ASTM D5391-99(2005)
(Test Method)Standard Test Method for Electrical Conductivity and Resistivity of a Flowing High Purity Water Sample
Standard Test Method for Electrical Conductivity and Resistivity of a Flowing High Purity Water Sample
SIGNIFICANCE AND USE
Conductivity measurements are typically made on samples of moderate to high ionic strength where contamination of open samples in routine laboratory handling is negligible. Under those conditions, standard temperature compensation using coefficients of 1 to 3 % of reading per degree Celsius over wide concentration ranges is appropriate. In contrast, this test method requires special considerations to reduce trace contamination and accommodates the high and variable temperature coefficients of pure water samples that can range as high as 7 % of reading per degree Celsius. In addition, measuring instrument design performance must be proven under high purity conditions.
This test method is applicable for detecting trace amounts of ionic contaminants in water. It is the primary means of monitoring the performance of demineralization and other high purity water treatment operations. It is also used to detect ionic contamination in boiler waters, microelectronics rinse waters, pharmaceutical process waters, etc., as well as to monitor and control the level of boiler and power plant cycle chemistry treatment chemicals. This test method supplements the basic measurement requirements for Test Methods D 1125, D 2186, and D 4519.
At very low levels of alkaline contamination, for example, 0–1 μg/L NaOH, conductivity is suppressed, and can actually be slightly below the theoretical value for pure water. (13,14)4 Alkaline materials suppress the highly conductive hydrogen ion concentration while replacing it with less conductive sodium and hydroxide ions. This phenomenon is not an interference with conductivity or resistivity measurement itself but could give misleading indications of inferred water purity in this range if it is not recognized.
SCOPE
1.1 This test method covers the determination of electrical conductivity and resistivity of high purity water samples below 10 S/cm (above 0.1 Mohm-cm). It is applicable to both continuous and periodic measurements but in all cases, the water must be flowing in order to provide representative sampling. Static grab sampling cannot be used for such high purity water. Continuous measurements are made directly in pure water process lines, or in side stream sample lines to enable measurements on high temperature or high pressure samples, or both.
1.2 The values stated in SI units are to be regarded as the standard.
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:D 5391–99 (Reapproved 2005)
Standard Test Method for
Electrical Conductivity and Resistivity of a Flowing High
Purity Water Sample
This standard is issued under the fixed designation D 5391; 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.
1. Scope and Steam in Closed Conduits
D 1193 Specification for Reagent Water
1.1 This test method covers the determination of electrical
D 2186 Test Methods for Deposit-Forming Impurities in
conductivityandresistivityofhighpuritywatersamplesbelow
Steam
10 µS/cm (above 0.1 Mohm-cm). It is applicable to both
D 2777 Practice for Determination of Precision and Bias of
continuous and periodic measurements but in all cases, the
Applicable Methods of Committee D19 on Water
water must be flowing in order to provide representative
D3370 PracticesforSamplingWaterfromClosedConduits
sampling. Static grab sampling cannot be used for such high
D 3864 Guide for Continual On-Line Monitoring Systems
purity water. Continuous measurements are made directly in
for Water Analysis
pure water process lines, or in side stream sample lines to
D 4519 Test Method for On-Line Determination of Anions
enable measurements on high temperature or high pressure
and Carbon Dioxide in High Purity Water by Cation
samples, or both.
Exchange and Degassed Cation Conductivity
1.2 The values stated in SI units are to be regarded as the
standard.
3. Terminology
1.3 This standard does not purport to address all of the
3.1 Definitions:
safety concerns, if any, associated with its use. It is the
3.1.1 electricalconductivity—refertoTestMethodsD1125.
responsibility of the user of this standard to establish appro-
3.1.2 electrical resistivity—refer to Test Methods D1125.
priate safety and health practices and determine the applica-
3.1.3 For definitions of other terms used in these test
bility of regulatory limitations prior to use.
methods, refer to Terminology D1129.
2. Referenced Documents 3.2 Definitions of Terms Specific to This Standard:
3.2.1 cell constant—the ratio of the length of the path, L
2.1 ASTM Standards:
(cm) and the cross-sectional area of the solution, A (cm ),
D 1066 Practice for Sampling Steam
between the electrodes of a conductivity/resistivity cell, with
D 1125 Test Methods for Electrical Conductivity and Re-
−1
units of cm . In high purity water measurements, the cell
sistivity of Water
−1
constant is normally between 0.001 and 0.1 cm to prevent
D 1129 Terminology Relating to Water
−1
electrical interference. This is lower than the 1 cm of the
D 1192 Specification for Equipment for Sampling Water
standard centimetre cube and is taken into account by direct
reading instrument ranges that are matched with specific cell
constants.
This test method is under the jurisdiction ofASTM Committee D19 on Water
and is the direct responsibility of Subcommittee D19.03 on Sampling of Water and
4. Summary of Test Method
Water-Formed Deposits, Surveillance of Water, and Flow Measurement of Water.
4.1 Conductivity or resistivity is measured with a cell and
Current edition approved April 1, 2005. Published April 2005. Originally
approved in 1993. Last previous edition approved in 1999 as D 5391–99. temperaturesensororcompensatorinaflowing,closedsystem
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 ASTM website. Withdrawn.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D 5391–99 (2005)
to prevent trace contamination from wetted surfaces and from but could give misleading indications of inferred water purity
the atmosphere. Specialized temperature compensation cor- in this range if it is not recognized.
rects the measurement to 25°C, taking into account the
6. Interferences
temperature effects on the ionization of water, the contami-
6.1 Exposure of the sample to the atmosphere may cause
nants, and interactions between the two. In the absence of
changes in conductivity/resistivity due to loss or gain of
specialized temperature compensation, the sample temperature
dissolved ionizable gases. Carbon dioxide, normally present in
is controlled to 25 6 0.2°C.
the air, can reach an equilibrium concentration in water of
4.2 To determine the cell constant of a high purity conduc-
about 1 mg/L and add approximately 1 µS/cm to the conduc-
tivity cell with an instrument capable of accurate measurement
tivity due to formation of carbonic acid. Closed flow-through
over the range of pure water to 150 µS/cm with a single cell
or sealed in-line cell installation is required for this reason.
constant, Test Methods D1125 are used directly. Manufactur-
6.2 Power plant installations utilizing long sample lines can
ers’certificationofcellconstanttraceabilitybythismeansisan
experience significant sampling problems. New sample lines
acceptable alternative.
normally require longterm conditioning. Iron oxides and other
4.3 To determine the cell constant of a high purity conduc-
deposits accumulate in slow flowing horizontal sample lines
tivity cell with an instrument which does not accurately cover
andcandevelopchromatograph-likeretentionofionicspecies,
the range from pure water to 150 µS/cm with a single cell
resultinginverylongdelaytimes.Precautionsaredescribedin
constant, a secondary standard cell is used that has an inter-
Section 9.
mediate cell constant with precise value determined by Test
6.3 Cell and flow chamber surfaces will slowly leach trace
Methods D1125. That secondary standard cell is then used in
ioniccontaminants,evidencedbyincreasingconductivityread-
low conductivity water (not a standard) and readings are
ings with very low or zero flowrate. There must be sufficient
comparedwiththoseofthelowconstantcellundertest.Inthis
flowtokeepthesecontaminantsfromaccumulatingtothepoint
manner, the cell constant of the latter is determined. Manufac-
that they affect the measurement. The high and convoluted
turers’certificationofcellconstanttraceabilitybythismeansis
surface area of platinized cells precludes their use for high
an acceptable alternative.
purity measurements for this reason.
5. Significance and Use
6.4 Capacitance of the cell and extension leadwire, espe-
ciallyinhighpurityrangescanaddsignificantpositiveerrorto
5.1 Conductivity measurements are typically made on
conductance readings (negative error to resistance readings).
samples of moderate to high ionic strength where contamina-
The measuring instrument must be designed to accommodate
tion of open samples in routine laboratory handling is negli-
cell and leadwire characteristics in high purity water as
gible. Under those conditions, standard temperature compen-
described in 7.1.1 and Annex A1. In addition, the instrument
sation using coefficients of 1 to 3% of reading per degree
manufacturers’ recommendations on cell leadwire must be
Celsius over wide concentration ranges is appropriate. In
carefully followed.
contrast, this test method requires special considerations to
6.5 Conductivity and resistivity measurements are refer-
reduce trace contamination and accommodates the high and
enced to 25°C. Either samples must be controlled to 25.0 6
variable temperature coefficients of pure water samples that
0.2°C or specialized temperature compensation must be em-
can range as high as 7% of reading per degree Celsius. In
ployedthataccountsforthecharacteristicsofhighpuritywater
addition, measuring instrument design performance must be
with specific contaminants, as described in 7.1.2.
proven under high purity conditions.
6.6 Samplescontainingdissolvedgasesmusthavesufficient
5.2 This test method is applicable for detecting trace
flow through the cell that bubbles cannot accumulate and
amountsofioniccontaminantsinwater.Itistheprimarymeans
occupy sample volume within the cell, causing low conductiv-
of monitoring the performance of demineralization and other
ity (high resistivity) readings. This problem is typical in
high purity water treatment operations. It is also used to detect
makeupwatertreatmentsystemswherewaterwarmsup,drops
ionic contamination in boiler waters, microelectronics rinse
in pressure, and is acidified by cation exchange operations.
waters, pharmaceutical process waters, etc., as well as to
This releases dissolved air and converts carbonates to carbon
monitor and control the level of boiler and power plant cycle
dioxide gas.
chemistry treatment chemicals. This test method supplements
6.7 High purity conductivity measurement must not be
the basic measurement requirements forTest Methods D1125,
made on a sample downstream of pH sensors since they
D2186, and D4519.
invariably contaminate the sample with traces of reference
5.3 At very low levels of alkaline contamination, for ex-
electrolyte salts. Use a dedicated sample line or place the
ample, 0–1 µg/L NaOH, conductivity is suppressed, and can
conductivity cell upstream from the pH sensors.
actually be slightly below the theoretical value for pure water.
6.8 Conductivity cells mounted downstream from ion ex-
(13,14) Alkaline materials suppress the highly conductive
changers are vulnerable to catching resin particles between the
hydrogen ion concentration while replacing it with less con-
cell electrodes. Resin particles are sufficiently conductive to
ductivesodiumandhydroxideions.Thisphenomenonisnotan
short the cell and cause high off-scale conductivity or ex-
interferencewithconductivityorresistivitymeasurementitself
tremely low resistivity readings. Resin retainers must be
effectiveandcellsmustbeaccessibleforcleaning.Celldesigns
with electrode spacing greater than 0.06 in. (1.5 mm) have
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
this test method. been found to be less likely to trap such particles.
D 5391–99 (2005)
6.9 Conductivity cells, if subjected to demineralizer regen-
eration reagents, would require excessive rinse time to obtain
satisfactory results. Therefore, locate cells where they will be
isolated during regeneration cycles.
7. Apparatus
7.1 Measuring Instrument:
7.1.1 The instrument shall be continuously reading in either
conductivity or resistivity units. It shall be specifically de-
signed to measure in high purity ranges, measuring with ac of
appropriate voltage, frequency, wave shape, phase correction,
andwavesamplingtechniquetominimizeerrorsduetoparallel
andseriescapacitanceofcellandleadwireaswellasminimiz-
ing electrode polarization errors and effects of small direct
current(dc)potentials.Acellsimulationtechniquetoverifythe
performance of an unproven measuring circuit design under
high purity conditions is provided in Annex A1.
7.1.2 Themeasurementshallincludealgorithmstotempera-
ture compensate conductivity or resistivity values to 25.0°C.
Thealgorithmshallcompensateforchangesinwaterionization
as well as for solute ion mobility for neutral salt contaminants.
Theconductivityofpurewaterhasbeendocumentedwithhigh
accuracy (15,1).
7.1.3 In the case of samples containing acidic or basic
solutes (such as power plant treatment using ammonia, mor-
pholine, etc., or acidic cation conductivity samples or micro-
electronicsacidetchrinsemonitoring),specialalgorithmsshall
beemployedthataccountfortheinteractionofacidsandbases
withtheionizationofwater(2,3,4).Theuseriscautionedthat
accuracy of temperature compensation algorithms for these
solutes may vary significantly. The user must determine the
applicability and accuracy for a particular sample in the
anticipatedtemperaturerange.Fig.1illustratesthevariationin
temperature effects on conductivity representative of neutral
NOTE—Curves represent concentrations of impurities given in Table 1.
salts,ammonia,morpholine,andacids.Wherespecializedhigh
FIG. 1 Temperature Effects on the Conductivity of High Purity
purity temperature compensation algorithms are not provided
Water
to accurately compensate for these effects, sample temperature
shall be controlled to 25.06 0.2°C. (Note that conductivity
TABLE 1 Concentrations of Trace Contaminants Plotted in Fig. 1
temperature coefficients exceed 7% of reading per degree
Conductivity Concentration (µg/L)
Celsius in the temperature range of 0 to 10°C.)
µS/cm at 25°C
NaCl HCl NH Morpholine
7.1.4 Output signal(s) from the instrument, if provided,
shall be electrically isolated from the cell and from the earth
5.00 2295 430 638 13 755
ground to prevent ground loop problems when the instrument
2.00 903 172 177 2632
is connected to grounded external devices.
1.00 439 86 76 837
0.500 207 42.6 34.4 298
7.2 Cell:
0.250 91 20.7 16.1 117
7.2.1 Flow-through or in-line conductivity/resistivity cells
0.100 21.0 6.5 5.4 34.6
shall be used to prevent contamination from the atmosphere
0.055 0.0 0.0 0.0 0.0
andwettedsurfacesasdescribedin6.1and6.3.Flowratesshall
be maintained within the manufacturer’s recommendations.
The cell shall retain its constant calibration under the condi-
tions of flowrate, temperature, and pressure of the installation. valid measurements in high purity ranges.Ahigh purity cell in
The cell shall incorporate an integral precision temperature a demineralizer system shall not be located where it can be
sensor to ensure that it accurately senses the sample tempera- exposed to regeneration reagents.
ture where the conductivity/resistivity is being detected to 7.2.3 Electrodes of the cell shall not be platinized for pure
ensure accurate temperature compensation. water measurements since the microscopically rough, porous
7.2.2 The cell for high purity water measurements shall not surfacewouldretainioniccontaminantsandproduceexcessive
be used for measuring higher ionic content samples (greater downscale response times. Only a trace or flash of platinum
than 20 µS/cm, less than 0.05 Mohm-cm) since it would retain black is permissible on electrode surfaces. Electrodes of
ionic contaminants and require excessive rinse-down time for titanium,nickel,monel,stainlesssteel,orplatinumaresuitable
D 5391–99 (2005)
for high purity measurement. However, extra care must be 9.2.1 Donotexposethesampletotheatmospheretoprevent
taken using platinum cells not to exceed manufacturers’ absorption or loss of gases, particularly carbon dioxide, that
recommended flowrate and not to permit rough handling that affect conductivity.
could bend the electrodes and change the cell constant. 9.2.2 Th
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