analisis electroquimico sartorius

8/12/2019 Analisis Electroquimico Sartorius

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ManualofElectrochemicalAnalysisPart1Terminologyof

ElectrochemicalAnalysis


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TerminologyofElectrochemicalAnalysis

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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analisis electroquimico sartorius

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