1、Designation: D 4127 06Standard Terminology Used withIon-Selective Electrodes1This standard is issued under the fixed designation D 4127; 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 parenth
2、eses indicates the year of last reapproval. Asuperscript 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. Scope1.1 This terminology covers those terms recommended bythe International Un
3、ion of Pure and Applied Chemistry (IU-PAC),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:3D 1129 Terminology Relating to Water3. Terminology3.1 DefinitionsFor
4、other definitions used in this terminol-ogy, refer to Terminology D 1129.3.2 Definitions of Terms Relevant to Ion-Selective ElectrodeTechnology:acid errorin very acid solutions, the activity of water isreduced (less than unity) causing a non-Nernstian responsein glass electrodes.Apositive error in t
5、he pH reading results.activitythe thermodynamically effective concentration of afree ion in solution. In dilute solutions, ionic activity, andconcentration are practically identical, but in solutions ofhigh ionic strength, or in the presence of complexing agents,activity may differ significantly fro
6、m concentration. Ionicactivity, not concentration, determines both the rate and theextent of chemical reactions.activity coefficienta factor, g, that relates activity, A,totheconcentration, C of a species in solution:A 5gCThe activity coefficient is dependent on the ionic strength ofthe solution. Io
7、ns of similar size and charge have similaractivity coefficients.alkaline errorin alkaline solutions, where hydrogen ionactivity becomes very small, some glass electrodes respondto other cations, such as sodium. A negative error in the pHreading results. By changing the composition of the glass,the a
8、ffinity of the glass for sodium ion can be reduced. Suchelectrodes are known as lithium glass, high-pH, or full-rangeelectrodes.asymmetry potentialthe potential across a glass pH elec-trode membrane when the inside and outside of the mem-brane are in contact with solutions of identical pH. This term
9、has also been used to define the observed potential differ-ences between identical electrode pairs placed in identicalsolutions.calibration curvea plot of the potential (emf) of a givenion-selective electrode cell assembly (ion-selective electrodecombined with an identified reference electrode) vers
10、us thelogarithm of the ionic activity (concentration) of a givenspecies. For uniformity, it is recommended that the potentialbe plotted on the ordinate (vertical axis) with the morepositive potentials at the top of the graph and that paA(logactivity of the species measured, A)orpcA(log concentra-tio
11、n of species measured, A) be plotted on the abscissa(horizontal axis) with increasing activity to the right.IUPACactivity standarda standardizing solution whose value isreported in terms of ionic activity. If the electrode iscalibrated using activity standards, the activity of the free,unbound ion i
12、n the sample is determined.concentration standarda standardizing solution whosevalue is reported in terms of total concentration of the ion ofinterest. If the electrode is calibrated using pure-concentration standards and measurements made on un-treated samples, results must be corrected for the sam
13、pleionic strength and the presence of complexing agents. Morecommonly, a reagent is added to all standards and samplesbefore measurement in order to fix the ionic strength, thusavoiding the need for bination electrodean electrochemical apparatus thatincorporates an ion-selective electrode and a refe
14、renceelectrode in a single assembly thereby avoiding the need fora separate reference electrode. IUPACconcentrationthe actual amount of a substance in a givenvolume of solution. When measuring ionic concentrations by1This terminology is under the jurisdiction of ASTM Committee D19 on Waterand is the
15、 direct responsibility of Subcommittee D19.05 on Inorganic Constituentsin Water.Current edition approved Dec. 15, 2006. Published January 2007. Originallyapproved in 1982. Last previous edition approved in 2002 as D 4127 02.2Recommendations for Nomenclature of Ion-Selective Electrodes, IUPAC Com-mis
16、sion on Analytical Nomenclature, Pergamon Press, Oxford, 1976.3For referenced ASTM standards, visit the ASTM 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
17、.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.electrode, a distinction is made between the concentration ofthe free, unbound ion, and total concentration that includesions bound to complexing agents.dissociation constanta number i
18、ndicating the extent towhich a substance dissociates in solution. For a simpletwo-species complex AB, the constant is given by theproduct of the molar concentrations of A and of B divided bythe molar concentrations of the undissociated species AB.For example, with hydrofluoric acid:H1# F2#!/HF!5K 5
19、6.7 3 1024at 25CThe smaller the value of K, the less the complex isdissociated. K varies with temperature, ionic strength, and thenature of the solvent.driftthis is the slow nonrandom change with time in thepotential (emf) of an ion-selective electrode cell assemblymaintained in a solution of consta
20、nt composition and tem-perature. IUPACelectrode lifethe length of time that an electrode functionsusefully. Life-time of solid-state and glass electrodes islimited by mechanical failure of the electrode body orchemical attack on the sensing membrane, and can rangefrom a few days, if the electrode is
21、 used continuously in hotor abrasive flowing streams, to several years under normallaboratory conditions. The life-time of liquid membraneelectrodes is limited by loss of ion exchanger with use, andis generally 1 to 6 months.electrode paira sensing electrode and a reference electrode;the reference e
22、lectrode may be separate or combined in onebody with the sensing electrode.electrolytea substance that ionizes in aqueous solution; also,a solution containing ions. Weak electrolytes are onlyslightly dissociated into ions in solution (acetic acid), andstrong electrolytes are highly dissociated (HCl,
23、 NaCl).equitransferenceequal diffusion rates of the positively andnegatively charged ions of an electrolyte across a liquidjunction.equitransferent filling solutiona reference electrode fillingsolution in which the diffusion rates of negatively andpositively charged ions are equal.filling solutionth
24、e solution inside a sensing or referenceelectrode that is replenished periodically. Solutions that arepermanently sealed within the electrode (like the bufferinside a pH electrode) are usually called internal referencesolutions to differentiate them from filling solutions.internal filling solution o
25、f sensing electrodein liquidmembrane electrodes, an aqueous internal filling solutioncontacts the internal reference element and the membrane,which is saturated with ion exchanger. The filling solutionnormally contains a fixed level of chloride and of the ion forwhich the electrode was designed; the
26、 concentration of thision determines the zero potential point of the electrode. Inaddition, the filling solution is saturated with silver chlorideto prevent the silver chloride of the internal referenceelement from dissolving.reference electrode filling solutiona concentrated saltsolution contacting
27、 the internal reference element and thesample solution. The composition of the filling solution ischosen to maximize stability of the potentials developed atthe internal reference element/filling solution interface andthe filling solution/sample junction. In general, filling solu-tions for AgCl inte
28、rnal construction reference electrodesshould: (1) contain Cland be saturated with AgCl toprevent the reference element from dissolving; (2) be at leastten times higher in total ionic strength than the sample; (3)be equitransferent; (4) not contain the ion being measured oran ion that interferes with
29、 the measurement.flowthrough electrodesion-selective and reference elec-trodes designed for anaerobic measurements. The two elec-trodes are connected by plastic tubing to a syringe orperistaltic pump, and the sample is pumped through theelectrodes at a constant rate. Ion-selective electrodes can bem
30、ade in a flow through configuration for the measurement ofvery small samples (0.2 to 0.3 mL) or samples that must bemeasured anaerobically.Grans plotsa method of plotting apparent concentration (asderived from the electrode potential) versus the volume ofreagent added to the sample. Grans plots are
31、especiallyuseful for plotting titrations that would give poor end-pointbreaks if plotted conventionally. They can also be used todetermine concentration by known addition with greaterprecision than can be obtained by a single addition measure-ment.hysteresis (electrode memory)hysteresis is said to h
32、aveoccurred if, after the concentration has been changed andrestored to its original value, there is a different potentialobserved. The reproducibility of the electrode will conse-quently be poor. The systematic error is generally in thedirection of the concentration of the solution in which theelec
33、trode was previously immersed. IUPACinterfering substanceany species, other than the ion beingmeasured, whose presence in the sample solution affects themeasured potential of a cell. Interfering substances fall intotwo classes: “electrode” interferences and “method” inter-ferences. Examples of the f
34、irst class would be those sub-stances which give a similar response to the ion beingmeasured and whose presence generally results in an appar-ent increase in the activity (or concentration) of the ion to bedetermined (for example, Na+for the Ca+electrode), thosespecies which interact with the membra
35、ne so as to change itschemical composition (that is, organic solvents for the liquidor poly(vinyl chloride) (PVC) membrane electrodes) orelectrolytes present at a high concentration giving rise toappreciable liquid-junction potentials. The second class ofinterfering substance is that which interacts
36、 with the ionbeing measured so as to decrease its activity or apparentconcentration, but where the electrode continues to reportthe true activity (that is, CNpresent in the measurement ofAg+). IUPACinternal reference electrodea reference electrode that iscontained inside an ion-selective electrode a
37、ssembly. Com-ment: The system frequently consists of a silver-silverchloride electrode in contact with an appropriate solutioncontaining chloride and a fixed concentration of the ion forwhich the membrane is selective. IUPACion-selective electrodeelectrochemical sensors, the poten-tials of which are
38、 linearly dependent on the logarithm of theD 4127 062activity of a given ion in solution. Such devices are distinctfrom systems that involve redox reactions.DISCUSSIONThe term ion-specific electrode is not recommended.The term specific implies that the electrode does not respond toadditional ions. S
39、ince no electrode is truly specific for one ion, the termion-selective is recommended as more appropriate. Selective ion-sensitive electrode is a little-used term to describe an ion-selectiveelectrode. IUPACionic strengththe weighted concentration of ions in solu-tion, computed by multiplying the co
40、ncentration (c) of eachand every ion in solution by the corresponding square of thecharge (Z) on the ion, summing and dividing by 2: ionicstrength = (12)(Z2C. Conductivity measurements give arough estimate of ionic strength. The ionic strength (and toa lesser extent, the concentration of nonionic di
41、ssolvedspecies) largely determines the activity coefficient of eachion in the solution.ionic-strength adjustment buffera pH buffered solution ofhigh ionic strength added to samples and calibration solu-tions before measurement in order to achieve identical ionicstrength and hydrogen ion activity. In
42、 addition, complexingagents and other components are often added to minimizethe effects of certain interferences. IUPACisopotential pointfor a cell containing an ion-selectiveelectrode and a reference electrode there is often a particularactivity of the ion concerned for which the potential of thece
43、ll is independent of temperature. That activity, and thecorresponding potential, define the isopotential point. Theidentity of the reference electrode, and the composition ofthe filling solution of the measuring electrode, must bespecified. IUPACjunction potentialthe portion of the total observed po
44、tentialdeveloped between the sensing and reference electrodes thatis formed at the liquid/liquid junction between the referenceelectrode filling solution and the sample solution. Foraccuracy, the junction potential should be as low and asconstant as possible despite variations in the composition oft
45、he sample solution. Reference electrode filling solutionsshould be judiciously chosen to minimize liquid junctionpotential.limit of detectiona calibration curve ordinarily has the shapeshown in Fig. 1.By analogy with definitions adopted in other fields, the limitof detection should be defined as the
46、 concentration for which,under the specified conditions, the potential E deviates fromthe average potential in region I by some arbitrary multiple ofthe standard error of a single measurement of the potential inregion I.In the present state of the art, and for the sake of practicalconvenience, a sim
47、ple (and more convenient) definition isrecommended at this time. The practical limit of detection maybe taken as the activity (or concentration) of A at the point ofintersection of the extrapolated linear segments of the calibra-tion curve, as shown in Fig. 2. Since many factors affect thedetection
48、limit, the experimental conditions used should bereported, that is composition of the solution, the history andpreconditioning of the electrode, strirring rate, etc. IUPACmean ionic activity coefficientfor a salt that is composed oftwo monovalent ions, the geometric mean of the individualionic activ
49、ity coefficients. (The geometric mean is obtained,in this case, by multiplying the two individual ionic activitycoefficients and then taking the square root.) It is importantbecause, unlike individual ionic activity coefficients, it canbe measured by a variety of techniques, such as freezingpoint depression and vapor pressure, as well as by pairedsensing electrodes.membranea thin space of material covering a structure ofseparating solutions and permitting selection transport of achemical species between the two solutio
