REDOX TITRATION.pdf

Redox titration, Reduction & Oxidation, Theory & Principle, Redox indicator, Permanganometry, Iodimetry, Iodometry, Dichrometry, Cerimetry, Bromimetry, Titanometry, Titration involving Potassium iodate

2. CONTENTS
•INTRODUCTION –CONCEPT OF OXIDATION AND REDUCTION
•THEORY AND PRINCIPLE
•TYPES OF REDOX TITRATION
•REDOX INDICATORS
2

3. INTRODUCTION
•Chemicalreactionsinwhichelectronsaretransferredfromoneatomtoanother
atomareknownasOxidation–Reductionreactions.
•Titrationwhichinvolvetransferofelectronsbetweenthetitrantandanalyteare
referredtoasOxidation-Reductionreactions.
•Titrationsinvolvingbothoxidationandreductionreactionsarereferredtoas
Redoxtitrations.
3

4. •Originally,Oxidationmeanadditionofoxygentoasubstanceandalsousedfor
removalofhydrogen.
•Reductionmeanremovalofoxygentoasubstanceandadditionofhydrogen.SO
2O SO
3+ H
2
SO S H
2
O+ +
-Addition of Oxygen
-Removal of hydrogenCuO 2H Cu H
2
O+ + C
2
H
2
2H C
2
H
4+
-Removal of oxygen
-Addition of Hydrogen
4

5. •Oxidation–reductionreactionasreactioninwhichelectronsaretransferredbetween
speciesorinwhichatomschangeoxidationnumber.
•Inanyreactioninwhichoxidationoccurs,reductionmustalsooccursimultaneously.
•Whenasubstancegivesupelectrons,theremustbeanothersubstancetoreceive
them.
•ThefirstsubstanceisoxidisedandthesecondsubstanceisreducedinanOxidation–
Reductionreaction.
5

6. OXIDATION
oOxidationisdefinedasthepartofanoxidation–reductioninwhichthereisalossof
electronsbyaspeciesoranincreaseintheoxidationnumberofanatom.
oAnoxidisingagentisaspeciesthatoxidisesanotherspecies;thus,oxidisingagentis
itselfgettingreducedintheprocessandgainselectronsfromthereducingagents.
oExample:
Potassiumpermanganate
Potassiumbromate
Potassiumiodate
Cericammoniumsulphate
Iodine
6

7. REDUCTION
oReductionisdefinedasthepartofanOxidation–Reductionreactioninwhichthereis
againofelectronsbyaspeciesoradecreaseinoxidationnumberofanatom.
oAreducingagentisaspeciesthatreducesanotherspeciesnamelyoxidisingagent,and
thereducingagentisitselfoxidised.
oInthisreactionthereducingagentloseselectronswhicharegainedbytheoxidising
agent.
oExamples:
Sodiumthiosulphate
Ferrousammoniumsulphate
Titanouschloride,sulphateetc.
7

8. ExamplesofOxidation–Reductionreactions
•FerricsaltisreducedbyaTitanoussalt
Thisequationmaybewrittenintheionicformas
Separatedinto2halfequations,onerepresentstheoxidation,
Andotherisreduction.TiCl
3
FeCl
3
TiCl
4
FeCl
2+ + Ti
3+
Fe
3+
Ti
4+
Fe
2+
+ + Ti
3+
Ti
4+
e+ Fe
3+
e Fe
2+
+
8

9. 9

10. 10

11. THEORY AND PRINCIPLE
•InOxidation–ReductionreactionsorRedoxtitrations,whenonesubstanceis
oxidised,someothersubstancemustbecorrespondinglyreducedandwhenone
substanceisreduced,someothersubstancemustbecorrespondinglyoxidised.
•ThisrelationshipmaybeexpressedaccordingtoFaraday’slaw:Achangein
chargeofoneisequivalenttothegainorlossof96,500Cofelectricityforeach
formulaweightofelementorgroupofelementsinvolved.
11

12. •Sinceineveryoxidation–reductionreactionthechargelostorgainedbyone
substancemustnecessarilybegainedorlostbyanother,iffollowsthatthereis
alwaysatransferofelectronsinoxidation–reductionreactions.
•Thereactantwhichloseselectronsinanoxidation–reductionreactionisthe
Reducingagentsandcanbeidentifiedintheequationforthereactionasthereactant
containingaconstituentatomoratomsconvertedtoahigherstateofoxidation.Fe
2+
Fe
3+
e+ 2I
-
I
2 2e+
12

13. •TheelectronslostbythereducingagentaregainedbytheOxidisingagent,the
reactantcontainingaconstituentatomoratomswhichareconvertedtoalowerstate
ofoxidation.
OXIDATIONNUMBER OROXIDATION STATE
oOxidationnumberisdefinedtothechargeanatominasubstancewouldhaveifthe
pairsofelectronsineachbondbelongedtothemoreelectronegativeatom.Ce
4+
e Ce
3+
+ Fe
3+
e Fe
2+
+
13

14. TYPES OF REDOX TITRATION
1. Titration involving KMnO
4(Permanganometry/ permanganometrictitration)
Eg: Estimation of H
2O
2
2. Titration involving Iodine
(a) Direct titration/ Iodimetry
Eg: Estimation of ascorbic acid
(b) Indirect titration/ Iodometry
(i)Backtitrationofexcessiodinewithsodiumthiosulphate
eg:Estimationofanhydroussodiumsodiumsulphite
14

15. (ii)Releaseofiodineanditstitrationwithsodiumthiosulphate
eg:Estimationofcoppersulphate,chlorinatedlimeetc..
3.TitrationinvolvingDichromate/Dichrometry
Eg:Estimationofiron,chromiumetc..
4.Titrationinvolvingbromine/Bromimetry
Eg:EstimationofPhenol,isoniazidetc..
5.TitrationinvolvingPotassiumiodate
Eg:EstimationofKI,weakiodinesolution
6.TitrationinvolvingCericammoniumsulphate(Cerimetry/Ceriometrictitration)
Eg:EstimationofFerroussulphateetc..
7.Titanometry
15

16. PERMAGNOMETRY
•PremanganometrictitrationsarethosethatuseasolutionofKMnO
4asanoxidizingagent.
•ThesolutionofKMnO
4isintenselypurplecoloured.
•AsKMnO
4isastrongoxidant,itreducestheintenselypurplecolouredMnO
4
-
ionstocolourless
Mn
2+
ions.
•Duringthetitration(i.ewhileKMnO
4addedtosample)thesolutionbecomescolourlessdueto
thepresenceofexcessMn
2+
ions.
•OncealltheMn
2+
ionshavecompletelyreacted,additionofKMnO
4changesthecolourof
solutiontopink.
•Thischangeincolourindicatestheendpoint,atwhichboththeionsareatequilibrium.
16

17. •KMnO
4actasaselfindicator,hencethereactiondoesnotrequiretheadditionofanyotherindicatorto
detecttheendpoint.
•KMnO
4actasanoxidisingagentnotonlyinacidicsolutionsbutalsoinalkalineandneutralmedia.It
exhibitsdifferentoxidationstatesindifferentmedia.
1.Acidicmedia
KMnO
4itselfactasanindicator.
MnO
4
-
isreducedtoMn
2+MnO
4
-
8H
+
5e
-
Mn
2+
4H
2
O++ +
Reduction 2KMnO
4
3H
2
SO
4
K
2
SO
4
2MnSO
4
3H
2
O 5[O]+ + + +
17

18. 2.BasicandNeutralMedia
MnO
4
-
isreducedtoMnO
2.
Diphenylamineorphenylanthranilicacid–usedasindicatorsinbasicandneutralsolutions,
becauseofformationofMnO
2.
KMnO
4ismorestableinneutralsolutionthaninacidicandbasicsolutions.
pHdeterminesthestabilityofKMnO
4.
(a)Moderatebasicmedium
(b)NeutralmediumMnO
4
-
2H
2
O 3e
-
MnO
2
4OH
-
+ + +
Reduced MnO
4
-
4H
+
3e
-
MnO
2
2H
2
O++ +
18

19. Preparation of 0.1N KMnO
4Solution
3.161 gm of potassium permanganate is needed to prepare 1 litre of 0.1N solution.
Standardisation of 0.1N KMnO
4Solution
Weight accurately about 0.63gm oxalic acid + dissolve in 30ml distilled water and make
up the volume to 100ml in volumetric flask.
Transfer 20 ml in to conical flask + 20ml of dil. Sulphuric acid, heated to 70
o
C
Titrated hot soln. with 0.1N KMnO
4
End point: Appearance of permanent pink colour
19

20. Detection of End-point
•Potassium permanganate is a good example for self indicator in redox titration.
Examples of Permanganometry
•Hydrogen peroxide
•Ferrous sulphate
•Benzoyl peroxide
•Lithium oxalate
20

21. IODIMETRY
•Determinationinvolvingdirecttitrationwithiodineduetotheoxidisingpower
ofiodineinaqueoussolution,suchtypeoftitrationsarecalledasIodimetry.
•Iodimetrictitrationisatypeofredoxtitrationwherewedeterminethe
concentrationofareducingagentsbytitratingagainstastandardiodine
solution.
21

22. PRINCIPLE
•I
2is a weak oxidant and it can be reduced by the reductants.
•Potassium iodide is used to dissolve iodine.
•The excess of iodide ion is present react to form tri-iodide.
•The tri-iodide react with the strong and weak reducing agents like sodium thiosulphate etc..I
2 2I
- KI K
+
I
-
+ I
2I
-
I
3
-
+ 2S
2
O
3
2-
I
2
S
4
O
6
2-
2I
-
As
3+
I
2
As
5+
2I
-
+ +
+ +
22

23. Preparation of Standard 0.05M Iodine solution
•0.335gm of iodine dissolves in 20ml KI and make up volume to 1000ml by water at 25
o
C.
Standardization of 0.05M Iodine solution
•Standardization using Arsenic trioxide
Primary standard substance for standardizing iodine solution.
Substance present in aqu. Soln. as Arsenous acid is oxidized by Iodine.
Arsenic trioxide is slowly soluble in cold water more rapidly soluble in boiling water and
readily soluble in NaOH solution form sodium arsenite.H
3
AsO
3I
2
H
2
O H
3
AsO
4
2HI
H
3
AsO
3I
2H
2
O H
3
AsO
4 2H
+
2I
-
++ +
++ ++ As
2
O
36NaOH 2Na
3
AsO
33H
2
O+ +
23

24. •If iodine is added to this alkaline solution –forms sodium hypoiodite.
•The excess of sodium hydroxide is neutralized with HCl using methyl orange as the indicator
until yellow colouris changed to pink.
•Sodium bicarbonate is added to neutralize the hydriodic acid, formed in the reversible reaction.
•NaOH or Na
2CO
3cannot remove HI because they react with iodine.
•Finally the reaction is,2NaOH I
2
NaIO NaIH
2
O+ ++
Sodium
hypoiodide H
3
AsO
3H
2
O I
2
H
3
AsO
4 2HI++ + 6NaOH 3I
2
5NaINaIO
3
3H
2
O
3Na
2
CO
33I
2
5NaINaIO
33CO
2
+
+
+ +
+ + Na
3
AsO
3
I
22NaHCO
3
Na
3
AsO
42NaI2CO
2
H
2++ + + +
24

25. •Procedure
Weight 0.15gm of arsenic trioxide previously dired@105
o
C for 1hr + 20ml of 1M NaOH, warming
Dilute with 40ml water + 0.1ml methyl orange + add dilHCl drop wise
Until yellow colour changed to pink.
Add 2gm of Na
2CO
3+ 50ml water + 3ml starch solution
Titrated with 0.05M Iodine
Permanent blue colour produced
25

26. DetectionofEndpoint
1.Selfindicator:Asolutionofiodineinaqu.Iodidehasanintenseyellowtobrowncolour.
Onedropof0.05MIodinesolutionimpartsaperceptiblepaleyellowcolourto100mlif
water.
2.Starchsolution:StarchimpartsadeepbluecolourtoIodinesolutionevenatverylow
concentration.Butstarchcannotbeusedinstronglyacidicsolutionduetohydrolysis
ofstarchmolecule.Starchindicatorshouldbefreshlypreparedorstabilizedbyaddition
ofapreservativesuchasMercuriciodide(HgI)orFormamide.
26

27. Examples of Iodimetry
(i) Direct Titration with Iodine solution
•Ascorbic acid
•Acetarsol
•Carbarsone
•Analgin
(ii) Back Titration
•Sodium metabisulphite
•Dimercaprol
Assay of Ascorbic acid I.P
•DeterminationdependsuponthequantitativeoxidationofAscorbicacidto
dehydroascorbicacidwithiodineinacidsolutioninpresenceofH
2SO
4.
27

28. Procedure
Weigh accurately abt0.1g dissolve in mixture of 100ml of freshly boiled and cooled water + 25ml
of 1M H
2SO
4.
Titrated immediately with 0.05M Iodine using starch solution as indicator.O
C
CH
2
H OH
OH OH
O
OH
2HI
O
C
CH
2
H OH
O
O O
OH
I
2+ +
H
2
SO
4
Ascorbic acid Dehydro
ascorbic acid
28

29. IODOMETRY
•Thetitrationsinwhichtheequivalentamountofiodineisliberatedfromthe
potassiumiodidebythesampleandtheliberatediodineistitratedagainst
standardsodiumthiosulphatesolution,suchatypeofindirectdeterminationof
strongoxidisingagentiscalledasIodometry.
29

30. PRINCIPLE
•Iniodometricanalysisofoxidisingagents,anequivalentamtofIodineisproducedwhenthe
sampleoxidiseswithpotassiumiodideinthepresenceofmineralacid.
•Theequivalentamtofiodineliberatedismeasuredbytitrationwithstandardsodiumthiosulphate
solutionusingstarchmucilagesolutionasindicatoraddedtowardstheendofthetitration.
Preparationof0.1NSodiumThiosulphate
•25gmofA.Rcrystallisedsodiumthiosulphateisdissolvedinonelitreofwater.
•Sodiumthiosulphateisreadilyobtainableinastateofhighpurityandefflorescentinnature.
•It’sareducingagent.2S
2
O
3
2-
S
4
O
6
2-
2e+
30

31. Standardization of 0.1N Sodium Thiosulphate
•Standardization using Potassium bromate
Potassium bromate is a primary standard and can be obtained in high state of purity.
This can be determined by the addition of potassium iodide and dil. HCl with potassium
bromate.
After reaction between potassium bromate and potassium iodide, excess KI in acid solution
result in liberation of Iodine .
Its titrated with sodium thiosulphate solution using starch solution as indicator which is added
towards the end of the titration.KIH
2
O HIKOH+ + 2KBrO
3
HI HIO
3
KBrKIO
3HBr+ ++ +
31

32. •Procedure
Dissolve 0.278gm in 100ml standard flask with water and pipette 20ml from the solution + 2gm of
KI + 3ml of 2N HCl
Titrate with 0.1N Sodium thiosulphate
Using starch solution as indicator added towards the end of titration.
End point: disappearance of blue colour.HIO
3
5KI6HCl 3I
2 6KCl 3H
2
O++ + + KBrO
3
= KIO
3
= 3I
2
32

33. Detection of End point
1. Starch solution
•Most commonly used indicator in titration involving iodine.
•2 types of starch preparations available as indicator :
(i) Starch mucilage
(ii) Starch solution
i. Preparation of Starch mucilage
Triturate 0.5gm of starch or soluble starch with 5ml water + sufficient water to produce 100ml
Boil for few mins, cool and filter
•It must be freshly prepared to produce blue colour with free iodine
33

34. ii. Preparation of Starch solution
Triturate 1gm of soluble starch + 5ml water , stirring continuously
Add to 100ml of boiling water containing 10mg of mercuric iodide
•Aqustarch suspension decompose within few days because of bacterial action –adding
Mercuric iodide or chloroform as a bacteriostat.
•Starch indicator solution –added towards end of the titration.
•Disadvantage
Insolubility in cold water.
Instability of suspensions in water
Gives water insoluble complex with iodine.
Sometimes drift endpoint which is marked when solutions are dilute.
34

35. 2. Sodium starch glycollate indicator
•Preparation
Dissolve 5gm of finely powdered solid sodium starch glycollate by mixing 1 to 2ml of ethanol + 100ml
of cold water
Boiling for few mins with vigorous shaking
Finely opalescent solution.
•Advantage
Does not form a water insoluble complex with iodine.
With excess of iodine the colour of solution containing 1ml of indicator is green –when iodine
conc. Diminishes the colour changes to blue.
35

36. ExamplesforIodometry
1.Coppersulphate
2.Chlorinatedlime
3.Phenol
4.Chloramine
36

37. TITRATION INVOLVING POTASSIUM IODATE
•KIO
3isapowerfuloxidizingagent.
•Reactwithreducingagentslikeiodide,iodine,arsenictrioxide.
•Iodatetitrationperformedinpresenceofalcoholandstandardorganicacids.
37

38. PRINCIPLE
1.ReactionbetweenKIO
3andreducingagentsinmoderatelyacidicsolutionstopatstateiodate
reducedtoiodine.
2.ReactionwithmorepowerfulreductantlikeTitaniumchloride Iodatereducedto
Iodide.
3.Inmorestronglyacidsolution reductiontoIodinemonochioride.[morewidelyused]
KIO
3–actasmorepowerfuloxidizingagents.KIO
35KI6HCl 3I
26KCl 3H
2
O
2KIO
3
5H
3
AsO
3
2HCl I
2
5H
3
AsO
4
H
2
O2KCl
++ + +
+ + + ++ IO
3
-
6Ti
3+
6H
+
I
-
6Ti
4+
3H
2
O+ + + + IO
3
-
6H
+
Cl
-
4e ICl3H
2
O+++ +
38

39. Preparation of 0.05M KIO
3
Dry A.R KIO
3@ 120
o
C for 1hr cool in desiccator
Weigh 10.7gm and dissolve in water to 1000ml
Standardization of 0.05M KIO
3Solution
•KIO
3obtained in high state of purity.
•2 methods:
1.Using Sod thiosulphate
2.Using KI
39

40. 1. Using Sodium thiosulphate
KIO
3+ excess KI + dil. HCl
Liberate Iodine
Titrated with Sodium thiosulphate solution using starch solution as indicator
End point: Disappearenceof blue colour
Detection of End Point
•Starchsolutioncannotbeused characteristicbluecolourofstarch-iodinecomplexisnot
[email protected].
40

41. •Immisciblesolventchloroform/carbontetrachlorideareadded disappearanceoflast
traceofvioletcolourduetoiodinefromsolventistakenasendpoint.
•Dyesusedinsteadofimmisciblesolvent
•p-ethoxy chrysoidine–satisfactory reversible indicator.
Dyes Colour change
Amaranth Red to colourless
p-ethoxy chrysoidine Red to orange
Naphthalene black Green to faint pink
41

42. Examples of Titration Involving Potassium Iodate
•Potassium iodide IP’69
•Weak Iodine solution
•Aqueous Iodine solution
•Sodium diatrizoate
42

43. DICROMETRY
•TitrationinvolvingPotassiumdichromateinacidicsolutionbyusingdilutesulphuricacid.
•Itisadirecttitrationmethod.
•MainlyusedfortitrationofFerroussulphate.
•Potassiumdichromateisaverystrongoxidizingagent.
43

44. PRINCIPLE
•Potassiumdichromateishighpurityandhighstablecompound.
•Aqueoussolutionnotattackedbyoxidisableimpurities compositiondoesnotchangeon
keeping becauseofthispropertyitsanalternativeforpotassiumpermanganate.
•Aqueoussolutionstabletowardslight.
•Excellentprimarystandard.
•Titration only take place in acidic solution.
•In neutral solution Potassium dichromate turns out to be a mixture of dichromate and chromate
ion–Hydrolysis of dichromate ions to orange yellow chromate ions [weak oxidising agents].Cr
2
O
7
2-
14H
+
6e
-
2Cr
3+
7H
2
O
Cr
2
O
7
2-
6Fe
2+
14H
+
2Cr
3+
6Fe
3+
7H
2
O
+ + +
+ + + +
44

45. •Thus oxidising strength of dichromate is reduced in neutral solutions.
Preparation of 0.0167M Potassium Dichromate
•4.9g Potassium dichromate dried and kept in dessicatorfor 4hrs and dissolved in 1000ml distilled
water.
Standardization of 0.0167M Potassium Dichromate
20ml solution + 1g KI + 7ml 2M HCl + 250ml water
Liberated iodine is titrated with 0.1M Sodium thiosulphate
Using 3ml starch solution as indicator added towards end pointCr
2
O
7
2-
H
2
O 2CrO
4
2-
2H
+
orange
yellow colour
+ +
45

46. End point: colour change from blue to light green.
Detection of End Point
oDichromatenotusedasaselfindicator–becauseitsreductionproduct(Cr
3+
)isgreenwhich
hindersinthevisualdetectionofendpointbymaskingdichromatecolour.
oIndicators used in dichrometry:
1.Diphenyl amine in presence of ortho phosphoric acid
2.N-phenyl anthranilic acid
3.Potassium ferricyanide
oEnd point: appearance of reddish pink colour.Cr
2
O
7
2-
14H
+
6e
-
2Cr
3+
7H
2
O+ + +
46

47. AdvantageofDichromateoverPermanganate
•Itisobtainableinastateofhighpurityandcanbeusedasprimarystandardsolutionsof
dichromateinwater.
•Verystabletowardslightandatmosphericconditions.
ExamplesofDichrometry
•Determinationofironinsampleofironwire.
•Determineferrousandferricionsinasolution.
•Determineferricioninsolutionofferricalum.
•Determinevarioussaltsofiron.
47

48. CERIMETRY
•RedoxtitrationsinvolvingcericammoniumsulphateasanoxidizingagentiscalledasCerimetry.
•AlsoknownasCerateoximetry.
•Ceriumsulphateisastrongoxidizingagentandcanbeusedonlyinacidicsolution–H
2SO
4.
•Sincecericammoniumsulphateisastrongoxidizingagentwecandeterminevariousreducing
substancesbysimpletitration.
48

49. PRINCIPLE
•Sulphuricacidisusedtopreventhydrolysisandprecipitationofbasicsaltsasinneutralsolution
cerichydroxideorinbasicmediumasbasicsaltsprecipitate.
•CanreplaceKMnO
4asitsverystable.
•Useonlyinacidsolution
•Solutionisintenseyellowcolourinhotsolution–actasselfindicator.
•Oxidationreaction
•SimpleoneelectronreactionisthemainadvantageofCerimetry–complicationduetounstable
intermediateseliminated.Ce
3+
Ce
4+reduction Ce
4+
e Ce
3+
+
49

50. •InthepresenceofreducingagentitundergoesreductiontotheCerrousstate.
Preparationof0.1MCericAmmoniumSulphate
65gm Ceric ammonium sulphate + 30ml H
2SO
4, gentle heat + 500ml water
Cool solution and dilute to 1000ml with waterCe
4+
Fe
2+
Ce
3+
Fe
3+
Ce(SO
4
)
2
2FeSO
4
Ce(SO
4
)
3 2Fe(SO
4
)
3
+ +
+ +
50

51. Standardizationof0.1NCericAmmoniumSulphate
1.UsingArsenicTrioxide(As
2O
3)
•ReactionbetweenCeric(IV)SulphateandAs
2O
3isveryslow@ordinarytemperature Add
Osmiumtetroxidesolnwhichcatalysttoincreasethereactionrate.
•OxidationofSodiumarsenite[As
2O
3+NaOH] arsenatebycericcompound.
•Ferroinusedasindicatorgiveendpointascolourchangefrompinktopaleblue.As
2
O
3
6NaOH 2Na
3
AsO
3
3H
2
O+ + Ce(SO
4
)
2
Na
3
AsO
3
H
2
O 2Ce
2
(SO
4
)
3
Na
3
AsO
4 H
2
SO
4
4[Ce
4+
e Ce
3+
]
4Ce
4+
Na
3
AsO
3
+ + + +
+
=
51

52. •Procedure
0.2g Arsenic trioxide, dried @ 105
o
C, 1hr + 25ml 8% NaOH + 100ml water + 30ml dilH
2SO
4 +
0.15ml Osmic acid
Titrate with 0.1M Ceric ammonium sulphate
Using 0.1ml Ferroin sulphate solution as indicator
End point: Pink to pale blue
AdvantagesofCeric(IV)Sulphate
Stableoverprolongedperiods.
Neednotbeprotectedfromlight
52

53. EmployedindeterminationofreducingagentsinpresenceofahighconcentrationofHCl.
Cerium(III)ioniscolourlessanddoesnotobscusetheindicatorendpoint.
Veryversatileoxidisingagent.
BeststandardisedwithprimarystandardsArsenictrioxide.
0.1MofCerium(IV)sulphatesolutionnothighlycolouredanddoesnotobscusevisionwhen
readingmeniscusinburettes.
Nointermediateproductsareformed.
Ferroinisverysatisfactoryindicatorinfiltrationwithcericsalts.
53

54. DisadvantageofCeric(IV)Sulphate
Untilrecentlyceratemethodswerelimitedtoelectrometrictitrationstillthedevelopmentofa
satisfactoryindicator.
ThechangeofcolourofCerictoCerrousisfromlightyellowtocolourlessthustheendpointis
noteasilydiscriminable.
Ceratemethodsarelesseconomical.
Detection of End Point
Self indicator –bright yellow in colour
Internal indicators used:
a. N-phenyl anthranilic
54

55. b. Ferroin
c. 5,6 –dimethyl Ferroin
Examples for Cerimetry
oFerrous fumarate
oFerrous gluconate
oParacetamol
oFerrous sulphate
55

56. BROMIMETRY
•Redox titration which involve Potassium bromate is known as Bromimetry.
•Titration process in which bromination of a chemical indicator is observed.
•Potassium bromate –strong oxidising agent.
•Reaction take place in presence of acidic medium 1M HCl.
56

57. PRINCIPLE
•PotassiumbromateisreducedsmoothlytobromideinpresenceofHClandwhichistheoxidised
togivefreebrominebymorebromate.
•Attheendofthetitrationfreebromineappears.
•Bromineisveryvolatile,suchoperationshouldbeconducted@lowtemperatureinconicalflask
fittedwithgroundglassstopper.BrO
3
-
6H
+
6e Br
-
3H
2
O
BrO
3
-
5Br
-
6H
+
3Br
23H
2
O
++ +
++ + KBrO
3 5KBr 6HCl 3Br
2 6KCl 3H
2
O+ + + +
57

58. •ExcessbromineisdeterminediodometricallybyadditionofexcessofKIandtitrationofliberated
iodinewithstandardthiosulphatesolution.
Preparationof0.0167MPotassiumbromatesolution
•Dissolve3.34gofKBrO
3whichisdried@120
o
Cfor1–2hrs+15gKBrin1000mlofdistilled
water.
Standardizationof0.0167MPotassiumbromatesolution
Pipette 25ml of solution in iodine flask + 120ml water + 5ml HCl2I
-
Br
2
I
2 2Br
-
I
2
Na
2
S
2
O
3
Na
2
S
2
O
62NaI
+ +
+ +
58

59. Insert stopper in flask and shake it gently + 5ml KI, shake gently
Allow to stand for 5 mins in dark
Liberated iodine titrated with 0.1M Sodium thiosulphate solution
Using starch solution as indicator added towards the end
End point: disappearance of blue colour.
DetectionofEndPoint
•Indicatorsused:
a.Methylorange
59

60. b. Methyl red
c. Naphthalene black
d. Xylidine ponceau
Example of Bromimetry
•Assay of Isoniazide
•Hydroxyl amine
•Determination of arsenic or antimony
1. Assay of Isoniazid I.P
Determined by addition of Potassium bromide and direct titration with potassium bromate in
presence of HCl.
60

61. Bromine is released and reacts with isoniazid
Azo dye Methyl red solution used as indicator becomes decolourised by oxidation at the endpoint.KBrO
3 5KBr 6HCl 3Br
2 6KCl 3H
2
O+ + + + N
CO NHNH
2
BrBr OH
2
N
COOH
N
2
4HBr
Isoniazid
+ + ++
61

62. TITANOMETRY
•TitrationwhichinvolveastrongreducingagentTitanouschloride(TiCl
3)iscalledas
Titanometry.
•TiCl
3–strongreducingagents.
•IntroducedbyKnechtandHibbert.
62

63. PRINCIPLE
1. It is used for direct titration of ferric salts.
2. Analysis of Dye stuffs and in analysis of organic compounds.
•Dye stuffs –Azo dyes
Eg: Indigo carmine
Methylene blue
Brilliant green
Crystal violet
•Nitro compounds: Nitroglycerine, Nitroguanidine
63FeCl
3
TiCl
3
FeCl
2
TiCl
4+ + RNO
2
6TiCl
3
6HCl RNH
2
6TiCl
4
2H
2
O+ + + +

64. •Nitrasoand Azo compounds and Quinones
•Oxidation potential
H
+
ions involved in oxidation and potential of system is strongly dependent on acidity of solution.
Smaller the hydrogen ion conc. greater reducing power
64RNO 4TiCl
3
4HCl RNH
2
4TiCl
4
H
2
O
RN=NR' 4TiCl
3
4HCl RNH
2
R'NH
2
4TiCl
4
+ + + +
+ + + + Ti
3+
3H
2
O TiO
2+
2H
+
e+ ++ 2H
+
2Ti
3+
2Ti
4+
H
2
O+ +

65. •Greatdisadvantageerrorcannotbeexactlydeterminedinblankdetermination.
•TiCl
3–rapidlyoxidisedonexposuretoairduetoatmosphericoxygen.
•Precaution:MaintainCO
2orHaboveliquid.
PreparationofTitanouschloridesolution
InmarketTiCl
3availableas15–20%.
Solution mixed with equal volume of conc. HCl
To remove traces of sulphides boiled for a min and diluted with water to about 10 times the original
volume.
65

66. Give a solution approximately N/10 and which should be immediately transferred to the apparatus
in such quantity as to fill the bottle up to the neck.
Burette can be filled as required by releasing the upper of the two clips.
As solution is drawn from the bottle, its place is taken automatically by CO
2from a kippapparatus
charged with marble and HCl.
First burette –full of solution be rejected and a free current of CO
2should then be allowed to flow
through the apparatus with the lower clip open for 2 mins, in order to make sure that all oxygen has
been displaced
66

67. Standardization of 0.1N TiCl
3solution
•Standardized against Ferric ammonium sulphate
•Procedure
Prepare 0.1N solution of ferric ammonium sulphate in 10 % H
2SO
4.
67

68. Pipette 25ml this solution into a conical flask
Pass a current of CO
2through the liquid to replace the air in the flask and maintain a stream of CO
2
through the liquid throughout the titration
Add 10% NH
4SCN (indicator), solution turns intense red colour
Titrate with 0.1N TiCl
3till red colour disappears.
Stream of CO
2must be maintained throughout the titration
68

69. ExamplesforTitanometry
•EstimationofFerricion
•EstimationofMethyleneblue
1.EstimationofFerricIon
Thiocyanateisasuitableindicator
Ferricironhasbeenreduced,thered-browncolourofferricthiocyanatecomplexdisappears.
Procedure
50ml ferric solution + 5-10ml of 4N HCl or H
2SO
4, heat to 50 –60
o
C
69

70. Add 0.5 –1ml of 10% KSCN solution + 3 portions of about 300mg of NaHCO
3to drive air from
solution
Titrate with Titanouschloride solution
End point: colour intensity disappears
70FeCl
3
TiCl
3
FeCl
2
TiCl
4+ +

71. REDOX INDICATORS
•Theidealoxidation–reductionindicatorwillbeonewithanoxidationpotentialintermediate
betweenthatofthesolutiontitratedandthatofthetitrantandwhichexhibitsasharp,readily
detectablecolourchange.
•Anoxidation–reductionindicatorisacompoundwhichexhibitsdifferentcoloursinthe
oxidizedandreducedforms.
•Ideally,thereactionsshouldbereversibleandshouldgiveapreciseandeasilyobservablecolour
changetheend-pointanddonotdependonthespecificpropertiesoftheoxidizingorreducing
agentsused–suchindicatorsareRedoxindicators.
71

72. A. Internal Indicators
•Ferroin sulphate
•Diphenylamine
1. Ferroin Sulphate
o1,10 –phenanthroline –Iron (II) complex sulphate.
oFerroin sulphate is a bright red complex formed by combination of the base.
oFerroin sulphate complex indicator is readily oxidized reversibly with strong oxidizing agents.
oEnd point –pale blue in colour.
72N N
Ferroin sulphate

73. oSubstituted Ferroin indicators: Nitroferroin, Dimethyl ferroin.
2. Diphenyl amine
o1%w/v solution of diphenylamine in conc. Sulphuricacid.
oIt will give intense blue –violet colourationat the endpoint.
73NH
Diphenyl amine

74. B.ExternalIndicators
1.PotassiumhexacyanoFerrate(III)
oUsedintitrationofIron(II)bypotassiumdichromate.
oDropsofthesolutionremovedtoaspottingtileduringthetitrationwillgiveadeepPrussianblue
colourwithpotassiumhexacyanoferrate(III)becauseferrousionsarestillpresent.
C.SelfIndicator
1.Potassiumpermanganate
oVariousreducingagentsareoxidisedbypotassiumpermanganateinaacidsolution.
2.IodineSolution
oReducingagentscanbetitratedwithIodinesolutionwithoutusingindicator,becausethedark
74

75. browncolourofiodinedisappearsastheresultofreductionofI
2toI
-
ions.
3.Cerium(IV)Sulpate
oYellowincolourreducedtocolourlessions.
75