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AnalyticaChimicaActa539(2005)

311–316

Acolorimetricmethodfor?uoridedeterminationinaqueoussamples

basedonthehydroxyldeprotectionreactionofacyaninedye

Chang-QingZhua,Jin-LongChenb,HongZhenga,Yu-QinWub,Jin-GouXua,?

a

TheKeyLaboratoryofAnalyticalScienceofMOE,DepartmentofChemistry,XiamenUniversity,Xiamen361005,China

bCollegeofChemistryandMaterialsScience,AnhuiNormalUnversity,Wuhu241000,China

Received6January2005;receivedinrevisedform3March2005

Availableonline30March2005

Abstract

Anewhighlysensitiveandselectivecolorimetricmethodfor?uoridedeterminationinwaterisdescribed.Thenovelreagentforthismethodisacyaninedye(C1)onwhichthehydroxylgrouphasbeenprotectedbyreactionwithtert-butyldimethylsilane(TBS)toformthesilanateddye,C2.C2isselectivelyattackedby?uorideionstoreformC1.C1hasanabsorptionmaximumat600nmwithamolarabsorptivityofabout200,000.Underoptimizedconditions,absorbanceat600nmisproportionalto?uorideconcentrationuptoabout1×10?4mol/Lwithadetectionlimitof1.0×10?7mol/L.Becauseofthespeci?caf?nityof?uoridefortheTBSfunctionalgrouponC2,thereislittleinterferencebyotherions.Themethodhasbeensuccessfullyappliedfor?uoridedeterminationsinlocalrainwatersamples.Resultsdeterminedbytheproposedmethodagreefavorablywiththosedeterminedbya?uorideionselectiveelectrodemethod.©2005ElsevierB.V.Allrightsreserved.

Keywords:Fluorideion;Cyanine;Hydroxyldeprotectionreaction;Colorimetricmethod

1.Introduction

Theeffectof?uorideonbothhumansandanimalshasadualroleasanessentialelementandathighlevelsasatoxicsubstance[1].Excess?uorideionresultsin?uorosisandrenal,gastrointestinalandimmunologicaltoxicity[2].Also,plantsandfruitssuchasapricots,plumsgrapes,tulips,irisandsweetcornaresensitiveto?uoride[3].Hydrogen?uorideisoftenfoundinrainwater[4]asaresultofvariousindustrialprocesses,suchasphosphatefertilizerproduction,aluminiumproductionandmetalsmelting.Therefore,anac-curateandrapidmethodforthedeterminationof?uorideinrainwatersamplesisimportantforassessmentofairandwaterquality.

Severalanalyticaltechniqueshavebeendevelopedanddescribedfor?uoridedeterminationsinwaterincludingspec-trophotometry[5–7],?uorometry[3,8–10],potentiometry

?

Correspondingauthor.Tel.:+865922180307;fax:+865922188054.E-mailaddress:jgxu@http://wendang.chazidian.com(J.-G.Xu).

(ion-selectiveelectrodes(ISE)[11–13],ionchromatography(IC)[14],gaschromatography(GC)[15],capillaryzoneelectrophoresis(CZE)[16]andradioanalysis[17].Amongthese,ISEmethodsaremostwidelyappliedbecausetheyareeasytouse,selectiveandprovideawidedynamicrange[11].Determinationoflow-concentration?uoridebyISEcanbedif?cultduetolongequilibrationtimes,electrodedriftanddissolutionofthelanthanum?uoridemembranecrystal[18,http://wendang.chazidian.comparedtoISE?uoridedeterminationmethods,chromatographicmethodsaretimeconsuming[15]andrequiremoreexpensiveinstrumentation.Severalspectrometricmethods[20–22]arewidelyacceptedforthedeterminationoftrace?uorideinnaturalwatersduetotheirsimplicity,lowcostandreliability.Inmostofthesespectro-metricmethods,?uorideionsdisplaceorganicligandsfromcoloredcomplexesofmetalionssuchaszirconium,thorium,titanium,aluminiumandferriciron[20–22].Displacementofthemetalionororganicligandby?uoridethereforecausestheabsorbanceofthereactionmixturetodecrease.Often,however,Beer’slawisnotobeyedinsuchmethods

0003-2670/$–seefrontmatter©2005ElsevierB.V.Allrightsreserved.doi:10.1016/j.aca.2005.03.002

312C.-Q.Zhuetal./AnalyticaChimicaActa539(2005)311–316

duetothediversityofco-existingcomplexesof?uoridewiththemetalioninthesolution.Furthermore,manyanionsorcationscompetewith?uorideorthecentralmetalionforcoordination[7],whichresultsinseriousinterference.

Recently,Descalzoetal.[23],KimandSwager[24]re-portedanewreactionsystemfortheselectivedetectionof?uorideionthatexploitstheuniquechemicalreactivityof?uorideionswithsilicon.Longreactiontimesandorganicsolventrequirementsofthesereactionsystems[24]greatlylimitedtheirutilityforwateranalysisapplications.Inthispaper,wedescribeusinganewreagentfor?uoridedetermi-nationinaqueoussolution,basedontheselectiveattackof?uorideontheSi–Obondofthecyaninedye,C2(1-ethyl-4-(p-tert-butyldimethylsilaneetherstyryl)quinoliniumiodide).C2isconvenientlyobtainedin98%yieldbyreactionofcya-ninedye,C1(1-ethyl-4-(p-hydroxystyryl)quinoliniumio-dide)withtert-butyldimethylsilane(TBS)asshowninFig.1.ThelargemolarabsorptivityofC1(≈200,000Lcmmol?1)[25]makesitanexcellentchromophoricreporter.WhenC2isincubatedwith?uorideionsin7:3v/vTHF:watersolutions,thecharacteristicabsorptionbandofC1at600nmgraduallyappears.Underoptimizedconditions,alinearrelationshipbe-tweentheabsorbanceat600nmand?uorideionconcentra-tionisobserved.Thisanalyticalapproachprovidesahighlysensitiveandselectivecolorimetricmethodforthedetermi-nationof?uorideioninaqueoussolutionthatwasappliedtothedetectionof?uorideioninlocalrainwatersamples.In-terferencebyforeignanionsisnegligibleandresultsbytheproposedmethodcomparefavorablywithresultsbystandardF?ISEmethods.

2.2.Reagents

The?uoridestandardsolutionswerepreparedbyase-rialdilutionofa10?2mol/Lsodium?uoridestocksolu-tion.CyaninedyeC1wassynthesizedandpuri?edaccord-ingtoBlazsek-Bodoetal.[26,27]andsilanatedwithTBSasdescribedbySartorietal.[28](Fig.1),toformtheindicator,C2in98%yield.C2wasdissolvedinchloro-formsolutiontomakea1.7×10?3mol/Lstocksolution.ThepurityofC2wascon?rmedby1HNMRandHRMSspectra.Allchemicalswereanalyticalreagentgradeandusedasreceived.Doublydeionizedwaterwasusedforallexperiments.

2.3.Pretreatmentofsamples

Twolocalrainwatersampleswereobtainedafterapre-treatmentaccordingtotheliterature[29];twotapwatersam-plesweredilutedby10-foldwithdoublydeionizedwaterforcomparativeexperiment.

2.4.Standardprocedureforthedetectionof?uorideAddsequentiallyto5.0mLvolumetric?askscontaining3.5mLofTHFsolution,0.1mLofC2workingsolution,1.3mLofstandardsodium?uoridesolutionorsampleand0.1mLofNaH2PO4–NaOHbuffersolution(pH9.0).Mixthecontentsofthe?askthoroughly.After10min,measuretheabsorbanceat600nmusingTHF–water(7:3,v/v)solutionasthereference.Afterdeterminationsarecomplete,recycleTHFbysimplecollectionanddistillationtoreducethepos-siblepollution.

2.Experimental2.1.Apparatus

AHitachiU-3010spectrophotometer(Tokyo,Japan)wasusedforrecordingabsorptionspectraandmakingabsorptionmeasurements.ThepHwasmeasuredwithaModelpHs-3cmeter(Shanghai,China).A201?uorideIonSelectiveElec-trode(JiangshuElectroanalysisApparatusFactory,China)wasusedforthedetectionof?uorideinrainwatersamples.

3.Resultsanddiscussion3.1.Spectralcharacteristics

ThevisibleabsorptionspectraofC1inpH9.0bufferdis-playstwobandsasshowninFig.2b.AtlowpH,however,thespectraofC1displayonlyonevisiblebandat440nmasshowninFig.3thatcorrsespondstotheabsorptionof

the

Fig.1.Protectionanddeprotectionofcyanine’shydroxyl.

C.-Q.Zhuetal./AnalyticaChimicaActa539(2005)311–316

313

Fig.2.AbsorptionchangeofC2uponadditionofNaF:(a)aserialvisualcolorchange,the?uorideconcentrations(fromthelefttotheright)are:0.0,2.8×10?6,6.6×10?6,9.4×10?6,2.8×10?5,4.7×10?5,6.6×10?5and9.4×10?5mol/L.(b)UV–visabsorptionspectrainTHF–water(70:30,v/v)solvent.Concentrationsof?uorideare:0.0,4.8×10?6,9.4×10?6,2.8×10?5,4.8×10?5,6.6×10?5and9.4×10?5mol/L.Otherconditionsarethesameasthosedescribedinthe

procedure.

Fig.3.UV–visabsorptionchangeofthecyaninewithpHbefore(C1)andafter(C2)itshydroxywasprotected.Solvent:THF–water(70:30,v/v);con-centrationofC1andC2:3.4×10?5mol/L;NaH2PO4–NaOHbuffer:pH6.0–9.7.

specieswithnon-ionizedhydroxylgroup.Thebandat440nmwasassignedtoanintramolecularchargetransfertransition[30].ItsintensitydecreasesaspHincreasesabove7.Thein-tensityofthebandat600nmincreasesasthepHincreasesabovepH7.Thebehaviorcanbeexplainedasfollows.WiththeincreaseofpHthehydroxylprotonbeginstoionize,whichincreasestheproportionoftheionizedforminsolutionduetomakingoftheenergybarrieroftheintramolecularCTsmaller.

Theabsorptionbandat600nmofC2atpH9.0disap-peared,withalittleblueshiftingofthemainabsorptionbandat440nm(Fig.2b)duetotheincreaseoftheenergybar-rieroftheintramolecularCTafterthehydroxylgroupofC1wasprotected.Atthesametime,thedependencetopHdis-appearedsinceonlyonecyanineformexisted.AsshowninFig.3,C2didnotshowtheabsorptionatabout600nmevenatthepHupto9.7.

WhenC2wasincubatedwithvariousamountsof?uorideion,adramaticcolorchangefromyellowtogreenwasob-served(Fig.2a)andthecharacteristicabsorptionofinitialcyanineC1atabout600nmgraduallyreturnedtothesolu-tion(Fig.2b).Undertheoptimumconditions,therewasagoodlinearrelationshipbetweentheabsorbanceat600nmandtheconcentrationof?uorideion.3.2.EffectsofreactionmediumandpH

THFgreatlyincreasestherateof?uoridereactionwithC2.Experimentsshowedthat70%THFwasoptimalforspeci?creactionof?uoridewithC2.BecauseC2wassta-bleandsolubleinchloroformatroomtemperature,chlo-roformwasusedtopreparethestocksolutionofC2andthissmallamountofchloroform(<3%)isunavoid-ablyintroducedintothedetectionsystem.Experimentsshowedthatitseffectonanalyticalresultswasnegligi-ble.

Twobuffersystems,Tris–HClandNaH2PO4–NaOHbuffer,wereinvestigated.Thelattershowedaverylowblankabsorbanceat600nm(seeFig.2b).Therefore,0.1mol/LpH9.0NaH2PO4–NaOHbuffersolutionwaschoseninordertohavehighersensitivityandlowerbackground.3.3.ProbeC2concentrationandincubationtime

TheeffectofconcentrationofC2onthedeterminationof?uoridewasinvestigated.Whenconcentrationsofotherreagentswerekeptconstant,theabsorbancedifferencebe-tweentheabsenceandthepresenceof?uorideionin-creasedwithincreasingtheamountofC2.However,whentheconcentrationofC2exceeded3.4×10?5mol/L,thebackgroundabsorbance(without?uorideion)increasedac-cordingly.Thus,C2concentrationof3.4×10?5mol/Lwasrecommended.

Undertheseoptimizedconditions,completecleavageoftheSi–Obond(ortheassociationof?uoridewiththesiliconatom)onC2requiredabout1.5h(Fig.4).Althoughanalyti-

314C.-Q.Zhuetal./AnalyticaChimicaActa539(2005)

311–316

Fig.4.Effectofreactiontime.Concentrationsof?uoride(fromthebottomtothetop):0.0,4.8×10?6and4.8×10?5mol/L.Otherconditionsarethesameasthosedescribedintheprocedure.

calsensitivitycouldhavebeenincreasedwithlongerreactiontimes,anincubationtimeof10minwaschosenforthisworkasapracticalmatter.Ourexperimentsdemonstratethataccu-rateresultscouldbeobtainedatthisrelativelyshortreactiontime.

3.4.Interferenceofforeignions

Theeffectsofforeignionsonthedeterminationof1.6×10?6mol/L?uorideindeionizedwaterwerestudied.Foreignionconcentrationsthatresultinrelativeerrorslessthan±5%arelistedinTable1.FromTable1,itcanbeseenthatthetestedionshaveverylittleinterferencewith?thedeter-mination.2?TherelativelyhighconcentrationsofCl,Br?,I?,SO4,SCN?andPO43?thatcanbetoleratedareexplained

Table1

ToleranceofforeignsubstancesForeignionsConcentrationRelativeerror(10?6mol/L)caused(%)Cl?280?5.0Br?125?1.7I?

78?1.0SO42?80+2.5SCN?90+3.5PO43?200+1.0Fe3+50?3.0NH4+200?2.0Cr3+50+3.5Co2+70+2.5Ni2+45+3.5Ca2+80?1.5Ba2+100+2.8Cu2+100?3.5Al3+40?2.0Mn2+

60

+4.0

Concentrationof?uoride:1.6×10?6mol/L.Otherconditionsarethesameasthosedescribedinthe

procedure.

Fig.5.Calibrationgraphsfordifferentincubationtime(10minand30min).Absorbancewasmeasuredat600nm;solvent:THF–water(7:3,v/v).Otherconditionsarethesameasthosedescribedintheprocedure.

bythespeci?citywithwhich?uoridereactswiththesiliconatomonC2.

3.5.Calibrationgraphsandanalysisofsamples

Thecalibrationgraphsfortwokindsofincubationtime,i.e.10minand30min,wereconstructed(Fig.5)undertheoptimumconditions.FromFigs.4and5,itcanbeseenthatreasonablyprolongingthereactiontimecanimprovethean-alyticalsensitivity.

Thelimitof7detection(LOD)fortheproposedmethodwas1.0×10?mol/Lgivenbytheequation,LOD=KS0/S,whereKisanumericalfactorchosenaccordingtothecon-?denceleveldesired,S0thestandarddeviationoftheblankmeasurements(n=6)andSthesensitivityofthecalibrationgraph.Hereavalueof3forKwasused.Andtherelativestandarddeviation(n=6)was3.3%forthedeterminationof1.6×10?6mol/LNaF.

Theproposedmethodwasappliedtothedeterminationof?uorideinrainwatersamples.Twolocalrainwatersampleswerecollectedand?lteredaccordingtotheliterature[29].ThedeterminationresultsbythismethodandcomparisonwiththatobtainedbytheF?ISEstandardmethodareshowninTable2.AllthedeterminationresultsshowninTable2werethemeanofsixmeasurements.Therecoverytestwascarriedoutbyadding1.0×10?6mol/L?uoridestandardsolutioninthesamplesolutions.Therecoverieswerefoundtobe98–102%.Atthesametime,thedatashowninTable2,whichareveryclosetothatobtainedbytheF?ISEstandardmethod,arealsoinagreementwiththatprovidedbylocalenvironmentmonitorstation.Furthermore,twotapwatersamplesdilutedby10-foldwithdoublydeionizedwaterweredetectedbytheproposedmethodandF?ISEmethod,theresultsareshowninTable3.FromthedataofTables2and3,itcanbeseenthattheproposedmethodissensitiveandreliable.

C.-Q.Zhuetal./AnalyticaChimicaActa539(2005)311–316

Table2

AnalyticalresultsofrainwatersamplesRainwatersamplesno.12

ThismethodF?,founda(n=6)3.78±0.103.55±0.11

R.S.D.(%)2.53.0

F?,added(10?6mol/L)1.01.0

F?,found(10?6mol/L)4.804.53

Recovery(%)10298

F?ISEmethodF?,found(10?6mol/L)3.853.50

315

t-testb1.811.15

Theanalyticalresultisexpressedasx¯±(tp,n?1S/),wheretp,n?1=t0.95,5=2.57,x¯andSarethemeanandstandarddeviation,respectively.Theunitfortheconcentrationof?uorideinrainwaterisexpressedin10?6mol/L.bt

0.95,5=2.57;t0.95,5=2.02.

a

Table3

ComparativedeterminationofF?intapwatersamplesSample

ThismethodF?,founda(10?6mol/L)F?founda

Tapwater1

1.21

Tapwater2

1.10

ab

F?ISEmethodF?,found(10?6mol/L)

Recovery(%)1049998101

F?foundaUndetectedUndetected

F?added1.04.01.04.0

F?foundbUndetected4.25Undetected4.16

Recovery(%)–106–104

F?added1.04.01.04.0

F?foundb2.255.202.085.15

The?uorideintapwaterwith10-folddilution.Meanofsixdeterminations.

Thetotal?uoridein10-folddilutedtapwaterwiththeadditionof?uoride.Meanofsixdeterminations.

4.Conclusions

Anovelcolorimetricmethodforthedeterminationof?uorideionhasbeendevelopedbasedontheselectiveat-tackof?uorideonasilanatedcyaninedye.Themethodhasseveralmeritsthatinclude:(i)highsensitivityandspeci?city;(ii)semi-quantitativedeterminationsbyopti-calcomparisonispossible;and(iii)synthesisofC1andC2arestraightforwardandspectrophotometricoropticalcomparisondetectionmethodsareconvenientandeconomi-cal.

Acknowledgements

ThisworkwassupportedbytheNationalNaturalScienceFoundationofChina(NNSFC,No.29775021)andtheEd-ucationCommissionNaturalScienceFoundationofAnhuiProvince(No.2001KJ114ZD);alltheauthorswishtoex-presstheirgratitude.

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