
or by external pressure. This type of electrode produces the
7.2 Process Electrodes—Although measuring and reference
most consistent junction potential under extreme process
electrodes and the temperature compensator are described
conditions and therefore is recommended especially for very
individually below, they may also be constructed into a single
high or low pH samples.
probe housing, frequently called a combination electrode. The
7.2.6 Temperature Compensator:
different types of measuring electrodes and reference elec-
7.2.6.1 The temperature compensator shall provide rapid
trodes below are options: only one measuring electrode and
one reference electrode are used for measurement. temperature response corresponding to the temperature of the
glass membrane. It’s temperature signal is used to compensate
7.2.1 Glass Measuring—The pH glass measuring electrode
is by far the most common type of pH sensor. It shall have a for output variations of the measuring electrode due to
D6569 − 23
temperature—called the Nernst effect. Where process tempera- temperature at high pH will cause chemi
...