ASTM D2791-19

Standard Test Method for On-Line Determination of Sodium in Water

SIGNIFICANCE AND USE
4.1 Sodium is a pervasive contaminant and it is the first cation to break through deionization equipment. This test method allows measuring sodium in low-sodium waters at micrograms per litre (parts per billion) concentrations to identify contamination or to monitor equipment performance. Applications include monitoring makeup systems, condensers, condensate polishers, feedwater, boilerwater, and steam.  
4.2 This test method is more sensitive and selective than conductivity measurements on high purity samples.
SCOPE
1.1 This test method covers the on-line determination of trace amounts of sodium in water using an ion-selective electrode.  
1.2 This test method is based on on-line application of the sodium ion electrode as reported in the technical literature (1-3).2 It is generally applicable over the range of 0.01 to 10 000 μg/L.  
1.3 The analyst should be aware that adequate collaborative data for precision and bias statements as required by Practice D2777 are not provided. See Section 16 for details.  
1.4 The values stated in SI units are to be regarded as standard. The values given in parentheses after SI units are provided for information only and are not considered 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. For specific hazard statements see Section 6.  
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: 14-Dec-2019
• ICS: 13.060.50 - Examination of water for chemical substances
• 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

• Replaces: ASTM D2791-13 - Standard Test Method for On-line Determination of Sodium in Water
• Effective Date: 15-Dec-2019
• Refers: ASTM D1129-13(2020)e2 - Standard Terminology Relating to Water
• Effective Date: 01-May-2020
• Refers: ASTM D1066-18 - Standard Practice for Sampling Steam
• Effective Date: 01-Aug-2018
• Refers: ASTM D1066-18e1 - Standard Practice for Sampling Steam
• Effective Date: 01-Aug-2018
• 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 D1066-11 - Standard Practice for Sampling Steam
• Effective Date: 15-Jun-2011
• 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 D3370-08 - Standard Practices for Sampling Water from Closed Conduits
• Effective Date: 01-Oct-2008
• Refers: ASTM D5540-08 - Standard Practice for Flow Control and Temperature Control for On-Line Water Sampling and Analysis
• 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 D1066-06 - Standard Practice for Sampling Steam
• Effective Date: 15-Dec-2006
• Refers: ASTM D1129-06ae1 - Standard Terminology Relating to Water
• Effective Date: 01-Sep-2006

Overview

ASTM D2791-19 is the widely recognized standard test method for the on-line determination of sodium in water using an ion-selective electrode. Developed by ASTM International, this standard provides a sensitive and selective approach for measuring trace amounts of sodium at micrograms per litre (µg/L, parts per billion) concentrations. Detecting sodium at such low levels is crucial for identifying contamination and monitoring the performance of deionization and other water purification equipment. This method is especially relevant in high purity water applications, where sodium is often the first cation to break through purification barriers, making it an essential quality and process control parameter in power generation, industrial, and laboratory environments.

Key Topics

• Measurement Principle: Utilizes a sodium ion-selective electrode for continuous, on-line monitoring of sodium concentration in water.
• Detection Range: Capable of identifying sodium across a broad range, from 0.01 to 10,000 µg/L (parts per billion).
• Sensitivity and Selectivity: More sensitive and specific than conventional conductivity measurements, particularly suited for high-purity samples.
• Operational Guidelines:
  • Stresses the importance of using high purity reagents and water to prevent contamination.
  • Outlines steps for sampling, calibration, and maintenance to ensure reliable results.
  • Recommends regular electrode cleaning and calibration checks for consistent accuracy.
• Interference Considerations: Addresses potential interference from other monovalent cations and the need for specific reagent and sample handling to minimize errors.
• Equipment Maintenance: Highlights proper care for electrodes, use of suitable flow chambers, and temperature control measures.
• Quality Assurance: Provides guidance on instrument calibration, sample handling, and validation methods appropriate for low-level sodium detection.

Applications

The on-line sodium determination method specified in ASTM D2791-19 is vital for a variety of industrial and laboratory applications, including:

• Power Generation Plants: Monitoring sodium in makeup water, condensers, condensate polishers, feedwater, boiler water, and steam to prevent scaling, corrosion, and equipment damage.
• High-Purity Water Systems: Quality assurance in semiconductor manufacturing, pharmaceuticals, and laboratories that require ultra-pure water.
• Deionization System Monitoring: Early detection of sodium breakthrough ensures effective operation and timely maintenance of deionization units.
• Industrial Process Control: Provides real-time data, enabling automatic control and optimization of water treatment and purification processes.
• Environmental Monitoring: Used to assess sodium contamination in natural and treated waters where precise, low-level measurement is required.

Related Standards

Several ASTM standards support or complement the requirements of ASTM D2791-19 for sodium measurement in water systems:

• ASTM D1066 - Practice for Sampling Steam
• ASTM D1129 - Terminology Relating to Water
• ASTM D1193 - Specification for Reagent Water
• ASTM D1293 - Test Methods for pH of Water
• ASTM D2777 - Practice for Determination of Precision and Bias of Applicable Test Methods of Committee D19 on Water
• ASTM D3864 - Guide for On-Line Monitoring Systems for Water Analysis
• ASTM D3370 - Practices for Sampling Water from Flowing Process Streams
• ASTM D5540 - Practice for Flow Control and Temperature Control for On-Line Water Sampling and Analysis

Conclusion

ASTM D2791-19 establishes a highly effective method for on-line sodium analysis in water, ensuring rapid detection of trace contamination and robust monitoring of water treatment systems. Adhering to this standard helps industries maintain water quality, protect critical equipment, and comply with stringent regulatory and operational requirements. For professionals concerned with water purity and process control, implementing ASTM D2791-19 offers reliable, real-time sodium measurement essential for operational excellence.

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: D2791 − 19
Standard Test Method for
On-Line Determination of Sodium in Water
This standard is issued under the fixed designation D2791; 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* D1129 Terminology Relating to Water
D1193 Specification for Reagent Water
1.1 This test method covers the on-line determination of
D1293 Test Methods for pH of Water
trace amounts of sodium in water using an ion-selective
D2777 Practice for Determination of Precision and Bias of
electrode.
Applicable Test Methods of Committee D19 on Water
1.2 This test method is based on on-line application of the
D3864 Guide for On-Line Monitoring Systems for Water
sodium ion electrode as reported in the technical literature
Analysis
(1-3). It is generally applicable over the range of 0.01 to
D3370 Practices for Sampling Water from Flowing Process
10 000 µg/L.
Streams
D5540 Practice for Flow Control and Temperature Control
1.3 The analyst should be aware that adequate collaborative
data for precision and bias statements as required by Practice for On-Line Water Sampling and Analysis
D2777 are not provided. See Section 16 for details.
3. Terminology
1.4 The values stated in SI units are to be regarded as
3.1 Definitions:
standard. The values given in parentheses after SI units are
3.1.1 For definitions of terms used in this standard, refer to
provided for information only and are not considered standard.
Terminology D1129.
1.5 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
4. Significance and Use
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter- 4.1 Sodium is a pervasive contaminant and it is the first
cation to break through deionization equipment. This test
mine the applicability of regulatory limitations prior to use.
method allows measuring sodium in low-sodium waters at
For specific hazard statements see Section 6.
micrograms per litre (parts per billion) concentrations to
1.6 This international standard was developed in accor-
identify contamination or to monitor equipment performance.
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the Applications include monitoring makeup systems, condensers,
condensate polishers, feedwater, boilerwater, and steam.
Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
4.2 This test method is more sensitive and selective than
Barriers to Trade (TBT) Committee.
conductivity measurements on high purity samples.
2. Referenced Documents
5. Reagents and Materials
2.1 ASTM Standards:
5.1 Purity of Reagents—Reagent grade chemicals shall be
D1066 Practice for Sampling Steam
used in all tests. Unless otherwise indicated, it is intended that
all reagents shall conform to the specifications of the Commit-
teeonAnalyticalReagentsoftheAmericanChemicalSociety.
This test method is under the jurisdiction of ASTM Committee D19 on Water
In many instances, reagent grade chemicals contain higher
and is the direct responsibility of Subcommittee D19.03 on Sampling Water and
levels of sodium contamination than are compatible with this
Water-Formed Deposits, Analysis of Water for Power Generation and Process Use,
On-Line Water Analysis, and Surveillance of Water. test method. It must be ascertained that the reagent is of
Current edition approved Dec. 15, 2019. Published January 2020. Originally
approved in 1969. Last previous edition approved in 2013 as D2791 – 13. DOI:
10.1520/D2791-19.
2 4
The boldface numbers in parentheses refer to the list of references at the end of ACS Reagent Chemicals, Specifications and Procedures for Reagents and
this test method. Standard-Grade Reference Materials, American Chemical Society, Washington,
For referenced ASTM standards, visit the ASTM website, www.astm.org, or DC. For suggestions on the testing of reagents not listed by theAmerican Chemical
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Society, see Analar Standards for Laboratory Chemicals, BDH Ltd., Poole, Dorset,
Standards volume information, refer to the standard’s Document Summary page on U.K., and the United States Pharmacopeia and National Formulary, U.S. Pharma-
the ASTM website. copeial Convention, Inc. (USPC), Rockville, MD.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D2791 − 19
sufficiently high purity to permit its use without lessening the 7.6 Where speed of response is not critical, sequential
accuracy of the determination. sampling of multiple streams may be effected with 3-way
solenoid valves for sample selection. The 3-way valves allow
5.2 Purity of Water—Unless otherwise indicated, references
samples not being measured to continue flowing (to drain) and
towatershallbeunderstoodtomeanreagentwaterconforming
to be current when they are selected. Automatic selection
to Specification D1193, Type I. In addition, the sodium or
shouldincludeanadjustabletimingdevicefortypicalsampling
potassium content shall not exceed 10 µg/L(10 ppb) or 1 % of
times near 10 min per point. It is not advisable to sequence
the lowest concentration to be determined, whichever is lower.
sample streams of significantly different concentrations using
5.2.1 Single-distilled water passed through a mixed bed
sequential sampling.
deionizing unit composed of strong cation and anion resins can
produce an effluent containing less than 1.0 µg/L (1.0 ppb) of
8. Summary of Test Method
sodium. If such water is stored in a closed alkali metal-free
container, such as one made of polyethylene, TFE- 8.1 Sodium ion electrodes provide consistent logarithmic
response over many orders of magnitude of concentration
fluorocarbon, or stainless steel, subsequent increases in
conductivity, usually due to absorption of carbon dioxide, will using the same principles as pH electrodes but with different
ion selectivity. The electrode signal has a slope of approxi-
not invalidate its use for this purpose.
mately 59 mV/decade change in sodium ion concentration at
25°C (77°F).
6. Hazards
8.2 This test method includes provision for the addition of
6.1 pH adjusting reagents are strongly alkaline and volatile.
pH adjusting reagent to suppress hydrogen ion concentration
Use normal eye and skin protection when handling ammonia,
and assure accurate electrode response to sodium.
ammonium hydroxide, dimethylamine, diisopropylamine,
monoethylamine, or morpholine. Extra care is needed in
8.3 This test method is particularly adaptable to high purity
handling the gas-permeable tubing exposed to reagents used
water and is relatively free of interferences (1). The overall
with the passive diffusion reagent delivery system. Keep
operating cost of this system is considerably less than that of
reagents in the open wherever possible and take necessary
on-line flame photometry, and it is more sensitive than electri-
precautions to keep them from the respiratory tract in event of
cal conductivity.
a spill or leak. Under certain conditions these reagents can
8.4 The repeatability of this test method is 65 % of the
produce an explosive mixture with air. OSHA standards must
reading.
be followed.
9. Interferences
7. Sampling
9.1 The sodium ion electrode, like all potentiometric elec-
7.1 Sample the water for on-line sodium ion measurements
trode measuring systems, is responsive to changes in ion
in a flowing stream in accordance with Practice D1066, Guide
activity and not true concentration changes (that is, the
D3864, Practices D3370, and Practice D5540 as applicable.
response is to changes in concentration multiplied by an
7.2 Regulate the pressure of samples within the instrument
activity coefficient). However, as concentrations approach
manufacturer’s requirements.
infinite dilution, activity coefficients approach unity and ion
concentration and active ion concentration become very nearly
7.3 Regulate the temperature of samples that must be
equal.
condensed, or cooled, or both, to a level between 15 and 40°C
(59 and 104°F) or within manufacturer’s requirements. For
9.2 The activity coefficient of sodium ion will vary with
highest accuracy, bring the sample temperature close to the
changes in the total ionic strength of the solution. Therefore, it
temperature of the standards during calibration. is important to maintain either a low or constant ionic strength.
Aconstant flow of pH adjusting reagent generally establishes a
7.4 When sample system plumbing has been newly
consistent ionic strength.
installed, or has not been carrying process stream water for
some time, or has been open to the atmosphere, it may take 24
9.3 ThesodiumcontentofpHadjustingreagent,ifdelivered
h of purging to bring the sodium content at the receiving end directly to the sample, must not be significant compared with
down to the same level as the sample point, especially when
the lowest concentration being measured. Any air contacting
the process stream is less than 1.0 µg/L (1.0 ppb). In the case the sample must be sodium-free.
of lines that are very dirty or have been subject to biological
9.4 The sodium ion electrode is responsive to certain other
fouling, pumping a 25 % solution of nitric acid is effective for
monovalent cations. Interference by silver, lithium, hydrogen,
plastic and stainless lines. About 30 line-volumes of acid
potassium,ammonium,andotherionsmustbeconsidered.The
should be pumped through slowly, followed by the fastest
selectivity to interfering ions varies by electrode manufacturer.
practical purge of process water in the amount of 300 volumes.
Inthelow-solidswatertowhichthistestmethodapplies,silver
When using an acid-cleaning procedure, confine the acid to the
and lithium ions are usually absent. Potassium ion, often
dirty part of the system. Under no circumstances should the
contributed to the sample by the reference electrode, must be
acid enter the measuring instrument.
carried downstream away from the sodium ion electrode.
7.5 Adjust the sample flow in accordance with the manu- Ammonium ion, present in many power plant samples, gener-
facturer’s recommendation. ally does not interfere with measurements greater than 1 µg/L
D2791 − 19
(1 ppb). Measurements below 1 µg/L use a stronger base interference from the reference electrode and construct the
reagent that suppresses the ionization of ammonia. chamber of inert materials such as plastic or stainless steel.
10.5.1 If a plastic is used, cast or machine from a solid
9.5 Elevation of pH so that hydrogen ion concentration is 3
block.Gasketelectrodestopreventin-leakageofair.Protection
to 4 orders of magnitude lower than that for sodium generally
of electrodes shall be in accordance with the manufacturer’s
makes the electrode response independent of variations in
recommendations. Connections to the flow chamber must be
hydrogen ion concentration of the original sample. Any of the
solution or earth grounded. No glass or copper is permissible
reagents mentioned is satisfactory to increase the pH to a level
in flow chamber construction.
such that the electrode is essentially insensitive to hydrogen
10.6 Analyzer Fluidics—Unattended automatic calibration
ion, within sodium ranges specified by the manufacturer. The
may be provided using relays and valves for accurately adding
apparatus for sodium measurement may include pH measure-
sodium standard solution with timing and sequencing con-
ment of the conditioned sample to assure absence of this
trolled by the measuring instrument.
interference.
9.6 The sodium ion electrode is not subject to interference
11. Reagents and Materials
fromcolor,turbidity,colloidalmatter,oxidants,andreductants.
11.1 pH Adjusting Reagents:
11.1.1 Ammonia Gas—Commercial anhydrous grade am-
10. Apparatus
monia (NH ) having a minimum purity of 99.9 %. Gas is
10.1 Measuring Instrument—Use commercially available
absorbed directly by the sample.
potentiometric specific ion monitors that have expanded-scale 11.1.2 Ammonium Hydroxide—Commercial ammonia
operation with adjustable ranges calibrated directly in sodium
solution,approximately29 %NH inwater.Vapordiffusesinto
ion concentration units of micrograms per litre (parts per the sample through gas-permeable tubing.
billion). Electrical output signals must be isolated from ground
11.1.3 Diisopropylamine—Commercial grade liquid is va-
and from electrode input and may be scaled for logarithmic, porized and transported by an inert carrier gas to the sample
linear, or bilinear ranges.
stream, or vapor diffuses into the sample through gas-
permeable tubing.
10.2 Sodium Ion Electrode—Use a commercially available
11.1.4 Dimethylamine Gas—Commercial grade having a
sodium-sensitive electrode (sodium ion electrode). Because
minimum purity of 99 %. Gas is absorbed directly by the
electrode selectivities vary among manufacturers, care must be
sample.
taken that the electrode, reagent and sample conditions are
11.1.5 Monoethylamine—Commercial grade vapor diffuses
compatible (see 9.4 and 9.5).
into the sample through gas-permeable tubing.
10.3 Reference Electrode—Use a reference electrode com-
11.2 Sodium Chloride Stock Solution (1.00 mL = 0.100 mg
patible with the measuring electrode.
Na)—Dissolve in water 0.2542 g of sodium chloride (NaCl),
10.3.1 When the sodium ion electrode has a silver-silver
dried to constant weight at 105°C in water, and dilute to 1 Lin
chloride internal half cell, the reference electrode should be
a thoroughly cleaned polyethylene flask. Store sodium stock in
silver-silver chloride. When the sodium ion electrode has a
a polyethylene or equally alkali metal-free container. Prepare
calomel internal half cell, the reference electrode should be
standards of lesser concentrations by dilutions of the stock
calomel. Dissimilar reference electrodes may be used provided
solution.
adequatecompensationismadeelectronicallytocorrectforthe
difference between the measuring electrode and the reference
12. Calibration and Standardization
electrode. If the reference electrode filling solution is an
12.1 All procedures described in this section are subject to
interference in the measurement of sodium, then the reference
zero and calibration drift. Determine the frequency of calibra-
electrode must be downstream from the measuring electrode
tion checks required to obtain the desired accuracy based on
(see 9.4 and 10.3.2).
experience. Whenever a major shift in sodium concentration is
10.3.2 If calomel electrodes are used, refer to Test Methods
noted, a calibration check is advisable.
D1293. The electrolyte used in reference electrodes and all
12.2 Low sodium standards are subject to contamination
maintenance shall conform to the manufacturer’s recommen-
from many sources. Calibrate using standards above 100 µg/L
dations. With flowing junction reference electrodes to ensure
(100ppb)toreducetheeffectsofcontamination.Toavoidmost
the desired slow outward flow of electrolyte, the solution
contamination connect the standard container to the measuring
pressureinsidethejunctionshallbekeptsomewhathigherthan
equipment so that the standard can be introduced without
that outside the junction.
exposing it to nonfiltered air. Use manual or automatic valves
10.4 Temperature Compensation—Use an automatic tem-
to introduce the standard for calibration checks. Some analyz-
perature compensator in accordance with the manufacturer’s
ers use a known addition method that employs higher level
recommendation.
sodium standards.
10.5 Flow Chamber—For best results install the electrodes 12.3 Run the sample and
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM 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.
Designation: D2791 − 13 D2791 − 19
Standard Test Method for
On-lineOn-Line Determination of Sodium in Water
This standard is issued under the fixed designation D2791; 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 on-line determination of trace amounts of sodium in water using an ion-selective electrode.
1.2 This test method is based on on-line application of the sodium ion electrode as reported in the technical literature (1-3).
It is generally applicable over the range of 0.01 to 10 000 μg/L.
1.3 The analyst should be aware that adequate collaborative data for precision and bias statements as required by Practice D2777
are not provided. See Section 16 for details.
1.4 The values stated in SI units are to be regarded as standard. The inch-pound units values given in parentheses are for
information only.after SI units are provided for information only and are not considered 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. For specific hazard statements see Section 6.
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.
2. Referenced Documents
2.1 ASTM Standards:
D1066 Practice for Sampling Steam
D1129 Terminology Relating to Water
D1193 Specification for Reagent Water
D1293 Test Methods for pH of Water
D2777 Practice for Determination of Precision and Bias of Applicable Test Methods of Committee D19 on Water
D3864 Guide for On-Line Monitoring Systems for Water Analysis
D3370 Practices for Sampling Water from Flowing Process Streams
D5540 Practice for Flow Control and Temperature Control for On-Line Water Sampling and Analysis
3. Terminology
3.1 Definitions—Definitions: For definitions of terms used in this test method, refer to Terminology D1129.
3.1.1 For definitions of terms used in this standard, refer to Terminology D1129.
4. Significance and Use
4.1 Sodium is a pervasive contaminant and it is the first cation to break through deionization equipment. This test method allows
measurement of measuring sodium in low-sodium waters at micrograms per litre (parts per billion) concentrations of sodium in
water to monitor low-sodium water sources for indications of contamination or proper operation. to identify contamination or to
monitor equipment performance. Applications include monitoring makeup systems, condensers, condensate polishers, feedwater,
boilerwater, and steam.
This test method is under the jurisdiction of ASTM Committee D19 on Water and is the direct responsibility of Subcommittee D19.03 on Sampling Water and
Water-Formed Deposits, Analysis of Water for Power Generation and Process Use, On-Line Water Analysis, and Surveillance of Water.
Current edition approved Jan. 1, 2013Dec. 15, 2019. Published February 2013January 2020. Originally approved in 1969. Last previous edition approved in 20072013
as D2791 – 07.D2791 – 13. DOI: 10.1520/D2791-13.10.1520/D2791-19.
The boldface numbers in parentheses refer to the list of references at the end of this test method.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’sstandard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D2791 − 19
4.2 This test method is more sensitive and selective than conductivity measurements on high purity samples.
5. Reagents and Materials
5.1 Purity of Reagents—Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all
reagents shall conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society. In many
instances, reagent grade chemicals contain higher levels of sodium contamination than are compatible with this test method. It must
be ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination.
5.2 Purity of Water—Unless otherwise indicated, references to water shall be understood to mean reagent water conforming to
Specification D1193, Type I. In addition, the sodium or potassium content shall not exceed 10 μg/L (10 ppb) or 1 % of the lowest
concentration to be determined, whichever is lower.
5.2.1 Single-distilled water passed through a mixed bed deionizing unit composed of strong cation and anion resins can produce
an effluent containing less than 1.0 μg/L (1.0 ppb) of sodium. If such water is stored in a closed alkali metal-free container, such
as one made of polyethylene, TFE-fluorocarbon, or stainless steel, subsequent increases in conductivity, usually due to absorption
of carbon dioxide, will not invalidate its use for this purpose.
6. Hazards
6.1 pH adjusting reagents are strongly alkaline and volatile. Use normal eye and skin protection when handling ammonia,
ammonium hydroxide, dimethylamine, diisopropylamine, monoethylamine, or morpholine. Extra care is needed in handling the
gas-permeable tubing exposed to reagents used with the passive diffusion reagent delivery system. Keep reagents in the open
wherever possible and take necessary precautions to keep them from the respiratory tract in event of a spill or leak. Under certain
conditions these reagents can produce an explosive mixture with air. OSHA standards must be followed.
7. Sampling
7.1 Sample the water for on-line sodium ion measurements in a flowing stream in accordance with Practice D1066, Guide
D3864, PracticePractices D3370, and Practice D5540 as applicable.
7.2 Regulate the pressure of samples within the instrument manufacturer’smanufacturer’s requirements.
7.3 Regulate the temperature of samples that must be condensed, or cooled, or both, to a level between 15 and 40°C (59 and
104°F) or within manufacturer’smanufacturer’s requirements. For highest accuracy, bring the sample temperature close to the
temperature of the standards during calibration.
7.4 When sample system plumbing has been newly installed, or has not been carrying process stream water for some time, or
has been open to the atmosphere, it may take 24 h of purging to bring the sodium content at the receiving end down to the same
level as the sample point, especially when the process stream is less than 1.0 μg/L (1.0 ppb). In the case of lines that are very dirty
or have been subject to biological fouling, pumping a 25 % solution of nitric acid is effective for plastic and stainless lines. About
30 line-volumes of acid should be pumped through slowly, followed by the fastest practical purge of process water in the amount
of 300 volumes. When using an acid-cleaning procedure, confine the acid to the dirty part of the system. Under no circumstances
should the acid enter the measuring instrument.
7.5 Adjust the sample flow in accordance with the manufacturer’smanufacturer’s recommendation.
7.6 Where speed of response is not critical, sequential sampling of multiple streams may be effected with 3-way solenoid valves
for sample selection. The 3-way valves allow samples not being measured to continue flowing (to drain) and to be current when
they are selected. Automatic selection should include an adjustable timing device for typical sampling times near 10 min per point.
It is not advisable to sequence sample streams of significantly different concentrations using sequential sampling.
8. Summary of Test Method
8.1 Sodium ion electrodes provide consistent logarithmic response over many orders of magnitude of concentration using the
same principles as pH electrodes but with different ion selectivity. The electrode signal has a slope of approximately 59 mV/decade
change in sodium ion concentration at 25°C (77°F).
8.2 This test method includes provision for the addition of pH adjusting reagent to suppress hydrogen ion concentration and
assure accurate electrode response to sodium.
8.3 This test method is particularly adaptable to high purity water and is relatively free of interferences (1). The overall
operating cost of this system is considerably less than that of on-line flame photometry, and it is more sensitive than electrical
conductivity.
ACS Reagent Chemicals, American Chemical Society Specifications Specifications and Procedures for Reagents and Standard-Grade Reference Materials, , American
Chemical Society, Washington, DC. For suggestions on the testing of reagents not listed by the American Chemical Society, see Analar Standards for Laboratory Chemicals,
BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia and National Formulary, U.S. PharmaceuticalPharmacopeial Convention, Inc. (USPC), Rockville, MD.
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8.4 The repeatability of this test method is 65 % of the reading.
9. Interferences
9.1 The sodium ion electrode, like all potentiometric electrode measuring systems, is responsive to changes in ion activity and
not true concentration changes (that is, the response is to changes in concentration multiplied by an activity coefficient). However,
as concentrations approach infinite dilution, activity coefficients approach unity and ion concentration and active ion concentration
become very nearly equal.
9.2 The activity coefficient of sodium ion will vary with changes in the total ionic strength of the solution. Therefore, it is
important to maintain either a low or constant ionic strength. A constant flow of pH adjusting reagent generally establishes a
consistent ionic strength.
9.3 The sodium content of pH adjusting reagent, if delivered directly to the sample, must not be significant compared with the
lowest concentration being measured. Any air contacting the sample must be sodium-free.
9.4 The sodium ion electrode is responsive to certain other monovalent cations. Interference by silver, lithium, hydrogen,
potassium, ammonium, and other ions must be considered. The selectivity to interfering ions varies by electrode manufacturer. In
the low-solids water to which this test method applies, silver and lithium ions are usually absent. Potassium ion, often contributed
to the sample by the reference electrode, must be carried downstream away from the sodium ion electrode. Ammonium ion, present
in many power plant samples, generally does not interfere with measurements greater than 1 μg/L (1 ppb). Measurements below
1 μg/L use a stronger base reagent that suppresses the ionization of ammonia.
9.5 Elevation of pH so that hydrogen ion concentration is 3 to 4 orders of magnitude lower than that for sodium generally makes
the electrode response independent of variations in hydrogen ion concentration of the original sample. Any of the reagents
mentioned is satisfactory to increase the pH to a level such that the electrode is essentially insensitive to hydrogen ion, within
sodium ranges specified by the manufacturer. The apparatus for sodium measurement may include pH measurement of the
conditioned sample to assure absence of this interference.
9.6 The sodium ion electrode is not subject to interference from color, turbidity, colloidal matter, oxidants, and reductants.
10. Apparatus
10.1 Measuring Instrument—Use commercially available potentiometric specific ion monitors that have expanded-scale
operation with adjustable ranges calibrated directly in sodium ion concentration units of micrograms per litre (parts per billion).
Electrical output signals must be isolated from ground and from electrode input and may be scaled for logarithmic, linear, or
bilinear ranges.
10.2 Sodium Ion Electrode—Use a commercially available sodium-sensitive electrode (sodium ion electrode). Because
electrode selectivities vary among manufacturers, care must be taken that the electrode, reagent and sample conditions are
compatible (see 9.4 and 9.5).
10.3 Reference Electrode—Use a reference electrode compatible with the measuring electrode.
10.3.1 When the sodium ion electrode has a silver-silver chloride internal half cell, the reference electrode should be
silver-silver chloride. When the sodium ion electrode has a calomel internal half cell, the reference electrode should be calomel.
Dissimilar reference electrodes may be used provided adequate compensation is made electronically to correct for the difference
between the measuring electrode and the reference electrode. If the reference electrode filling solution is an interference in the
measurement of sodium, then the reference electrode must be downstream from the measuring electrode (see 9.4 and 10.3.2).
10.3.2 If calomel electrodes are used, refer to Test Methods D1293. The electrolyte used in reference electrodes and all
maintenance shall conform to the manufacturer’smanufacturer’s recommendations. With flowing junction reference electrodes to
ensure the desired slow outward flow of electrolyte, the solution pressure inside the junction shall be kept somewhat higher than
that outside the junction.
10.4 Temperature Compensation—Use an automatic temperature compensator in accordance with the
manufacturer’smanufacturer’s recommendation.
10.5 Flow Chamber—For best results install the electrodes in a flow chamber and take the measurement on a flowing stream.
Use a flow chamber as recommended by the manufacturer. If otherwise, design the flow chamber to minimize interference from
the reference electrode and construct the chamber of inert materials such as plastic or stainless steel.
10.5.1 If a plastic is used, cast or machine from a solid block. Gasket electrodes to prevent in-leakage of air. Protection of
electrodes shall be in accordance with the manufacturer’smanufacturer’s recommendations. Connections to the flow chamber must
be solution or earth grounded. No glass or copper is permissible in flow chamber construction.
10.6 Analyzer Fluidics—Unattended automatic calibration may be provided using relays and valves for accurately adding
sodium standard solution with timing and sequencing controlled by the measuring instrument.
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11. Reagents and Materials
11.1 pH Adjusting Reagents:
11.1.1 Ammonia Gas—Commercial anhydrous grade ammonia (NH ) having a minimum purity of 99.9 %. Gas is absorbed
directly by the sample.
11.1.2 Ammonium Hydroxide—Commercial ammonia solution, approximately 29 % NH in water. Vapor diffuses into the
sample through gas-permeable tubing.
11.1.3 Diisopropylamine—Commercial grade liquid is vaporized and transported by an inert carrier gas to the sample stream,
or vapor diffuses into the sample through gas-permeable tubing.
11.1.4 Dimethylamine Gas—Commercial grade having a minimum purity of 99 %. Gas is absorbed directly by the sample.
11.1.5 Monoethylamine—Commercial grade vapor diffuses into the sample through gas-permeable tubing.
11.2 Sodium Chloride Stock Solution (1.00 mL = 0.100 mg Na)—Dissolve in water 0.2542 g of sodium chloride (NaCl), dried
to constant weight at 105°C in water, and dilute to 1 L in a thoroughly cleaned polyethylene flask. Store sodium stock in a
polyethylene or equally alkali metal-free container. Prepare standards of lesser concentrations by dilutions of the stock solution.
12. Calibration and Standardization
12.1 All procedures described in this section are subject to zero and calibration drift. Determine the frequency of calibration
checks required to obtain the desired accuracy based on experience. Whenever a major shift in sodium concentration is noted, a
calibration check is advisable.
12.2 Low sodium standards are subject to contamination from many sources. Calibrate using standards above 100 μg/L (100
ppb) to reduce the effects of contamination. To avoid most contamination connect the standard container to the measuring
equipment so that the standard can be introduced without exposing it to nonfiltered air. Use manual or automatic valves to introduce
the standard for calibration checks. Some analyzers use a known addition method that employs higher level sodium standards.
12.3 Run the sample and pH adjusting reagent through the flow chamber for at least 12 h before attempting measurements or
calibration (24 h when the equipment is new) to purge sodium contamination from all parts of the sampling and measuring
equipment, especially valves. In systems where serious damage to the measuring electrode may occur if the sample flow is
interrupted, provide means to automatically discontinue the pH adjusting reagent flow.
12.4 Sodium Ion Measurement with pH Adjustment by Addition of Anhydrous Ammonia:
12.4.1 In this procedure, pH adjustment is by on-line addition of ammonia to the continuously flowing liquid sample. The pH
level is maintained relatively constant at pH 11.0 to 11.1. Provide a reasonably stable sample flow rate, sufficient to attain
reasonable response time, but not so great as to unduly consume ammonia. Follow manufacturer’smanufacturer’s flow rate
recommendations.
12.4.2 Pipe the standard sodium solution into the system to prevent contamination during calibration.
12.4.3 Turn on the instrument and allow it to warm up in accordance with the manufacturer’smanufacturer’s instructions.
12.4.4 Connect the analyzer to a water source expected to have sodium ion concentration below 10 μg/L (10 ppb). This may
be demineralizer effluent or high quality process water. Start the water flow. Maintain the temperature of the sample below 40°C
(104°F).
12.4.5 Allow the water to flow until a stable reading is obtained. For a new instrument this may be 24 h or more. It is extremely
important that all new equipment and sample lines be thoroughly flushed to remove any residual sodium (see 12.3).
12.4.6 After a stable reading is obtained, calibrate the instrument according to the manufacturer’smanufacturer’s procedure.
12.5 Sodium Ion Measurement with pH Adjustment by Passive Diffusion of Monoethylamine, Ammonium Hydroxide, or
Diisopropylamine, and Calibration by Double Known Addition (DKA):
12.5.1 In this procedure, pH adjustment is accomplished by passive diffusion, wherein the sample passes through a gas
permeable tubing coil present in a reagent bottle containing monoethylamine, diisopropylamine, or ammonium hydroxide, which
diffuses through the tubing wall and redissolves,
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