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SUSANA.BROWN
AustralianNuclearScienceandTechnology Organisation,Sydney,NSW,Australia
PAULL.BROWN
RioTintoGrowthandInnovation,Melbourne, VIC,Australia
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Poloniumwasdiscoveredin1898byPierreandMarieCuriefollowing theobservationthattheradioactivityofpitchblendewasaboutfivetimes greaterthanexpectedonthebasisofitsuraniumcontent.Largequantities ofpitchblendefromJoachimsthalwereprocessedandanintenselyradioactivesubstancewascarrieddownwithbismuthsulfideprecipitatedfrom hydrochloricacidsolution.Tracerexperimentsindicatedthattheradioactivesubstancewasanewelement,polonium,namedafterMarieCurie’ s birthcountry,Poland(WahlandBonner,1951).Fouryearslater, Marckwalddemonstratedthattheradioactivitythathadbeenconcentratedwithbismuthcouldbeseparatedfromthelatterusingbothcathodic andspontaneousdepositionandbyprecipitationfromaqueoussolution usingstannouschloride(Bagnall,1957).Thenamesuggestedforthe enhancedradioactivesubstancewasradio-tellurium,duetothechemical resemblanceoftheseparatedmaterialtotellurium.Within2moreyears, Rutherfordidentified,fromthenatureoftheresidualactivityobserved fromthedecayofradon,asubstancethatwasidentifiedasradium-F (originallyidentifiedasradium-E)(Bagnall,1957;FryandThoennessen, 2013).Rutherfordlateridentifiedthatpolonium,radio-tellurium,and radium-Fwereallthesame,beingoneofthedaughterproductsof 238U, theisotope 210Poofthenewelementpolonium.
Therearealargenumberofisotopesofpolonium,butonlysevenare producedwithintheuranium,actinium,andthoriumdecaychainsthat occurnaturally.Thelongestlivednaturallyoccurringisotopeofpolonium is 210Powithahalf-lifeof138.378days(Kocher,1977).Polonium-210is verydangeroustohandleasaresultofintenseradiationreleaseand requiresspecialequipmentandstrictcontrolevenwithmilligramor microgramquantities.Thelongestlivedisotopeis 209Po[t1/2 5 102years (FryandThoennessen,2013)],whichissyntheticandisproducedby bombardmentofleadorbismuthinacyclotron.
Theothersixnaturallyoccurringpoloniumisotopeswerealldiscoveredwithin20yearsofthediscoveryof 210Po.Theywereallidentified fromtheiruniqueradioactivitydecayenergies.Syntheticpoloniumisotopescouldnotbeproduceduntilthedevelopmentofcyclotronsand
2 TheAqueousChemistryofPoloniumandthePracticalApplicationofitsThermochemistry
nuclearreactors,almost50yearsaftertheidentificationofpolonium.The mostrecentisotopes, 223Poto 227Po,werefirstproducedasrecentlyas 2010(FryandThoennessen,2013).
Poloniumisabouteightordersofmagnitudemoretoxicthanhydrogencyanide(McFeeandLeiken,2009).However,poloniumisonlya healthhazardifitistakeninternally.Externalexposuredoesnotrepresent anissuesincepoloniumisanalphaemitter(althoughitdoeshavephoton emissions,theseareextremelylowinabundance)(Ansoborloetal.,2012). Thereforepoloniumisonlyahealthissueifitisinhaledoringested. Polonium-210emitsalphaparticlesofhighenergy(5MeV)whichare capableoftravelingaround50 µminwaterandbiologicaltissues.Cellsin thehumanbodyaretypicallybetween10and30 µmindiameterand,as such,alphaparticlesreleasedfromthedecayofpoloniumwillhavedevastatingeffectsoncellstructuresandDNA(Ansoborloetal.,2012).
Polonium-210toxicitycausessymptomswhicharesimilar,butnotidentical,toacuteradiationsyndromethatiscausedbywholebodygammaradiation.Thesesymptomsarebonemarrowsyndrome,gastrointestinal syndrome,andcentralnervoussystemsyndrome(Ansoborloetal.,2012).
Afteringestionorinhalation,poloniumisdepositedprimarilyinsofttissues,withthegreatestconcentrationsinthereticuloendothelialsystem, principallytheliver,spleen,andbonemarrow,aswellasinthekidneys andskin(hairfollicles)(McFeeandLeiken,2009).Typically,fourphases ofillnessarefollowed:prodromeincludingnauseaandvomiting(which occursinminutestodays),hematopoieticlatency(nonetoweeks)where whitecellsandplateletsdecrease,illness(daystoweeks),andfinallydeath orrecovery(weekstomonths).Theseverityofthesesymptomswill dependontheextentofexposureandtheabilitytodetecttheexposure andadministeranappropriatechelatordrugquicklyenough(McFeeand Leiken,2009).Theeffectsofpoloniumaresimilarinarangeofanimals includingman.Thelethaldose(LD50)hasbeendeterminedtobe 6 15ngkg 1 bodymass.Thusforan80kghuman,thelethaldoseis approximately1 µg(thisamountofpoloniumwouldinitiallyemitin excessof25billionalphaparticleseverysecond).
Therehavebeenanumberofcasesofpoloniumpoisoning.Thefirst documentedcasewasthatofCotelle,whowasatechnicianoftheCurie’ s, andwasworkingwithIreneJoliot-Curiewhenavesselcontainingpoloniumexploded.Sheinhaledalethaldoseofpoloniumanddied2weeks later(Ansoborloetal.,2012).Joliot-CuriewasstandingbehindCotelle andfeltnoimmediateeffects;however,sheeventuallycontractedleukemia
anddied10yearslaterasaconsequenceofthepoloniumexposure.In anotherincident,aRussianworkerinhaledanestimateddoseof530MBq. Symptomsincludedvomiting,severefever,andadecreaseofredblood cells.Theworkerdied13daysaftercontamination(Ansoborloetal., 2012).Themostfamouscaseoccurredin2006.AlexanderLitvinenko,a formerRussianspy,waspoisonedwithpoloniuminaLondonrestaurant. Itwasnotinitiallyrecognizedthatpoloniumhadbeenadministered,and consequently,ittooksomeperiodoftimebeforethecorrecttestswere carriedoutthatrevealedsubstantialcontentsofalpharadiationinaurine sampleintheformof 210Po.Litvinenkodiedafewdayslater.Ithasalso beensuspectedthatthePalestinianleaderYasserArafatmayhavebeen poisonedwithpolonium,butnoconclusiveevidenceforthisexists.
Polonium-210alsooccursintobacco.Thepresenceofpolonium resultsfromthewidespreadusageofphosphatefertilizerswhichnaturally containsomeuranium.TobaccogrownincountriessuchasTurkey, India,andIndonesia,whereorganicfertilizersareused,containareduced concentrationofpolonium(Rego,2009).Thepoloniumcontainedin tobaccoisvolatizedintosmoke,andconsequently,canenterthesmoker’ s lungs.Researchconductedinthe1960sdemonstratedthatpoloniumin smokeconcentratedinbranchingpointsofthebronchialepithelium (so-calledhotspots)thatonlyaccountfor2% 3%ofthelung(Rego, 2009).Testsconductedonhamsterswithpoloniumforcedintothe tracheashowedthat94%developedlungtumorswithsuchsmalldoses thatinflammationdidnotoccur.Alaterstudyonthesameanimalsdemonstratedthatexposuretolowdosesofpoloniumresultedin10% 36% developingmalignanttumors.Thiscomparestoapproximately15%of lifelongsmokerswhodeveloplungcancer(Rego,2009).Consequently,it appearscertainthatthepresenceofpoloniumintobaccoisonesignificant causeofthedevelopmentofcancersinsmokers.
Poloniumhasfewapplications;however,asaconsequenceofits intenseradiation,poloniumhasbeenusedasathermoelectricgenerator andinsatellitesandmoonroversasaheatsource.Poloniumisalsoused toeliminatestaticchargesinphotographicplates,textilemills,paperrolls, andsheetplastics;however,thepoloniumneedstobereplacedfrequently duetotheshorthalf-lifeof 210Po.
Uranium-richorestypicallycontainabout100 µgof 210Popertonneof ore.Manyotherorescancontainuraniumand,assuch,willalsocontain 210Po,butatconcentrationssubstantiallylowerthanthosecontainedin typicaluraniumores.Nevertheless,whenorescontaininguraniumare
4 TheAqueousChemistryofPoloniumandthePracticalApplicationofitsThermochemistry
processed,poloniumpartitionsbetweenthesolidandliquidphasesaccordingtoitschemistry.Thiscanoftenleadtounacceptablyhighlevelsofcontaminationwhichaffectthequalityofthefinalproductsandalsoposean occupationalhealthandsafetyrisktoplantoperatorsandmaintenancestaff.
Althoughtechniqueshavebeendevelopedfortheremovaland/or controlofpoloniuminmanyoreprocessingstreams,littleisknownabout thechemistryoftheelementand,inparticular,itsaqueouschemistryis poorlyunderstood.Theefficacyofthedevelopedtechniquescould, therefore,besignificantlyimprovedbyanincreasedunderstandingofthe aqueouschemistryofpolonium.
Tounderstandtheaqueouschemistryofanyelementrequirestheformulationofalistofspecieswhicharelikelytoformforaparticularsetof elements/ionsunderspecifiedconditions.Therelativeformationofeach individualspeciesisdependentonitsstability,theconcentrationsofreactingelements/ions,andsolutioncharacteristics(e.g.,pH,redoxpotential). Forpolonium,untilnow,nosuchdatabasehasbeenavailable.
Mostcommonly,theacquisitionofathermochemicaldatabasecanbe achievedbyassessingsuchdatafromliteraturesources.Onoccasions, however,andparticularlyforradioactiveelements,theextentoftheliteraturedataislimitedornonexistentandofteninconsistent.Thiscanbe overcome,tosomedegree,bytheuseofthermochemicaldataderived fromtheuseoftheoreticalequationsaimedsolelyatpredictingsuchdata. Theseequationsarevalidated,inthefirstinstance,bycomparisonofpredictedandliteraturedata,afterwhichtheequationscanbeusedtopredict thermochemicaldataofotherspecies.
Oncethespecieslisthasbeenestablished,thedata,togetherwithits concomitantsuiteofthermochemicalparameters,canbeusedtopredict thespeciationcharacteristicsoftheelementforagivensetofsolution conditions.Thesecharacteristicscanbeusedbeforeaparticularexperimentisconductedtoindicatelikelyoutcomesortoassess,forexample,a particularindustrialprocess.Inindustry,suchinformationisoftenessential inunderstandingtheenvironmentalconsequencesofaprocess.
Uraniumcanbepresentinanumberoftypesofores.Theprocessing oftheseorescanoftenconcentratepoloniumleadingtopotentialissues withelevatedconcentrationsinproductsandprocessstreams(e.g.,fumes, duetothehighvolatilityofpolonium).Forexample,thereareanumber ofsignificantglobalcopperproducerswherethepresenceofuraniumcan beproblematic.Theseoperationsoftenhaveintegratedprocessestoproducehigh-qualityrefinedcopperand,attimes,refinedgoldandsilver
and,morerarely,uraniumoxideascoproducts.Anexampleofsuchan operationisBHP’sOlympicDamOperationsatRoxbyDowns,South Australia,whichistheeighthlargestcopperandthelargesturaniumore bodyintheworld.
Poloniumconcentrationsinprocessliquorsatsomecopperoperations have,fromtimetotime,resultedinunacceptablyhighcontaminationlevels inthefinalproducts.Companiesrunningtheseoperationshave,inthepast, undertakenprojectstoaddressspecific problemsinseveralareasoftheirplant circuits.Theseinvestigationswereingeneralsuccessful,andtheyservedto highlightthelackofunderstandingofthechemistryofpoloniumintheprocesscircuits.Althoughpoloniumisstrictlycontrolled,operatingexperience hasshownthatpoloniumconcentrationscanvarysignificantly,andthereasonsforthesechangesarepoorlyunderstood.Itfollowsthatinareasofthe plantcircuitwhereconcentrationsareclosetoallowedlimits,changesto processconditionscouldresultinunacceptablepoloniumlevels.
Rareearthmineralconcentrates,particularlythoseofmonazite,canalso containthoriumanduraniumand,assuch,willalsocontainpolonium. Thereislimitedunderstandingofthe deportmentofradionuclidesinthe processingofmineralscontainingrareearthelements.Aswithcopperoperations,whereuraniumisaby-productorispresentinquantitiesthatneed carefulmanagement,companiesprocessingrareearthelementconcentrates willalsoneedtoimplementstrictcontrolpracticessothatthefinalproducts containradioactiveconcentrationslowerthanregulatorylimits.
Tobeabletopredictthebehaviorofpoloniumandtodevelopeffectivemethodsforitsremovalorcontrol,itisnecessarytoacquirean understanding,orindication,ofitsaqueousspeciationinthesetypesof circuits.Astudyhasbeenundertakentoaddresstheseissues.Athermochemicaldatabaseforpoloniumhasbeendeveloped,andthenusedto derivepH-potentialdiagramsforvariousprocessingcircuits.Theaimwas togainabetterunderstandingofpoloniumbehaviorinthesecircuits whichwouldultimatelyallowthemtodrivepoloniuminthemostappropriatedirection,asdictatedbyprocessneeds.
References
Ansoborlo,E.,Berard,P.,DenAuwer,C.,Leggett,R.,Menetrier,F.,Younes,A.,etal., 2012.Reviewofchemicalandradiotoxicologicalpropertiesofpoloniumforinternal contaminationpurposes.Chem.Res.Toxicol.25,1551 1564.
Bagnall,K.W.,1957.ChemistryoftheRareRadioelements.ButterworthsScientific. Publications,London.
6 TheAqueousChemistryofPoloniumandthePracticalApplicationofitsThermochemistry
Fry,C.,Thoennessen,M.,2013.Discoveryofactinium,thorium,protactinium,anduraniumisotopes.Atom.DataNucl.DataTables99,345 364.
Kocher,D.C.,1977.NuclearDecayDataforRadionuclidesOccurringinRoutine ReleasesFromNuclearFuelCycleFacilities.OakRidgeNationalLaboratory, ORNL/NUREG/T102.
McFee,R.B.,Leiken,J.B.,2009.Deathbypolonium-210:lessonslearnedfromthemurderofformerSovietspyAlexanderLitvinenko.Semin.Diagn.Pathol.26,61 67.
Rego,B.,2009.Thepoloniumbrief.Ahiddenhistoryofcancer,radiationandthe tobaccoindustry.Isis100,453 484.
Wahl,A.C.,Bonner,N.A.(Eds.),1951.RadioactivityAppliedtoChemistry.JohnWiley &Sons,NewYork.
Thereareonlysevennaturallyoccurringisotopesofpolonium;twointhe thorium(4n)decayseries(212Poand 216Po),twointheactinium(4n 1 1) decayseries(211Poand 215Po)andthreeintheuranium(4n 1 2)decay series(210Po, 214Po,and 218Po).Thetwoisotopesinthethoriumchain haveveryshorthalf-lives,withthelongestbeingthatfor 216Poof 0.145seconds(FryandThoennessen,2013).Polonium-211and 215Poare alsobothshort-lived,withbothagainhavinghalf-livesoflessthan1second.Incomparison,twooftheuraniumseriesisotopesaremuchlonger lived(210Poand 218Po),withhalf-livesof138.378days(Kocher,1977) and3.098minutes(FryandThoennessen,2013),respectively.Thehalflifeofthethirdpoloniumisotopeintheuraniumseries(214Po)isvery short.Polonium-210wastheformofpoloniumdiscoveredbytheCuries in1898.Fouryearslaterradio-telluriumwasidentified,butthiswaslater showntobeidenticalto 210Po(FryandThoennessen,2013).
Inadditiontothesevennaturalisotopes,thereareafurther35syntheticisotopesofpoloniumthathavebeenproduced.Theseare 186Poto 209Po, 213Po, 217Po,and 219Poto 227Po(FryandThoennessen,2013). Thevastmajorityoftheseisotopeshaveveryshorthalf-lives,buttwo, 208Poand 209Po,havehalf-liveslongerthananyofthenaturallyoccurring poloniumisotopes.Thecurrentlyacceptedhalf-livesofthesetwoisotopes are2.898and102years,respectively(FryandThoennessen,2013).Asa consequenceoftherelativelylonghalf-lifeof 209Po,itisusedasatracer intheanalysisofnaturalpolonium.Polonium-208wasinitiallyproduced bythebombardmentof 207Pbwithalphaparticles(helium)and 209Poby thebombardmentofbismuthwithdeuterons,withbothisotopesproducedinacyclotron.
Duetotheshorthalf-livesofnaturallyoccurringpolonium,the amountthatcouldbeseparatedininitialexperimentalworkwasexceedinglysmallanditspresencecouldonlybefollowedbyitsradioactivity (Bagnall,1957b).Largeamountsofuraniumoreresidueswereprocessed bytheCuriesandaproductwasproducedthatwasordersofmagnitude
8 TheAqueousChemistryofPoloniumandthePracticalApplicationofitsThermochemistry
moreradioactivethanuranium.Thisproduct,containing 210Po,couldbe concentratedinabismuthfractionandprecipitatedfromsolutionusing hydrogensulfide(Bagnall,1957b).Poloniumsulfidewasfoundtobemore volatilethanbismuthsulfideandthetwosulfidescouldbeseparatedusing vacuumsubmilation.Thuspolonium(as 210Po)hadbeendiscovered.
2.2Elementalpolonium
2.2.1Physicalproperties
Elementalpoloniumisreportedtobemetallicandresemblesleadandbismuthinitsphysicalproperties,whilechemicallyitissimilartothesulfur groupelements,seleniumandtellurium(Bagnall,1957a).Theelectron configurationofneutralpoloniumatomsintheirgroundstateisprobably (Xe) 4f145d106s26p4 (3P2),analogoustoseleniumandtellurium.The meltingpointof α-poloniumis254°C(Maxwell,1949)withadensityof 9.196gcm 3 (Goode,1956).Themeltingpointandboilingpoint(962° C)aremuchlowerthanthecorrespondingvaluesfortelluriumandare comparabletothoseofthallium,lead,andbismuth(Bagnall,1983).The boilingpointhasbeenextrapolatedfromthevaporpressuredataof Brooks(1955).Brooksalsodeterminedthatthevaporizationenthalpyhas avalueof102.9 6 0.1kJmol 1.Somepropertiesofpoloniumarelisted in Table2.1.
X-raydiffraction(XRD)studieshaveindicatedthatthemetalexistsin atleasttwocrystallineforms; “low-temperature” α-poloniumwithasimplecubiclatticeand “high-temperature” β-poloniumwithasimple rhombohedrallattice.Thelatticeparameterof α-poloniumwasmeasured by BeamerandMaxwell(1949) andwasfoundtobe a 5 (3.345 6 0.002)Å.Thesameauthorsalsomeasuredthelatticeparameterof β-polonium,withvaluesdeterminedof a 5 (3.359 6 0.002)Åandan angle α 5 98°130 6 30 .Thephasetransformationoccursatabout75°C (BeamerandMaxwell,1949). StullandSinke(1956) indicatedthatthe sluggishnatureofthetransitionsuggestsasmallheatoftransitionand,as such,itcanbeneglected.Thedensityof β-poloniumis9.398gcm 3 (Goode,1956).Thedensitiesofboth α-poloniumand β-poloniumwere indicatedtohaveuncertaintiesof0.006gcm 3 andwerebasedonX-ray data,theunitcellvolume,andthenumberofatomsintheunitcell (Goode,1956).Bothcrystalmodificationsoftheelementaremetallicin characterwithapositivetemperaturecoefficientofresistivityincontrast tosulfur,selenium,andtellurium.
Table2.1 Somepropertiesofpolonium.
PropertyValueReference
Atomicnumber84
StandardstateSolidat298K
ColorSilvery
CrystalstructureCubicat298K
Electronic configuration (Xe) 4f145d106s2 6p4 (groundstate)
Meltingpoint527K Maxwell(1949)
Boilingpoint1235K Bagnall(1983)
Soliddensity9.196gcm 3 (α-polonium) Goode(1956)
Molarvolume22.54cm3 (α-polonium)
Electronaffinity183.3kJmol 1
Ionizationenthalpy (first)
Bondlength (Po Po)
813kJmol 1
334.5pm(α-polonium)
Atomicradius164pm
Winter(2000)
Greenwoodand Earnshaw(1998)
BeamerandMaxwell (1949)
BeamerandMaxwell (1949)
Ionicradii94pmPo(IV)6-coordinate Shannon(1976)
108pmPo(IV)8-coordinate Shannon(1976)
67pmPo(VI)6-coordinate
228.4pmPo(II)
Shannon(1976)
Bagnall(1983)
Electricalresistivity42 μΩ cm(α-polonium) Maxwell(1949)
Poloniumplatedontoplatinumfoilfromnitricacidsolutionhasbeen studiedusingaBairdgratingspectrograph.Fromtheresults,anionization potentialof8.43Vwasreportedfortheneutralpoloniumatom(Charles etal.,1955). FinkelnburgandStern(1950) studiedthechangeofthe screeningconstantfortheoutermostelectronofanatom, Z Zeff,from elementtoelement,fromwhichtheydeterminedanionizationpotential forpoloniumof8.4 6 0.3Vinexcellentagreementwiththevalueof Charlesetal.(1955).Theelectricalresistivityof α-poloniumwasmeasuredby Maxwell(1949) andfoundtobe42 6 10 μΩ cm.
Ionicradiifortetravalentandhexavalentpoloniumhavebeenreported by Shannon(1976) being0.94and0.67Å,respectively(bothsixcoordinates).Eightcoordinatepolonium(IV)hasanionicradiusof1.08Å
(Shannon,1976). Bagnall(1983) listsanionicradiusforthepolonideion (Po2 )of2.284Å.Theatomicradiusforpoloniumwasreportedby BeamerandMaxwell(1949) tohaveavalueof1.64Å.
Themetalisprecipitatedwhenpoloniumcompoundsaretreatedwith anhydrousliquidorconcentratedaqueousammonia,orwithprimaryor secondaryaliphaticamines,areactionthatapparentlyresultsfromtheformationofatomichydrogenfromtheammoniaoramineunderalpha bombardment.Preparedinthisway,themetalisobtainedasagray-black powder,whereasthevacuum-sublimedmetalhasabright,silveryappearance.Acommonmethodofpreparationisbythethermaldecomposition ofpoloniumsulfide(PoS)ordioxide(PoO2)inavacuum.
Thechemistryofpoloniumatlowconcentrations(108 105 and 5000 40atoms)wasstudiedby Reischmannetal.(1984,1986) using electrodeposition,coprecipitation,andchromatographytechniques.They obtainedcompleterecoveryofpoloniumoverthewholeconcentration rangeineachofthesystemsandconcludedthatpoloniuminextremely smallamountsdisplaysnormalchemicalbehavior. BorgandDienes(1981) conductedatheoreticalstudyonthevalidityofsingleatomchemistry andfoundthatabout10atomsshouldbesufficienttoestablishchemical identityundernormalconditions.
2.3Oxidationstatesofpolonium
2.3.1Chemicalproperties
Valenciesof 2, 1 2,and 1 4arecomparativelywellknownandhave beenwellestablishedbycharacterizationofpolonidesandahydride( 2), thehalides(1 2),andthedioxide(1 4).Thereisalsosomeevidencefora 1 6stateandastable,volatilehexafluorideisthemostlikelycompound ofthisvalency;however,ithasnotbeenprepared.Thereisnoconclusive evidenceforthe 1 3statecharacteristicofmanybismuthcompounds (Bagnall,1957a).
Poloniummetalisrapidlydissolvedby2molL 1 hydrochloricacid giving,first,apalepinksolutionbelievedtocontainthePo2 1 ion.This solutiongraduallybecomesyellowand,onevaporation,yieldspolonium tetrachloride.Additionofhydrogenperoxideorchlorinewatertothe pinksolutionacceleratesthecolorchange,whiletheyellowsolution 10 TheAqueousChemistryofPoloniumandthePracticalApplicationofitsThermochemistry
containingthetetrachlorideisreducedtothepinkchloridebysulfur dioxideorhydrazineinthecold,andbyarsenic(III)oxideonwarming. Themetalreactsvigorouslywithconcentratednitricacidgivingayellow solution[whenconcentrated,10curiesof 210PomL 1 (i.e.,2.2gL 1)], whichbecomescolorlessondilution.Itisprobablethattheconcentrated solutioncontainstetravalentpoloniumnitrate(BagnallandD’Eye,1954).
Hydroxylamineandoxalicacidhavenoeffectandelectrochemical observationsonthelattermay,therefore,beduetotheformationofanoxalatecomplex.Reducedsolutionsoxidizebacktothetetravalentstateafter excessreducingagentiseliminated.Thechangeofpotentialwithtimeata platinumelectrodehasbeenmeasuredtodeterminethevalencystateofthe reducedpolonium.Thevalueswere620mVat4minutes[Po21 (pink)]and 470mVat12minutes[Po41 (yellow)].Thebreakinthepotential time curve(510mVat7.5minutes)couldcorrespondtotheformationofan intermediatetrichloride[Po31 (pink)].However,thereisnootherevidence fortheexistenceofthiscompound(Bagnalletal.,1955a).
Electrochemicaldepositionexperimentshaveshownthatpolonium (IV)canbereadilyreducedbychemicalagentssuchassulfurous,oxalic, andnitrousacids(WahlandBonner,1951). Bagnall(1957b) reportedthat polonium,unliketellurium,wasnotreducedtothemetalbysulfurdioxideorhydrazine.Inthepresenceofselenium;however,poloniumwas precipitatedquantitativelyfromhydrochloricorhydrofluoricacidsby reductionusingbothhydrazineorsulfurdioxide,anditseemspossible thatapoloniumselenidemayform.
Poloniumcanbeprecipitatedfromsolutionbystannouschlorideand fromsulfuricacidusingsulfurdioxide.Poloniumisreducedinbothacetic acidandalkalinesolutionusinghydrazineinthecold,andinsulfuricacid attheboilingpointwhereashydroxylaminereducespoloniumtoalower oxidationstateinaceticacidorsulfuricacidattheboilingpoint.Trivalent titaniumandhypophosphorousacidhavebeenusedtoprecipitatepoloniummetalfromacidsolutionsofsaltsinthecold.Sodiumdithionite reducestracepoloniumfromacidsolution,probablyasthemetal,while reductionwithdithioniteinalkalinesolutioninanatmosphereofhydrogenwasreportedtoyieldapolonide.Traceamountsofsodiumnitritedid notreactwithpoloniumindilutenitricacid(0.1molL 1);however,in moreconcentratedsolutionsofthenitrite[(0.2 4)molL 1],mostofthe poloniumpresentisprecipitated.Thenatureoftheprecipitateis unknown.Nitriteshavealsobeenusedtodecomposepoloniumtetraiodideforanalysiswithouttheprecipitationofpolonium(Bagnall,1957b).
12 TheAqueousChemistryofPoloniumandthePracticalApplicationofitsThermochemistry
Theoxidationoftetravalentpoloniumwith0.5% 5%potassiumpermanganatein1 8molL 1 nitricacidunderrefluxyieldsasludgeof hydratedmanganesedioxidecontainingallthepolonium.Thepolonium inthesludgeisnotdissolvedby2molL 1 hydrochloricacid,aqueous ammonia,or2molL 1 potassiumhydroxide,whereasthepoloniumdissolvesimmediatelyinamixtureof2molL 1 hydrochloricacidand20% (byvolume)hydrogenperoxide,yieldingasolutioncontainingtetravalent polonium.Thereisnoreactionwhenpoloniumhydroxideisboiledwith potassiumpersulfatein2molL 1 potassiumhydroxideorwhenpoloniumnitratein2molL 1 nitricacidisboiledwith0.002molL 1 chromiumtrioxide.Theadditionofexcesscericsaltin2molL 1 nitricacid tosolutionsofpoloniumnitrateinthesameacidalsohasnoeffect,and poloniumhydroxidesuspendedin2molL 1 potassiumhydroxideis notoxidizedbychlorine,incontrasttothebehavioroftellurium (Bagnalletal.,1958a).
TheelectrodepotentialforPo/Po2 hasbeencalculatedtobe 1.0Vbyextrapolationofthecorrespondingpotentialsoftheother Group6Belements.Thevaluesobservedfortheanodicdeposition potentialindicatethatdepositiondoesnotoccurbydischargeofthePo2 ion(Bagnall,1957b).Ahigheroxide,probablyPoO3,isconsideredtobe theproductobtainedbyanodicdepositionoftracepoloniumandchemicalevidenceseemstosupportthisview.Afterpoloniumisextractedfrom nitric,hydrochloric,orsulfuricacidsolutionsintomethylisobutylketone (MIBK),theadditionofastrongoxidant(Ce4 1 ,Cr6 1 )displacestheequilibriuminfavoroftheaqueousphase.ReductionoftheoxidantreestablishestheinitialdistributioninthecaseofCe4 1 andleadstoan intermediatedistributionwithdichromate,probablyduetoincomplete reduction.Anapproximatevalueof1.5VisassignedtothePo4 1 /Po6 1 couple(Figgins,1961).FromtheobservedvaluesoftheelectrodepotentialofPo/Po4 1 ,itappearsthatpoloniumliesbetweentellurium (Te/Te4 1 5 0.63V)andsilver(Ag/Ag 1 5 0.7996V)intheelectrochemicalseries,whichisinagreementwithitsbehaviorinsolutiontoward reducingagents(Bagnall,1957b;Figgins,1961).
Atableofredoxpotentialsinvarioussolutionsproposedbyanumber ofauthorshasbeensummarizedby Moyer(1956).Fromdatapublishedin theliterature, WahlandBonner(1951) proposedtentativeoxidation potentialschemesforpolonium,assumingthelowerpositivestatetobe 1 2,andalsoaschemeforsolutionsat10 9 molL 1 atwhichmuchof theexperimentalworkwasperformed.
Adetailedsummaryoftheelectrochemistryofpoloniumhasbeen givenby Bagnall(1957b).Theelectrodepotential, E°,forthePo/Po41 couplewasgivenas0.75V.Inextremelydilutesolution,thenormalelectrodepotentialcouldonlybeobtainedbyextrapolationofthecritical depositionpotential,determinedfromaplotoftherateofdeposition againstthecathodepotential.Forsolutionsoftracepolonium (B10 9 molL 1)indilutenitricacid,avalueofabout0.6V(referredto thenormalcalomelelectrode)wasobtained.Poloniumwas,however, expectedtoliebetweenbismuthandtelluriumintheelectrochemical series,anditwasthoughtthattheabovevaluewasincompatiblewiththe failureofhydrazinetoprecipitatepoloniumfromsolutionasthemetal. Furtherworkdidnotconfirmthisview.Thecriticaldepositionpotential ofpolonium(B10 9 molL 1)in0.1normalsulfuric,nitric,oraceticacids or1normalphosphoricacidis0.37V(referredtonormalcalomelelectrode)anditisevidentthatthesamepoloniumcationsarepresentineach oftheseacids.Inthepresenceofreducingagents,thisvaluefallsto0.10V. SomeoftheworkattracelevelssuggestedthatthemodifiedNernstequationmightbeinvalid(i.e., E 5 E° 1 (RT/nF)ln a).Laterandmoreprecise measurementsdoshowsomedeviation;however,itshouldbeemphasized thatattheverylowconcentrationsofpoloniumusedformeasurements (10 8 10 13 molL 1),theelectrodewillbeonlypartiallycoveredbythe poloniumdepositandtheactivityofthesolutionwillnolongerbeunity. Thereisasyetnocomprehensiveandquantitativetheorythatwillpredict theelectrochemicalbehaviorofanelementinsuchextremelydilutesolutions.Representativevaluesforelectrodepotentialsaregivenin Table2.2 ElectrodepotentialsforthePo/Po4 1 couple(indilutenitricacid),the Po=PoCl2 6 couple,thePoCl2 4 =PoCl2 6 couple,andthePo=PoCl2 4 couple,havebeenobtained.Valuesforhydrochloricacidat 6 3 10 6 10 4 molL 1 poloniumreferredtothecomplexionsPoCl2 6 (Po(IV))andPoCl2 4 (Po(II)),butcould,however,beequallyapplicable tothecorrespondingcomplexionsPoCl5 andPoCl3 . Bagnall(1957b) wasnotabletoascertainwhichofthesecomplexionswerepresentin hydrochloricacidsolution. Moyer(1956) alsoreportedthatexperiments todeterminetheoxidationstatesofpoloniuminacidsolutionswerenot conclusive.Inhydrochloricacid,twoequalreductionstepswerefound (i.e., 1 4to 1 2and 1 2to0),whileonlyoneoccurredinnitricacid.
Otherevidenceindicatedthattheoxidationstatesinhydrochloricacid were 1 4and 1 3,withtheseconclusionsbasedonthetimerequiredto completethesecondoxidationreactionofpolonium.
Table2.2
Theelectrodepotentialsforpolonium.
CoupleElectrodepotential(V)Reference
Acid
aMeasurementmadeinHClandreferstocomplexions.
TheoxidationpotentialsforthePo=PoCl2 4 couple,at25°C,determinedby Eichelbergeretal.(1965) in1.0,1.5,2.0,3.0,and4.0molL 1 hydrochloricacidwere0.417,0.387,0.367,0.342,and0.297V,respectively.ThepotentialsforthePoCl2 4 =PoCl2 6 coupleat25°Cin1.0and 1.5Mhydrochloricacidwere0.717and0.702V,respectively. Bagnall andFreeman(1956a) measuredapotentialforthiscoupleof0.72Vin 1molL 1 hydrochloricacidat22°Cwhile Power(1949a,b) measureda
potentialof0.582Vin4.7molL 1 hydrochloricacid.Thepotentials obtainedby Eichelbergeretal.(1965) forthePo=PoCl2 6 coupleat25°C were0.567and0.545Vin1.0and1.5molL 1 hydrochloricacid, respectively,whereasavalueof0.55Vin1molL 1 hydrochloricacidat 22°Cwasobtainedby BagnallandFreeman(1956a)
ThecathodicdepositionofPo2 1 wasstudiedby Joliot(1930).A potentialof0.63Vin0.25molL 1 sulfuricacidwasobtained.The potentialofpoloniumperoxidewasdeterminedattracelevelsandvalues of0.89and0.82V(referredtonormalcalomelelectrode)wereobtained. Avalueof0.55VwascalculatedforthePoO3 =PoO2 3 electrode.Marked changesinthecriticaldepositionpotentialoftracepoloniuminthepresenceofoxalicacidhavebeenascribedtothereductionofpoloniumtoa lowervalencystate.Itseemsmorelikelythattheseresultsareduetocomplexionformation,andmigrationexperimentshaveshownthatnearlyall thepoloniumistransportedtotheanodeinthepresenceofoxalicacid.It hasbeenobservedthatanodicdepositionofpoloniumisrepressedbyoxalicacid,althoughthemechanisminvolvedisuncertain.Itshouldbenoted thattracepoloniumissolubleinneutraloxalatesolutionandhydrolysis doesnotappeartooccur,suggestingthepresenceofanoxalato-complex. Thepresenceofasinglychargedcomplexion,(PoIII(C2O4)2(H2O)2) , hasbeenpostulatedtoexplaintheresultsofdiffusionstudies.However, poloniumwouldprobablybeinthetetravalentstateundertheconditions oftheexperiment.Cocrystallizationstudieswithlanthanum,yttrium,and scandiumoxalatessuggestthepresenceoftrivalentpolonium,although thisseemsunlikely.Furtherworkonamilligramscaleestablishedthe divalentandtetravalentstatesonly(Bagnall,1957b).
2.4Poloniumoxides,hydroxides,andhydrides
2.4.1Physicalproperties
XRDpatternsofpoloniumdioxide(PoO2),preparedfromthereaction ofpoloniummetalandoxygen,andthosepreparedbythedecomposition ofpoloniumnitrateareidentical.Inbothcases,itwasfoundthatthe oxideexistedsuccessivelyindifferentsolidphases.Thefirst,whichlasted onlyafewdaysafterthecompoundwasprepared,istetragonal.Thesecondhadacubicfluorite(CaF2)typestructure.Theradiusratio, r/x,of thecompoundis0.73,whichisthelowerlimitofstabilityforthecubic configuration(Bagnall,1957b).Thecubicformhasalatticeparameterof a 5
5.59Åandatheoreticaldensityof9.18gcm 3 (Moyer,1956).
Poloniumdioxidebecomeschocolatebrownat885°Cinoxygenat 105 Paandsublimesundertheseconditions;however,itdecomposesto theelementsatabout500°Cundervacuum(Bagnall,1983).Thedioxide reactsat250°Cwithbothammoniagasandhydrogensulfidetogive blacksolids,whosecompositionsarenotknown.Whenpoloniumdioxideisheatedinsulfurdioxideat250°C,awhitesolidremains,presumably asulfite,sincethecorrespondingsulfatesarepurpleoryellowatthistemperature.Thedioxidedoesnotreactwithliquidsulfurdioxide(Bagnall etal.,1958a).
Thedioxideisappreciablymorebasicincharacterthantellurium dioxide,formingcompoundssuchasthedisulfate(Po(SO4)2 H2O),for whichnotelluriumanalogisknown;however,itstillshowssomeacidic character.Thehydrateddioxide,apaleyellow,flocculantprecipitate obtainedwhendiluteaqueousalkaliisaddedtoanaqueoussolutioncontainingpolonium(IV),isfeeblyacidicandacolorlessproduct,probablya polonite(IV),formswhenthedioxideisfusedwithpotassiumhydroxide ornitriteinair(Bagnall,1983).
Poloniumdioxideisreadilysolubleindilutehydrochloricorhydrobromicacidsgivingyellowandorange-redsolutions,respectively,which yieldyellowpoloniumtetrachloride,andcarmine-redtetrabromideon evaporationtodryness.Asolutionofaqueous,iodinefreehydroiodicacid reactswiththeoxidetogiveaveryvolatileblack,insolublesolid,believed tobepoloniumtetraiodide(BagnallandD’Eye,1954).
Themonoxideisproducedinthespontaneousdecompositionof poloniumsulfite,orselenite,PoXO3 (X 5 S,Se).Thecorresponding hydratedoxideorhydroxideformsasadarkbrownprecipitateonthe additionofalkalitosolutionsofthedihalidesinacid.Itisrapidlyoxidized tothe 1 4state(Bagnall,1957a).
Thetrioxideisthoughttobeformedonthetracerscalebytheanodic depositionofpoloniumfromacidicmedia,thoughithasnotbeenfully characterized.Fusionofpoloniumdioxidewithamixtureofpotassium hydroxideandchlorateyieldsabluishcoloredsolid,whichismoresolubleinwaterthanthedioxidealoneandmaywellcontainpolonate(VI) (Bagnall,1983).
Tracersolutionchemistryindicatesthatpoloniumhydroxideisacidic andanalogoustotellurousacid.Thereportedsolubilityforthiscompoundinwaterorexcessalkaliof0.075mgofPoL 1
given)(Bagnalletal.,1955a)appearedtobemuchlargerthanthevalues suggestedbytracerwork,andhence BagnallandFreeman(1957) investigatedthedependenceofthesolubilityin0.26 1.73molL 1 potassium hydroxide.
Thesolubilityinalkaliincreasedslowlyfor24hoursandthenremained constant,andwasnotaffectedbytheadditionofhydrogenperoxide.The equilibriumwasapproachedfrombothhigherandlowerconcentrationsof alkali,withresultsconsistenttowithin 6 1%.Activitycorrectionswere notappliedandaplotoflog(KOH)versuslog(solubility)waslinearwith aslopecloseto2. Eq.(2.1) describestheprobablereaction.
PoOOHðÞ2 1 2KOH"K2 PoO3 1 2H2 O(2.1)
Theequilibriumconstant, K,is8.2 6 0.4 3 10 5 at22°C,which suggeststhatpoloniumhydroxideis,therefore,muchlessacidicthantellurousacid.Likethelatter,itssolubilityinaqueousammoniaislittledifferentfromitssolubilityinwater(4.2 3 10 7 molL 1).
Valuesof0.044and0.22mgL 1 havebeenreportedforthesolubility ofthehydroxidein18molL 1 ammoniumhydroxideandthatof3.7 3 10 5 mgL 1 forthesolubilityin0.1molL 1 ammoniumhydroxide.
ValuesforthesolubilityproductofPo(OH)4 rangebetween10 37 and 10 38.2 (Figgins,1961;SillénandMartell,1964).
Fromanalogieswithseleniumandtellurium,electricalmigration experimentsandtheincreasingsolubilityofpoloniumhydroxidewith increasinghydroxylionconcentration,thepredominantionicspeciesof polonium(IV)inbasicsolutionsseemstobethePoO32 ion.Asacidityis increased,thereisprobablyashifttospeciesofthetype PoO2 xH2 O; PoOHðÞ1 3 andPoO21 ,andthentovariousanioniccomplexes,forexample,Po(NO3)62 andPoCl62 (WahlandBonner,1951).
Althoughthedegreeofhydrolysisdoesnotseemtochangewithperchloricacidconcentrationintheabsenceofcomplexingagent,thechange isveryrapidin0.01molL 1 hydrochloricacid.Thefollowingequilibria havebeenpostulatedtoexplaintheobservations.
Hydrogenpolonide(H2Po)hasbeenpreparedandinvestigatedata tracerscale(inpooryield)bytheactionof2molL 1 hydrochloricacid onmagnesiumfoiluponwhichpoloniumhadbeendepositedchemically orelectrolytically,andalsobytheadditionofmagnesiumpowdertoa solutionoftracerpoloniumindilutehydrochloricacid.Astreamofdry nitrogenwaspassedthroughtheacidduringtheexperimentandthe emerginggaswasscrubbedinaqueoussolutionsofsilvernitrate,lead nitrate,and2molL 1 hydrochloricacid.Nosignificantquantitiesof poloniumwerecarriedoverinthegasstream.Thehydrideisnotformed fromtheelementsonamilligramscale,isveryvolatile(boilingpoint37° C),andmoreunstablethanbismuthtrihydride.Itisdecomposedbymoist airormoisthydrogen(asishydrogentelluride),bydesiccantssuchascalciumchlorideandphosphorouspentoxide,bypassagethroughalkaline solutionsandsilvernitratesolutionandbycondensationatlowtemperatures(WahlandBonner,1951;Bagnall,1957a;Bagnalletal.,1958a).
2.5.1Physicalproperties
Thebehaviorofpoloniumwiththehalidesissomewhatsimilartothatof tellurium.Poloniumformsbothasoliddichlorideandtetrachlorideand divalentanionsofbothoxidationstates.TheanionoftetravalentpoloniumisknowntoformsolidphaseswiththestoichiometryM2(PoX6) (withM 5 Cs,Rb,KandX 5 Cl,Br,I).Poloniumtetrachlorideliquefies atabout300°Cformingapaleyellowmelt(Abakumov,1982)thatboils ataround390°C.Thedichlorideismorevolatilethanthetetrachloride andisknowntosublimeatabout190°C(Bagnall,1957b).
Poloniumdichloridehasbeenfoundtohaveanorthorhombiccrystal structure,withtheunitcellapparentlycontainingonlyasinglemolecule ofthesaltand,assuch,ithasbeensuggestedthatitisapseudo-cell (Bagnall,1957b).Thereisrelativelypooragreementinrelationtothelatticeparameters(Bagnalletal.,1955a;Moyer,1956)andthedensityofthe solidhasbeencalculatedtobe6.50gcm 3.Poloniumtetrachloridehasa monoclinicortriclinicstructure(Bagnall,1957b).
Anumberofhexachloropoloniteshavebeenpreparedassolidphases includingcesium,ammonium,andtetramethylammonium.Allhavebeen showntohaveaface-centeredcubicstructure.Thecesiumphasehasa latticeparameterof a 5 10.59Åandadensityof3.82gcm 3,whereas
theammoniumphasehasalatticeparameterof a 5 10.33Åandadensity of2.76gcm 3 (Bagnall,1957b).
Poloniumdibromidemeltsataround275°Candsublimesinavacuum at110°C(Bagnall,1957b).Thetetrabromide,whichisbrightredincolor, meltsatabout330°C.Italsohasaface-centeredcubicstructurewitha latticeparameterof5.60Å.Aswithchlorine,thephasesCs2PoBr6(s)and (NH4)2PoBr6(s)havebeenpreparedandagainhaveaface-centeredcubic structurewithlatticeparametersof10.99and10.82Å,respectively,and densitiesof4.75and3.78gcm 3 (Bagnalletal.,1955b).
Poloniumtetraiodideisavolatileblacksolid.Itcanbeusedwithcesium iodideinasolutionofhydriodicacidtoproducethesolidphaseCs2PoI6(s). Thislattersolidhasaface-centeredcubicstructurewithalatticeparameter of11.77Å(Bagnalletal.,1956).ItisisostructuralwithCs2TeI6(s)andpoloniumcancoprecipitatewiththisphase(Bagnall,1957b).
Thepoloniumhalidesarecovalent,volatile,readilyhydrolyzedcompoundsofwhichthosebeingtetravalentareratherless,andthedivalent muchmore,stablethantheirtelluriumanalogs.Complexsaltsoftheform M2PoX6 (M 5 Cs,Rb,K;X 5 Cl,Br,I)havebeenpreparedfromthe tetravalenthalidesandcloselyresemblethecorrespondingtelluriumderivatives,withwhichtheyareisomorphous.Ofthealkalimetals,cesium givestheleastsolublecomplexsaltineachcase.Saltscontainingthe ammoniumandtetramethylammoniumionshavealsobeenprepared (Staritzky,1951;Bagnall,1957a).Absorptionspectralstudiesofpolonium chloridesolutionshaveshownthatatleasttwocomplexesinvolving poloniumandchlorideionsexistinhydrochloricaciddependingonconcentration(Moyer,1956).Solventextractiontechniqueshavebeenused tostudytheformationofanioniccomplexesofthetype PoX3 orPoX3 H2 O ðÞ ðÞ,whereX 5 Cl,Br,I,inweaklyacidicsolution, andfortheformationofthePoX42 anioninmoreconcentratedacid. Thereiselectrochemicalevidencewhichsuggestspolonium(III)species maybetransientintermediatesintheoxidationofpolonium(II)topolonium(IV)inaqueoushydrochloricacid.However,solidtrihalideshave notbeenrecordedandareveryunlikelytobestablewithrespecttodisproportionation.Thereisalsoevidencefortheexistenceofafewpolonium(IV)mixedhalides(e.g.,salmon-pinkdichlorodibromide,PoCl2Br2). Polonium(VI)halidesareunknown(Bagnall,1983).
Poloniumtetrachlorideishygroscopicandreadilyhydrolyzedtoa whitesolidofvariablecompositionwhichispossiblyamixtureofabasic chlorideandahydroxideorhydratedoxide.Asimilarresultwasobtained byhydrolysisofasolutionofthetetrachlorideinboilingwater.From tracerexperiments,itwasconsideredthatthisproductwasanoxychloride (Bagnalletal.,1955a).Thetetrachlorideissolubleinhydrochloricacid, water,withslowhydrolysis,andthionylchloride,andmoderatelysoluble inethylalcohol,acetone,andotherketones.With0.1molL 1 nitric acid,itgivesawhiteinsolublesolidcontainingnochlorine(Bagnalletal., 1955a).Itisslightlysolubleinliquidsulfurdioxidebutdoesnotappearto reactwiththissolvent.Itsreactionwithliquidnitrosylchloridehasalso beeninvestigated.Nitrogenanalysissuggeststhattheproducthasfour atomsofnitrogentooneofpolonium((NO)2PoCl6 2NOClor (NO)2PoCl6 N2O4)andthecompoundisreadilydecomposedby 0.5molL 1 potassiumhydroxide(Bagnalletal.,1958a).Theadsorption enthalpyandentropyofthetetrachlorideontosilicahavebeenreportedas 9kJmol 1 and 11JKmol 1,respectively,overthetemperature range300 1100K(RudolphandBächmann,1979).
Solutionsinhydrochloricacidarebrightyellowatconcentrationsas lowas5 3 10 5 molL 1 andtheadditionofasolutionofcesium chlorideinethylalcoholyieldsagr eenish-yellowprecipitateofcesium hexachloropolonite(Cs2 PoCl6 (s)).Theadditionofammoniumor sodiumhydroxidetosolutionsindilutehydrochloricacidprecipitatesa bufftopalebrownflocculentsolid,withasolubilityof75 μgL 1210 Po inwaterorexcessalkaliatambienttemperature.Whenthesuspensionis boiled,theprecipitatebecomescrystallineandyellow-brown,and thesolubilityincreasesto12mgofPoL 1 .Theprecipitate,which isprobablyahydratedoxide,appearstobefeeblyamphoteric (Bagnalletal.,1955a ).
Inhydrochloricacidsolution,thetetrachlorideisrapidlyreducedto thepinkdivalentstatebysulfurdioxideorhydrazineinthecold,andby arsenic(III)oxideonwarming.Hydroxylamineandoxalicacidhaveno effectandelectrochemicalobservationswhenusingthelattermay,therefore,beduetotheformationofacomplexoxalate.Thereducedsolutions oxidizebacktothetetravalentstateafterexcessreducingagentiseliminated.Thechangeofpotentialwithtimeataplatinumelectrodewas measuredtodeterminethevalencystateofthereducedpolonium.The valuesare620mVat4minutes[Po2 1 (pink)]and470mVat12minutes [Po4 1 (yellow)].Thebreakinthepotential timecurve(510mVat
7.5minutes)couldcorrespondtotheformationofanintermediate trichloride[Po3 1 (pink)].However,thereisnootherevidenceforthe existenceofthiscompound(Bagnalletal.,1955a).
Incoprecipitationexperiments,itwasfoundthatsaltsofPoCl62 are isomorphouswithchloroplumbates,chloroplatinates,andchlorotellurites. Potassium,rubidium,cesium,ammonium,andtetramethylammoniumsalts areinsolubleandPoCl62 hasbeendemonstratedastheprincipalspecies presentinsolutionin2.5molL 1 hydrochloricacid(Cairo,1958).
Dark,ruby-redpoloniumdichlorideisformedbythereductionofthe solidtetrachloridewithsulfurdioxideat25oC.Thesolidishygroscopic andmildlyvolatile(Bagnall,1983).Thedichloridedissolvesreadilyin dilutehydrochloricacidtoformapinksolutionthatrapidlyoxidizes,oris immediatelyoxidizedbyhydrogenperoxideorchlorinewater,tothetetravalentstate.Theadditionofpotassiumhydroxidetothissolutiongivesa darkbrownprecipitate(solubility1.4mgofPoL 1),whichmaybethe hydrateddivalentoxideorhydroxide,andwhichisveryrapidlyoxidized tothetetravalentstate(Bagnalletal.,1955a).Whenthedichlorideis heatedinammoniagasat200oC,abrownammineforms(Bagnall,1983). In0.1molL 1 nitricacid,thedichlorideformsadarkredsolutionand then,rapidly,awhiteflocculentprecipitate,thecompositionofwhichis notknown(Bagnalletal.,1955a).Theadsorptionenthalpyandentropy ofthedichlorideontosilicahavebeenstudiedusinggaschromatography andwerefoundtobe 133kJmol 1 and 74JKmol 1,respectively, overthetemperaturerange300 1100K(RudolphandBächmann,1979).
Solutionsofbothpoloniumtetra-anddichloridesyieldblackprecipitateswithhydrogensulfide.Heatingtheseprecipitatesinavacuumgives sulfurandmetallicpolonium.Althoughtheseprecipitatesmaybesulfides, theirexactcompositionhasnotbeendetermined(Bagnalletal.,1955a).
Poloniumtetrachlorideisconvertedtoawhitesolidby15%hydrobromicacid,whichgivespoloniumdioxideonheating.Thesolidisprobablyaproductofthehydrolysisofthetetrachloridebythehydrobromic acidratherthanabromate(Bagnalletal.,1958a).
Polonium(IV),togetherwithselenium(IV)andtellurium(IV),are knowntoformchloridecomplexanionsofthetypeMCl5 orMCl62 inconcentratedhydrochloricacid.Tracepoloniumcanbeseparatedfrom asolutionofthegroup6BelementsonDowex1X-4anionexchange resin(chlorideform).Poloniumiselutedusingeithernitricorperchloric acids,althoughtheeffectivenessofperchloricacidasaneluantistimedependent(Sasaki,1955;Bagnall,1957b).
Anionexchangehasalsobeenusedindeterminingtheaverageanionic chargeofpoloniumspeciesinchloridesolution.Themethodwasbased onmeasurementsofthedistributionratioofpoloniumataconstantinternalchlorideionconcentrationoftheanionexchangerphase.In 1molL 1 sodiumchloride/hydrochloricacidaroundpH1,polonium (IV)wasfoundtoexistinbothchemicalformsPoCl4 OH ðÞ2 2 (80%)and PoCl3 OH ðÞ2 (20%)(Suganuma,1995).
Poloniumtetrabromideishygroscopicandeasilyhydrolyzed,yielding awhitesolidofindefinitecomposition,presumablyabasicbromide.Itis solubleinhydrobromicacid,ethylalcohol,acetone,andsomeother ketones,sparinglysolubleinliquidbromineandinsolubleinbenzene, chloroform,andcarbontetrachloride.Thetetrabromidedissolvesindilute hydrobromicacidtogiveanorange-red(10 3 molL 1)orcarmine-red (0.025molL 1)solution.Additionofaqueouscesiumbromidetoa 10 3 molL 1 solutionyieldsanimmediateprecipitateofdarkredcesium hexabromopolonite(Cs2PoBr6(s)),whichisimmediatelyhydrolyzedby coldwater.Itisrapidlyreducedinsolutiontothepinkdivalentstateby sulfurdioxideorhydrazineinthecold.Reducedsolutionsarereoxidized tothetetravalentstate,andtheoxidationpotential timecurveshows noevidencefortheexistenceofanintermediatetrivalentbromide. Withammoniaatroomtemperature,thetetrabromideformsan unstableyellowammineandgivessomeindicationofavolatile,colorless phase(Bagnalletal.,1955b).
Poloniumdibromideisapurple-brownsolid,formedbythereduction ofthesolidtetrabromidewithsulfurdioxideat25°C.Thereduction, however,isincomplete.Byanalogytothedichloride,thesolidishygroscopicandsomewhatvolatile(Bagnall,1983).Itissolubleinanumberof ketonesanddilutehydrobromicacid,givingpurplesolutionsthatarerapidlyoxidizedtothetetravalentstate.Solutionsofthedibromideinhydrobromicacidareobtainedbysimilarmethodstothoseofthedichloride (i.e.,reducingthetetrabromideinhydrobromicacidwithsulfurdioxide orhydrazineinthecoldorwitharsenic(III)oxideonwarming)(Bagnall etal.,1955a).Indilutenitricacid,thedibromidedecomposestoforma whiteprecipitateofunknowncomposition(Bagnalletal.,1955b).
Poloniumtetraiodideistheonlyiodideknownandispreparedby treatingthehydroxideordioxidewith0.1molL 1 hydroiodicacidorby precipitationfromahydrochloricacidsolutionofthetetrachloridewith 0.1molL 1 hydroiodicacid.Itisinsolublein2molL 1 hydrochloric acid,1or2molL 1 nitricacid,aceticacid,chloroform,benzene,carbon
tetrachloride,anddiethylanddibutylether,andisslightlysolubleinethyl alcoholandacetone.Itisslowlyhydrolyzedtoawhitesolidofindefinite compositioninwaterandisdecomposedbyhot,concentratednitricacid, orsodiumhypochlorite(NaOCl),orslowlybyconcentratedpotassium hydroxide.Itissolublein2molL 1 hydriodicacid,givingasolutionthat isred-brownat20°Candgreenat0°C(Bagnalletal.,1956).Thetetraiodideisoxidizedbyacidifiedpotassiumnitriteandotheroxidizingagents (Bagnall,1957a).Asuspensionofthesolidin0.1molL 1 hydriodicacid isnotreducedbysulfurdioxideorhydrazine,andnoprecipitateis obtainedwhendilutesolutionsofhydriodicacidorpotassiumiodideare addedtosolutionsofpoloniumdichlorideinhydrochloricacid(Bagnall etal.,1956).Solidtetraiodide,however,isreducedtothemetalby hydrogensulfide(Bagnall,1983).
Theadditionofasolutionofcesiumiodidein0.1molL 1 hydroiodic acidyieldsanimmediateprecipitateofblackcesiumhexaiodopolonite (Cs2PoI6(s)),whichisreadilyhydrolyzedbywater.Thecorresponding potassiumandrubidiumsaltsareinsoluble.Solubilitystudiesofthetetraiodidein0.02 0.5molL 1 hydroiodicacid,carriedoutfrom0°Cto 50°C,indicatethatthesolubilityisproportionaltothesquareoftheacid concentration.Forafixedconcentrationof0.3molL 1 hydroiodicacid, therelationshipofsolubilitytoacidconcentrationat0°C,22°C,and 50°Cwasfoundtobeidentical,andtherefore,thesamecomplexion mustbeinvolved(Bagnalletal.,1956).
Insolutionscontaininghydroiodicacidandlithiumiodideofconstant totaliodideionconcentrationandvaryinghydrogenionconcentration, theresultsshowthatthesolubilityisindependentofthehydrogenion concentration,andtherefore,thereactioninvolvedmustbethatshown by Eq.(2.5).
Theequilibriumconstant, K,determinedwas5.9 6 0.2 3 10 3 at 22°C.Thisrelationship,however,doesnotholdfor(HI) , 0.02molL 1 anditispossiblethatthedeviationsatlowacidconcentrationsareduetoreaction (2.6).
Theequilibriumconstantforreaction (2.6) canbeestimatedbycalculatingthesolubilityduetoreaction (2.5) andsubtractingthisresultfrom theobservedvalue.Thevalueobtainedfor K is6.7 6 0.5 10 5 at22°C.
SolventextractiondataobtainedwithtracerlevelpoloniumalsoindicatetheformationofthehalocomplexanionsPoX5 orPoX5 H2 O ðÞ ðÞ in dilutehalogenacidsolutionandthehexahaloanions,PoX62 inmore concentratedacidsolutions(X 5 Cl,Br,I).Theequilibriumconstantfor theformationofthePoCl62 ion(reaction 2.7)is B1014,asdetermined fromelectrochemicaldata(Bagnall,1983).
Po41 1 6Cl "PoCl2 6 (2.7)
Awhitesolid,presumablypoloniumtetrafluorideorabasiccompound,resultsfromtheactionofdiluteaqueoushydrofluoricacidon poloniumhydroxideortetrachloride.Ontreatmentwithsulfurdioxide, thesolidbecomesbluishgray(possiblyduetoreductiontothedivalent state),whileonstanding,thematerialrevertstothewhitesolid.Thesolubilityoftetravalentpoloniuminaqueoushydrofluoricacidincreasesrapidlywithincreasingconcentration,indicatingcomplexionformation (Bagnalletal.,1958a).Theformationofavolatilepoloniumfluoride compoundthatsubsequentlydecomposesduetochemicalorradioactive decompositionhasbeendescribedby WeinstockandChernick(1960) Theformationofadifluoridehasnotbeenrecorded(Bagnall,1983).
Moyer(1956) summarizedstudiesonthereactionofhalogenvapors withpurepoloniummetal.Themetalisheatedindrychlorineat1atm from125°Cto200°Candthecolorchangesprogressivelyfromgrayto browntoyellow.Theyellowtetrachloride,PoCl4,volatilizesatabout 390°Cinachlorineatmosphere.Atlowertemperaturesandpressures,a redcompoundforms,whichwasshowntobepoloniumdichloride, PoCl2.Whenthemetalistreatedwithdrybrominevaporat200mm Hg,areactionoccursonstandingovernightatroomtemperature.This reactionisthencompletedbyheatingfor1hourat250°Cand,aftervolatilizingat360°Cinabrominevaporatmosphere,darkredcrystalsof poloniumtetrabromide(PoBr4)condense.Noconclusiveevidenceforthe formationofpoloniumtetraiodide(PoI4)wasobtainedandnovolatile fluorideswereformedwhenthemetalwastreatedwithfluorinefrom roomtemperatureto700°C.
MIBKandacetylacetoneextractedallthepoloniumfromtheaqueous phaseofhalogenacidsolutionsoverawiderangeofacidconcentrations.
Evaporationoftheextractsleftayellowoil,whichismisciblewith60/80 petroleumetheronwarming.Withstrongcooling,paleyellowneedles separatefromthesolutionandthepoloniumcanonlyberecoveredfrom thiscrystallineproductbydestroyingtheorganiccomponentwitha
strongoxidizingagent(Bagnalletal.,1958a).Thereactionofpolonium tetrachlorideortetrabromidewithMIBKalsoyieldsdihalopolonium(IV) compounds,possiblyoftheform(iC4H9COCH2)2PoX2 (Bagnall,1983).
Thereisverylittlepublishedinformationoncomplexesofthetetrahalideswithneutraldonorligands.Aminesofunknowncompositionare formedwhenpoloniumtetrachlorideortetrabromideareexposedtogaseousammoniaandastudyofthesolubilityofthetetrachlorideintributyl phosphate(TBP)indicatestheformationofthecomplex(PoCl4(TBP)2). Thisareaofpoloniumchemistry,however,requiresfurtherinvestigation (Bagnall,1983).
Arelativelylargenumberofpolonidephaseshavebeenpreparedandtheir crystalstructuresdetermined.Latticeparametersforsodiumpolonide werepredictedonthebasisofthoseforNa2O,Na2S,Na2Se,andNa2Te andtheparametersthatweredeterminedfollowingthepreparationof Na2Powereingoodagreementwiththeprediction.Thephasehasa face-centeredcubicstructure(fluoritetype)withalatticeparameterof a 5 7.473Å.Sodiumpolonidehasacalculateddensityof4.08gcm 3 (Moyer,1956).IthasalsobeendemonstratedthatNa2Poisisomorphous withNa2Te(KhlopinandSamartseva,1934).
Beryllium,magnesium,calcium,strontium,andbariumpolonide phaseshaveallbeenpreparedby Wittemanetal.(1960).Berylliumpolonidehasaface-centeredcubicstructureofthesphaleritetypewithalatticeparameterof a 5 5.838Åandadensityof7.3gcm 3 (Witteman etal.,1960).Itisvolatileattemperaturesabove600°C.Unliketheother alkalineearthpolonides,magnesiumpolonidehasahexagonalstructureof theNiAstype.Ithaslatticeparametersof a 5 4.345and c 5 7.077Åanda calculateddensityof6.7gcm 3.Calcium,strontium,andbariumpolonideallhaveface-centeredcubicstructuresofthehalitetype.Theyhave latticeparametersanddensitiesof a 5 6.514Åand6.04gcm 3 , a 5 6.796Åand6.3gcm 3,and a 5 7.119Åand6.3gcm 3 ,respectively.Theselatterphasesarevolatileaboveabout600°C 650°C (Wittemanetal.,1960).
Thepolonidephasesofalltherareearthelements(Sc,Y,andthe lanthanides)havebeenstudied.Twophaseshavebeenreportedforeach
rareearthmetal[MPo(s)andM2Po3(s)].Thelatterphasesareobtainedby heatingtherareearthmetalsinpoloniumvaporforaperioduptoabout 11hoursat1000°C.Themonopolonidephasesareobtainedatlower temperaturesinashorterperiod(upto2hours).Themeltingpointofthe M2Po3(s)phasesrangebetween1442°Cand . 2400°Cwhereasthoseof themonopolonidephasesareslightlylowerat1235°C 2212°C.More detailonthesephasesisdescribedby Abakumov(1982).
Nickelpolonidehasbeenproducedwhennickelisheatedinpoloniumvapor.Althoughitsstoichiometryandlatticeparametersarevariable, ithasbeenreportedtohaveahexagonalstructure(Moyer,1956; Abakumov,1982)andameltingpointnear625°C(Wittemanetal., 1960).
Polonidephaseshavebeenreportedforzinc,cadmium,andmercury. Asmightbeexpected,zincpolonidehasthesphaleritestructurewitha latticeparameterof a 5 6.309Åandadensityof7.2gcm 3.Cadmium polonide(CdPo)alsohasthesphaleritestructurewithalatticeparameter of a 5 6.665Åandadensityof7.2gcm 3.Mercurypolonidediffersin itsstructurebeingofthehalitetype.Ithasalatticeparameterof a 5 6.25Åandadensityof11.1gcm 3.Thesepropertieswerereported byWittemanetal.(1960).
Otherpolonidephaseswhosephysicalpropertieshavebeenstudied arethoseofsilver,platinum,lead,andbismuth.Silverpolonidewaspreparedasearlyas1950(Moyer,1956).Anorthorhombicstructurewas assignedbutthiswasnotentirelysatisfactoryanditwassuggestedthatthe structuremaybeeitherorthorhombicormonoclinic,whichcorrespond tothestructuresofsilverselenideandsilvertelluride,respectively.Lead polonidehasthehalitestructurewithalatticeparameterof a 5 6.59Å andadensityof9.6gcm 3 (Wittemanetal.,1960).Thevaporpressure ofleadpolonidehasalsobeenmeasuredandthevaporizationenthalpyhas beenreportedtobe139kJmol 1.Platinumpolonideisbelievedtohave thestoichiometryPtPo2(s)(Moyer,1956).ItisthoughttohaveahexagonalstructureoftheCd(OH)2 type.Bismuthpolonidehasbeenassigned twostructures;arhombohedral(a 5 4.456 4.503Å; c 5 3.602Å)anda cubicstructure(a 5 3.602Å),butnodefinitivestructurehasbeenassigned (Abakumov,1982).
Studiesofotherpolonidephaseshavebeenundertaken,butnodefinitivedatahavebeenreported.Discussionofthesephasesisprovidedinthe reviewof Abakumov(1982)
Poloniummonosulfide,PoS,formsasablackprecipitatebytheactionof hydrogensulfideonsolutionsofpoloniumdi-ortetrachlorideindilute hydrochloricacid.Itissolubleinconcentratedhydrochloricacid,insolubleinethylalcohol,acetone,andtolueneanddecomposedbybromine, sodiumhypochlorite,aquaregia,andammoniumsulfide(Bagnall,1957a; BagnallandRobertson,1957b).Thesulfidealsoformsbytreatingpolonium(IV)hydroxidewithaqueousammoniumsulfideandisdecomposed totheelementsat275°Cundervacuum(Bagnall,1983).
Thesolubilityproductwasdeterminedbyprecipitatingthecompound fromsolutionsofvaryinghydrochloricacidconcentrationwhichhad beensaturatedwithhydrogensulfide.Thesulfideionconcentrationwas calculatedfromthesolubilitydataof KendallandAndrews(1921) andthe knowndissociationconstantsofhydrogensulfide,butactivitycorrections werenotapplied.Thereproducibilitywasnotgoodandattainmentof equilibriumrequiredsometime,duringwhichaconsiderableamountof sulfurwasprecipitated,probablyduetotheoxidationofhydrogensulfide byalpharadiation.Thesolubilityproductwasfoundtobe5.5 6 0.1 3 10 29 in1 5.5molL 1 hydrochloricacidatambienttemperature (BagnallandRobertson,1957b).
Theformationofpolonium(IV)sulfateprovidesfurtherevidencefor themoremarkedlybasiccharacterofpoloniumdioxideascomparedwith telluriumdioxide,whichistobeexpectedfromitspositioninthe PeriodicTable.Thedisulfateisofparticularinterestsincenotellurium analogisknown.Thewhite,hydrateddisulfate,Po(SO4)2 xH2O,is obtainedwhenpoloniumtetrachlorideorthehydrateddioxideistreated withsulfuricacid( . 0.25molL 1).Removalofthesupernatantleadsto aseriesofirreversiblecolorchangesatroomtemperature.Thesearealso observedonheating(pinkat200°C,deeppurpleat380°C)andareprobablyduetoprogressivedehydration.Thesoliddecomposestopolonium dioxideat550°C.Thedisulfatephasesareverysolubleindilutehydrochloricacidandinsolubleinacetoneandethylalcohol.Thesolubilityin dilutesulfuricacidisremarkablylow(420 μgofanhydroussaltL 1 in 0.25molL 1 sulfuricacid;notemperaturegiven);theobservedincrease insolubilitywithacidconcentrationsuggestscomplexformation.The solidloseswaterat100°Candwhenwashedwithanhydrousdiethylether, yieldingthedeeppurple,anhydrousdisulfate(BagnallandFreeman, 1956b;Bagnall,1957a,1983).