experiment c6- cations experiment c6: qualitative analysis of cations learning outcomes upon...

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EXPERIMENTC6:QUALITATIVEANALYSISOFCATIONS

LearningOutcomesUponcompletionofthislab,thestudentwillbeableto:

1) Analyzeagivensampleofanioniccompoundandidentifywhichofthefollowingcationsarepresent:silver,lead(II),bismuth(III),iron(III),manganese(II),aluminum,chromium(III),barium,strontium,calcium,nickel(II),copper(II),magnesium,zinc

IntroductionThecationsbeingtestedinthequalitativeanalysisschemeareorganizedintogroupslabeledAthroughD(Table1)

Group CationsA Ag+,Pb2+B Bi3+,Fe3+,Mn2+,Al3+,Cr3+C Ba2+,Sr2+,Ca2+D Ni2+,Cu2+,Mg2+,Zn2+

TABLE1

Thisgroupingofionswasarrivedatbasedonthedifferencesinthesolubilityoftheseions.GROUPA:Ag+,Pb2+TheKSPofthechloridesaltsofthetwoionsinthisgroupisgiveninTable2.

Salt KSPAgCl 1.7×10-10PbCl2 1.6×10-5

TABLE2

Thechloridesaltsofalltheothercationsthatwillbeanalyzedinthisexperimentarehighlysoluble.Therefore,additionofaqueousHClwillresultinaprecipitatecontainingchloridesoftheGroupAcations.

Ag+(aq)+Cl−(aq)!AgCl(s)

Pb2+(aq)+2Cl−(aq)!PbCl2(s)

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Thechloridesofallothercationswillremaininsolution.Whenthismixtureiscentrifuged:theprecipitatewillcontainthegroupAcationsandthesupernatantwillcontainthecationsfromtheothergroups.

Ofthetwochlorideprecipitates,PbCl2issolubleinhotwaterandAgClissolubleinaqueousammonia.TestforPb2+WhenthechlorideprecipitatesoftheGroupAcationsaremixedwithhotwater,PbCl2willdissolve.ThisisseparatedfromAgClbycentrifugingthemixture.ThePbCl2inthesupernatantcanbeconfirmedbyreactingthesupernatantwithasolutionofKI,whichresultsinayellowprecipitateofPbI2.

PbCl2(s)+H2O(l)+heat!Pb2+(aq)+2Cl−(aq)

Pb2+(aq)+2KI(aq)!PbI2(s)+2K+(aq)

TestforAg+TheAgClprecipitatedissolvesinaqueousammoniaduetotheformationofacomplexion.

AgCl(s)+2NH3(aq)!Ag(NH3)2(aq)+Cl−(aq)

AdditionofHNO3resultsinthedecompositionofthesilverammoniumcomplex.ThefreesilverionscombinewiththechlorideionsfromtheabovereactiontoformtheAgClprecipitateagain.

GroupsA,B,C,D

Add6MHCl

Precipitate:GroupA-AgCl,PbCl2

Supernatant:GroupsB,C,D

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GROUPB:(Bi3+,Fe3+,Mn2+,Al3+,Cr3+)Thehydroxidesofthecationsinthisgrouparealmostcompletelyinsoluble.TheKSPofthehydroxidesaltsaregiveninTable3.

Salt KSPMn(OH)2 4.5×10-14Fe(OH)3 1.0×10-36Bi(OH)3 InsolubleAl(OH)3 3.7×10-15Cr(OH)3 6.7×10-31

TABLE3

ThehydroxidesaltsoftheGroupCcationsarefarmoresoluble.Although,thehydroxidesaltsoftheGroupDcationsarealsoquiteinsoluble,thedistinctionbetweenthehydroxidesaltsofGroupBcationsvs.GroupDcationsisthat,thehydroxidesaltsoftheGroupDcationsaresolubleinaqueousammonia,whereasthoseoftheGroupBcationsarenot.RecallthattheoriginalmixtureofcationswastreatedwithHCl.ThiswouldnotonlylowerthepHbutalsoresultintheprecipitationoftheGroupAcationsaschlorides.AfterseparatingtheGroupAcationsbycentrifugation,thesupernatantsolutioncontainingalltherestofthecationsshouldbetreatedwithaqueousNH3.ThiswouldraisethepH(i.e.increasethe[OH−])andprecipitatetheGroupBcations.ThesupernatantcontainingthecationsfromGroupsCandDcanbeobtainedbycentrifugation.Ofthefivecationsinthisgroup,thehydroxidesofAl3+andCr3+arebothsolubleinanalkali.So,thisgroupofcationsissubdividedintotwocatagories:B1,whichcontainsMn2+,Fe3+,andBi3+andB2,whichcontainsAl3+andCr3+.

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*NOTE:TheMn2+isoxidizedtoMn4+bytheH2O2TheB1cationsareallsolubleinHCl.TheprecipitatecontainingtheB1cationsisdissolvedinhotHClsolution.ThissolutionisthentestedforeachofthethreeB1cations.TestforMn2+InanacidicenvironmenttheMn4+isreducedtoMn2+usingH2O2.TheMn2+isconvertedtoMnO4−usingbismuthate,BiO3−.Theformationofapurplecoloredsolution(fromtheMnO4−)confirmsthepresenceofMn2+.Thebismuthateisastrongoxidizingagentandcausesavigorousreactionandmustthereforebeaddedinsmallportions.

Mn4+(aq)+H2O2(l)

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⇔Mn2+(aq)+O2(aq)+H+(aq)

Mn2+(aq)+H+(aq)+BiO3−(aq)

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⇔Bi3+(aq)+MnO4−(aq)

TestforBi3+Bi3+hasahigherreductionpotential(E°RED=0.286V)thanSn2+(E°RED=-0.136V).ThereforeSn2+reducesBi3+.Thereactionhappensunderbasicconditions.Theformationofablackprecipitate,whichistheelementBi,confirmsthepresenceofBi3+.

Bi3+(aq)+Sn2+(aq)+7OH−(aq)!Bi(s)+Sn(OH)72-(aq)

GroupsB,C,D

AddaqueousNH3

Precipitate:GroupB-Mn(OH)2,Fe(OH)3,Bi(OH)3,Al(OH)3,Cr(OH)3

Add6MNaOH+3%H2O2

Precipitate:Mn4+*,Fe3+,Bi3+ Supernatant:Al(OH)4-,CrO42-

Supernatant:GroupsC,D

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TestforFe3+Fe3+formsacomplexwiththiocyanate,SCN−.AdditionofpotassiumthiocyanatetoFe3+producesareddish-browncolorduetotheformationofthiscomplex.Theformationofthereddish-browncolorconfirmsthepresenceofFe3+.

Fe3+(aq)+KSCN(aq)

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⇔[FeSCN]2+(aq)+K+(aq)B2cationsTreatmentwith6MNaOHand3%H2O2convertstheB2cationstoAl(OH)4−andCrO42-.Underacidicconditionsthefollowingreactionsoccur.

Al(OH)4−(aq)+4H+(aq)!Al3+(aq)+4H2O(l)

2CrO42-(aq)+2H+(aq)!Cr2O72-(aq)+H2O(l)TestforAl3+ThesolutioncontainingtheAl3+istreatedwithaqueousNH3.Thisresultsintheformationofawhitealuminumhydroxideprecipitate.Thedyealuminonwhenaddedtotheprecipitate,changesthecoloroftheprecipitate.TheappearanceofaredcolorconfirmsthepresenceofAl3+.

Al3+(aq)+3NH3(aq)+3H2O(l)!Al(OH)3(s)+3NH4+(aq)

Al(OH)3(s)+aluminondye!redprecipitate

TestforCr3+AnyCr3+presentinthetestsolutionisfoundasCr2O72-atthispoint.InanacidicmediumthedichromateisoxdizedbyH2O2toperoxychromate,CrO5.TheappearanceofadarkbluecoloredCrO5confirmsthepresenceofCr3+.However,theCrO5isunstableanddecomposesquickly.Thiscausesthecolortodisappear.

Cr2O72-(aq)+4H2O2(aq)+2H+(aq)

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⇔2CrO5(aq)+5H2O(l)GROUPC:(Ba2+,Sr2+,Ca2+)ThesimilaritybetweencationsofGroupsCandDisthattheybothforminsolublesaltswithoxalateanion.However,theGroupDoxalatesaltsaresolubleinaqueousammoniawhereastheGroupCoxalatesaltsareinsolubleinaqueousammonia.NotethatafterseparatingtheGroupBcations,thesolutionisalreadyinamediumcontainingaqueousammoniaandatahighpH.Therefore,whenammoniumoxalate

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isaddedtothismixturecontainingdissolvedionsofGroupsCandD,theGroupCoxalateionsprecipitate.

TheoxalateintheprecipitateisremovedbyheatingtheprecipitatewithHNO3inacrucible.TheoxalateisconvertedtoCO2(g).TheprecipitateisthendissolvedinHCl.Thethreecationsinthisgroupwillbeanalyzedbyperformingtheflametestwiththissolution.IftheunknownmixtureisknowntocontainmorethanoneofBa2+,Sr2+,orCa2+,thentheymustbeseparatedfirstpriortoconductingtheflametest.TestforBa2+,Sr2+,Ca2+AflametestisperformedtoanalyzethepresenceofBa2+,Sr2+,orCa2+.

• Ayellow-greenlightconfirmsthepresenceofBa2+.

• AcrimsoncolorconfirmsthepresenceofSr2+.

• Abrick-redcolorconfirmsthepresenceofCa2+GROUPD:(Ni2+,Cu2+,Zn2+,Mg2+)ThesupernatantsolutionfollowingtheremovaloftheGroupCcations,containstheoxalatesaltsoftheGroupDcationsthathavebeendissolvedinaqueousammonia.Theoxalateaswellastheammoniawillfirstberemovedbyheatingthesampleto

GroupsBC,D

AddaqueousNH3

Precipitate:GroupB-Mn(OH)2,

Fe(OH)3,Bi(OH)3,Al(OH)3,Cr(OH)3

Add6MNaOH+3%

H2O2

Precipitate:Mn4+*,Fe3+,Bi3+

Supernatant:Al(OH)4-,CrO42-

Supernatant:GroupsC,D

Add(NH4)2C2O4

Precipitate:GroupC-BaC2O4,SrC2O4,CaC2O4

Supernatant:GroupD

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dryness.OxalatesareremovesasCO2gasandammoniumionsareremovedasN2Ogas.TheresidueatthisstageisdissolvedinHClwhichresultsinasolutionthatmaybebluishgreenincolorandwillbeusedforanalysisoftheGroupDcations.TestforNi2+Underbasicconditions,Ni2+formsacherryrdcolorprecipitatewithdimethylglyoxime(DMG).TheformationoftheredcolorprecipitateconfirmsthepresenceofNi2+.

[Ni(H2O)6]2+(aq)+6NH3(aq)![Ni(NH3)6]2+(aq)+6H2O(l)

[Ni(NH3)6]2+(aq)+2DMG!Ni(DMG)2(s)+6NH3(aq)

TestforCu2+Underslightlyacidiccondition,Cu2+formsamaroonprecipitatewithhexacyanoferrate.TheformationofthemaroonprecipitateconfirmsthepresenceofCu2+.

2[Cu(NH3)4]2+(aq)+4H+(aq)+[Fe(CN)6]4-(aq)!Cu2[Fe(CN)6](s)+4NH4+(aq)

RemovalofNi2+andCu2+SulfidesofNi2+andCu2+areblackinsolublesolids.TheseprecipitatesarecentrifugedandremovedpriortotheanalysisforMg2+andZn2+TestforMg2+ThetestsolutioncontainingMg2+and/orZn2+istreatedwithaqueousNH3andNa2HPO4.Zn2+,ifpresentprecipitatesasZn3(PO4)2andMg2+,ifpresentprecipitatesasMgNH4PO4.

Mg2+(aq)+NH4+(aq)+HPO42-(aq)!MgNH4PO4(s)

3Zn2+(aq)+2NH3(aq)+2HPO42-(aq)!Zn3(PO4)2(s)+2NH4+(aq)

TheprecipitatesaredissolvedinNaOH;onlytheZn3(PO4)2dissolves.PresenceofawhiteprecipitateafteradditionofNaOHconfirmsthepresenceofMg2+.

Zn3(PO4)2(s)+12OH−(aq)!3Zn(OH)42-(aq)+2PO43-(aq)

TestforZn2+

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ThesupernatantsolutionattheendofthelasttestisanalyzedforthepresenceofZn2+.Potassiumhexacyanoferrateisaddedtoaslightlyacidictestsolution.FormationofawhiteprecipitateconfirmsthepresenceofZn2+.

2Zn(OH)42-(aq)+8H+(aq)+K4[Fe(CN)6](aq)!Zn2Fe(CN)6(s)+4K+(aq)+8H2O(l)

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ExperimentalDesignAnunknownsolutioncontainingSIXcationsbelongingtoGroupsA,B,C,andDwillbeprovided.TheSIXcationsmustbeidentifiedusingtheproceduresdescribedbelow.Whiletestingtheunknownsample,itisrecommendedtoperformatestofknownsamplesofionswithineachgroup.Theproceduresdescribedbelowuseasetofstandardprotocolsthatwillbeusedduringvariousstagesoftheanalyses.Thesestandardprotocolsandthepreparationoftheknownsamplesforeachanalysisaredescribedinthissection.1. Centrifugingthesample

Centrifugeswillbeusedtoseparateamixtureintoasolidprecipitateandaliquidsupernatant.Theinstructorwilldemonstratetheproperuseofacentrifuge.Thecentrifugetubesareprovidedinthe“CHEM1CAdditionalKit”providedbythestockroom.Onlythesetubesmustbeusedinsidethecentrifuges.Atubeofapproximatelythesamemassintheoppositeslotofthecentrifugemustbeusedtobalancethecentrifuge.Besurethatcentrifugetubesareneithercrackednorchipped.Thestressappliedbythecentrifugecancausedamagedtubestoshatter,resultinginchemicalsandpiecesofglassbeingscatteredinsidethecentrifuge.Acentrifugewithoutatopisdangerous;alwaysclosethetop.Donotslowcentrifugesdownwithyourhands.Theyarespinningatahighrateofspeed,andifthereisanyimperfectiononthespinningsurface,itcancatchthefleshanddoagreatdealofdamageinonlyaninstant.

2. SeparatingaPrecipitatefromaSupernatant

Aftercentrifuging,asupernatantcanbeseparatedfromaprecipitatebydecanting.Carefullytipthecentrifugetube,andpouroffthesupernatantwithoutdisturbingthepelletofsolid.Thesupernatantmaybepoureddirectly.Alternatively,thesupernatantcanberemovedbycarefullysuctioningitupintoadropper.

3. Washingtheprecipitate

Afterseparationfromthesupernatant,aprecipitateisoftenwashedtofreeitfromreagentsthatmightinterfereatalaterstage.Usually,therinseisdeionizedwater,butotherliquidsorsolutionsmaybeused.Addtheindicatedamountofthewashliquidandstirthecontentsofthetesttubethoroughly.Thepelletof

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solidmustbebrokenupandmixedwellwiththewashliquid.Afterthoroughstirring,centrifugethesampleanddecantthewashsolution.

4. AdjustingandtestingthepH

WhendirectedtocheckthepHofasolution,stirthesolutionthoroughlywithacleanglassstirringrodandthentouchthetipoftherodtoapieceofpHpaper.Severalsuchtestsmaybeperformedoneachstripofpaper.Neverinsertthetestpaperintothetesttube,sincethechemicalsonthepapercouldcontaminatethecontents.

5. Heating

Duetothesmallquantityofmaterialbeingheated,testtubescontainingsamplesshouldNEVERbeheateddirectlyinaflame.Asolutionintesttubecanreachitsboilingpointwithinafewseconds,andmaybeejectedviolentlyfromthetesttube.Allheatingshouldbedoneusingawaterbath.Becarefulthatthetopsofthetesttubesarewellabovethewater.Thewatermaybeboilingattimesandcouldspatterintothetesttubes,contaminatingthecontents.Labelingtapewillfalloffinaboilingwaterbath;therefore,itisbesttolabeltesttubesthataretobeplacedinawaterbathwithasharpiemarker.

6. Performingaflametest

Whenexposedtoaflame,certainelementsemitlightofacharacteristiccolor.Individualknownsamplesolutionscanbeflametesteddirectlyorthecationcanbeprecipitatedandaflametestperformedontheprecipitate.Ineithercase,awireloopisusedtointroducethesampleintotheflame.Theloopoftheflametestwiremustfirstbethoroughlycleanedofanytracecontamination.BeginbylightingaBunsenBurnerandadjustingtheflamesothatitburnshot;thatisitappearsblue,notyellow.InsertthewireloopintothehottestpartofaBunsenBurnerflame;thetipoftheinnerbluecone.Ifthewireiscontaminated,theflamewillexhibitacolorcharacteristicofthecontaminant.RepeattheprocessofdippingthewireloopintoDIwaterandthenintothehotflameuntilnocontaminationisevident.Notethatuponsufficientheatingthetestwireitselfwillturntheflameorange.Aftercleaningthewire,itcanbeusedtotestasolutionoraprecipitate.Totestasolution,useacleandroppertoremoveonedropoftherestsolutionandplacethisdropinthewirelooptheninsertitintotheflame,observingthecolorthatisemitted.Fortheknownsolutions,DONOTinsertthewireloopdirectlyintothereagentbottle.Instead,placeasmallamountofthesolutionintoatesttubeforuse.Eachmetalcationmaynotemitatthesameburnertemperature.Therefore,whenperformingflametestsbestresultsareoftenobtainedbyslowlybringingthewireloopcontainingthesampleintotheflame

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fromtheside.Asthewiremovesintotheflame,itissubjectedtoarangeoftemperatureswithintheflame.Thismethodofbringingtheloopslowlyintotheflameismoreimportantfortheunknownsamples,whereconcentrationsoftheemittingelementstendtobelowerthanintheknownsamples.Toflametestaprecipitate,centrifugeanddecantthesupernatant.WettheprecipitateslightlywithDIwater.Dipacleanwireloopinthemixturecontainingthesample.Ifsolidispresent,attempttogetsomeofittosticktotheloop.Insertthewireloopintotheflame,bringingtheloopslowlyintotheflamefromtheside.

7. Knownsamplepreparation:GroupA

Combine10dropseachof0.1MsolutionsofAg+andPb2+inacentrifugetube.Addthreedropsof6MHClintothetube.Stir/shakethecontentsofthetubeandwaitfortwominutes.Centrifugethetubefortwominutes.DiscardthesupernatantandtesttheprecipitatefortheGroupAcations.

8. Knownsamplepreparation:GroupB

Obtain10dropseachof0.1MsolutionsofBi3+,Fe3+,Mn2+,Al3+,Cr3+inacentrifugetube.Addfourdropsof6MHClfollowedby6MNH3untilthepHofthesolutionisbetween9and10.Centrifugethetubefortwominutes.Discardthesupernatant.Washtheprecipitatetwicewith10dropsofdeionizedwater.Add10dropsof6MNaOHandtwodropsof3%H2O2andplacethemixtureinaboilinghotwaterbathfortwominutes.Centrifugethemixturefortwominutes.AnalyzetheprecipitatefortheB1cationsandthesupernatantfortheB2cations.

9. Knownsamplepreparation:GroupC

Obtain10dropseachof0.1MsolutionsofBa2+,Ca2+,Sr2+inacentrifugetube.Addfourdropsof6MHClfollowedby6MNH3untilthesolutionisneutral.AddanequalamountofNH3inexcess.Centrifugethemixturefortwominutes.AnalyzethesupernatantsolutionfortheGroupCcations.

10. Knownsamplepreparation:GroupD

Obtain10dropseachof0.1MsolutionsofNi2+,Cu2+,Mg2+,Zn2+inacentrifugetube.Addfourdropsof6MHClfollowedby6MNH3untilthesolutionisneutral.AddanequalamountofNH3inexcess.Addfivedropsof0.5Mammoniumoxalate.Stirthemixtureandcentrifugefortwominutes.AnalyzethesupernatantsolutionfortheGroupDcations.

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ReagentsandSupplies0.1MsolutionsofAg+,Pb2,Bi3+,Fe3+,Mn2+,Al3+,Cr3+,Ba2+,Sr2+,Ca2+,Ni2+,Cu2+,Zn2+,Mg2+Acids:6MHCl,6MHNO3,6MCH3COOHBases:6MNH3,6MNaOHOtherreagents:1MKI,3%H2O2,NaBiO3,SnCl2,0.1MKSCN,1MNaHCO3,0.5M(NH4)2C2O4,3M(NH4)2CO3,0.1MK2CrO4,0.1%dimethylglyoxime,0.1MK4Fe(CN)6,Na2S2O3,0.1MNa2HPO4,0.1%aluminon(SeepostedMaterialSafetyDataSheets)

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ProcedurePART1:GROUPA(Ag+,Pb2+)1. Obtain10dropsoftheunknownsolutioninacentrifugetube.2. Addthreedropsof6MHClintotheabovecentrifugetube.3. Ifaprecipitateforms,thenoneofthegroupAcationsinpresentintheunknown.

Inthiscaseproceedtothenextstep.Ifnoprecipitateforms,thennoneofthegroupAcationsarepresentandinthiscaseproceedtoPart2.

4. Centrifugethemixturecontainingtheprecipitatefortwominutes.Decantthe

precipitateandsavethesupernatantforanalysisinPart2.5. Washtheprecipitatewith10dropsofdeionizedwatertwotimes.6. Add20dropsofdeionizedwatertotheprecipitateandheatthemixtureina

boilinghotwaterbathforthreeminutes.7. Centrifugethemixtureagain.Ifthereisnoprecipitate,thenconfirmthepresence

ofPb2+usingthetestdescribedinStep8.Ifthereisaprecipitate,thenconfirmthepresenceofAg+usingthetestdescribedinStep9.

8. TestforPb2+:Addthreedropsof1MKItotheclearsolutionfromStep7.The

formationofayellowprecipitateconfirmsthepresenceofPb2+.9. TestforAg+:Add10dropsof6MNH3totheprecipitateinStep7.Stirthe

mixture.IfAg+ispresent,theprecipitatewilldissolve.Add6MHNO3tothesamesolutionuntilawhiteprecipitateofAgClformstoconfirmthepresenceofAg+.

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PART2:GROUPB(Bi3+,Fe3+,Mn2+,Al3+,Cr3+)1. ObtainthesupernatantfromStep3ofPart1.Ifadditionaltestsampleisneeded,

thenrepeatSteps1-3ofPart1.2. Add6MNH3totheabovesampleuntilthepHisbetween9and10.Centrifuge

themixturefortwominutes.TheprecipitatewillcontaintheGroupBcationsandthesupernatantwillcontainthecationsfromGroupsC&D.SavethesupernatantforanalysisinPart3.

3. WashtheprecipitatefromStep2abovetwicewithdeionizedwater.4. Add10dropsof6MNaOHandtwodropsof3%H2O2totheprecipitate,mix

thoroughlyandplaceinaboilinghotwaterbathfortwominutes.5. Centrifugetheabovemixturefortwominutes.Theprecipitatewillcontainthe

cationsfromGroupB1andthesupernatantwillcontainthecationsfromGroupB2.TestfortheB1cationsaccordingtoproceduresdescribedinSteps6-10.TestfortheB2cationsaccordingtoproceduresdescribedinSteps11-14.

6. TestforB1cations:Add2mLofdeionizedwatertotheprecipitatefromStep5.

Mixthoroughlyandheatfor10minutesinaboilinghotwaterbath.ThiswillremoveanyexcessH2O2fromStep4.Centrifugethemixtureanddiscardthesupernatant.

7. TestforB1cations:Add10dropsof6MHCltotheprecipitate.Mixandheatfor

10minutesinaboilinghotwaterbath.Centrifugethemixture.TheB1cationswillbefoundinthesupernatant.UsethesupernatantinthenextthreestepstotestforeachoftheB1cations.Discardtheprecipitate.

8. TestforMn2+:TofivedropsofthesupernatantsolutionfromStep7,addseveral

smallamountsofNaBiO3untilnofurtherreactionisobserved.Centrifugethemixtureandobservethecolorofthesupernatantsolution.ApurplesupernatantsolutionconfirmsthepresenceofMn2+.

9. TestforBi3+:TofivedropsofthesupernatantsolutionfromStep7,addthree

dropsof6MNaOHfollowedbyasmallamountofSnCl2.FormationofablackprecipitateconfirmsthepresenceofBi3+.

10. TestforFe3+:TofivedropsofthesupernatantfromStep7,addthreedropsof0.1

MKSCN.FormationofareddishbrowncomplexconfirmsthepresenceofFe3+.11. TestforB2cations:ObtainthesupernatantsolutionfromStep5aboveandheat

thesolutionfor10minutesinaboilinghotwaterbath.Now,coolthesolutioninacoldbath.Add6MHCluntilthesolutionisacidicandobserveanycolor

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changes.Add1MNaHCO3toneutralizethesolution.CentrifugethesolutionanddividethesupernatantintotwopartstobeusedinSteps12and13.

12. TestforAl3+:TotwodropsofthesupernatantfromStep11,addtwodropsof6

MHCl.Addtwodropsofaluminonfollowedby6MNH3untilthesolutionisbasic.Centrifugethemixtureforoneminute.TheappearanceofaredprecipitateconfirmsthepresenceofAl3+.

13. TestforCr3+:TotwodropsofthesupernatantfromStep11,addonedropof3%

H2O2.Nowadd6MHCluntilthesolutionisacidic.TheappearanceofadarkbluecomplexconfirmsthepresenceofCr3+.However,thiscomplexdecomposesquicklytherebycausingthecolortodisappear.

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PART3:GROUPC(Ba2+,Sr2+,Ca2+)1. ObtainthesupernatantsolutionfromStep2ofPart2.Thiscontainsthecations

fromGroupsC&D.Ifadditionaltestsolutionisrequired,thenrepeatStep2ofpart2.

2. TothesolutionfromStep1above,addfivedropsof0.5Mammoniumoxalate

andmix.ThecationsfromGroupCwillprecipitateandthecationsfromGroupDwillstaydissolved.Centrifugethismixturefortwominutes.SavethesupernatantsolutionforanalysisinPart4.

3. Washtheprecipitatewith1mLofdeionizedwater.4. Transfertheprecipitateintoacrucible.UseconcentratedHNO3todothe

transfer.Coverthecruciblepartiallywithalid.Heatthecrucibleoveralowflameandevaporatetheliquid.

5. CoolthecrucibleforfiveminutesandaddsixdropsofconcentratedHNO3.Cover

thecruciblepartiallywithalid.Heatthecrucibleoveralowflameandevaporatetheliquid.

6. Dissolvetheprecipitateinfourdropsof6MHCl.Transferthesolutiontoaclean

testtube.7. TestforBa2+:PerformtheflametestwiththesolutionfromStep6.Emissionof

yellow-greenlightconfirmsthepresenceofBa2+.8. TestforSr2+:PerformtheflametestwiththesolutionfromStep6.Emissionofa

crimsonlightconfirmsthepresenceofSr2+.9. TestforCa2+:PerformtheflametestwiththesolutionfromStep6.Emissionofa

brick-redlightconfirmsthepresenceofCa2+.

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PART4:GROUPD(Ni2+,Cu2+,Zn2+,Mg2+)1. ObtainthesupernatantsolutionfromStep2ofPart3.Thiswillcontainthe

GroupDcations.2. PourthesupernatantliquidcontainingtheGroupDcationsintoacleancrucible.

Placethelidonthecrucibleslightlyajar,andheatthesolutiontodrynessoveralowflame.

3. Coolthecrucibleforfiveminutes,andthenaddsixdropsofconcentratednitric

acid,washingtheinsideofthecrucible.Replacethelidandheattodrynessonceagain.

4. Coolthecrucibleforfiveminutes,repeattheadditionofconcentratednitricacid

andheattodryness.5. Coolthecruciblefor5minutes,anddissolvetheresiduebyadding5-10dropsof

6MHCI.(Itisalrightifallofthesoliddoesnotdissolve.)UsingacleanPasteurpipet,transferthesolutiontoacleantesttube,labelthetesttubeas“D”.

6. Rinsethecruciblewith5dropsofdeionizedwater.Addthisrinsetothetesttube

labeled“D”.7. TestforNi2+:Transferonedropofthesolutionfromtesttube“D”toacleantest

tube.Add6MNH3,withstirring,untilthesolutiontestsbasictolitmuspaper.Addonedropof1%dimethylglyoxime.TheappearanceofacherryredprecipitateconfirmsthepresenceofNi2+.

8. TestforCu2+:Transferonedropofthesolutionfromtesttube“D”toacleantest

tube.Add6MNH3untilthesolutiontestsonlyweaklyacidic.Ifthesolutionbecomesbasic,use6Maceticacidtomakeitslightlyacidic.Add3-4dropsof0.1MK4[Fe(CN)6],solution.TheappearanceofamaroonprecipitateconfirmsthepresenceofCu2+.

9. SeparationofCu2+/Ni2+fromMg2+/Zn2+:Add6MNH3totheremainderofthe

solutionintesttubeDuntilittestsslightlybasic(pH8-9).Thenadd6Maceticaciduntilthesolutionisweaklvacidic(pH4-5).Addabout0.2gofsolidsodiumthiosulfate,Na2S2O3,andheatforfiveminutesinaboilingwaterbath.IfCu2+and/orNi2+arepresent,thenablackprecipitateshouldform.Ifablackprecipitatedoesnotformundertheseconditions,addonetotwomoredropsof6Maceticacidandcontinueheatingafewmoreminutes.(FormationoftheblackprecipitateispHsensitive.)Coolfor1minutebyswirlingthetesttubeincoldtapwater.CentrifugeanddecantthesupernatantintoacleantesttubelabeledD1.Theprecipitatemaybediscarded.

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10. TestforMg2+:TothesolutionlabeledD1add6MNH3untilthepHisabout8.Add6-8dropsofsaturated(about0.1M)Na2HPO4solution.Stirandthencoolthesolutioninicewaterforseveralminutes.Ifaprecipitatedoesnotform,add6MHClindropstolowerthepHslightly.Watchcarefullyforprecipitateformation;stopaddingtheHCluponformationofaprecipitate.Centrifugeanddiscardthesupernatant.TheformationofawhiteprecipitateindicatesthepossibilityofMg2+.

11. Washtheprecipitatewith10dropsofdeionizedwater.Discardthewash.Tothe

washedprecipitate,add6dropsof6MNaOHandstirthoroughly.CentrifugeanddecantthesupernatantintoacleantesttubelabeledD2.

12. TestforZn2+:TothesolutioninthetesttubelabeledD2add6Maceticaciduntil

thesolutiontestsonlyweaklyacidic(pH4-5).Ifthesolutionbecomestooacidic,use6MNH3tomakeitslightlyacidic.Add4dropsof0.1MK4[Fe(CN)6].TheformationofawhiteprecipitateconfirmsthepresenceofZn2+.

19

DataTableIneachinstance,clearlydescribetheobservationsandtheappropriateinferences.PART1:GROUPA(Ag+,Pb2+)CationAnalyzed

Observationsfromknown

Observationsfromunknown

Inferenceaboutunknown

Pb2+

Ag+

20

PART2:GROUPB(Bi3+,Fe3+,Mn2+,Al3+,Cr3+)CationAnalyzed

Observationsfromknown

Observationsfromunknown

Inferenceaboutunknown

Bi3+

Fe3+

Mn2+

Al3+

Cr3+

21

PART3:GROUPC(Ba2+,Sr2+,Ca2+,Co2+)CationAnalyzed

Observationsfromknown

Observationsfromunknown

Inferenceaboutunknown

Ba2+

Sr2+

Ca2+

22

PART4:GROUPD(Ni2+,Cu2+,Zn2+,Mg2+)CationAnalyzed

Observationsfromknown

Observationsfromunknown

Inferenceaboutunknown

Ni2+

Cu2+

Zn2+

Mg2+

23

Results

TheunknownprovidedconsistsofthefollowingSIXcations:

Name

Formula

1

2

3

4

5

6

STUDENTNAME:________________________________________________UNKNOWNNUMBER:___________________________________________