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PIIS0016-7037(02)01044-XProton-bindingstudyofstandardandreferencefulvicacids,humicacids,andnaturalorganicmatterJASOND.RITCHIEandE.MICHAELPERDUE*SchoolofEarthandAtmosphericSciences,GeorgiaInstituteofTechnology,Atlanta,GA30332-0340,USA(ReceivedSeptember17,2001;acceptedinrevisedformMay6,2002)Abstract—Theacid–basepropertiesof14standardandreferencematerialsfromtheInternationalHumicSubstancesSociety(IHSS)wereinvestigatedbypotentiometrictitration.Titrationswereconductedin0.1MNaClunderanitrogenatmosphere,averaging30minfromstarttofinish.Concentrationsofcarboxylgroupsandphenolicgroupswereestimateddirectlyfromtitrationcurves.TitrationdatawerealsofittoamodifiedHenderson-Hasselbalchmodelfortwoclassesofproton-bindingsitestoobtain“bestfit”parametersthatdescribeproton-bindingcurvesforthesamples.Themodelwaschosenforitssimplicity,itseaseofimplementationincomputerspreadsheets,anditsexcellentabilitytodescribetheshapesofthetitrationcurves.ThecarboxylcontentsoftheIHSSsamplesareinthegeneralorder:terrestrialfulvicacids
aquaticfulvicacids
SuwanneeRivernaturalorganicmatter(NOM)
aquatichumicacids
terrestrialhumicacids.Overall,fulvicacidsandhumicacidshavesimilarphenoliccontents;however,alloftheaquaticallyderivedsampleshavehigherphenoliccontentsthantheterrestriallyderivedsamples.Theacid–basepropertiesofreferenceSuwanneeRiverNOMaresurprisinglysimilartothoseofstandardSuwanneeRiverhumicacid.Resultsfromtitrationsinthisstudywerecomparedwithotherpublishedresultsfrombothdirectandindirecttitrations.Typically,carboxylcontentsfortheIHSSsampleswereinagreementwiththeresultsfrombothmethodsoftitration.PhenoliccontentsfortheIHSSsampleswerecomparabletothosedeterminedbydirecttitrations,butweresignificantlylessthanestimatesofphenoliccontentthatwerebasedonindirecttitrationswithBa(OH)2andCa(OAc)2.Theaveragephenolic-to-carboxylicratiooftheIHSSsamplesisapproximately1:4.Modelsthatassumea1:2ratioofphenolic-to-carboxylicgroupsmayoverestimatetherelativecontri-butionofphenolicgroupstotheacid–basechemistryofhumicsubstances.Copyright©2003ElsevierScienceLtd1.INTRODUCTIONSince1981,theInternationalHumicSubstancesSociety(IHSS)hascollected,maintained,anddistributedstandardandreferencesamplesofhumicacidsandfulvicacidstoscientiststhroughouttheworld.Suchacollectionofstandardandrefer-encehumicsubstancesfacilitatesintercomparisonsoflabora-torymeasurementsandthedevelopmentofnewmethodologiesforstudyingthephysicalandchemicalpropertiesofhumicsubstances.Additionally,scientiststhroughouttheworldhavesubjectedtheIHSSsamplestoavarietyofmeasurementsandanalyses.Todate,however,therehasbeennosystematicanal-ysisorcomparisonoftheacid–basepropertiesoftheentirecollectionofthesestandardandreferencematerials.Theobjectiveofthisstudyistopresentandanalyzetheresultsofacid–basetitrationsofIHSSstandardandreferencefulvicandhumicacidsandtherecentlycollectednaturalor-ganicmatter(NOM)fromtheSuwanneeRiver.The14samplesinthiscollectionincludesixfulvicacids,sevenhumicacids,andonesampleofNOM.Alternatively,thesamplesmaybeclassifiedaccordingtothesourcematerialsfromwhichtheywereisolated.Onthisbasis,therearesixaquaticsamples(isolatedfromriversandlakes)andeightterrestrialsamples(isolatedfromsoil,peat,andleonardite).Primaryemphasisisplacedoncomparisonoftheconcentrationsofcarboxylandphenolicgroupsinthesesamples.Theseresultsarealsocom-paredwithalargenumberofpreviouslypublishedmeasure-mentsofcarboxylandphenoliccontents.Theacid–basepropertiesofhumicsubstancesareofintrinsicinterestbecausethesesubstancescontributetotheacid–basebalanceofnaturalwaters.Inaddition,thecomplexationofmetalcationsbyhumicsubstancesisstronglydependentonpH(RandhawaandBroadbent,1965;VanDijk,1971;Spositoetal.,1978;SpositoandHoltzclaw,1979;PerdueandLytle,1983;MarinskyandEphraim,1986;CabanissandShuman,1988).Anysuccessfulthermodynamicmodelofmetalcompl-exationbyhumicsubstancesmustthereforealsobeabletomodelprotonbinding.Numerousmodelshavebeendevelopedtodescribemetal-bindingandproton-bindingequilibriawithhumicsubstancesandNOM.AfewofthemorecommonmodelsaretheNon-IdealCompetitiveAdsorption(NICA)(Benedettietal.,1995)andNICA-Donnanmodels(Koopaletal.,1994;Benedettietal.,1996;Milneetal.,2001),ModelsVandVI(TippingandHurley,1992;Tipping,1998),andthecompetitivegaussiandistributionmodel(PerdueandParrish,1987;Dobbsetal.,1989).Thetitrationsinthispaperwereallconductedatthesametemperature,ionicstrength,andconcen-trationofdissolvedorganiccarbon,andanymathematicalmodelthatreproducesthetitrationcurvescan,inprinciple,beusedtodescribethem.Giventhatthemainobjectiveofthisstudyisnotmodelingperse,averysimplemodelhasbeenused,primarilytoobtainfittingparametersthatcanbeusedtogeneratetitrationcurvesoftheIHSSsamplesusingonlyasimplecomputerspreadsheet.Thefittingparametersalsopro-*Authortowhomcorrespondenceshouldbeaddressed([e-mail]).PergamonGeochimicaetCosmochimicaActa,Vol.67,No.1,pp.85–96,2003Copyright©2003ElsevierScienceLtdPrintedintheUSA.Allrightsreserved0016-7037/03$22.00.0085vide,atleasttosomeextent,abasisforcomparingtheacid–basepropertiesofthesamples.2.BACKGROUNDTheconce