ASTM_D_4127_-_12.pdf

Designation:D412712Standard 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 parentheses indicates the year of last reapproval.Asuperscript epsilon()indicates an editorial change since the last revision or reapproval.1.Scope1.1 This terminology covers those terms recommended bythe International Union 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 DefinitionsForotherdefinitionsusedinthisterminology,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 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 significantly fromconcentration.Ionic activity,not concentration,determines both therate and the extent of chemical reactions.activity coefficient,na factor,that relates activity,A,to theconcentration,C of a species in solution:A 5 CDISCUSSIONThe activity coefficient is dependent on the ionicstrength 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 changingthe 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 from 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 define 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 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)or pcA(log concentration of species 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 longerdemonstrates 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 standardizing solution whosevalue is reported in terms of total concentration of the ion ofinterest.DISCUSSIONIf 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 in order to fix the ionic strength,thusavoiding the need for correction.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 August 2012.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 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 States1 combin