ASTM D4127-2018a Standard Terminology Used with Ion-Selective Electrodes.pdf

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1、Designation: D4127 18D4127 18aStandard 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

2、parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope*1.1 This terminology covers those terms recommended by the International Union of Pure and Applied Chemistry (IUPAC),2and is intended to provide gui

3、dance in the use of ion-selective electrodes for analytical measurement of species in water,wastewater, and brines.1.2 This international standard was developed in accordance with internationally recognized principles on standardizationestablished in the Decision on Principles for the Development of

4、 International Standards, Guides and Recommendations issuedby the World Trade Organization Technical Barriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:3D1129 Terminology Relating to Water3. Terminology3.1 Definitions:3.1.1 For definitions of terms used in this standard, ref

5、er to Terminology D1129.3.2 Definitions of Terms Relevant to Ion-Selective Electrode Technology:acid error, nin very acid solutions, the activity of water is reduced (less than unity) causing a non-Nernstian response in glasselectrodes.DISCUSSIONA positive error in the pH reading results.activity, n

6、the thermodynamically effective concentration of a free ion in solution.DISCUSSIONIn dilute solutions, ionic activity, and concentration are practically identical, but in solutions of high ionic strength, or in the presence of complexingagents, activity may differ significantly from concentration. I

7、onic activity, not concentration, determines both the rate and the extent of chemicalreactions.activity coefficient, na factor, , that relates activity, A, to the concentration, C of a species in solution:A 5CDISCUSSIONThe activity coefficient is dependent on the ionic strength of the solution. Ions

8、 of similar size and charge have similar activity coefficients.1 This terminology is under the jurisdiction ofASTM Committee D19 on Water and is the direct responsibility of Subcommittee D19.05 on Inorganic Constituents in Water.Current edition approved Feb. 1, 2018May 1, 2018. Published May 2018Jun

9、e 2018. Originally approved in 1982. Last previous edition approved in 20122018 asD4127 12.D4127 18. DOI: 10.1520/D4127-18.10.1520/D4127-18a.2 Recommendations for Nomenclature of Ion-Selective Electrodes, IUPAC Commission on Analytical Nomenclature, Pergamon Press, Oxford, 1976.3 For referencedASTM

10、standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at serviceastm.org. For Annual Book of ASTM Standardsvolume information, refer to the standards Document Summary page on the ASTM website.This document is not an ASTM standard and is intended only to provide the user of

11、an ASTM standard an indication of what changes have been made to the previous version. Becauseit may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current versionof the standard as publ

12、ished by ASTM is to be considered the official document.*A Summary of Changes section appears at the end of this standardCopyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1alkaline error, nin alkaline solutions, where hydrogen ion activi

13、ty becomes very small, some glass electrodes respond to othercations, such as sodium.DISCUSSIONA negative error in the pH reading results. By changing the composition of the glass, the affinity of the glass for sodium ion can be reduced. Suchelectrodes are known as lithium glass, high-pH, or full-ra

14、nge electrodes.analate, nthe sample being analyzed; used in the terms “analate addition” and “analate subtraction.”DISCUSSIONThis term differs from the term “analyte,” which describes the chemical species of interest in an analytical test.asymmetry potential, nthe potential across a glass pH electro

15、de membrane when the inside and outside of the membrane arein contact with solutions of identical pH.DISCUSSIONThis term has also been used to define the observed potential differences between identical electrode pairs placed in identical solutions.calibration curve, na plot of the potential (emf or

16、 E) of a given ion-selective electrode cell assembly (ion-selective electrodecombined with an identified reference electrode) versus the logarithm of the ionic activity (or concentration) of a given species.DISCUSSIONFor uniformity, it is recommended that the potential be plotted on the ordinate (ve

17、rtical axis) with the more positive potentials at the top of the graphand that pa A (log activity of the species measured, A) or pcA (log concentration of species measured, A) be plotted on the abscissa (horizontal axis)with increasing activity to the right. See Fig. 1 and Fig. 2. Region I of Fig. 1

18、 represents the segment of the curve where the potential no longer changesin response to changes of the measured species activity and the electrode no longer demonstrates Nernstian response.IUPACactivity standard, na standardizing solution whose value is reported in terms of ionic activity.DISCUSSIO

19、NIf the electrode is calibrated using activity standards, the activity of the free, unbound ion in the sample is determined.concentration standard, na standardizing solution whose value is reported in terms of total concentration of the ion of interest.DISCUSSIONIf the electrode is calibrated using

20、pure-concentration standards and measurements made on untreated samples, results must be corrected for the sampleionic strength and the presence of complexing agents. More commonly, a reagent is added to all standards and samples before measurement in orderto fix the ionic strength, thus avoiding th

21、e need for bination electrode, nan electrochemical apparatus that incorporates an ion-selective electrode and a reference electrode ina single assembly thereby avoiding the need for a separate reference electrode.FIG. 1 Example Response Curve (log activity on the horizontal axis)D4127 18a2concentrat

22、ion, nthe actual amount of a substance in a given volume of solution.DISCUSSIONWhen measuring ionic concentrations by electrode, a distinction is made between the concentration of the free, unbound ion, and total concentrationthat includes ions bound to complexing agents.dissociation constant, na nu

23、mber indicating the extent to which a substance dissociates in solution.DISCUSSIONFor a simple two-species complex AB, the constant is given by the product of the molar concentrations of A and of B divided by the molarconcentrations of the undissociated species AB. For example, with hydrofluoric aci

24、d:H1#F2#!/HF#!5K 56.731024 at 25CThe smaller the value of K, the less the complex is dissociated. K varies with temperature, ionic strength, and the nature of the solvent.drift, nthis is the slow nonrandom change with time in the potential (emf) of an ion-selective electrode cell assembly maintained

25、in a solution of constant composition and temperature.electrode life, nthe length of time that an electrode functions usefully.DISCUSSIONLife-time of solid-state and glass electrodes is limited by mechanical failure of the electrode body or chemical attack on the sensing membrane, andcan range from

26、a few days, if the electrode is used continuously in hot or abrasive flowing streams, to several years under normal laboratory conditions.The life-time of liquid membrane electrodes is limited by loss of ion exchanger with use, and is generally 1 to 6 months.FIG. 2 Example Calibration Curve (log con

27、centration on the horizontal axis)D4127 18a3electrode pair, na sensing electrode and a reference electrode; the reference electrode may be separate or combined in one bodywith the sensing electrode.electrolyte, na substance that ionizes in aqueous solution; also, a solution containing ions.DISCUSSIO

28、NWeak electrolytes are only slightly dissociated into ions in solution (acetic acid), and strong electrolytes are highly dissociated (HCl, NaCl).equitransference, nequal diffusion rates of the positively and negatively charged ions of an electrolyte across a liquid junction.equitransferent filling s

29、olution, na reference electrode filling solution in which the diffusion rates of negatively and positivelycharged ions are equal.filling solution, nthe solution inside a sensing or reference electrode that is replenished periodically.DISCUSSIONSolutions that are permanently sealed within the electro

30、de (like the buffer inside a pH electrode) are usually called internal reference solutions todifferentiate them from filling solutions.internal filling solution of sensing electrode, nin liquid membrane electrodes, an aqueous internal filling solution contacts theinternal reference element and the m

31、embrane, which is saturated with ion exchanger.DISCUSSIONThe filling solution normally contains a fixed level of chloride and of the ion for which the electrode was designed; the concentration of this iondetermines the zero potential point of the electrode. In addition, the filling solution is satur

32、ated with silver chloride to prevent the silver chloride ofthe internal reference element from dissolving.reference electrode filling solution, na concentrated salt solution contacting the internal reference element and the samplesolution.DISCUSSIONThe composition of the filling solution is chosen t

33、o maximize stability of the potentials developed at the internal reference element/filling solutioninterface and the filling solution/sample junction. In general, filling solutions for AgCl internal construction reference electrodes should: (1) containCl and be saturated withAgCl to prevent the refe

34、rence element from dissolving; (2) be at least ten times higher in total ionic strength than the sample;(3) be equitransferent; (4) not contain the ion being measured or an ion that interferes with the measurement.flowthrough electrodes, nion-selective and reference electrodes designed for anaerobic

35、 measurements.DISCUSSIONThe two electrodes are connected by plastic tubing to a syringe or peristaltic pump, and the sample is pumped through the electrodes at a constant rate.Ion-selective electrodes can be made in a flow through configuration for the measurement of very small samples (0.2 to 0.3 m

36、L) or samples that mustbe measured anaerobically.Grans plots, na method of plotting apparent concentration (as derived from the electrode potential) versus the volume ofreagent added to the sample.DISCUSSIONGrans plots are especially useful for plotting titrations that would give poor end-point brea

37、ks if plotted conventionally. These plots can also be usedto determine concentration by known addition with greater precision than can be obtained by a single addition measurement.hysteresis (electrode memory), nhysteresis is said to have occurred if, after the concentration has been changed and res

38、toredto its original value, there is a different potential observed.DISCUSSIONThe reproducibility of the electrode will consequently be poor. The systematic error is generally in the direction of the concentration of the solutionin which the electrode was previously immersed.D4127 18a4interfering su

39、bstance, nany species, other than the ion being measured, whose presence in the sample solution affects themeasured potential of a cell.DISCUSSIONInterfering substances fall into two classes: “electrode” interferences and “method” interferences. Examples of the first class would be those substancest

40、hat give a similar response to the ion being measured and whose presence generally results in an apparent increase in the activity (or concentration)of the ion to be determined (for example, Na+ for the Ca+ electrode), those species that interact with the membrane so as to change its chemicalcomposi

41、tion (that is, organic solvents for the liquid or poly(vinyl chloride) (PVC) membrane electrodes) or electrolytes present at a high concentrationgiving rise to appreciable liquid-junction potentials. The second class is a substance that interacts with the ion being measured so as to decrease itsacti

42、vity or apparent concentration, but where the electrode continues to report the true activity (for example, CN present in the measurement of Ag+).internal reference electrode, na reference electrode that is contained inside an ion-selective electrode assembly.DISCUSSIONThe system frequently consists

43、 of a silver-silver chloride electrode in contact with an appropriate solution containing chloride and a fixed concentrationof the ion for which the membrane is selective.ion-selective electrode, nan electrochemical sensors, the potentials of which is linearly dependent on the logarithm of theactivi

44、ty of a given ion in solution.DISCUSSIONSuch devices are distinct from systems that involve redox reactions. The term ion-specific electrode is not recommended. The term specific impliesthat the electrode does not respond to additional ions. Since no electrode is truly specific for one ion, the term

45、 ion-selective is recommended as moreappropriate. Selective ion-sensitive electrode is a little-used term to describe an ion-selective electrode.ionic strength, nthe weighted concentration of ions in solution, computed by multiplying the concentration (c) of each ion insolution by the corresponding

46、square of the charge (Z) on the ion, summing and dividing by 2: ionic strength = (12)Z2C.DISCUSSIONConductivity measurements give a rough estimate of ionic strength. The ionic strength (and to a lesser extent, the concentration of nonionic dissolvedspecies) largely determines the activity coefficien

47、t of each ion in the solution.ionic-strength adjustment buffer, na pH buffered solution of high ionic strength added to samples and calibration solutionsbefore measurement in order to achieve identical ionic strength and hydrogen ion activity.DISCUSSIONIn addition, complexing agents and other compon

48、ents are often added to minimize the effects of certain interferences.isopotential point, nfor a cell containing an ion-selective electrode and a reference electrode there is often a particular activityof the ion concerned for which the potential of the cell is independent of temperature.DISCUSSIONT

49、hat activity, and the corresponding potential, define the isopotential point. The identity of the reference electrode, and the composition of the fillingsolution of the measuring electrode, must be specified.junction potential, nfor the total observed potential developed between the sensing and reference electrodes, the portion thatis formed at the liquid/liquid junction between the reference electrode filling solution and the sample solution.DISCUSSIONFor accuracy, the junction potentia