ASTM D6569-00

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An American National Standard
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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope near zero and is stable. However, in samples with extreme pH
it becomes larger by an unknown amount and is a zero offset.
1.1 This test method covers the continuous determination of
pH of water by electrometric measurement using the glass, the
4. Summary of Test Method
antimony or the ion-selective field-effect transistor (ISFET)
4.1 pH is measured as a voltage between measuring elec-
electrode as the sensor.
trode and reference electrode elements. The sensor assembly
1.2 This test method does not cover measurement of
typically includes a temperature compensator to compensate
samples with less than 100 µS/cm conductivity. Refer to Test
for the varying output of the measuring electrode due to
Method D 5128.
temperature.
1.3 This test method does not cover laboratory or grab
4.2 The sensor signals are processed with an industrial pH
sample measurement of pH. Refer to Test Method D 1293.
analyzer/transmitter.
1.4 This standard does not purport to address all of the
4.3 The equipment is calibrated with standard pH buffer
safety concerns, if any, associated with its use. It is the
solutions encompassing or in close proximity to the anticipated
responsibility of the user of this standard to establish appro-
pH measurement range.
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use.
5. Significance and Use
2. Referenced Documents 5.1 pH is a measure of the hydrogen ion activity in water. It
is a major parameter affecting the corrosivity and scaling
2.1 ASTM Standards:
properties of water, biological life in water and many applica-
D 1129 Terminology Relating to Water
2 tions of chemical process control. It is therefore important in
D 1193 Specification for Reagent Water
water purification, use and waste treatment before release to
D 1293 Test Methods for pH of Water
the environment.
D 2777 Practice for Determination of Precision and Bias of
5.2 On-line pH measurement is preferred over laboratory
Applicable Methods of Committee D-19 on Water
measurement to obtain real time, continuous values for auto-
D 3370 Standard Practices for SamplingWater from Closed
2 matic control and monitoring purposes.
Conduits
D 3864 Guide for Continual On-Line Monitoring Systems
6. Interferences
for Water Analysis
6.1 Pressure and temperature variations may force process
D 5128 TestMethodforOn-LinepHMeasurementofWater
2 sample into the liquid junction of non-flowing junction refer-
of Low Conductivity
ence electrodes and cause changes in the junction potential.
3. Terminology Estimates of 0.2 to 0.5 pH errors from this source have been
cited. (1)
3.1 Definitions—For definitions of terms used in this test
6.2 Liquid junction potentials at the reference electrode can
method, refer to Terminology D 1129, Method D 1293 and
varydependingonthecompositionofthesample.Strongacids,
Practice D 3864.
bases and extremely high and low ionic strength samples
3.2 Definitions of Terms Specific to This Standard:
develop liquid junction potentials different from typical cali-
3.2.1 liquid junction potential—the dc potential which ap-
brating buffer solutions.(2) Where these conditions exist, the
pears at the point of contact between the reference electrode’s
most stable junction potential is obtained using a flowing
salt bridge and the sample solution. Ideally this potential is
junction reference electrode—one that requires refilling with
electrolyte solution. However, providing positive flow of
electrolyte through the reference junction places limitations on
This test method is under the jurisdiction ofASTM Committee D-19 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. The boldface numbers given in parentheses refer to a list of references at the
Annual Book of ASTM Standards, Vol 11.01. end of this standard.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D6569–00
the sample pressure that can be tolerated. Follow manufactur- provide some self-cleaning effects. Cleaning may be accom-
ers’ recommendations. plished manually using solvents, acids, detergents, etc. Clean-
6.3 pH reference electrodes must not be allowed to dry. ing may be automated by a number of approaches. See
Electrolyte salts can crystallize in the liquid junction and
Appendix X1.
produce a high liquid junction impedance. Subsequent pH
6.8 Abrasion of measuring electrode surfaces from particles
measurements could be noisy, drifting or off-scale. When pH
in the sample can shorten sensor life. Where abrasive particles
sensors are not in use, they should be typically stored wet per
are present, the flow velocity past the electrode surface should
manufacturers’ instructions.
be controlled low enough to minimize abrasion and provide
6.4 There are several temperature effects on pH measure-
satisfactory electrode life yet high enough to prevent particles
ment. The pH electrode signal is described by the Nernst
from accumulating into a coating as in 6.7.
equation with its output proportional to the absolute tempera-
6.9 High pH conditions can produce an alkaline error as the
ture times the pH deviation from the isopotential point—
glass pH sensor responds to sodium or other small cations in
usually 7 pH for glass electrodes. Compensation for this effect
addition to hydrogen. This type of error is greater at higher
may be accomplished automatically with a temperature sensor
temperatures. The result is always a negative error in the range
integral to the combination pH probe and an algorithm in the
of 0 to -1 pH depending on the pH, temperature, sodium
instrument. Alternatively, some instruments may be set manu-
concentration and sensor glass formulation. Some manufactur-
ally for a fixed temperature when a temperature signal is not
ers have characterized the alkaline or sodium error sufficiently
available. Errors caused by deviations from the manual setting
to closely estimate those errors. Some process ISFET elec-
may be calculated from the following (for a conventional glass
trodes do not experience these errors.
electrode system with 7 pH isopotential point).
6.10 While fluorides in the sample do not interfere with the
pH–7! 3 T – Tf!
~ ~
Glass Electrode pH error 5 (1)
measurement, if present at pH below 5, they attack silica,
Tf 1 273
greatly shortening the life of glass and ISFET electrodes.
where:
6.11 Antimonyelectrodemeasurementsaresubjecttomajor
pH = uncorrected process pH
interferences from oxidizing or reducing species, non-linearity,
T = process temperature (°C)
irregular temperature characteristics and the physical condition
Tf = temperature setting of fixed compensation (°C)
of the electrode surface. However, the antimony electrode can
Other types of electrodes, (antimony, ISFET) have different
withstand hydrofluoric acid which other electrodes cannot and
isopotential points and therefore different corrections. Consult
this application is its primary use. The typical useful range of
the manufacturer.
the antimony electrode is 3-9 pH. Performance is very
6.5 Solution temperature effects may be caused by changes
application-dependent and should be carefully evaluated.
in the sample, such as ionization of constituents, off-gassing,
6.12 Electrical noise induced on the pH sensor-to-
and precipitation, which occur with changes in temperature.
These are generally small for many samples over moderate instrument cable can cause erratic and offset readings. Route
pH signal cables separately from AC power and switching
temperature ranges. In waste streams with variation in compo-
sition, such effects are usually not predictable. However, for circuit wiring.
samples with uniform or predictable composition with tem-
6.13 Electrical insulation leakage in electrode connectors
perature changes > 5°C, one may determine the effect for the
and cable or cracking of a glass electrode membrane can cause
samples being measured and make the correction on all
the high impedance pH signal to be attenuated or completely
measurements. The pH to be reported is referenced to 25°C
lost. This results in a dead response where the measurement
unless another temperature is specified. Some process instru-
system will not give response away from the calibration point.
ments have built-in solution temperature compensation which
Keep pH signal cables and connectors clean and dry. Pream-
allows entry of a user-defined linear temperature coefficient
plifiers are normally located close to pH sensors to minimize
into instrument memory for on-line correction of this effect.
the distance high impedance signals must be transported—a
The temperature of the solution measured for pH should be
help in minimizing noise interference in 6.12 as well.
monitored and recorded since this information may be critical
6.14 Ground loop interference can occur if the pH measur-
to understanding the base state of the solution.
ing circuit is not galvanically isolated from earth ground,
NOTE 1—For regulatory monitoring, correction for solution tempera-
except for the electrodes themselves. Such interference can
tureeffectsshouldnotbedonewithoutconsultingthegoverningauthority.
give an offset or off-scale reading when measuring in a
grounded process installation but will give satisfactory re-
6.6 A small temperature influence can occur due to differ-
sponse in grab samples or calibration solutions that are not
encesinthecompositionofmeasuringandreferencehalf-cells.
grounded. Sources of ground loops include improper wiring of
This is not compensated by any instrumentation. For this
sensor cables, lack of isolation of analog or digital output
reason it is advisable to calibrate as near the measuring
temperature as possible. signals from the measuring circuit, or a leaking sensor body
which allows electrical contact of the sample to a part of the
6.7 Coating of the measuring electrode may produce a slow
or erroneous response since the sensing surface is in contact measuring circuit beyond the external electrode surfaces.
Remove output wiring, check sensor wiring and observe
with the coating layer rather than the bulk sample. Flat surface
electrodes and high sample flow velocity have been found to readings to locate the cause of grounding problems.
D6569–00
6.15 Measurements on samples with conductivity less than as a trace amount of it diffuses through the junction into the
100 µS/cm are vulnerable to streaming potentials, large junc- sample. The only opening of the electrode is its interface with
tion potentials and other difficulties and are beyond the scope the process through its liquid junction—a small passage of
of this method. Use Method D 5128. porous ceramic, polymer, wood, fiber, ground glass surfaces or
other material that allows electrical continuity with the sample
7. Apparatus
while limiting loss of electrolyte. Some non-flowing reference
7.1 Process instrument
electrodes are refillable.
7.1.1 The measuring system shall use a high impedance
7.2.4.2 For fouling processes containing sulfides, or other
preamplifier, preferably located near the electrode but may be
species that could react with the electrolyte, a second or double
contained within the instrument, capable of measuring the high
liquid junction shall be provided as a barrier to contamination
impedancepHsensorvoltage.Whenlocatedneartheelectrode,
or dilution of the inner electrolyte. A long path between the
the preamplifier shall be sealed against moisture intrusion. A
liquid junction and the inner half-cell is also helpful. Some
glass pH electrode measuring circuit must have at least 10
electrode systems use another pH glass membrane within the
Megohm input impedance to preserve the signal. Some mea-
reference electrode in place of a second junction. In that case,
suring circuits use a differential input and solution ground
the intermediate electrolyte is a concentrated pH buffer which
which can tolerate a higher reference junction impedance and
holds the reference potential constant.
reduce liquid junction potential errors.
7.2.4.3 For oil, grease or suspended solids-bearing samples,
7.1.2 The instrument shall provide indication, alarms, re-
the liquid junction should have a relatively large surface area,
lays, isolated analog outputs and digital outputs as needed for
typically greater than 15 mm , to reduce the chances of
the application. Where output signal isolation from the mea-
becoming completely blocked.
surement circuit is not provided within the instrument, the
7.2.5 Flowing Junction Reference
signal must pass through an external signal isolator before
7.2.5.1 The flowing junction reference electrode shall con-
connection to a grounded computer, data acquisition or control
tain an electrode half-cell similar to the glass measuring
system. This will prevent ground loop errors in the measure-
electrode, if used, to cancel the temperature effects of the
ment as described in 6.14.
half-cells. It shall have a reservoir of electrolyte solution that is
7.1.3 Some instruments provide as a part of their measuring
continuouslyforcedthroughtheliquidjunctionbygravityhead
circuit, sensor diagnostics which check the impedance of the
or by external pressure. This type of electrode produces the
glass electrode, reference electrode or both to assure their
most consistent junction potential under extreme process
integrity.
conditions and therefore is recommended especially for very
7.2 Process electrodes—Although measuring and reference
high or low pH samples.
electrodes and the temperature compensator are described
7.2.6 Temperature Compensator
individually below, they may also be constructed into a single
7.2.6.1 The temperature compensator shall provide rapid
probe housing, frequently called a combination electrode. The
temperature response corresponding to the temperature of the
different types of measuring electrodes and reference elec-
glass membrane. It’s temperature signal is used to compensate
trodes below are options: only one measuring electrode and
for output variations of the measuring electrode due to
one reference electrode are used for
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