1
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)
2
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
3
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+.
4
*NOTE:TheMn2+isoxidizedtoMn4+bytheH2O2TheB1cationsareallsolubleinHCl.TheprecipitatecontainingtheB1cationsisdissolvedinhotHClsolution.ThissolutionisthentestedforeachofthethreeB1cations.TestforMn2+InanacidicenvironmenttheMn4+isreducedtoMn2+usingH2O2.TheMn2+isconvertedtoMnO4−usingbismuthate,BiO3−.Theformationofapurplecoloredsolution(fromtheMnO4−)confirmsthepresenceofMn2+.Thebismuthateisastrongoxidizingagentandcausesavigorousreactionandmustthereforebeaddedinsmallportions.
Mn4+(aq)+H2O2(l)
€
⇔Mn2+(aq)+O2(aq)+H+(aq)
Mn2+(aq)+H+(aq)+BiO3−(aq)
€
⇔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
5
TestforFe3+Fe3+formsacomplexwiththiocyanate,SCN−.AdditionofpotassiumthiocyanatetoFe3+producesareddish-browncolorduetotheformationofthiscomplex.Theformationofthereddish-browncolorconfirmsthepresenceofFe3+.
Fe3+(aq)+KSCN(aq)
€
⇔[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)
€
⇔2CrO5(aq)+5H2O(l)GROUPC:(Ba2+,Sr2+,Ca2+)ThesimilaritybetweencationsofGroupsCandDisthattheybothforminsolublesaltswithoxalateanion.However,theGroupDoxalatesaltsaresolubleinaqueousammoniawhereastheGroupCoxalatesaltsareinsolubleinaqueousammonia.NotethatafterseparatingtheGroupBcations,thesolutionisalreadyinamediumcontainingaqueousammoniaandatahighpH.Therefore,whenammoniumoxalate
6
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
7
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+
8
ThesupernatantsolutionattheendofthelasttestisanalyzedforthepresenceofZn2+.Potassiumhexacyanoferrateisaddedtoaslightlyacidictestsolution.FormationofawhiteprecipitateconfirmsthepresenceofZn2+.
2Zn(OH)42-(aq)+8H+(aq)+K4[Fe(CN)6](aq)!Zn2Fe(CN)6(s)+4K+(aq)+8H2O(l)
9
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
10
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
11
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.
12
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)
13
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+.
14
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
15
changes.Add1MNaHCO3toneutralizethesolution.CentrifugethesolutionanddividethesupernatantintotwopartstobeusedinSteps12and13.
12. TestforAl3+:TotwodropsofthesupernatantfromStep11,addtwodropsof6
MHCl.Addtwodropsofaluminonfollowedby6MNH3untilthesolutionisbasic.Centrifugethemixtureforoneminute.TheappearanceofaredprecipitateconfirmsthepresenceofAl3+.
13. TestforCr3+:TotwodropsofthesupernatantfromStep11,addonedropof3%
H2O2.Nowadd6MHCluntilthesolutionisacidic.TheappearanceofadarkbluecomplexconfirmsthepresenceofCr3+.However,thiscomplexdecomposesquicklytherebycausingthecolortodisappear.
16
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+.
17
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.
18
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:___________________________________________