1、Designation: D4127 12Standard Terminology Used withIon-Selective Electrodes1This standard is issued under the fixed designation D4127; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parenthes
2、es indicates the year of last reapproval. Asuperscript 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. Scope1.1 This terminology covers those terms recommended bythe International Union
3、 of Pure and Applied Chemistry(IUPAC),2and is intended to provide guidance in the use ofion-selective electrodes for analytical measurement of speciesin water, wastewater, and brines.2. Referenced Documents2.1 ASTM Standards:3D1129 Terminology Relating to Water3. Terminology3.1 DefinitionsFor other
4、definitions used in thisterminology, refer to Terminology D1129.3.2 Definitions of Terms Relevant to Ion-Selective ElectrodeTechnology:acid error, nin very acid solutions, the activity of water isreduced (less than unity) causing a non-Nernstian responsein glass electrodes.DISCUSSIONA positive error
5、 in the pH reading results.activity, nthe thermodynamically effective concentration ofa free ion in solution.DISCUSSIONIn dilute solutions, ionic activity, and concentration arepractically identical, but in solutions of high ionic strength, or in thepresence of complexing agents, activity may differ
6、 significantly fromconcentration. Ionic activity, not concentration, determines both therate and the extent of chemical reactions.activity coefficient, na factor, , that relates activity, A,totheconcentration, C of a species in solution:A 5 CDISCUSSIONThe activity coefficient is dependent on the ion
7、icstrength of the solution. Ions of similar size and charge have similaractivity coefficients.alkaline error, nin alkaline solutions, where hydrogen ionactivity becomes very small, some glass electrodes respondto other cations, such as sodium.DISCUSSIONAnegative error in the pH reading results. By c
8、hangingthe composition of the glass, the affinity of the glass for sodium ion canbe reduced. Such electrodes are known as lithium glass, high-pH, orfull-range electrodes.analate, nthe sample being analyzed; used in the terms“analate addition” and “analate subtraction”.DISCUSSIONThis term differs fro
9、m the term “analyte,” whichdescribes the chemical species of interest in an analytical test.asymmetry potential, nthe potential across a glass pHelectrode membrane when the inside and outside of themembrane are in contact with solutions of identical pH.DISCUSSIONThis term has also been used to defin
10、e the observedpotential differences between identical electrode pairs placed in iden-tical solutions.calibration curve, na plot of the potential (emf or E) of agiven ion-selective electrode cell assembly (ion-selectiveelectrode combined with an identified reference electrode)versus the logarithm of
11、the ionic activity (or concentration)of a given species.DISCUSSIONFor uniformity, it is recommended that the potential beplotted on the ordinate (vertical axis) with the more positive potentialsat the top of the graph and that paA(log activity of the speciesmeasured, A)orpcA(log concentration of spe
12、cies measured, A)beplotted on the abscissa (horizontal axis) with increasing activity to theright. See Fig. 1 and Fig. 2. Region I of Fig. 1 represents the segmentof the curve where the potential no longer changes in response tochanges of the measured species activity and the electrode no longerdemo
13、nstrates Nernstian response.IUPACactivity standard, na standardizing solution whose value isreported in terms of ionic activity.DISCUSSION If the electrode is calibrated using activity standards,the activity of the free, unbound ion in the sample is determined.concentration standard, na standardizin
14、g solution whosevalue is reported in terms of total concentration of the ion ofinterest.1This terminology is under the jurisdiction of ASTM Committee D19 on Waterand is the direct responsibility of Subcommittee D19.05 on Inorganic Constituentsin Water.Current edition approved Aug. 1, 2012. Published
15、 January 2007. Originallyapproved in 1982. Last previous edition approved in 2006 as D4127 06. DOI:10.1520/D4127-12.2Recommendations for Nomenclature of Ion-Selective Electrodes, IUPAC Com-mission on Analytical Nomenclature, Pergamon Press, Oxford, 1976.3For referenced ASTM standards, visit the ASTM
16、 website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United
17、States1DISCUSSIONIf the electrode is calibrated using pure-concentrationstandards and measurements made on untreated samples, results mustbe corrected for the sample ionic strength and the presence ofcomplexing agents. More commonly, a reagent is added to all standardsand samples before measurement
18、in order to fix the ionic strength, thusavoiding the need for bination electrode, nan electrochemical apparatus thatincorporates an ion-selective electrode and a referenceelectrode in a single assembly thereby avoiding the need fora separate reference electrode.concentration, nthe actual amount of a
19、 substance in a givenvolume of solution.DISCUSSIONWhen measuring ionic concentrations by electrode, adistinction is made between the concentration of the free, unbound ion,and total concentration that includes ions bound to complexing agents.dissociation constant, na number indicating the extent tow
20、hich a substance dissociates in solution.DISCUSSIONFor a simple two-species complex AB, the constant isgiven by the product of the molar concentrations of A and of B dividedby the molar concentrations of the undissociated species AB. Forexample, with hydrofluoric acid:H1#F2#!/HF#! 5 K 5 6.7 31024at
21、25CThe smaller the value of K, the less the complex is dissociated. Kvaries with temperature, ionic strength, and the nature of the solvent.drift, nthis is the slow nonrandom change with time in thepotential (emf) of an ion-selective electrode cell assemblymaintained in a solution of constant compos
22、ition and tem-perature.electrode life, nthe length of time that an electrode functionsusefully.DISCUSSIONLife-time of solid-state and glass electrodes is limitedby mechanical failure of the electrode body or chemical attack on thesensing membrane, and can range from a few days, if the electrode isus
23、ed continuously in hot or abrasive flowing streams, to several yearsunder normal laboratory conditions. The life-time of liquid membraneelectrodes is limited by loss of ion exchanger with use, and is generally1 to 6 months.electrode pair, na sensing electrode and a reference elec-trode; the referenc
24、e electrode may be separate or combinedin one body with the sensing electrode.electrolyte, na substance that ionizes in aqueous solution;also, a solution containing ions.DISCUSSIONWeak electrolytes are only slightly dissociated into ionsin solution (acetic acid), and strong electrolytes are highly d
25、issociated(HCl, NaCl).equitransference, nequal diffusion rates of the positivelyand negatively charged ions of an electrolyte across a liquidjunction.equitransferent filling solution, na reference electrodefilling solution in which the diffusion rates of negatively andpositively charged ions are equ
26、al.filling solution, nthe solution inside a sensing or referenceelectrode that is replenished periodically.DISCUSSION Solutions that are permanently sealed within theelectrode (like the buffer inside a pH electrode) are usually calledinternal reference solutions to differentiate them from filling so
27、lutions.internal filling solution of sensing electrode, nin liquidmembrane electrodes, an aqueous internal filling solutioncontacts the internal reference element and the membrane,which is saturated with ion exchanger.DISCUSSIONThe filling solution normally contains a fixed level ofchloride and of t
28、he ion for which the electrode was designed; theconcentration of this ion determines the zero potential point of theelectrode. In addition, the filling solution is saturated with silverchloride to prevent the silver chloride of the internal reference elementfrom dissolving.reference electrode fillin
29、g solution, n a concentrated saltsolution contacting the internal reference element and thesample solution.DISCUSSIONThe composition of the filling solution is chosen tomaximize stability of the potentials developed at the internal referenceelement/filling solution interface and the filling solution
30、/sample junc-tion. In general, filling solutions for AgCl internal construction refer-ence electrodes should: (1) contain Cland be saturated with AgCl toprevent the reference element from dissolving; ( 2) be at least ten timeshigher in total ionic strength than the sample; (3) be equitransferent; (4
31、)not contain the ion being measured or an ion that interferes with themeasurement.flowthrough electrodes, nion-selective and reference elec-trodes designed for anaerobic measurements.DISCUSSIONThe two electrodes are connected by plastic tubing to asyringe or peristaltic pump, and the sample is pumpe
32、d through theelectrodes at a constant rate. Ion-selective electrodes can be made in aflow through configuration for the measurement of very small samples(0.2 to 0.3 mL) or samples that must be measured anaerobically.Grans plots, na method of plotting apparent concentration(as derived from the electr
33、ode potential) versus the volumeof reagent added to the sample.DISCUSSIONGrans plots are especially useful for plotting titrationsthat would give poor end-point breaks if plotted conventionally. Theseplots can also be used to determine concentration by known additionwith greater precision than can b
34、e obtained by a single additionmeasurement.hysteresis (electrode memory), nhysteresis is said to haveoccurred if, after the concentration has been changed andrestored to its original value, there is a different potentialobserved.DISCUSSIONThe reproducibility of the electrode will consequentlybe poor
35、. The systematic error is generally in the direction of theconcentration of the solution in which the electrode was previouslyimmersed.FIG. 1 Example Response Curve (log activity on the horizontalaxis)D4127 122interfering substance, nany species, other than the ionbeing measured, whose presence in t
36、he sample solutionaffects the measured potential of a cell.DISCUSSION Interfering substances fall into two classes: “electrode”interferences and “method” interferences. Examples of the first classwould be those substances that give a similar response to the ion beingmeasured and whose presence gener
37、ally results in an apparent increasein the activity (or concentration) of the ion to be determined (forexample, Na+for the Ca+electrode), those species that interact withthe membrane so as to change its chemical composition (that is, organicsolvents for the liquid or poly(vinyl chloride) (PVC) membr
38、aneelectrodes) or electrolytes present at a high concentration giving rise toappreciable liquid-junction potentials. The second class is a substancethat interacts with the ion being measured so as to decrease its activityor apparent concentration, but where the electrode continues to reportthe true
39、activity (for example, CNpresent in the measurement ofAg+).internal reference electrode, na reference electrode that iscontained inside an ion-selective electrode assembly.DISCUSSIONThe system frequently consists of a silver-silver chlo-ride electrode in contact with an appropriate solution containi
40、ngchloride and a fixed concentration of the ion for which the membraneis selective.ion-selective electrode, nan electrochemical sensors, thepotentials of which is linearly dependent on the logarithm ofthe activity of a given ion in solution.DISCUSSIONSuch devices are distinct from systems that invol
41、veredox reactions. The term ion-specific electrode is not recommended.The term specific implies that the electrode does not respond toadditional ions. Since no electrode is truly specific for one ion, the termion-selective is recommended as more appropriate. Selective ion-sensitive electrode is a li
42、ttle-used term to describe an ion-selectiveelectrode.ionic strength, nthe weighted concentration of ions insolution, computed by multiplying the concentration ( c)ofeach ion in solution by the corresponding square of thecharge (Z) on the ion, summing and dividing by 2: ionicstrength = (12)Z2C.DISCUS
43、SIONConductivity measurements give a rough estimate ofionic strength. The ionic strength (and to a lesser extent, the concen-tration of nonionic dissolved species) largely determines the activitycoefficient of each ion in the solution.ionic-strength adjustment buffer, n a pH buffered solutionof high
44、 ionic strength added to samples and calibrationsolutions before measurement in order to achieve identicalionic strength and hydrogen ion activity.DISCUSSION In addition, complexing agents and other componentsare often added to minimize the effects of certain interferences.isopotential point, nfor a
45、 cell containing an ion-selectiveelectrode and a reference electrode there is often a particularactivity of the ion concerned for which the potential of thecell is independent of temperature.FIG. 2 Example Calibration Curve (log concentration on the horizontal axis)D4127 123DISCUSSIONThat activity,
46、and the corresponding potential, definethe isopotential point. The identity of the reference electrode, and thecomposition of the filling solution of the measuring electrode, must bespecified.junction potential, nfor the total observed potential devel-oped between the sensing and reference electrode
47、s, theportion that is formed at the liquid/liquid junction betweenthe reference electrode filling solution and the samplesolution.DISCUSSION For accuracy, the junction potential should be as lowand as constant as possible despite variations in the composition of thesample solution. Reference electro
48、de filling solutions should be judi-ciously chosen to minimize liquid junction potential.mean ionic activity coefficient, n for a salt that is composedof two monovalent ions, the geometric mean of the indi-vidual ionic activity coefficients.DISCUSSION (The geometric mean is obtained, in this case, b
49、ymultiplying the two individual ionic activity coefficients and thentaking the square root.) This mean coefficient is important because,unlike individual ionic activity coefficients, it can be measured by avariety of techniques, such as freezing point depression and vaporpressure, as well as by paired sensing electrodes.membrane, na thin space of material covering a structure ofseparating solutions and permitting selection transport of achemical species between the two solutions.liquid ion-
