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ManualofElectrochemicalAnalysisPart1Terminologyof
ElectrochemicalAnalysis
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Accuracy Measureofthedeviationofthevaluemeasured(displayed)fromthetruevalueAcid Substancethatcanreleaseprotons;alsoanaqueoussolutionofsuchasubstance.
Substancesthathaveamarkedtendencytoreleaseprotonsarecalledstrongacids(e.g.HCl,H2SO4).Substances,suchasaceticacid,whichhavealowertendencytoreleaseprotonsarecalledmedium-strengthacids.Substanceswhich,likecarbonicacidorsilicicacid,showonlyaslighttendencytoreleaseprotonsareweakacids.
Aciderror
Instronglyacidicsolutions,theswellinglayerofthepHmembranetakesupundissociatedacidmolecules,theH+activityfalls,toohighapHisdisplayed.
Acidic pH9,H+ionsintheglassmembranearereplacedbyalkalimetalions.ThisgivesthefalsereadingofahigherH+activity(lowerpH).ThiseffectcanbesuppressedbyusingsuitableglassesforpHelectrodes.
Alternatingvoltageappliedtotheelectrode Theconductivitymeasurementisbasedonthemobilityoftheionsinthesolution.Topreventelectrolyticdecompositionattheelectrode,analternatingvoltageis
used.
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Ampholytes Substancesorparticleswhichcanoccurbothasanacidandasabase(e.g.HSO4-)Example1:HSO4
-reactsasprotonacceptorHSO4-+H3O
+->H2SO4+H2O
Example2:HSO4-reactsasprotondonatorHSO4-+OH-->SO42-+H2O
Base Substancethatcanacceptprotons;alsoanaqueoussolutionofsuchasubstance.Substancesthathaveamarkedtendencytoacceptprotonsarecalledstrongbases(e.g.OH-ions,theactiveconstituentof"conventional"basessuchasNaOHorKOH).SubstanceslikeammoniaNH3thathavealowertendencytoacceptprotonsarecalledmedium-strengthbases.Theanionsofhighlydissociatedacidshaveonlyalowtendencytoacceptprotonsandarecalledweakorveryweakbases.
Basic pHvalue>7,lowconcentrationofH+orH3O+ionsBuffer BuffersarestablesolutionswhichretaintheirpHevenifsmallamountsofwater,
acidorbasegetintothebuffersolution.BuffersarethepH-standardsolutionsfor
calibrationofthecombinationelectrodeandequipment.Buffercapacity Thebuffercapacity("buffervalue")indicatestheeffectofabuffersolution.For
this,thechangeinpHonadditionofasmallamountofacidorbaseisdetermined.
Calibration Calibrationisnecessarytodeterminedeviationsoftherealelectrodefromtheidealbehaviourinrespectofthezeropointandtheslopeoftheelectrodeandtocompensateforthesemathematically.InpHmeasurement,theoperationcalledcalibrationisactuallyanadjustment.
StandardsolutionsofdefinedpHareusedforthis.ThezeropointoftheelectrodesisnowadaysnormallyatapHof7.00.Theslopeofthecurrentelectrodeisdeterminedwiththeaidoffurtherbuffersolutions(usuallypH=4.00andpH=10.00)andusedbythemeasuringequipmentforlaterevaluationofexperiments.
Calibrationfrequency Anewcalibrationshouldusuallybeperformedonceaday,preferablybeforeanalysisofeachseriesofsamples.WithSartoriusProfessionalmeters,anintervaloftimecanbeset,afterwhichacalibrationpromptappearsonthedisplay.
Careoftheelectrode Bycaringforandcleaningtheelectrodeaccordingtothemanufacturer'sinstructions,thelifeoftheelectrodecanbeincreasedsignificantly,especiallyifsampleswhichareaggressiveorcanblockthereferencejunctionaremeasured.
Cellvoltage Potentialofthesensingelectrodeminuspotentialofthereferenceelectrode(V=VS-VR)
Cell,voltaiccell Asysteminwhichtwoelectrodesareconnectedviaanelectrolyte.Theelectrodesareusuallymetalrods.Inso-calledgalvaniccells,thereactionsbetweentheelectrodesandthesolutionorsolutionsresultinapotentialdifferencewhichcanbe
utilizedformeasurement-orservestogeneratecurrente.g.inbatteries.
Changeintheslope Achange(usuallydecrease)intheslopeofanelectrodecausedbyageingorcontaminationoftheelectrode.Ifthechangeinslopeistobeattributedtocontamination,theelectrodecanusuallybeusedagainaftercleaning.(seeCleaningoftheelectrode).
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Cleaningoftheelectrode Contaminatedreferencejunctionsarecleanedbysuctionwithawaterjetpump.Blockagesduetoproteinscanberemovedbymeansofanacidicpepsinsolution
(seePepsinsolution).Silversulphideprecipitatescanberemovedwithasolutionofthioureainhydrochloricacid(seeThiourea).Organiccontaminantscanberemovedbyrinsingbrieflywithalcoholoracetone.ContaminantswhicharesolubleinacidandalkalicanberemovedbyrinsingbrieflywithNaOHorHCl(0.1mol/l).Afterrinsing,theelectrodemustbestoredinstandardsolutionforafewhoursandthemeasuringequipmentmustbere-calibratedwiththeelectrode.
Combinationelectrode Combinationofreferenceelectrodeandsensingelectrodeinonesensor-oftenadditionallycombinedwithatemperaturesensor
Combinationelectrode Combinationelectrode=Sensingelectrode+ReferenceelectrodeCombinationelectrodevoltage TheactualmeasurementsignalConcentration Theamountofdissolvedsubstancebasedontheamountofsolventortheentire
solution.
Therearevariouspossibilitiesforstatingtheconcentrationofsolutions:
Concentrationinmol/l Solutionwiththeconcentrationofaparticularionicspeciesof1mol/l(moleperlitre)
Concentrationinpercent Termfor1outof100derivedfromLatin.Usuallybasedonweightcontents.Usualforstatingthecontentofconcentratedsolutions.Example:A10%sugarsolutioncontains10gsugarand90gwater.
Symbol:%
Concentrationinppb Partsperbillion.Thetermrelatestotheweightcontentandiscommonforsolutionsofverylowconcentration.
Symbol:ppb
Concentrationinppm Partspermillion.Thetermrelatestotheweightcontentandiscommonforsolutionsoflowconcentration.Atadensityof1g/ml(1kg/l),ppmcorrespondsto
theconcentrationstatedinmg/lorg/ml.Appliese.g.todiluteaqueoussolutions.
Symbol:ppm
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Dissociationconstantofwater WaterdissociatestoaverysmallextentinaccordancewithH20H++OH-.
Theequilibriumofthisreactionliesfaronthesideofundissociatedwater.At22Conly110-7mol/lwaterisdissociated.Forcomparison,1lwaterat22Ccontainsabout55molofwatermolecules
Thefollowingrelationshipexistsbetweentheactivitiesofhydroxideionsandhydroniumionsinaqueoussystems:
[H+][OH-]=110-14mol2/l2
Thisconstant(foragiventemperature)valueiscalledtheionicproductor
dissociationconstantofwater.Itis0.1310-14mol2/l2at0Cand5.9510-14mol2/l2at50C
SeeH+ion,H3O+ion
Drystorage Conductivitymeasuringcellsandmostion-selectivemeasuringcellsarestoredinthdrystateafteruseandcleaning.FETsensorsshouldalsobestoredprotectedfromlight.
Electrochemicalmeasuringsystem Measurementarrangementwhichusesthechemicalreactionsandtheelectrochemicalpotentialwhichdevelopsasaresult;itisbasedonavoltage
measurement(measurementofapotentialdifferenceorcellvoltagebetweentwoelectrodes)
Electrochemicalresistancemeasurement Determinationofthespecificelectricalresistance(reciprocaloftheelectricalconductivity)Electrochemicalsensor
combinationelectrodeCombinationofreferenceelectrodeandsensingelectrode;thedifferencebetween
thedifferentpotentialsoftheelectrodesinthesolutionscanbemeasuredasanelectricalvoltage(e.g.inmV).
Electrode Combinationofametalimmersedinasaltsolution(oracidorbase).Adefinedelectrochemicalpotentialisestablished,dependingonthepropertiesofthematerialandtheambientconditions,suchase.g.temperature.
Colloquially,"electrode"usuallymeanstheentirecombinationelectrode.
Electrodezeropoint Theconcentrationofionstobemeasured(orthepH)atwhichthepotentialofanelectrodeis0mV.Foranidealelectrode,thecharacteristiclineswouldintersectattheelectrodezeropoint.Forthebehaviourofarealelectrode,seeIsothermalpointofintersection.
Electrolyte Substancewhichconductstheelectricalcurrent,primarilysaltsolutionsandacidsandbases.
Electrostaticcharging Electrodesshouldneverbecleanedorabradedmechanically,sincethesurfacethenbecomeselectrostaticallychargedandthemeasurementresultsarefalsified.Forcleaning,rinseelectrodesthoroughlywithdistilledwaterandpatdrywithsoftabsorbentpaper.Avoidtouchingtheglassmembranes.
Environmentalinfluences Allinfluenceswhichactexternallyonthemeasurementandmeasuringequipment,suchastemperature,atmospherichumidityandpressureorvibration,havethecombinedeffectwithsmalldeviationsinthehandlingofthemeasuringequipmentofscatterofcorrectlyperformedmeasurementsaroundameanvalue.
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FET Fieldeffecttransistorscanbeusedforselectivedetectionofionsifsuitablematerialsareusedforcoatingtheso-called"gate".SinceFETsarelight-sensitive,
theymustbestoredawayfromlightandalsoshouldnotbeexposedtodirectincidentlightduringthemeasurement.
ISFETsensors(ion-selectiveFET)areusedforpHmeasurementinareaswhereglasselectrodesmustnotbeused,e.g.foodstuffsproduction.
Fillingsolution Buffersolutionofdefinedconcentrationusedforfillingtheelectrode;theleadandreferenceelectrodeareimmersedinthissolution.
Gelelectrolyte Electrodeswithagelorpolymerelectrolyterequireparticularlylittlecareormaintenance.Theymustnotwhichmeanstheycannotbetoppedup-likeelectrodeswithliquidelectrolyte.
Glasselectrode SeeGlassmembraneelectrode.
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Glassmembrane TheglassmembraneofpHelectrodesisthemostcommonlyusedelectrochemicalsensor.Whenglasssurfacescomeincontactwithanaqueoussolution,thisresultsinadifferenceinelectricalpotentialbetweentheglassandsolution.ThepotentialattheglasssurfaceisdependentonthepHofthesolution.Thedifferenceinpotentialbetweentheglassmembraneandthesolutioncanonlybemeasuredusingacompletelectrochemicalelement(inthiscase,theglassmembraneandthemetallicreferenceelectrode).Anion-sensitiveglassmembranecanbeconsideredasolid-stateelectrolyte:processesinvolvingionexchangeandrearrangementareresponsibleforgeneratingthepotentiaThechemicalandphysicalpropertiesofglass,silicateglasstobemoreaccurate,arewhatmakesitpossibletoevenuseglassmembranestomeasurethepH.Silicateglass
consistsofanirregularnetworkofsilicon,oxygenandaluminumionsintowhichrelativelysmallandhighlymobilesodiumionsforchargeequalizationareincorporateThetwomostimportantrequirementsthatenableaglassmembranetofunctionasapHsensorare:
ThemobileNa+ionscanreacttochangesinconcentration(potentialgradients):theycanleaveorre-entertheglassnetwork.
Anionic(negativelycharged)groupsdirectlyattheglasssurfacebelongbothtotheglasstowhichtheyarechemicallyboundandtothesurroundingsolution(gellayer).Heterogeneousequilibriumprevailsattheglass/solutionboundarylayer(interface).
Theactivityoftheionsinthesolutiondeterminestheactivityofthesurfacegroups
andthustheoccupationofthesitesontheglasssurfacewithanionicgroups,suchas(SiOH)-,(SiONa)-and(SiO-)groups.Therefore,theionsinthesolutiondeterminethesurfacechargeoftheglassmembraneandthusitsdifferenceinpotentialfromthoftheliquid.Example:AsthepHofasolutionincreases,i.e.,astheactivityoftheH+ionsdecrease
(SiOH)-groupsincreasinglydissociateinto(SiO)-groups.Thismeansthattheactivityofthegroupsincreases.Asaresult,thedensityofthenegativesurfacechargeattheglassmembraneincreases(seerightsideofthediagram)and,hence,thepotentialdrops.
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Glassmembraneelectrode MostcommonelectrodeforpHdetermination.TheelectrochemicalpotentialoftheglassmembranechangesasafunctionofthepHofthemeasurementsolution.
Electrodesmadeofspecialglassesareusedfordeterminationofsodiumions.
H+ion,H3O+ion Waterdissociatestoaverysmallextentinaccordancewith
H20H++OH-;sincefreeprotons(H+ions)donotexistinaqueoussystems,thereactionisoftenalsodescribedas
2H20H3O++OH-.TheH3O
+ioniscalledtheoxoniumorhydroxoniumion,andtheOH-ioniscalledthehydroxideion.
Hydroniumion Hydratedproton,forexampleH9O4+orH5O2+Hydroxideion OH-ion,activeconstituentofbasesIonicproductofwater 110-14mol2/l2at22C.Productoftheactivityofthehydroniumionsandhydroxide
ionsofwater.Forallaqueoussolutionstheionicproductisconstantatagiventemperature.Theionicproductdependsgreatlyonthetemperature.
Ionicstrength Anothertermfortheactivityofionsinasolution.Ionicstrengthadjustmentsolution ChemicalreagentwhichisaddedwhenmeasuringwithISEsinordertocompensatetheinfluencesofthesamplemediumontheionicstrength.AfteradditionoftheISA
solutiontheionactivitycorrespondstotheconcentration.
Ionicvalency Ionscancarryoneormorepositive/negativecharges.Thenumberofchargesiscalltheionicvalency(n).
Ions Electricallychargedparticles,calledionsbecauseoftheirmovementinanelectricalfield(Greek"migrators").
Positivelychargedionsarecalledcations,negativelychargedionsanions.
Ion-selectiveelectrode(ISE) Electrochemicalsensorwhichrecordstheactivityofanionicspeciese.g.Cl-orNa+.Isopotentialpoint
Isothermalpointofintersection
Whenthevoltageofarealcombinationelectrodeisrecordedatvarioustemperatures,auniquecharacteristicline(isotherm)isobtainedforeachtemperature.Theisothermsdonotintersectwiththeidealcharacteristiclineonthe0mVaxis.Therealpointofintersectioniscalledtheisopotentialpointorisothermalpointofintersection.
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Junction Thesocalledjunctionordiaphragmisaporousmembraneseparatingtwoelectrolytes.+Thetaskofthereferencejunctionistoseparatethereferencesolution
fromthesamplesolutionandpreventmixing-atthesametimethereferencejunctionmustensuretheelectricalconnectionbetweenthetwohalf-cells(sensingelectrodeandreferenceelectrode).Variousdesignsarecommon:(synthetic)fibrereferencejunction,platinumreferencejunction,ground-glassreferencejunction,perforatedreferencejunction,ceramicreferencejunction
Lifeoftheelectrode Electrodes"age"duetoexposuretoe.g.temperature,polarizationandshort-circuitandchemicalinfluences.Thismanifestsitselfbylongerresponsetimes,anincreaseinresistanceandadecreaseintheslope.Thelifeofanelectrodecanbeincreasedbycorrectcleaning,careandstorage.SeeAgeingoftheelectrode
Measurementerror Sumofalltheinfluenceswhichleadtodeviationsofthemeasurementvaluefromthetruevalue.
Measuringcell
=2electrodes,=combinationelectrodeMole Baseunitoftheamountofsubstance.Symbol:mol
1molcorrespondsto6,0221023particles
(BeforetheintroductionofTheInternationalSystemofUnitsonemolewasdefinedasamassequaltoitsrelativemolecularmassexpressedingrams.)
Molarsolution Solutionwiththeconcentrationofaparticularionicspeciesof1mol/l(moleperlitre)
Nernstequation TheNernstequationdescribestherelationshipbetweentheactivityofasubstanceinasolutionandtheelectricalpotentialestablishedatanelectrodeimmersedinthesolution:
xazFRT ln0 +=
Ifthetemperatureiskeptconstant,theequationcanbeabbreviatedto:
zxalg
0 += E0:zeropointvoltage
EN:"Nernstvoltage"or"Nernstfactor";at25CEN=59.16mV
z:ionicvalency
a:ionactivity
T:temperature/K
R:generalgasconstant
(8.314J/molK=8.314VAs/molK)
F:Faradayconstant
9.648104C/mol=9.648104As/mol
Normalsolution A1normalsolutioncorrespondstoamolarsolutionfor1-valentions;forpolyvalentionsitcorrespondstoa(1/z)molarsolution.
z:ionicvalency
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Ohm'slaw Ohm'slawdescribesthelineardependencyofthecurrentstrength(I)onthevoltageapplied(V):
RVI=
TheresistanceRdependsonthegeometry(lengthlandareaA)oftheconductor
andthespecificel.conductivity():
Al
R
=!
ORPelectrode SeeRedoxelectrodeOverallvoltage Theoverallvoltageofacombinationelectrodeiscomposedofthesumofallthe
individualpotentialsatthesensingandreferenceelectrode.
Oxoniumion Hydratedproton:H3O+,alsohydroxoniumion.
Pepsinsolution Pepsinisanenzymewhichcleavesproteins.Inacidicsolutionitissuitableforcleaningtheelectrodeandespeciallythereferencejunctionifsamplescontainingproteinshavebeenmeasured.Theelectrodemustbeimmersedinthesolutionforseveralhours.
pH ThepHisdefinedasthenegativelogarithmtothebasetenofthehydrogenionactivity. += HapH log a=activity
ApH7correspondstoanalkalinesolution
pHcombinationelectrode SeecombinationelectrodepHmeter AmeasuringinstrumentwhichcalculatesapHfromavoltagemeasurementbetween
thereferenceandsensingelectrode-basedonthetheoreticalrelationshipbetweentheelectrodepotentialandtheactivityoftheH+ionsdescribedintheNernstequation.
pH=7correspondsto0mV
Polymerelectrolyte SeeGelelectrolytePotential Electricalchargedistributionestablishedatthetransitionbetweentwomediaof
differentcomposition.Forexample,atametalwhenimmersedinasolutionofmetalsalts.
Anindividualpotentialcannotbemeasured,onlythedifferencebetweentwopotential.
Potentiometricmeasurementmethods Generictermforthefollowinganalyticalmethodswhichdeterminepotentialdifferencesbetweenthereferenceandsensingelectrodeandgivethevoltage
directly(redoxvoltage)orcomputethisfurther:pHmeasurement,measurementwithion-selectiveelectrodes.
Protein-containingsolutions Solutionswhichcontainproteins,e.g.blood,milk,manyprocesswatersinthefoodstuffsindustry.Proteinscansettleinthereferencejunctionandblockthis.Anelectrodewithaground-glassreferencejunctionisthereforetoberecommendedformeasurementsinprotein-containingsolutions.Ifotherelectrodesareused,the
referencejunctionshouldbecleanedregularlywithacidicpepsinsolution(seePepsinsolution)
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Proton Elementalparticle.Usuallyasingleparticleinthenucleusofhydrogen,andthereforeusuallyappearsinchemistryasthehydrogenion(seeH+ion,H3O
+ion)
Quantitativeelectrochemicalmeasurements Methodfordeterminingconcentrationinwhichanelectrochemicalsensorisusedforendpointdetection(potentiometrictitration)orfordeterminationofachange
inconcentration(standardaddition/standardsubtraction)
Redoxelectrode
ORPelectrodeCombinationelectrode,forexample,withaplatinumdiscasasensorfordeterminationoftheredoxpotentialofsolutions.
Redoxpairs Oxidized/reducedformofasubstancee.g.Fe2+/Fe3+.Redoxreactions Reactionswhichproceedwithrelease(oxidation)oruptake(reduction)ofelectrons.
Sinceelectronsarenotpresentinthefreestate,thesereactionsarealwayscoupled.Theelectricalpotentialwhichexistsintheredoxreactioniscalledtheredoxpotential(ORP)andisameasureoftheoxidizingpowerofasystem.
Referencesolution
(Fillingsolution)Theaqueoussysteminwhichthereferenceelectrodeisimmersed.A3-molarpotassiumchloridesolution(KCl)isusuallyusedforsilver/silverchloridesystems.Thereferencesolutionshouldhavealowelectricalresistance,andmustnotreactwiththemeasurementsolution.
Referenceelectrode Electrodewithaconstantelectrochemicalpotential,whichisusedasthereferencepointwhenmeasuringthevoltagebetweenthereferenceandsensingelectrode.Everyelectrodeconsistsofametallicelementandanelectricallyconductivesolutioninwhichtheelementisimmersed.Nowadays,thesilver/silverchloridereferencesystemisalmostalwaysused.
Referenceelectrode Referenceelectrodewithaconstantpotential.Junction
Thetaskofthereferencejunctionistoseparatethereferencesolutionfromthesamplesolutionandpreventmixing-atthesametimethereferencejunctionmustensuretheelectricalconnectionbetweenthetwohalf-cells(sensingelectrodeandreferenceelectrode).Variousdesignsarecommon:(synthetic)fibrereferencejunction,platinumreferencejunction,ground-glassreferencejunction,perforatedreferencejunction,ceramicreferencejunction
Referencetemperature Referencetemperature,nowadaysusually25C(formerly20C)RelativemVmode Onepotentiale.g.ofastandardsolutionissetat0andallotherpotentialsare
relatedtothiszeropoint.
RelativepH TheconventionalpHrangeof0-14relatestodilutedaqueoussolutions.Ifasolventotherthanwaterispresent,otherconcentrationrangesfor"free"H+ionsresult.
Iftheelectrodeisconditionedintheenvisagedsolventforaprolongedperiodoftime,thesystemcanbecalibratedbyadditionofdefinedamountsofacidoralkali.
Reproducibilityofthemeasurementvalues Ameasureofthedeviationofseveralmeasurementsofthesamesamplefromoneanother.Evenameasurementthatiseasytoreproducecanbeincorrectifa
systematicerrorexists.
Resistance Themobilityofthechargecarriers(electronsorions)andthereforetheconductionofcurrentislimitedbythepropertiesoftheconductor(materialandconductorgeometry).Thequotientofvoltageandcurrentstrengthiscalledtheohmic
resistance(Ohmunit:=V/A)
Responsetimeoftheelectrode Thetimetakentoreachastablemeasurementvalue;itincreasese.g.duetocontaminationoftheelectrodeorreferencejunction.Atypicalresponsetimeofnew
electrodesinstandardbuffersis5s.AtextremepHvaluestheresponsetimeislonger.
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Salinity Thesalinityisasumparameterforthesaltcontentofseawater.Thesaltcontentisstatedhereing/kgandisthereforedimensionless.
Sensingelectrode Theelectrodeofacombinationelectrodewhosepotentialdependsontheactivityofthemeasurementsolution.
Sensor Generaltermforthatpartofameasuringinstrumentwhichreactstoexternalinfluencesandallowsthemtobedeterminedbymeasurement,suchase.g.theglassmembrane,whichreactstochangesinpHinthesolutionwithachangeinpotential,orexpandingmeasuringstrips,whichreacttoachangeintheloadonabalancewithachangeinshape.
Slope Thetheoretical"slope"ofapHelectrodeat25Cis59.16mVperpHunit(derivedfromtheNernstequation);theactualslopeisdeterminedfortheelectrodeduringcalibration.TheslopedescribesthegradientofthestraightlineinthegraphoftheelectrodevoltageasafunctionofthepH.
Iftheslopefallsbelowacertainvalue,theelectrodeisnolongersuitableformeasurementsandtheaccuracydecreases.InSartoriuspHmetersthecalibrationdataareacceptedaslongastheslopeoftheelectrodelieswithintherangeof90%to105%ofthetheoreticalslope.
Solution Solutionsarehomogeneousmixturesoftwoormoresubstances.Onesubstance(solvent)isusuallypresentinalargeexcess.Thedissolvedsubstanceispresentintheformofmoleculesorions.Electrochemistryusuallyconcernsaqueoussolutions.
Storageoftheelectrode ThesensitiveglassmembranemustalwaysbestoredmoistsothatthepH-sensitiveswellinglayeroftheglassisdevelopeduniformlyandtheelectrodeisreadyformeasurement.Theelectrodemustbestoredintheelectrolyteusedasthefillingsolution,i.e.usually3molarKClsolution.
TDS TotaldissolvedsolidsInthecaseofsamplesofknowncomposition,theconductivitycanbeconvertedintotheconcentrationofsolidsviaasample-specificfactor.Thisfactordescribestheinterrelationshipbetweentheconcentrationofthedissolvedsubstancesandtheelectricalconductivity.
Temperaturecoefficient Sinceboththemobilityoftheionsandthenumberofdissociatedmoleculesaretemperature-dependent,theelectricalconductivityofelectrolytesolutionsishighlydependentonthetemperature.
Withtheaidofthetemperaturecoefficient25,theconductivitymeasuredattemperature(inC)canbeconvertedtotheconductivityatthereferencetemperatureof25C:
+= 25
1001 2525 "!!
25isapprox.2%/Kfornaturalwater.
Inthecaseofanon-linearrelationshipbetweenand,25itselfistemperature-dependent.
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Temperaturecompensation ThetermtemperaturecompensationencompassesvariouscorrectioncalculationsinconductivitydeterminationandpHdetermination:
pHmeasurement:Thetemperature-dependencyofthepHofthecommonbuffersolutionsisstoredinthesoftwareofmodernpHmeters.TheinfluenceoftemperatureontheslopeofthepHcombinationelectrodeiscompensatedby
convertingthepHmeasuredtothepHofthebuffersolutionat25CandusingtheNernstfactorrelatingtothemeasurementtemperature.
Conductivitymeasurement:Conversionoftheconductivitydeterminedatany
desiredtemperaturetotheconductivityat25Cusingaspecifictemperaturecoefficient.Thepropertiesofthemeasuringcell(cellconstant)areindependentofthetemperatureinthecontextofmeasurementaccuracy.
Temperatureeffect Inallelectrochemicalmeasurementmethodsthereisasignificantdependencyonthemeasurementtemperature,sincethedissociationofweakelectrolyteschanges
withtemperature.TheslopeofpHcombinationelectrodes,themobilityofionsetc.alsodependonthetemperature.
Thetemperaturedependencyoftheionmobilityisthecauseofthetemperaturedependencyoftheconductivity.
Thiourea Organiccompoundwhoseaqueoussolutionconvertssilversulphideintoasolublecomplexwiththeformationofhydrogensulphide.
Asolutionofthioureainhydrochlorideacidisusedtocleanareferencejunctionofacombinationelectrodeblockedwithblacksilversulphide.
Toppinguptheelectrolyte Liquidelectrolytemustbetoppedupinglassmembraneelectrodesthroughtherefillingholeintheelectrodecap
whentheelementofthereferenceelectrodeisnolongercompletelyimmersedintheliquid;thelevelofthefillingsolutionmustalwaysbeabovethatofthesamplesolution;
ifthereferencesolutioniscontaminatedbypenetrationofsamplesubstanceoristoohighlyconcentratedduetoevaporation-removethe"old"solutionfirst.
Two-pointcalibration Calibrationofacombinationelectrodewithtwostandardsolutionse.g.bufferpH=7andpH=4orconductivitystandardwith=147S/cmand=1,413S/cm
Zeropointcalibration CalibrationofapHelectrodewithastandardwiththepH7.00Zeropointshift Deviationoftherealelectrodezeropointfromthetheoretical,e.g.dueto
concentrationofthereferencesolutionbyevaporationofwater
Zeropointvoltage VoltageatapHelectrodeatpH=7.00.Theoretically0mV,inpracticedeviationsoccur(e.g.30mV)
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