ASTM D4127-02
(Terminology)Standard Terminology Used with Ion-Selective Electrodes
Standard Terminology Used with Ion-Selective Electrodes
SCOPE
1.1 This terminology includes those terms recommended by the International Union of Pure and Applied Chemistry, 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:D 4127–02
Standard Terminology Used with
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Ion-Selective Electrodes
This standard is issued under the fixed designation D 4127; 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.
This standard has been approved for use by agencies of the Department of Defense.
1. Scope asymmetry potential—the potential across a glass pH elec-
trode membrane when the inside and outside of the mem-
1.1 Thisterminologyincludesthosetermsrecommendedby
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braneareincontactwithsolutionsofidenticalpH.Thisterm
theInternationalUnionofPureandAppliedChemistry, andis
has also been used to define the observed potential differ-
intended to provide guidance in the use of ion-selective
ences between identical electrode pairs placed in identical
electrodes for analytical measurement of species in water,
solutions.
wastewater, and brines.
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calibration curve —a plot of the potential (emf) of a given
2. Referenced Documents ion-selectiveelectrodecellassembly(ion-selectiveelectrode
combined with an identified reference electrode) versus the
2.1 ASTM Standards:
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logarithm of the ionic activity (concentration) of a given
D 1129 Terminology Relating to Water
species. For uniformity, it is recommended that the potential
3. Terminology
be plotted on the ordinate (vertical axis) with the more
positive potentials at the top of the graph and that pa (−log
3.1 Definitions—For other definitions used in this terminol- A
activity of the species measured, A)orpc (−log concentra-
ogy, refer to Terminology D1129. A
tion of species measured, A) be plotted on the abscissa
3.2 Definitions of Terms Relevant to Ion-Selective Electrode
(horizontal axis) with increasing activity to the right.
Technology:
activity standard—a standardizing solution whose value is
activity—the thermodynamically effective concentration of a
reported in terms of ionic activity. If the electrode is
free ion in solution. In dilute solutions, ionic activity, and
calibrated using activity standards, the activity of the free,
concentration are practically identical, but in solutions of
unbound ion in the sample is determined.
high ionic strength, or in the presence of complexing agents,
concentration standard—a standardizing solution whose
activity may differ significantly from concentration. Ionic
value is reported in terms of total concentration of the ion of
activity, not concentration, determines both the rate and the
interest. If the electrode is calibrated using pure-
extent of chemical reactions.
concentration standards and measurements made on un-
activity coefficient—a factor, g, that relates activity, A,tothe
treated samples, results must be corrected for the sample
concentration, C of a species in solution:
ionic strength. More commonly, a reagent is added to all
A5gC standardsandsamplesbeforemeasurementinordertofixthe
ionic strength, thus avoiding the need for correction.
The activity coefficient is dependent on the ionic strength of
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combination electrode —an electrochemical apparatus that
the solution. Ions of similar size and charge have similar
incorporates an ion-selective electrode and a reference
activity coefficients.
electrode in a single assembly thereby avoiding the need for
activity limit—the lowest activity that can be measured in a
a separate reference electrode.
solution that is well buffered with respect to the ion being
concentration—the actual amount of a substance in a given
measured. The activity limit will vary as a function of
volumeofsolution.Whenmeasuringionicconcentrationsby
solution pH.
electrode,adistinctionismadebetweentheconcentrationof
the free, unbound ion, and total concentration that includes
ions bound to complexing agents.
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This terminology is under the jurisdiction ofASTM Committee D19 on Water
dissociation constant—a number indicating the extent to
and are the direct responsibility of Subcommittee D19.05 on Inorganic Constituents
which a substance dissociates in solution. [For a simple
in Water.
Current edition approved Jan. 10, 2002. Published April 2002. Originally
two-species complex AB, the constant is given by the
published as D4127–82. Last previous edition D4127–92(1996).
productofthemolarconcentrationsof Aandof Bdividedby
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Recommendations for Nomenclature of Ion-Selective Electrodes, IUPAC Com-
the molar concentrations of the undissociated species AB.
mission on Analytical Nomenclature, Pergamon Press, Oxford, 1976.
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Annual Book of ASTM Standards, Vol 11.01. For example, with hydrofluoric acid:
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