Cobalt - Wikipedia, The Free Encyclopedia

10/10/2015 CobaltWikipedia,thefreeencyclopedia

https://en.wikipedia.org/wiki/Cobalt 1/15

Cobalt,27Co

electrolyticallyrefinedcobaltchips

Generalproperties

Name,symbol cobalt,Co

Appearance hardlustrousgraymetal

Pronunciation /koblt/KOHbolt

Cobaltintheperiodictable

CoRh

ironcobaltnickel

Atomicnumber 27

Standardatomicweight()(Ar)

58.933194(4)[1]

Elementcategory transitionmetal

Group,block group9,dblock

Period period4

Electronconfiguration

[Ar]3d74s2

pershell 2,8,15,2

Physicalproperties

Color metallicgray

Phase solid

Meltingpoint 1768K(1495C,2723F)

Boilingpoint 3200K(2927C,5301F)

Densitynearr.t. 8.90g/cm3

whenliquid,atm.p.

8.86g/cm3

Heatoffusion 16.06kJ/mol

CobaltFromWikipedia,thefreeencyclopedia

Thisarticleisaboutthechemicalelement.Forotheruses,seeCobalt(disambiguation).

CobaltisachemicalelementwithsymbolCoandatomicnumber27.Likenickel,cobaltintheEarth'scrustisfoundonlyinchemicallycombinedform,saveforsmalldepositsfoundinalloysofnaturalmeteoriciron.Thefreeelement,producedbyreductivesmelting,isahard,lustrous,silvergraymetal.

Cobaltbasedbluepigments(cobaltblue)havebeenusedsinceancienttimesforjewelryandpaints,andtoimpartadistinctivebluetinttoglass,butthecolorwaslaterthoughtbyalchemiststobeduetotheknownmetalbismuth.Minershadlongusedthenamekoboldore(Germanforgoblinore)forsomeofthebluepigmentproducingminerals;theyweresonamedbecausetheywerepoorinknownmetals,andgavepoisonousarseniccontainingfumesuponsmelting.In1735,suchoreswerefoundtobereducibletoanewmetal(thefirstdiscoveredsinceancienttimes),andthiswasultimatelynamedforthekobold.

Today,somecobaltisproducedspecificallyfromvariousmetalliclusteredores,forexamplecobaltite(CoAsS),butthemainsourceoftheelementisasabyproductofcopperandnickelmining.ThecopperbeltintheDemocraticRepublicoftheCongoandZambiayieldsmostofthecobaltminedworldwide.

Cobaltisprimarilyusedasthemetal,inthepreparationofmagnetic,wearresistantandhighstrengthalloys.Itscompoundscobaltsilicateandcobalt(II)aluminate(CoAl2O4,cobaltblue)giveadistinctivedeepbluecolortoglass,ceramics,inks,paintsandvarnishes.Cobaltoccursnaturallyasonlyonestableisotope,cobalt59.Cobalt60isacommerciallyimportantradioisotope,usedasaradioactivetracerandfortheproductionofhighenergygammarays.

Cobaltistheactivecenterofcoenzymescalledcobalamins,themostcommonexampleofwhichisvitaminB12.Assuchitisanessentialtracedietarymineralforallanimals.Cobaltininorganicformisalsoanactivenutrientforbacteria,algaeandfungi.

Contents



Heatofvaporization

377kJ/mol

Molarheatcapacity

24.81J/(molK)

vaporpressureP(Pa) 1 10 100 1k 10k 100k

atT(K) 1790 1960 2165 2423 2755 3198

Atomicproperties

Oxidationstates 3,1,+1,+2,+3,+4,+5[2](anamphotericoxide)

Electronegativity Paulingscale:1.88

Ionizationenergies

1st:760.4kJ/mol2nd:1648kJ/mol3rd:3232kJ/mol(more)

Atomicradius empirical:125pm

Covalentradius Lowspin:1263pmHighspin:1507pm

Miscellanea

Crystalstructure hexagonalclosepacked(hcp)

Speedofsoundthinrod

4720m/s(at20C)

Thermalexpansion

13.0m/(mK)(at25C)

Thermalconductivity

100W/(mK)

Electricalresistivity

62.4nm(at20C)

Magneticordering ferromagnetic

Young'smodulus 209GPa

Shearmodulus 75GPa

Bulkmodulus 180GPa

Poissonratio 0.31

Mohshardness 5.0

Vickershardness 1043MPa

Brinellhardness 4703000MPa

CASRegistryNumber

7440484

History

Discovery GeorgBrandt(1732)

Moststableisotopes

Ablockofelectrolyticallyrefinedcobalt(99.9%purity)cutfromalargeplate

Contents

1Characteristics2Compounds

2.1Oxygenandchalcogencompounds2.2Halides2.3Coordinationcompounds2.4Organometalliccompounds

3Isotopes4History5Occurrence6Production7Applications

7.1Alloys7.2Batteries7.3Catalysts7.4Pigmentsandcoloring7.5Radioisotopes7.6Otheruses

8Biologicalrole9Precautions10References11Externallinks

Characteristics

Cobaltisaferromagneticmetalwithaspecificgravityof8.9.TheCurietemperatureis1,115C(2,039F)[3]andthemagneticmomentis1.61.7Bohrmagnetonsperatom.[4]

Cobalthasarelativepermeabilitytwothirdsthatofiron.[5]Metalliccobaltoccursastwocrystallographicstructures:hcpandfcc.Theidealtransitiontemperaturebetweenthehcpandfccstructuresis450C(842F),butinpractice,theenergydifferenceissosmallthatrandomintergrowthofthetwoiscommon.[6][7][8]



iso NA halflife DM DE(MeV) DP

56Co syn 77.27d 4.566 56Fe

57Co syn 271.79d 0.836 57Fe

58Co syn 70.86d 2.307 58Fe

59Co 100% 59Coisstablewith32neutrons

60Co syn 5.2714y , 2.824 60Ni

Cobaltisaweaklyreducingmetalthatisprotectedfromoxidationbyapassivatingoxidefilm.Itisattackedbyhalogensandsulfur.HeatinginoxygenproducesCo3O4whichlosesoxygenat900C

(1,650F)togivethemonoxideCoO.[9]Themetalreactswithfluorine(F2)at520KtogiveCoF3;withchlorine(Cl2),bromine(Br2)andiodine(I2),thecorrespondingbinaryhalidesareformed.Itdoesnotreactwithhydrogengas(H2)ornitrogengas(N2)

evenwhenheated,butitdoesreactwithboron,carbon,phosphorus,arsenicandsulfur.[10]Atordinarytemperatures,itreactsslowlywithmineralacids,andveryslowlywithmoist,butnotwithdry,air.

Compounds

Seealso:Category:Cobaltcompounds.

Commonoxidationstatesofcobaltinclude+2and+3,althoughcompoundswithoxidationstatesrangingfrom3to+4arealsoknown.Acommonoxidationstateforsimplecompoundsis+2(cobalt(II)).Thesesaltsformthepinkcoloredmetalaquocomplex[Co(H2O)6]2+inwater.Additionof

chloridegivestheintenselyblue[CoCl4]2

.[2]

Oxygenandchalcogencompounds

Severaloxidesofcobaltareknown.Greencobalt(II)oxide(CoO)hasrocksaltstructure.Itisreadilyoxidizedwithwaterandoxygentobrowncobalt(III)hydroxide(Co(OH)3).Attemperaturesof600

700C,CoOoxidizestothebluecobalt(II,III)oxide(Co3O4),whichhasaspinelstructure.[2]Black

cobalt(III)oxide(Co2O3)isalsoknown.[11]Cobaltoxidesareantiferromagneticatlowtemperature:CoO(Neltemperature291K)andCo3O4(Neltemperature:40K),whichisanalogoustomagnetite

(Fe3O4),withamixtureof+2and+3oxidationstates.[12]

Theprincipalchalcogenidesofcobaltincludetheblackcobalt(II)sulfides,CoS2,whichadoptsapyritelikestructure,andcobalt(III)sulfide(Co2S3).

Halides

Fourdihalidesofcobalt(II)areknown:cobalt(II)fluoride(CoF2,pink),cobalt(II)chloride(CoCl2,blue),cobalt(II)bromide(CoBr2,green),cobalt(II)iodide(CoI2,blueblack).Thesehalidesexistinanhydrous

andhydratedforms.Whereastheanhydrousdichlorideisblue,thehydrateisred.[13]

Thereductionpotentialforthereaction

Co3+

+eCo2+



Cobalt(II)chloridehexahydrate

is+1.92V,beyondthatforchlorinetochloride,+1.36V.Asaconsequencecobalt(III)andchloridewouldresultinthecobalt(III)beingreducedtocobalt(II).Becausethereductionpotentialforfluorinetofluorideissohigh,+2.87V,cobalt(III)fluorideisoneofthefewsimplestablecobalt(III)compounds.Cobalt(III)fluoride,whichisusedinsomefluorinationreactions,reactsvigorouslywithwater.[9]

Coordinationcompounds

Asforallmetals,molecularcompoundsandpolyatomicionsofcobaltareclassifiedascoordinationcomplexes,thatismoleculesorionsthatcontaincobaltlinkedtoseveralligands.Theprinciplesofelectronegativityandhardnesssoftnessofaseriesofligandscanbeusedtoexplaintheusualoxidationstateofthecobalt.Forexample,Co+3complexestendtohaveammineligands.Asphosphorusissofterthannitrogen,phosphineligandstendtofeaturethesofterCo2+andCo+,anexamplebeingtris(triphenylphosphine)cobalt(I)chloride((P(C6H5)3)3CoCl).Themoreelectronegative

(andharder)oxideandfluoridecanstabilizeCo4+andCo5+derivatives,e.g.caesiumhexafluorocobaltate(Cs2CoF6)andpotassiumpercobaltate(K3CoO4).[9]

AlfredWerner,aNobelprizewinningpioneerincoordinationchemistry,workedwithcompoundsofempiricalformula[Co(NH3)6]Cl3.Oneoftheisomersdeterminedwascobalt(III)hexamminechloride.Thiscoordinationcomplex,a"typical"Wernertypecomplex,consistsofacentralcobaltatomcoordinatedbysixammineligandsorthogonaltoeachotherandthreechloridecounteranions.Usingchelatingethylenediamineligandsinplaceofammoniagivestris(ethylenediamine)cobalt(III)chloride([Co(en)3]Cl3),whichwasoneofthefirstcoordinationcomplexesthatwasresolvedintoopticalisomers.Thecomplexexistsasbotheitherrightorlefthandedformsofa"threebladedpropeller".ThiscomplexwasfirstisolatedbyWernerasyellowgoldneedlelikecrystals.[14][15]

Organometalliccompounds

Mainarticle:Organocobaltchemistry

Cobaltoceneisastructuralanalogtoferrocene,wherecobaltsubstitutesforiron.Cobaltoceneissensitivetooxidation,muchmorethanferrocene.[16]Cobaltcarbonyl(Co2(CO)8)isacatalystin

carbonylationandhydrosilylationreactions.[17]VitaminB12(seebelow)isanorganometalliccompound

foundinnatureandistheonlyvitamintocontainametalatom.[18]

Isotopes

Mainarticle:Isotopesofcobalt



EarlyChineseblueandwhiteporcelain,manufacturedcirca1335

59CoistheonlystablecobaltisotopeandtheonlyisotopetoexistnaturallyonEarth.22radioisotopeshavebeencharacterizedwiththemoststablebeing60Cowithahalflifeof5.2714years,57Cowithahalflifeof271.8days,56Cowithahalflifeof77.27days,and58Cowithahalflifeof70.86days.Alloftheremainingradioactiveisotopeshavehalflivesthatareshorterthan18hours,andthemajorityoftheseareshorterthan1second.Thiselementalsohas4metastates,allofwhichhavehalflivesshorterthan15minutes.[19]

Theisotopesofcobaltrangeinatomicweightfrom50u(50Co)to73u(73Co).Theprimarydecaymodeforisotopeswithatomicmassunitvalueslessthanthatofthemostabundantstableisotope,59Co,iselectroncaptureandtheprimarymodeofdecayforthoseofgreaterthan59atomicmassunitsisbetadecay.Theprimarydecayproductsbefore59Coareelement26(iron)isotopesandtheprimaryproductsafterareelement28(nickel)isotopes.[19]

History

Cobaltcompoundshavebeenusedforcenturiestoimpartarichbluecolortoglass,glazesandceramics.CobalthasbeendetectedinEgyptiansculptureandPersianjewelryfromthethirdmillenniumBC,intheruinsofPompeii(destroyedin79AD),andinChinadatingfromtheTangdynasty(618907AD)andtheMingdynasty(13681644AD).[20]

CobalthasbeenusedtocolorglasssincetheBronzeAge.TheexcavationoftheUluburunshipwreckyieldedaningotofblueglass,whichwascastduringthe14thcenturyBC.[21][22]BlueglassitemsfromEgyptarecoloredwithcopper,iron,orcobalt.TheoldestcobaltcoloredglasswasfromthetimeoftheEighteenthdynastyinEgypt(15501292BC).Thelocationwherethecobaltcompoundswereobtainedisunknown.[23][24]

ThewordcobaltisderivedfromtheGermankobalt,fromkoboldmeaning"goblin",asuperstitioustermusedfortheoreofcobaltby

miners.Thefirstattemptsatsmeltingtheseorestoproducemetalssuchascopperornickelfailed,yieldingsimplypowder(cobalt(II)oxide)instead.Also,becausetheprimaryoresofcobaltalwayscontainarsenic,smeltingtheoreoxidizedthearseniccontentintothehighlytoxicandvolatilearsenicoxide,whichalsodecreasedthereputationoftheorefortheminers.[25]

SwedishchemistGeorgBrandt(16941768)iscreditedwithdiscoveringcobaltcirca1735,showingittobeanewpreviouslyunknownelementdifferentfrombismuthandothertraditionalmetals,andcallingitanew"semimetal."[26][27]Hewasabletoshowthatcompoundsofcobaltmetalwerethesourceofthebluecoloringlass,whichpreviouslyhadbeenattributedtothebismuthfoundwithcobalt.Cobaltbecamethefirstmetaltobediscoveredsincetheprehistoricalperiod,duringwhichalltheknownmetals(iron,copper,silver,gold,zinc,mercury,tin,leadandbismuth)hadnorecordeddiscoverers.[28]

Duringthe19thcentury,asignificantpartoftheworld'sproductionofcobaltblue(adyemadewithcobaltcompoundsandalumina)andsmalt(cobaltglasspowderedforuseforpigmentpurposesinceramicsandpainting)wascarriedoutattheNorwegianBlaafarvevrket.[29][30]Thefirstminesfortheproductionofsmaltinthe16thto18thcenturywerelocatedinNorway,Sweden,SaxonyandHungary.



WiththediscoveryofcobaltoreinNewCaledoniain1864theminingofcobaltinEuropedeclined.WiththediscoveryoforedepositsinOntario,Canadain1904andthediscoveryofevenlargerdepositsintheKatangaProvinceintheCongoin1914theminingoperationsshiftedagain.[25]WiththeShabaconflictstartingin1978,themainsourceforcobalt,thecopperminesofKatangaProvince,nearlystoppedtheirproduction.[31][32]Theimpactontheworldcobalteconomyfromthisconflictwashoweversmallerthanexpected.Cobaltbeingararemetalandthepigmentbeinghighlytoxic,theindustryhadalreadyestablishedeffectivewaysforrecyclingcobaltmaterialsandinsomecaseswasabletochangetocobaltfreealternatives.[31][32]

In1938,JohnLivingoodandGlennT.Seaborgdiscoveredtheradioisotopecobalt60.[33]ThisisotopewasfamouslyusedatColumbiaUniversityinthe1950stoestablishparityviolationinradioactivebetadecay.[34][35]

AfterWorldWarII,theUSwantedtobesureitwasnevershortoftheoreneededformilitarycobaltuses(astheGermanshadbeenduringthatwar)andexploredforcobaltwithintheU.S.border.AgoodsupplyoftheoreneededwasfoundinIdahonearBlackbirdcanyoninthesideofamountain.ThefirmCaleraMiningCompanygotproductionstartedatthesite.[36]

Occurrence

Thestableformofcobaltiscreatedinsupernovasviatherprocess.[37]Itcomprises0.0029%oftheEarth'scrustandisoneofthefirsttransitionmetals.

Freecobalt(thenativemetal)isnotfoundinonEarthduetotheamountofoxygenintheatmosphereandchlorineintheocean.OxygenandchlorineareabundantenoughintheupperlayersoftheEarth'scrustsoastomakenativemetalcobaltformationextremelyrare.Exceptasrecentlydeliveredinmeteoriciron,purecobaltinnativemetalformisunknownonEarth(seebelow).Thoughtheelementisofmediumabundance,naturalcompoundsofcobaltarenumerous.Smallamountsofcobaltcompoundsarefoundinmostrocks,soil,plants,andanimals.

Innature,cobaltisfrequentlyassociatedwithnickel,andbotharecharacteristiccomponentsofmeteoriciron,thoughcobaltismuchlessabundantinironmeteoritesthannickel.Aswithnickel,cobaltinmeteoricironalloysmayhavebeenwellenoughprotectedfromoxygenandmoisturetooccurasthefreemetal,[38]astatewhichotherwiseisnotseenwitheitherelementintheancientterrestrialcrust.

Cobaltincompoundformoccursasaminorcomponentofcopperandnickelminerals.Itisthemajormetalliccomponentincombinationwithsulfurandarsenicinthesulfidiccobaltite(CoAsS),safflorite(CoAs2),glaucodot((Co,Fe)AsS),andskutterudite(CoAs3)minerals.[9]Themineralcattieriteissimilarto

pyriteandoccurstogetherwithvaesiteinthecopperdepositsoftheKatangaProvince.[39]Uponcontactwiththeatmosphere,weatheringoccursandthesulfidemineralsoxidizetoformpinkerythrite("cobaltglance":Co3(AsO4)28H2O)andspherocobaltite(CoCO3).[40][41]

Production

Seealso:Cobaltextractiontechniques



Cobaltore

Cobaltoutputin2005

Worldproductiontrend

Themainoresofcobaltarecobaltite,erythrite,glaucodotandskutterudite(seeabove),butmostcobaltisobtainednotbyactiveminingofcobaltores,butratherbyreducingcobaltcompoundsthat

occurasbyproductsofnickelandcopperminingactivities.[42][43]

In2005,thecopperdepositsintheKatangaProvince(formerShabaprovince)oftheDemocraticRepublicoftheCongowerethetopproducerofcobaltwithalmost40%worldshare,reportstheBritishGeologicalSurvey.[44]ThepoliticalsituationintheCongoinfluencesthepriceofcobaltsignificantly.[45]

TheMukondoMountainproject,operatedbytheCentralAfricanMiningandExplorationCompanyinKatanga,maybetherichestcobaltreserveintheworld.Itisestimatedtobeabletoproduceaboutonethirdoftotalglobalproductionofcobaltin2008.[46]InJuly2009CAMECannouncedalongtermagreementunderwhichCAMECwoulddeliveritsentireannualproductionofcobaltinconcentratefromMukondoMountaintoZhejiangGalicoCobalt&NickelMaterialsofChina.[47]

Severalmethodsexistfortheseparationofcobaltfromcopperandnickel.Theydependontheconcentrationofcobaltandtheexactcompositionoftheusedore.Oneseparationstepinvolvesfrothflotation,inwhichsurfactantsbindtodifferentorecomponents,leadingtoanenrichmentofcobaltores.Subsequentroastingconvertstheorestothecobaltsulfate,whereasthecopperandtheironareoxidizedtotheoxide.Theleachingwithwaterextractsthesulfatetogetherwiththearsenates.Theresiduesarefurtherleachedwithsulfuricacidyieldingasolutionofcoppersulfate.Cobaltcanalsobeleachedfromtheslagofthecoppersmelter.[48]

Theproductsoftheabovementionedprocessesaretransformedintothecobaltoxide(Co3O4).Thisoxideisreducedtothemetalbythealuminothermicreactionorreductionwithcarboninablastfurnace.[9]

Applications

Themainapplicationofcobaltisasthefreemetal,inproductionofcertainhighperformancealloys.[42][43]

Alloys

Cobaltbasedsuperalloysconsumemostoftheproducedcobalt.[42][43]Thetemperaturestabilityofthesealloysmakesthemsuitableforuseinturbinebladesforgasturbinesandjetaircraftengines,thoughnickelbasedsinglecrystalalloyssurpasstheminthisregard.[49]Cobaltbasedalloysarealso



corrosionandwearresistant.Thismakesthemusefulinthemedicalfield,wherecobaltisoftenused(alongwithtitanium)fororthopedicimplantsthatdonotweardownovertime.Thedevelopmentofthewearresistantcobaltalloysstartedinthefirstdecadeofthe19thcenturywiththestellitealloys,whicharecobaltchromiumalloyswithvaryingtungstenandcarboncontent.Theformationofchromiumandtungstencarbidesmakesthemveryhardandwearresistant.[50]SpecialcobaltchromiummolybdenumalloyslikeVitalliumareusedforprostheticpartssuchashipandkneereplacements.[51]Cobaltalloysarealsousedfordentalprosthetics,wheretheyareusefultoavoidallergiestonickel.[52]Somehighspeedsteeldrillbitsalsousecobalttoincreaseheatandwearresistance.Thespecialalloysofaluminium,nickel,cobaltandiron,knownasAlnico,andofsamariumandcobalt(samariumcobaltmagnet)areusedinpermanentmagnets.[53]Itisalsoalloyedwith95%platinumforjewelrypurposes,yieldinganalloythatissuitableforfinedetailedcastingandisalsoslightlymagnetic.[54]

Batteries

Lithiumcobaltoxide(LiCoO2)iswidelyusedinlithiumionbatterycathodes.Thematerialiscomposedofcobaltoxidelayersinwhichthelithiumisintercalated.Duringdischargingthelithiumintercalatedbetweenthelayersissetfreeaslithiumion.[55]Nickelcadmium[56](NiCd)andnickelmetalhydride[57](NiMH)batteriesalsocontainsignificantamountsofcobalt;thecobaltimprovestheoxidationcapabilitiesofnickelinthebattery.[56]

Catalysts

Severalcobaltcompoundsareusedinchemicalreactionsasoxidationcatalysts.Cobaltacetateisusedfortheconversionofxylenetoterephthalicacid,theprecursortothebulkpolymerpolyethyleneterephthalate.Typicalcatalystsarethecobaltcarboxylates(knownascobaltsoaps).Theyarealsousedinpaints,varnishes,andinksas"dryingagents"throughtheoxidationofdryingoils.[55]Thesamecarboxylatesareusedtoimprovetheadhesionofthesteeltorubberinsteelbeltedradialtires.

Cobaltbasedcatalystsarealsoimportantinreactionsinvolvingcarbonmonoxide.Steamreforming,usefulinhydrogenproduction,usescobaltoxidebasecatalysts.CobaltisalsoacatalystintheFischerTropschprocess,usedinthehydrogenationofcarbonmonoxideintoliquidfuels.[58]Thehydroformylationofalkenesoftenrelyoncobaltoctacarbonylasthecatalyst,[59]althoughsuchprocesseshavebeenpartiallydisplacedbymoreefficientiridiumandrhodiumbasedcatalysts,e.g.theCativaprocess.

Thehydrodesulfurizationofpetroleumusesacatalystderivedfromcobaltandmolybdenum.Thisprocesshelpstoridpetroleumofsulfurimpuritiesthatinterferewiththerefiningofliquidfuels.[55]

Pigmentsandcoloring

Beforethe19thcentury,thepredominantuseofcobaltwasasapigment.SincetheMiddleAges,ithasbeeninvolvedintheproductionofsmalt,abluecoloredglass.Smaltisproducedbymeltingamixtureoftheroastedmineralsmaltite,quartzandpotassiumcarbonate,yieldingadarkbluesilicateglasswhichisgroundaftertheproduction.[60]Smaltwaswidelyusedforthecolorationofglassandaspigmentforpaintings.[61]In1780,SvenRinmandiscoveredcobaltgreenandin1802LouisJacquesThnarddiscoveredcobaltblue.[62]Cobaltpigments,suchascobaltblue(cobaltaluminate),cerulean



Cobaltblueglass

Cobaltcoloredglass

blue(cobalt(II)stannate),varioushuesofcobaltgreen(amixtureofcobalt(II)oxideandzincoxide),andcobaltviolet(cobaltphosphate)areusedasartist'spigmentsbecauseoftheirsuperiorstability.[63][64]Aureolin(cobaltyellow)isnowlargelyreplacedbymorelightfastyellowpigments.

Radioisotopes

Cobalt60(Co60or60Co)isusefulasagammaraysourcebecauseitcanbeproducedinpredictablequantityandhighactivitybybombardingcobaltwithneutrons.Itproducestwogammarayswithenergiesof1.17and1.33MeV.[19][65]

Itsusesincludeexternalbeamradiotherapy,sterilizationofmedicalsuppliesandmedicalwaste,radiationtreatmentoffoodsforsterilization(coldpasteurization),[66]industrialradiography(e.g.weldintegrity

radiographs),densitymeasurements(e.g.concretedensitymeasurements),andtankfillheightswitches.Themetalhastheunfortunatehabitofproducingafinedust,causingproblemswithradiationprotection.Cobaltfromradiotherapymachineshasbeenaserioushazardwhennotdisposedofproperly,andoneoftheworstradiationcontaminationaccidentsinNorthAmericaoccurredin1984,afteradiscardedradiotherapyunitcontainingcobalt60wasmistakenlydisassembledinajunkyardinJuarez,Mexico.[67][68]

Cobalt60hasaradioactivehalflifeof5.27years.Thisdecreaseinactivityrequiresperiodicreplacementofthesourcesusedinradiotherapyandisonereasonwhycobaltmachineshavebeenlargelyreplacedbylinearacceleratorsinmodernradiationtherapy.[69]

Cobalt57(Co57or57Co)isacobaltradioisotopemostoftenusedinmedicaltests,asaradiolabelforvitaminB12uptake,andfortheSchillingtest.Cobalt57isusedasasourceinMssbauerspectroscopy

andisoneofseveralpossiblesourcesinXrayfluorescencedevices.[70][71]

Nuclearweapondesignscouldintentionallyincorporate59Co,someofwhichwouldbeactivatedinanuclearexplosiontoproduce60Co.The60Co,dispersedasnuclearfallout,createswhatissometimescalledacobaltbomb.[72]

Otheruses

Otherusesofcobaltareinelectroplating,owingtoitsattractiveappearance,hardnessandresistancetooxidation,[73]andasgroundcoatsforporcelainenamels.[74]

Biologicalrole



Cobalamin

Cobaltdeficientsheep

Cobaltisessentialtoallanimals.Itisakeyconstituentofcobalamin,alsoknownasvitaminB12,which

istheprimarybiologicalreservoirofcobaltasan"ultratrace"element.[75][76]BacteriainthegutsofruminantanimalsconvertcobaltsaltsintovitaminB12,acompoundwhichcanonlybeproducedbybacteriaorarchaea.Theminimumpresenceofcobaltinsoilsthereforemarkedlyimprovesthehealthofgrazinganimals,andanuptakeof0.20mg/kgadayisrecommendedforthem,astheycanobtainvitaminB12innootherway.[77]

Intheearly20thcenturyduringthedevelopmentforfarmingoftheNorthIslandVolcanicPlateauofNewZealand,cattlesufferedfromwhatwastermed"bushsickness".Itwasdiscoveredthatthevolcanicsoilslackedcobaltsalts,whichwasnecessaryforcattle.[78]TheailmentwascuredbyaddingsmallamountsofcobalttofertilizersintheformofSuperphosphate(atthetimederivedfromCanadiansources).

Inthe1930s"coastdisease"ofsheepintheNinetyMileDesertoftheSoutheastofSouthAustraliawasfoundtobeduetonutrientdeficienciesofthetraceelementscobaltandcopper.Thecobaltdeficiencywasovercomebythedevelopmentof"cobaltbullets",densepelletsofcobaltoxidemixedwithclay,whichareorallyinsertedtolodgeintheanimal'srumen.[79]

NonruminantherbivoresproducevitaminB12frombacteriaintheircolonswhichagainmakethevitaminfromsimplecobaltsalts.Howeverthevitamincannotbeabsorbedfromthecolon,andthusnonruminantsmustingestfecestoobtainthenutrient.AnimalsthatdonotfollowthesemethodsofgettingvitaminB12fromtheirowngastrointestinalbacteriaorthatofotheranimals,mustobtainthevitaminpremadeinotheranimalproductsintheirdiet,andtheycannotbenefitfromingestingsimplecobaltsalts.

Thecobalaminbasedproteinsusecorrintoholdthecobalt.CoenzymeB12featuresareactiveCCobond,whichparticipates

initsreactions.[80]Inhumans,B12existswithtwotypesofalkylligand:methylandadenosyl.MeB12promotesmethyl(CH3)grouptransfers.TheadenosylversionofB12catalyzesrearrangementsinwhichahydrogenatomisdirectlytransferredbetweentwoadjacentatomswithconcomitantexchangeofthesecondsubstituent,X,whichmaybeacarbonatomwithsubstituents,anoxygenatomofanalcohol,oranamine.MethylmalonylcoenzymeAmutase(MUT)convertsMMlCoAtoSuCoA,animportantstepintheextractionofenergyfromproteinsandfats.[81]

Althoughfarlesscommonthanothermetalloproteins(e.g.thoseofzincandiron),cobaltoproteinsareknownasidefromB12.Theseproteinsincludemethionineaminopeptidase2anenzymethatoccursinhumansandothermammalswhichdoesnotusethecorrinringofB12,butbindscobaltdirectly.Anothernoncorrincobaltenzymeisnitrilehydratase,anenzymeinbacteriathatareabletometabolizenitriles.[82]



Patchtest

Precautions

Mainarticle:Cobaltpoisoning

Cobaltisanessentialelementforlifeinminuteamounts.TheLD50valueforsolublecobaltsaltshasbeenestimatedtobebetween150and500mg/kg.Thus,fora100kgpersontheLD50forasingledosewouldbeabout20grams.[83]IntheUS,theOccupationalSafetyandHealthAdministration(OSHA)hasdesignatedapermissibleexposurelimit(PEL)intheworkplaceasatimeweightedaverage(TWA)of0.1mg/m3.TheNationalInstituteforOccupationalSafetyandHealth(NIOSH)hassetarecommendedexposurelimit(REL)of0.05mg/m3,timeweightedaverage.TheIDLH(immediatelydangeroustolifeandhealth)valueis20mg/m3.[84]

However,chroniccobaltingestionhascausedserioushealthproblemsatdosesfarlessthanthelethaldose.In1966,theadditionofcobaltcompoundstostabilizebeerfoaminCanadaledtoapeculiarformoftoxininducedcardiomyopathy,whichcametobeknownasbeerdrinker'scardiomyopathy.[85][86]

Afternickelandchromium,cobaltisamajorcauseofcontactdermatitis.[87]

Cobaltcanbeeffectivelyabsorbedbycharredpigsbones;howeverthisprocessisinhibitedbycopperandzinc;whichhavegreateraffinitiestobonechar.[88]

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Externallinks

NationalPollutantInventory(Australia)Cobaltfactsheet(http://www.npi.gov.au/database/substanceinfo/profiles/26.html)Londoncelebrates50yearsofCobalt60Radiotherapy(http://www.caro

acro.ca/caro/educ/publ/vig/vignettes/cobalt/Interactions.pdf)Cobalt(http://www.periodicvideos.com/videos/027.htm)atThePeriodicTableofVideos(UniversityofNottingham)CentersforDiseaseandPreventionCobalt(http://www.cdc.gov/niosh/topics/cobalt/)TheCobaltDevelopmentInstitute(http://www.thecdi.com)

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