1
EXPERIMENTA9:REDOXTITRATION
LearningOutcomes
Uponcompletionofthislab,thestudentwillbeableto:
1) Applytheprinciplesoftitrationthatwerepreviouslydiscussed&
performedtoanoxidation-reductionreaction.
2) Evaluatethepercentageofhypochloriteinbleach.
Introduction
Oxidation-Reductionreactions,knowninshortasredoxreactions,involvetransfer
ofelectrons.Onereactantloseselectrons,andisthereforeoxidized.Theelectrons
lostbythisreagentaretransferredtothesecondreagent,whichacceptsthese
electronsandisthereforereduced.Theoxidationnumberofanelementinthe
reagentgettingoxidizedincreasesandtheoxidationnumberofanelementinthe
reagentgettingreduceddecreases.
Thesetypesofreactionsarecommoninelectrochemistry,atopicthatwillbe
discussedingreaterdetailinalatersectionofGeneralChemistry.
Inthisexperiment,theamountofhypochloriteion-OCl−,theactiveingredientin
bleach,willbemeasuredbyperformingaredoxtitration.Asinanytitration,the
stoichiometryofthechemicalequationofthereactionwillenablethedetermination
oftheamountoftheunknownreagent.
Thehypochloriteionwillbetitratedwiththiosulfate(S2O32-)accordingtothe
followingoverallreactionwritteninitsionicform:
2S2O32-(aq)+OCl−(aq)+H2O(l)!S4O62-(aq)+Cl−(aq)+2OH−(aq)
Reaction1
Reaction1showsthatonemoleofhypochloritecompletelyreactswithtwomolesof
thiosulfate.Thisstoichiometricrelationshipbecomesthebasisfortherequired
calculations.
However,Reaction1doesnotprovidethecompletepictureofthechemistryofthe
reactionbetweenhypochloriteandthiosulfate.
Athirdimportantreagentthatisusedfortheelectrontransferprocess(keepin
mindthatthisisaredoxreactionandmustthereforeinvolveelectrontransfer)is
iodide(I−).Whenhypochloriteionsreactwithiodideions,thefollowingreaction
firsttakesplace:
OCl−(aq)+H2O(l)+2I−(aq)!Cl−(aq)+2OH−(aq)+I2(aq)
Reaction2
2
TheI2formedinReaction2reactswiththethiosulfateaccordingtothefollowing
reaction:
I2(aq)+2S2O32-(aq)!2I−(aq)+S4O62-(aq)
Reaction3
WhenReaction2andReaction3arecombined,theoverallreactionobtainedis
Reaction1.So,inrealityitistheI2thatistitratedwiththiosulfate.SincemolesofI2
formedinReaction2arethesameasthemolesofOCl−,thestoichiometrybetween
OCl−andS2O32-isthesameasthatbetweenI2andS2O32-.
Inthistitration,theiodideandbleach(containingOCl−)willbeintheErlenmeyer
flaskandtheburettewillcontainthethiosulfate.
Asisthecaseinanytitration,theendpointisdeterminedvisuallybyusing
indicators(seeExperimentVII:VinegarTitration).Theredoxtitrationdiscussed
hereusesstarchasanindicatorinaninterestingmanner.
Whenallthereagentsarecombined,thesolutionintheErlenmeyerflaskinitially
hasareddish-browncolor,whichisaresultofthecombinationofI−andI2(from
Reaction2).Oncethetitrationisbegun,theadditionofthethiosulfatedecreasesthe
amountofI2asitisconvertedtoI−(accordingtoReaction3)andthecolorofthe
solutionbecomesmoreyellowthanbrown.Ifstarch(whichisacolorlesssolution)
isaddedatthispoint,itformsacomplexwiththeremainingI2andthesolution
takesonadeepbluetovioletcolor.Whenthetitrationiscompleted,alltheI2will
havebeenconvertedtoI−,releasingthestarchinitsfreeformandrenderingthe
solutioncolorless.
Thereforethetransformationofthedeepbluecoloredsolutiontoacolorless
solutionisusedasavisualindicationoftheendpointofthistitration.
Asdiscussedabove,themolesofhypochloritepresentinbleachisdeterminedfrom
thestoichiometricrelationshipbetweenhypochloriteandthiosulfate(1:2
accordingtoReaction1).Themolesofthethiosulfatewillbedeterminedfromits
molarityandthevolumeusedforthetitration.Thereforeitisimportanttoknowthe
exactmolarityofthethiosulfate.
Themolarityofthepreparedthiosulfatesolutionmustfirstbedeterminedby
titratingitagainstaprimarystandard(seeExperimentVII).Theprimarystandard
usedinthecurrentexperimentisiodate(IO3−).Thereactionbetweenthiosulfate
andiodateisasfollows:
6S2O32-(aq)+IO3−(aq)+6H+(aq)!3S4O62-(aq)+I−(aq)+3H2O(l)
Reaction4
AccordingtoReaction4,sixmolesofthiosulfateareneededtocompletelyreactwith
onemoleofiodate.
3
AswasthecasewithReaction1,Reaction4doesnotprovidethecompletepictureof
thechemistryofthisreaction.InReaction4aswell,theiodate(likethehypochlorite
inReaction1)iscombinedwithiodide(I−)tofacilitatetheelectrontransferprocess.
Thechemicalreactionbetweeniodateandiodideisasfollows:
IO3−(aq)+5I−(aq)+6H+(aq)!3I2(aq)+3H2O(l)
Reaction5
TheI2formedinReaction5reactswiththethiosulfateaccordingtothefollowing
reaction:
I2(aq)+2S2O32-(aq)!2I−(aq)+S4O62-(aq)
Reaction3
WhenReaction5andReaction3arecombined,theoverallreactionobtainedis
Reaction4.Soonceagain,inrealityitistheI2thatistitratedwiththiosulfate.Since
threemolesofI2areformedinReaction5andeachmoleofI2istitratedwithtwo
molesofS2O32-accordingtoReaction3,itthenfollowsthatsixmolesofS2O32-are
neededtocompletelyreactwithonemoleofIO3−.
Inthistitration,theiodideandbleach(containingOCl−)willbeintheErlenmeyer
flaskandtheburettewillcontainthethiosulfate.
TheindicatorusedfortheendpointdeterminationofReaction4isalsostarch.The
colorofthemixtureintheErlenmeyerflask(IO3−andI−)willbereddishbrown.
OncethetitrationbeginsandastheI2isconvertedtoI−(accordingtoReaction3),
thecolorwillbecomemoreyellow.Starchaddedatthispointwillcomplexwiththe
remainingI2intheflaskandgivethesolutionadeepbluetovioletcolor.Onceallthe
I2hasbeenconvertedtoI−bytheS2O32-thesolutionwillbecomecolorless,asthe
starchisnotcomplexedwithI2.
ExperimentalDesign
4
Aprimarystandardofiodateofconcentration0.0500Mmustfirstbeprepared.This
solutionwillbeusedtotitratetheprovidedthiosulfatesolutionofunknown
molarity.Thedatafromthistitrationwillbeusedtodeterminetheexactmolarityof
thethiosulfatesolution.Thisthiosulfatesolutionwillthenbeusedtotitratethe
hypochloriteinthebleach.Themasspercentofhypochloriteinthebleachsolution
willthenbecalculated.
ReagentsandSupplies
Starchsolution,sodiumthiosulfatesolution(~molarity=0.2M),solidpotassium
iodate,2Msulfuricacid,10%potassiumiodidesolution,6Maceticacid,bleach
(SeepostedMaterialSafetyDataSheets)
Volumetricflask
Procedure
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PART1:STANDARDIZATIONOFAQUEOUSSODIUMTHIOSULFATESOLUTION
1. Calculatethemassofpotassiumiodate(KIO3)neededtomake25.00mLof
0.0500Msolution.
2. Weighanamountofpotassiumiodateasclosetothenumbercalculatedinstep1
aspossible.Recordtheexactmass.
3. Preparethe0.0500MKIO3solutionusinga25.00mLvolumetricflask.
4. Obtainabout10mLofeachofthefollowingsolutions:1%starch,2Msulfuric
acid,10%aqueouspotassiumiodide,andsodiumthiosulfatesolution.Clearly
labeleachcontainer.
5. Setuptwomicroburettesandlabeloneasthethiosulfateburetteandtheother
asiodateburette.
6. Rinse,condition,andfilleachburettewiththeappropriatereagent.
7. Recordtheinitialburettereadingsofboththeburettes.
8. Addabout1mLofiodatefromtheburetteintoa25-mLErlenmeyerflask.
Recordthefinalburettereading.
9. AddthefollowingreagentsintotheaboveErlenmeyerflask:
a. 5mLofdeionizedwater
b. Sixdropsof2MH2SO4
c. 2mLof10%KI
10. ThesolutionintheErlenmeyerflaskshouldbereddishbrownincolor.Begin
titratingthissolutionwiththethiosulfate.
11. Ifanyreagentisstucktothesidesoftheflask,rinsetheflaskwithdeionized
watertoensuremixingofallthereagents.
12. WhenthesolutionintheErlenmeyerflaskispaleyellow,add2mLofdeionized
waterand15dropsof1%starchsolution.Thissolutionshouldnowbedeepblue
tovioletincolor.
13. Continuetitratingwiththethiosulfateuntilthesolutionturnscolorless.Record
thefinalburettereadingofthethiosulfateburette.
14. Repeatsteps7to13twotothreemoretimes.
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PART2:TITRATIONOFTHEHYPOCHLORITEINBLEACHWITHTHESTANDARDIZEDSODIUM
THIOSULFATESOLUTION
1. Obtainabout10mLofthefollowingsolutions:1%starch,6Maceticacid,10%
aqueouspotassiumiodide,standardizedsodiumthiosulfatesolutionfrompart1,
andbleach.Clearlylabeleachcontainer.
2. Setuptwomicroburettesandlabeloneasthethiosulfateburetteandtheother
asbleach.
3. Rinse,condition,andfilleachburettewiththeappropriatereagent.
4. Recordtheinitialburettereadingsofboththeburettes.
5. Recordthemassofaclean,empty,anddry25-mLErlenmeyerflask.
6. Addabout0.1mLofbleach(abouttwotofourdrops)fromtheburetteintothe
aboveErlenmeyerflask.Recordthefinalburettereading.
7. MeasurethemassoftheErlenmeyerflaskcontainingthebleachandrecordthis
value.
8. AddthefollowingreagentsintotheaboveErlenmeyerflask:
a. 2mLofdeionizedwater
b. 1mLof2MH2SO4
c. 2mLof10%KI
9. ThesolutionintheErlenmeyerflaskshouldbereddishbrownincolor.Begin
titratingthissolutionwiththethiosulfate.
10. Ifanyreagentisstucktothesidesoftheflask,rinsetheflaskwithdeionized
watertoensuremixingofallthereagents.
11. WhenthesolutionintheErlenmeyerflaskispaleyellow,add10dropsof1%
starchsolution.Thissolutionshouldnowbedeepbluetovioletincolor.
12. Continuetitratingwiththethiosulfateuntilthesolutionturnscolorless.Record
thefinalburettereadingofthethiosulfateburette.
13. Repeatsteps4to12twotothreemoretimes.
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DataTable
PART1:STANDARDIZATIONOFAQUEOUSSODIUMTHIOSULFATESOLUTION
MassofKIO3(grams)
VolumeofKIO3solution(mL) 25.00
Iodatesolution
Trial1
Trial2
Trial3
Initialburette reading(mL)
Finalburette
reading(mL)
Thiosulfatesolution
Trial1
Trial2
Trial3
Initialburette reading(mL)
Finalburette
reading(mL)
Trial4
Trial4
PART2:TITRATIONOFTHEHYPOCHLORITEINBLEACHWITHTHESTANDARDIZEDSODIUM
THIOSULFATESOLUTION
Bleach
Trial1
Trial2
Trial3
Trial4
Massofempty Erlenmeyer
flask(grams)
Massof
Erlenmeyer
flask+bleach
(grams)
Thiosulfatesolution
Trial1
Trial2
Trial3
Trial4
Initialburette reading(mL)
Finalburette
reading(mL)
8
9
Calculations
PART1:STANDARDIZATIONOFAQUEOUSSODIUMTHIOSULFATESOLUTION
MolarmassofKIO3=
MassofKIO3=
MolesofKIO3=
VolumeofKIO3solutionprepared=25.00mL=0.02500L
moles
MolarityofKIO3=
=
Volume
Trial1
Trial2
Trial3
−
VolumeofIO3 (liters)
€
Volumeof
2S2O3 (liters)
Molarityofthiosulfate(showcalculationforeachtrial):
6S2O32-(aq)+IO3−(aq)+6H+(aq)!3S4O62-(aq)+I−(aq)+3H2O(l)
Trial1
Trial2
Trial3
Trial4
MolarityofS2O32-
Trial1
Trial2
Trial3
Trial4
Average Trial4
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PART2:TITRATIONOFTHEHYPOCHLORITEINBLEACHWITHTHESTANDARDIZEDSODIUM
THIOSULFATESOLUTION
2S2O32-(aq)+OCl−(aq)+H2O(l)!S4O62-(aq)+Cl−(aq)+2OH−(aq)
Atequivalencepoint:1moleofOCl−=2molesofS2O32-
AveragemolarityofS2O32-(frompart1)=
Trial1
Trial2
Trial3
Trial4
Volumeof
S2O32-(liters)
MolesofS2O32- (M×V)
MolesofOCl−
MolarMassof OCl−(g/mol)
MassofOCl−
(grams)
Massofbleach (grams)
Masspercent ofOCl−in
bleach(%)
Averagemasspercentofhypochloriteinbleach=______________________