ASTM D4127-92(1996)

NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: D 4127 – 92 (Reapproved 1996)
Standard Terminology Used with
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. A number 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 brane are in contact with solutions of identical pH. This term
has also been used to define the observed potential differ-
1.1 This terminology includes those terms recommended by
ences between identical electrode pairs placed in identical
the International Union of Pure and Applied Chemistry, and is
solutions.
intended to provide guidance in the use of ion-selective
calibration curve —a plot of the potential (emf) of a given
electrodes for analytical measurement of species in water,
ion-selective electrode cell assembly (ion-selective electrode
wastewater, and brines.
combined with an identified reference electrode) versus the
2. Referenced Documents logarithm of the ionic activity (concentration) of a given
species. For uniformity, it is recommended that the potential
2.1 ASTM Standards:
be plotted on the ordinate (vertical axis) with the more
D 1129 Terminology Relating to Water
positive potentials at the top of the graph and that pa (−log
A
3. Terminology
activity of the species measured, A)orpc (−log concentra-
A
tion of species measured, A) be plotted on the abscissa
3.1 Definitions—For other definitions used in this terminol-
(horizontal axis) with increasing activity to the right.
ogy, refer to Terminology D 1129.
activity standard—a standardizing solution whose value is
3.2 Definitions of Terms Relevant to Ion-Selective Electrode
reported in terms of ionic activity. If the electrode is
Technology:
calibrated using activity standards, the activity of the free,
activity—the thermodynamically effective concentration of a
unbound ion in the sample is determined.
free ion in solution. In dilute solutions, ionic activity, and
concentration standard—a standardizing solution whose
concentration are practically identical, but in solutions of
value is reported in terms of total concentration of the ion of
high ionic strength, or in the presence of complexing agents,
interest. If the electrode is calibrated using pure-
activity may differ significantly from concentration. Ionic
concentration standards and measurements made on un-
activity, not concentration, determines both the rate and the
treated samples, results must be corrected for the sample
extent of chemical reactions.
ionic strength. More commonly, a reagent is added to all
activity coefficient—a factor, g, that relates activity, A,tothe
standards and samples before measurement in order to fix the
concentration, C of a species in solution:
ionic strength, thus avoiding the need for correction.
A5gC combination electrode —an electrochemical apparatus that
incorporates an ion-selective electrode and a reference
The activity coefficient is dependent on the ionic strength of
electrode in a single assembly thereby avoiding the need for
the solution. Ions of similar size and charge have similar
a separate reference electrode.
activity coefficients.
concentration—the actual amount of a substance in a given
activity limit—the lowest activity that can be measured in a
volume of solution. When measuring ionic concentrations by
solution that is well buffered with respect to the ion being
electrode, a distinction is made between the concentration of
measured. The activity limit will vary as a function of
the free, unbound ion, and total concentration that includes
solution pH.
ions bound to complexing agents.
asymmetry potential—the potential across a glass pH elec-
dissociation constant—a number indicating the extent to
trode membrane when the inside and outside of the mem-
which a substance dissociates in solution. [For a simple
two-species complex AB, the constant is given by the
This terminology is under the jurisdiction of ASTM Committee D-19 on Water
product of the molar concentrations of A and of B divided by
and are the direct responsibility of Subcommittee D19.05 on Inorganic Constituents
the molar concentrations of the undissociated species AB.
in Water.
Current edition approved Oct. 15, 1992. Published December 1992. Originally
For example, with hydrofluoric acid:
published as D 4127 – 82. Last previous edition D 4127 – 82 (1987).
1 2 24
~@H #@F #!/~@HF#! 5 K 5 6.7 3 10 at 25°C
Recommendations for Nomenclature of Ion-Selective Electrodes, IUPAC Com-
mission on Analytical Nomenclature, Pergamon Press, Oxford, 1976.
3 The smaller the value of K, the less the complex is
Annual Book of ASTM Standards, Vol 11.01.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
D 4127
dissociated. K varies with temperature, ionic strength, and the trodes designed for anaerobic measurements. The two elec-
nature of the solvent.] trodes are connected by plastic tubing to a syringe or
drift —this is the slow nonrandom change with time in the peristaltic pump, and the sample is pumped through the
potential (emf) of an ion-selective electrode cell assembly electrodes at a constant rate. Ion-selective electrodes can be
maintained in a solution of constant composition and tem- made in a flow through configuration for the measurement of
perature. very small samples (0.2 to 0.3 mL) or samples that must be
electrode life—the length of time that an electrode functions measured anaerobically.
usefully. Life-time of solid-state and glass electrodes is Gran’s plots—a method of plotting apparent concentration (as
limited by mechanical failure of the electrode body or derived from the electrode potential) versus the volume of
chemical attack on the sensing membrane, and can range reagent added to the sample. Gran’s plots are especially
from a few days, if the electrode is used continuously in hot useful for plotting titrations that would give poor end-point
or abrasive flowing streams, to several years under normal breaks if plotted conventionally. They can also be used to
laboratory conditions. The life-time of liquid membrane determine concentration by known addition with greater
electrodes is limited by loss of ion exchanger with use, and precision than can be obtained by a single addition measure-
is generally 1 to 6 months. ment.
electrode pair—a sensing electrode and a reference electrode; Gran’s plot paper—a type of graph paper designed to
the reference electrode may be separate or combined in one eliminate calculations when measurements are made by the
body with the sensing electrode. Gran’s plot technique. Electrode potentials in millivolts are
electrolyte—a substance that ionizes in aqueous solution; also, plotted on the vertical antilogarithmic axis and either volume
a solution containing ions. Weak electrolytes are only of reagent added or sample concentration on the horizontal
slightly dissociated into ions in solution (acetic acid), and linear axis. Special Gran’s plot papers can be constructed
strong electrolytes are highly dissociated (HCl, NaCl). that automatically correct for volume changes occurring
equitransference—equal diffusion rates of the positively and during titrations.
negatively charged ions of an electrolyte across a liquid hysteresis (electrode memory) —hysteresis is said to have
junction. occurred if, after the concentration has been changed and
equitransferent filling solution—a reference electrode filling restored to its original value, there is a different potential
solution in which the diffusion rates of negatively and observed. The reproducibility of the electrode will conse-
positively charged ions are equal. For low liquid junction quently be poor. The systematic error is generally in the
potentials, the ionic strength of the filling solution must be direction of the concentration of the solution in which the
high compared to the sample solution. electrode was previously immersed.
filling solution—the solution inside a sensing or reference interfering substance —any species, other than the ion being
electrode that is replenished periodically. Solutions that are measured, whose presence in the sample solution affects the
permanently sealed within the electrode (like the buffer measured potential of a cell. Interfering substances fall into
inside a pH electrode) are usually called internal reference two classes: “electrode” interferences and“ method” inter-
solutions to differentiate them from filling solutions. ferences. Examples of the first class would be those sub-
internal filling solution of sensing electrode—in liquid stances which give a similar response to the ion being
membrane electrodes, an aqueous internal filling solution measured and whose presence generally results in an appar-
contacts the internal reference element and the membrane, ent increase in the activity (or concentration) of the ion to be
+ ++
which is saturated with ion exchanger. The filling solution determined (for example, Na for the Ca electrode),
normally contains a fixed level of chloride and of the ion for those species which interact with the membrane so as to
which the electrode was designed; the concentration of this change its chemical composition (that is, organic solvents
ion determines the zero potential point of the electrode. In for the liquid or poly(vinyl chloride) (PVC) membrane
addition, the filling solution is saturated with silver chloride electrodes) or electrolytes present at a high concentration
to prevent the silver chloride of the internal reference giving rise to appreciable liquid-junction potentials. The
element from dissolving. second class of interfering substance is that which interacts
reference electrode filling solution—a concentrated salt with the ion being measured so as to decrease its activity or
solution contacting the internal reference element and the apparent concentration, but where the electrode continues to
−
sample solution. The composition of the filling solution is report the true activity (that is, CN present in the measure-
+
chosen to maximize stability of the potentials developed at ment of Ag ).
the internal reference element/filling solution interface and internal reference electrode —a reference electrode that is
the filling solution/sample junction. In general, filling solu- contained inside an ion-selective electrode assembly. Com-
tions for AgCl internal construction reference electrodes ment: The system frequently consists of a silver-silver
−
should: (1) contain Cl and be saturated with AgCl to chloride electrode in contact with an appropriate solution
prevent the reference element from dissolving; (2) be at least containing chloride and a fixed concentration of the ion for
ten times higher in total ionic strength than the sample; (3) which the membrane is selective.
be equitransferent; (4) not contain the ion being measured or ion-selective electrode —electrochemical sensors, the poten-
an ion that interferes with the measurement. tials of which are linearly dependent on the logarithm of the
flowthrough electrodes—ion-selective and reference elec- activity of a given ion in solution. Such devices are distinct
D 4127
from systems that involve redox reactions. diate solution. This change is sensed by the ion-selective
electrode and is proportional to the partial pressure of the
(A) Primary Electrodes
gaseous species in the sample.
(1) crystalline electrodes—may be homogeneous or hetero-
(b) redox reaction type—a gas sensing electrode using a
geneous.
platinum redox electrode, typified by the hydrogen gas elec-
(a) homogeneous membrane electrodes—are ion-selective
trode. This responds to the redox equilibrium between hydro-
electrodes in which the membrane is a crystalline material
gen gas and hydrogen ion in solution.
prepared from either a single compound or a homogeneous
(c) amperometric detector—this is a type that uses an
mixture of compounds (that is, Ag S, AgI/Ag S).
2 2
amperometric detector, generally behind a gas permeable
(b) heterogeneous membrane electrodes—are formed when
membrane. Such devices have been used for oxygen and for
an active substance, or mixture of active substances, is mixed
free chlorine.
with an inert matrix, such as silicone rubber or PVC, or placed
(2) enzyme substrate electrodes—sensors in which an ion-
on hydrophobized graphite, to form the sensing membrane that
selective electrode is covered with a coating containing an
is heterogeneous in nature.
enzyme that causes the reaction of an organic or inorganic
(2) noncrystalline electrodes—In these electrodes a support,
substance (substrate) to produce a species to which the
containing an ionic (either cationic or anionic) species or an
electrode responds. Alternatively, the sensor could be covered
uncharged species, forms the ion-selective membrane which is
with a layer of substrate that reacts with the enzyme to be
usually interposed between two aqueous solutions. The support
assayed.
used can be either porous (for example, filter, glass frit, etc.) or
nonporous (for example, glass or inert polymeric material such
NOTE 1—The term ion-specific electrode is not recommended. The term
as PVC, yielding with the ion-exchanger and the solvent a
specific implies that the electrode does not respond to additional ions.
solidified homogeneous mixture). These electrodes exhibit a
Since no electrode is truly specific for one ion, the term ion-selective is
recommended as more appropriate. Selective ion-sensitive electrode is a
response due to the presence of the ion-exchange material in
little-used term to describe an ion-selective electrode.
the membrane.
(a) rigid matrix electrodes—Glass electrodes are ion-
ionic strength—the weighted concentration of ions in solu-
selective electrodes in which the sensing membrane is a thin
tion, computed by multiplying the concentration (c) of each
piece of glass whose chemical composition determines the
and every ion in solution by the corresponding square of the
selectivity of the electrode. In this group are: hydrogen
charge (Z) on the ion, summing and dividing by 2: ionic
ion-selective electrodes and monovalent cation-selective elec- 2
strength 5 ( ⁄2)(Z C. Conductivity measurements give a
trodes.
rough estimate of ionic strength. The ionic strength (and to
(b) electrodes with a mobile carrier:
a lesser extent, the concentration of nonionic dissolved
(1) positively charged—bulky cations (for example, those of
species) largely determines the activity coeffi
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