ASTM D2791-93(2001)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation:D 2791–93 (Reapproved 2001)
Standard Test Methods for
Continuous Determination of Sodium in Water
This standard is issued under the fixed designation D 2791; 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* D 1192 Specification for Equipment and Sampling Water
and Steam in Closed Conduits
1.1 These test methods cover the continuous determination
D 1193 Specification for Reagent Water
of trace amounts of sodium in water using an ion-selective
D 1293 Test Methods for pH of Water
electrode and flame photometry. Two test methods are in-
D 2777 Practice for Determination of Precision and Bias of
cluded:
Applicable Methods of Committee D-19 on Water
Sections
D 3864 Guide for Continual On-Line Monitoring Systems
Test Method A—Ion-Selective Electrode 8 to 16
for Water Analysis
Test Method B—Flame Photometry 17 to 26
3. Terminology
1.2 Test MethodAis based on continuous application of the
sodium ion electrode as reported in the technical literature 3.1 Definitions—For definitions of terms used in these test
(1-3). It is generally applicable over the range of 0.05 to methods, refer to Terminology D 1129.
10 000 µg/L.
4. Significance and Use
1.3 Test Method B is based on the use of flame photometry.
It is most applicable to measurements below 20 µg/L. Com- 4.1 Sodiumisapervasivecontaminantandthefirstcationto
break through deionization equipment. These test methods
pared with Test Method A, it is less vulnerable to interference
from other monovalent cations at low concentrations and can allow measurements of micrograms per litre (parts per billion)
concentrations of sodium in water for monitoring low-sodium
reach equilibrium with very low sample throughput.
1.4 The analyst should be aware that adequate collaborative water sources for indications of contamination or proper
operation. Applications include monitoring of demineralizer
data for precision and bias statements as required by Practice
D 2777 are not provided. See Sections 16 and 26 for details. system performance and power plant boiler carryover and
1.5 The values stated in SI units are to be regarded as condenser leakage.
4.2 Thesetestmethodsaremoresensitiveandselectivethan
standard. The inch-pound units given in parentheses are for
information only. conductivity measurements on high purity samples.
1.6 This standard does not purport to address all of the
5. Reagents and Materials
safety concerns, if any, associated with its use. It is the
5.1 Purity of Reagents—Reagent grade chemicals shall be
responsibility of the user of this standard to establish appro-
used in all tests. Unless otherwise indicated, it is intended that
priate safety and health practices and determine the applica-
all reagents shall conform to the specifications of the Commit-
bility of regulatory limitations prior to use. For specific hazard
teeonAnalyticalReagentsoftheAmericanChemicalSociety.
statements see Section 6.
In many instances, reagent grade chemicals contain higher
2. Referenced Documents
levels of sodium contamination than are compatible with these
2.1 ASTM Standards: test methods. It must be ascertained that the reagent is of
sufficiently high purity to permit its use without lessening the
D 1066 Practice for Sampling Steam
D 1129 Terminology Relating to Water accuracy of the determination.
5.2 Purity of Water— Unless otherwise indicated, refer-
ences to water shall be understood to mean reagent water
These test methods are under the jurisdiction of ASTM Committee D19 on
Water and are the direct responsibility of Subcommittee D19.03 on Sampling of
Water and Water-Formed Deposits, Surveillance of Water, and Flow Measurement
of Water. Reagent Chemicals, American Chemical Society Specifications, American
Current edition approved April 15, 1993. Published June 1993. Originally Chemical Society, Washington, DC. For suggestions on the testing of reagents not
published as D 2791 – 69T. Last previous edition D 2791 – 87. listed by the American Chemical Society, see Analar Standards for Laboratory
The boldface numbers in parentheses refer to the list of references at the end of Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia
these test methods. and National Formulary, U.S. Pharmaceutical Convention, Inc. (USPC), Rockville,
Annual Book of ASTM Standards, Vol 11.01. 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.
D 2791–93 (2001)
conforming to Specification D 1193, Type I. In addition, the dirty part of the system. Under no circumstances should the
sodium or potassium content shall not exceed 10 µg/L(10 ppb) acid enter the measuring instrument.
or1 %ofthelowestconcentrationtobedetermined,whichever 7.5 Adjust the sample flow in accordance with the manu-
is lower. facturer’s recommendation.
5.2.1 Single-distilled water passed through a mixed bed 7.6 Where speed of response is not critical, sequential
deionizing unit composed of strong cation and anion resins can sampling of multiple streams may be effected with 3-way
produce an effluent containing less than 1.0 µg/L (1.0 ppb) of solenoid valves for sample selection. The 3-way valves allow
sodium. If such water is stored in a closed alkali metal-free samples not being measured to continue flowing (to drain) and
container, such as one made of polyethylene, TFE- to be current when they are selected. Automatic selection
fluorocarbon, or stainless steel, subsequent increases in con- shouldincludeanadjustabletimingdevicefortypicalsampling
ductivity, usually due to absorption of carbon dioxide, will not times near 10 min per point.
invalidate its use for this purpose.
TEST METHOD A—SODIUM ION ELECTRODE
6. Hazards
6.1 Test Method A— pH adjusting reagents are strongly
8. Scope
alkaline and volatile. Use normal eye and skin protection when
8.1 This test method covers the continuous measurement of
handling ammonia, ammonium hydroxide, dimethylamine,
sodium in water using a sodium ion electrode.
diisopropylamine, monoethylamine, or morpholine. Extra care
is needed in handling the gas-permeable tubing immersed in
9. Summary of Test Method
liquid reagents used with the passive diffusion reagent delivery
9.1 Sodium ion electrodes provide consistent logarithmic
system. Keep reagents in the open wherever possible and take
response over many orders of magnitude of concentration
necessaryprecautionstokeepthemfromtherespiratorytractin
using the same principles as pH electrodes but with different
event of a spill or leak. Under certain conditions these reagents
ion selectivity. The electrode signal has a slope of approxi-
can produce an explosive mixture with air. OSHA standards
mately 59 mV/decade change in sodium ion concentration at
must be followed.
25°C (77°F).
6.2 Test Method B:
9.2 Where electrode selectivity and the sodium concentra-
6.2.1 Use normal safety precautions in handling hydrogen
tion and pH of the sample require it, this test method includes
and oxygen gas.
provision for the addition of pH adjusting reagent to suppress
6.2.2 Use extreme care to avoid contact of skin with the
hydrogen ion concentration and assure accurate electrode
extremely hot oxy-hydrogen flame as instant third-degree
response to sodium. The lower limit for accurate measurement
burns would result.
without reagent appears to be about 1 µg/L (1 ppb) in
6.2.3 Forunattendedoperationofflamephotometer,provide
ammonia-treated power plant samples (4).
means to ensure that flame gases are shut off quickly and
9.3 This test method is particularly adaptable to high purity
automatically if the flame should be extinguished for any
water and is relatively free of interferences (1). The overall
reason.
operating cost of this system is considerably less than that of
continuous flame photometry, and it is more sensitive than
7. Sampling
electrical conductivity.
7.1 Sample the water for continuous sodium ion measure-
9.4 The repeatability of this test method is 65 % of the
ments in a flowing stream in accordance with Practice D 1066,
reading.
Specification D 1192, and Guide D 3864, as applicable.
7.2 Regulate the pressure of samples within the instrument
10. Interferences
manufacturer’s requirements.
10.1 The sodium ion electrode, like all potentiometric
7.3 Regulate the temperature of samples that must be
condensed, or cooled, or both, to a level between 15 and 40°C electrode measuring systems, is responsive to changes in ion
activity and not true concentration changes (that is, the
(59 and 104°F) or within manufacturer’s requirements. For
highest accuracy, bring the sample temperature close to the response is to changes in concentration multiplied by an
temperature of the standards during calibration. activity coefficient). However, as concentrations approach
7.4 When sample system plumbing has been newly in- infinite dilution, activity coefficients approach unity and ion
stalled, or has not been carrying process stream water for some concentration and active ion concentration become very nearly
time, or has been open to the atmosphere, it may take 24 h of equal.
purging to bring the sodium content at the receiving end down 10.2 The activity coefficient of sodium ion will vary with
to the same level as the sample point, especially when the changes in the total ionic strength of the solution. Therefore, it
process stream is less than 1.0 µg/L (1.0 ppb). In the case of is important to maintain either a low or constant ionic strength.
lines that are very dirty or have been subject to biological Aconstant flow of pH adjusting reagent generally establishes a
fouling, pumping a 25 % solution of nitric acid is effective for consistent ionic strength.
plastic and stainless lines. About 30 line-volumes of acid 10.3 The sodium content of pH adjusting reagent, if deliv-
should be pumped through slowly, followed by the fastest ered directly to the sample, must not be significant compared
practical purge of process water in the amount of 300 volumes. with the lowest concentration being measured. Any air con-
When using an acid-cleaning procedure, confine the acid to the tacting the sample must be sodium-free.
D 2791–93 (2001)
10.4 The sodium ion electrode is responsive to certain other the desired slow outward flow of electrolyte, the solution
monovalent cations. Interference by silver, lithium, hydrogen, pressureinsidethejunctionshallbekeptsomewhathigherthan
potassium,ammonium,andotherionsmustbeconsidered.The that outside the junction.
selectivity to interfering ions varies by electrode manufacturer. 11.4 Temperature Compensation—Use an automatic tem-
Inthelow-solidswatertowhichthistestmethodapplies,silver perature compensator in accordance with the manufacturer’s
and lithium ions are usually absent. Potassium ion, often recommendation.
contributed to the sample by the reference electrode, must be 11.5 Flow Chamber— For best results install the electrodes
carried downstream away from the sodium ion electrode. in a flow chamber and take the measurement on a flowing
Ammonium ion, present in many power plant samples, gener- stream. Use a flow chamber as recommended by the manufac-
ally does not interfere with measurements greater than 1 µg/L turer. If otherwise, design the flow chamber to minimize
(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 Elevation of pH so that hydrogen ion concentration is 11.5.1 If a plastic is used, cast or machine from a solid
3to4ordersofmagnitudelowerthanthatforsodiumgenerally block.Gasketelectrodestopreventin-leakageofair.Protection
makes the electrode response independent of variations in of electrodes shall be in accordance with the manufacturer’s
hydrogen ion concentration. Any of the reagents mentioned is recommendations. Connections to the flow chamber must be
satisfactory to increase the pH to a level such that the electrode solution or earth grounded. No glass or copper is permissible
isessentiallyinsensitivetohydrogenion,withinsodiumranges in flow chamber construction.
specifiedbythemanufacturer.Exceptionalelectrodeselectivity
12. Reagents and Materials
allows some measurements in ammoniated power plant
12.1 pH Adjusting Reagents:
samples greater than 1 µg/L (1 ppb) sodium without further
reagent addition. 12.1.1 Ammonia Gas— Commercial anhydrous grade am-
monia (NH ) having a minimum purity of 99.9 %. Gas is
10.6 When this test method is used without pH adjusting
absorbed directly by the sample.
reagent, the sample pH and sodium concentration must be
12.1.2 Ammonium Hydroxide—Commercial ammonia solu-
within the manufacturer’s guidelines for the particular sodium
tion, approximately 29 % NH in water.Vapor diffuses into the
electrode to assure accurate measurement.
sample through an ion-impermeable membrane.
10.7 The sodium ion electrode is not subject to interference
12.1.3 Diisopropylamine— Commercial grade liquid is va-
fromcolor,turbidity,colloidalmatter,oxidants,andreductants.
porized and transported by an inert carrier gas to the sample
11. Apparatus
stream.
11.1 Measuring Instrument—Use commercially available
12.1.4 Dimethylamine Gas—Commercial grade having a
potentiometric specific ion monitors that have expanded-scale
minimum purity of 99 %. Gas is absorbed directly by the
operation with adjustable ranges calibrated directly in sodium
sample.
ion concentration units of micrograms per litre (parts per
12.1.5 Monoethylamine— Commercial grade vapor diffuses
billion). Electrical output signals must be isolated from ground
into the sample through an ion impermeable membrane.
and from electrode input and may be scaled for logarithmic,
12.1.6 Morpholine-Containing Proprietary Buffer
linear, or bilinear ranges.
Solution—From sodium analyzer manufacturer, maintained
11.2 Sodium Ion Electrode—Use a commercially available
freeofsodiumandnotstoredinglasspriortouse.Liquidisfed
sodium-sensitive electrode (sodium ion electrode). Because
directly to the sample.
electrode selectivities vary among manufacturers, care must be
12.2 Sodium Chloride Stock Solution (1.00 mL = 0.100 mg
taken that the electrode, reagent or lack of it, and sample
Na)—Dissolve in water 0.2542 g of sodium chloride (NaCl),
conditions are compatible (see 10.4, 10.5, and 10.6).
dried to constant weight at 105°C in w
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