ASTM D4127-06
(Terminology)Standard Terminology Used with Ion-Selective Electrodes
Standard Terminology Used with Ion-Selective Electrodes
SCOPE
1.1 This terminology covers those terms recommended by the International Union of Pure and Applied Chemistry (IUPAC), and is intended to provide guidance in the use of ion-selective electrodes for analytical measurement of species in water, wastewater, and brines.
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Designation: D4127 − 06
StandardTerminology Used with
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Ion-Selective Electrodes
This standard is issued under the fixed designation D4127; 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 (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the Department of Defense.
1. Scope of the solution. Ions of similar size and charge have similar
activity coefficients.
1.1 This terminology covers those terms recommended by
the International Union of Pure and Applied Chemistry
alkaline error—in alkaline solutions, where hydrogen ion
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(IUPAC), and is intended to provide guidance in the use of
activity becomes very small, some glass electrodes respond
ion-selective electrodes for analytical measurement of species
to other cations, such as sodium.Anegative error in the pH
in water, wastewater, and brines.
reading results. By changing the composition of the glass,
the affinity of the glass for sodium ion can be reduced. Such
2. Referenced Documents
3 electrodesareknownaslithiumglass,high-pH,orfull-range
2.1 ASTM Standards:
electrodes.
D1129Terminology Relating to Water
asymmetry potential—the potential across a glass pH elec-
3. Terminology
trode membrane when the inside and outside of the mem-
3.1 Definitions—For other definitions used in this
braneareincontactwithsolutionsofidenticalpH.Thisterm
terminology, refer to Terminology D1129.
has also been used to define the observed potential differ-
3.2 Definitions of Terms Relevant to Ion-Selective Electrode
ences between identical electrode pairs placed in identical
Technology:
solutions.
acid error—in very acid solutions, the activity of water is
calibration curve—a plot of the potential (emf) of a given
reduced (less than unity) causing a non-Nernstian response
ion-selectiveelectrodecellassembly(ion-selectiveelectrode
inglasselectrodes.ApositiveerrorinthepHreadingresults.
combined with an identified reference electrode) versus the
activity—the thermodynamically effective concentration of a
logarithm of the ionic activity (concentration) of a given
free ion in solution. In dilute solutions, ionic activity, and
species. For uniformity, it is recommended that the potential
concentration are practically identical, but in solutions of
be plotted on the ordinate (vertical axis) with the more
high ionic strength, or in the presence of complexing agents,
positive potentials at the top of the graph and that pa (−log
A
activity may differ significantly from concentration. Ionic
activity of the species measured, A)orpc (−log concentra-
A
activity, not concentration, determines both the rate and the
tion of species measured, A) be plotted on the abscissa
extent of chemical reactions.
(horizontal axis) with increasing activity to the right.
activity coefficient—a factor, γ, that relates activity, A, to the IUPAC
concentration, C of a species in solution:
activity standard—a standardizing solution whose value is
A 5 γC
reported in terms of ionic activity. If the electrode is
calibrated using activity standards, the activity of the free,
The activity coefficient is dependent on the ionic strength
unbound ion in the sample is determined.
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This terminology is under the jurisdiction ofASTM Committee D19 on Water
concentration standard—a standardizing solution whose
and is the direct responsibility of Subcommittee D19.05 on Inorganic Constituents
in Water. value is reported in terms of total concentration of the ion of
Current edition approved Dec. 15, 2006. Published January 2007. Originally
interest. If the electrode is calibrated using pure-
approved in 1982. Last previous edition approved in 2002 as D4127–02. DOI:
concentration standards and measurements made on un-
10.1520/D4127-06.
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treated samples, results must be corrected for the sample
Recommendations for Nomenclature of Ion-Selective Electrodes, IUPAC Com-
mission on Analytical Nomenclature, Pergamon Press, Oxford, 1976.
ionic strength and the presence of complexing agents. More
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For referenced ASTM standards, visit the ASTM website, www.astm.org, or
commonly, a reagent is added to all standards and samples
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
before measurement in order to fix the ionic strength, thus
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. avoiding the need for correction.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
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D4127 − 06
combination electrode—an electrochemical apparatus that contacts the inter
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