Complexometric titration-ppt

Complexometric titration

1. BY
Dr. G.C.Wadhawa
DEPARTMENT OF CHEMISTRY
K. B. P. College,Vashi,Navimumbai
Complexometric titration

2. Complexometrictitration
AComplexometrictitrationistheonein
whichthereactionbetweentheanalyte
andtitrantinvolvestheformationofa
soluble,undissociated complex.
Complexometric reactioninvolved
formationofco-ordinatecovalentbond
betweendoneratomandacceptor.This
leadstotheformationofastoichiometric
complexwhichformsthebasisof
quantitative Complexometric
determinations
2

3. Basic Terms:
Complexion:Itisagrouporentityin
whichametalatomorioniscoordinately
bondedtooneormoreelectrondonating
group.
Ligand:Themoleculeorionwhich
donatesatleastonelonepairofelectrons
toacceptorlikemetalatomorion.
MonodentateLigand:Aligandthathasa
singledonorgroup,suchasammonia,is
calledunidentate(single-toothed),
.
3

4. BidentateLigand:whereasonesuchas
glycine,whichhastwogroupsavailablefor
covalentbonding,iscalledbidenate.
Tridentate:Three doner group,
tetradentate:fourdentategroup,
pentadentate:fivedonergroupand
hexadentate:sixdonergroup.
StabilityConstant:Thecomplex
formationreactionisreversiblereaction
whichattainsequilibrium
4

5. EDTA as Chelating agent
5

6. ADVANTAGES OF EDTA
EDTAformsstablecomplexwithvariousmetal
ions.
Thecomplexationoccursinsinglestepand
hencethetitrationofametalproducesasharp
changeinthemetalionconcentrationatthe
equivalencepoint.
TheM-EDTAcomplexesarewatersolubleand
henceallstudiescanbecarriedoutin
aq.solutions.
EDTAforms1:1complexwithallmetalions
irrespectiveofchargeonthemetal.
TheStoichiometryforallmetalionsissame.
6

7. Limitations:
ManyM-EDTAtitrationsarecarriedoutin
solutionsofrelativelyathighpHwhichmay
leadstodifficultiesduetohydrolysisof
certainmetalions.
SinceEDTAformsstablecomplexwith
mostcationsthereagentslacksselectivity
ifitisusedtoestimatesinglecationin
mixture
7

8. DETERMINATION OF EQUIVALENCE
POINT
Theequivalencepointofacomplexationtitration
occurswhenstoichiometricallyequivalentamounts
ofanalyteandtitranthavereacted.
FortitrationsinvolvingmetalionsandEDTA,the
equivalencepointoccurswhentheconcentrations
ofthemetalionandEDTAareequal.
Theaccuracyoftheendpointdependsonthe
relativestrengthofthemetal–indicatorandmetal-
titrantcomplex.
Ifthemetal-indicatorcomplexistoostrong,the
colorchangeoccursaftertheequivalence
point.
Ifitistooweak,theendpointisobservedbefore
reachingtheequivalencepoint 8

9. The role of Metallochromic
Indicators
In
Complexometric titrations
9

10. Metallochromic
Indicators
metal-
indicator
complex
Chelate-metal
complex
Low stability
constant
10
High
stability
constant
At equivalence point Indicator get
free to produce colour

11. Mechanism of Indicator
11
Zn
+2
Colorless
EDTA
Colorless
EBT
Blue
Wine red
Color
Complex
is formed

12. Therequirementofametalionindicatorfor
useinthevisualdetectionofendpoints
include:
(a)Thecolourreactionmustbesuchthatbefore
theendpoint,whennearlyallthemetalionis
complexedwithEDTA,thesolutionisstrongly
coloured.
(b)Thecolourreactionshouldbespecificorat
leastselective.
(c)Themetal-indicatorcomplexmustpossess
sufficientstability,otherwise,becauseof
dissociation,asharpcolourchangeisnot
obtained.
(d)Themetal-indicatorcomplexmustbelessstable
thanthemetal-EDTAcomplextoensurethat,atthe
endpoint,EDTAremovesmetalionsfromthe
metalindicator-complex..
12

13. The change in equilibrium from the metaI
indicator complex to the metal-EDTA
complex should be sharp and rapid
(e) The colour contrast between the free
indicator and the metal-indicator complex
should be such as to be readily observed.
( f ) The indicator must be very sensitive to
metal ions (i.e. to PM) so that the colour
change occurs as near to the equivalence
point as possible.
(g) The above requirements must be fulfilled
within the pH range at which the titration is
performed.
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14. TYPES OF EDTA TITRATIONS
Direct
Titrations
Back
Titrations
Replacement
Titrations
Alkali metric
Titrations
14

15. Types of EDTA titrations
DirectTitration:Itisthesimplestandthe
mostconvenientmethodinwhichthe
standardsolutionofEDTAisslowlyaddedto
themetalionsolutiontilltheendpointis
achieved.
Forthismethodtobeusefultheformation
constantmustbelargeandtheindicatormust
provideaverydistinctcolorchange.
Someimportantelementswhichcouldbe
determineddirectlybythecomplexometric
titrationareCu,Mn,Ca,Ba,Br,Zn,Cd,Hg,
Al,Sn,Pb,Bi,Cr,Mo,Fe,Co,Ni,andPd,etc.
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16. Back Titration:
This methosis suitable when;
The reaction of metal ion with EDTA is
slow ,
The metal ion precipitates.
No suitable indicator is available
Inthismethod,anexcessofastandard
solutionofEDTAisaddedtothemetal
solutionbeingdeterminedsoasto
complexallthemetalionspresentinthe
solution.TheexcessofEDTAleftafterthe
complexformationwiththemetalisback
titratedwithastandardsolutionofa
secondmetalion.Usuallystandardmetal
ionsolutionusedareZnCl
2
,
ZnSO
4.MgCl
2.
16

17. Replacement Titration:
Whendirectorbacktitrationsdonotgivesharp
endpointsorwhenthereisnosuitableindicatorfor
theanalytethemetalmaybedeterminedbythis
method.
Themetaltobeanalyzedisaddedtoametal-
EDTAcomplex.Theanalyteion(withhigherKf_)
displacesmetalfromtheEDTAandthemetalis
subsequentlytitratedwithstandardEDTA.
Forexample,inthedeterminationofMnanexcess
ofMgEDTAchelateisaddedtoMnsolution.The
MnionsquantitativelydisplaceMgfromMg-EDTA
solutionbecauseMnformsamorestablecomplex
withEDTA.
Mn
+
+MgY
2-
(MY)
(n-4)+
+Mg
2+
ThefreedMgmetalisthendirectlytitratedwitha
standardsolutionofEDTAusingEriochromeblack
Tindicator.
Ca,PbandHgmayalsobedeterminedbythis
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Free metal ion

18. Indirect Titration:
Certainanionsthatformprecipitatewith
metalcationsanddonotreactwithEDTA
canbeanalyzedindirectly.SO
4
-2
,PO
4
-3
,
canbedeterminedbythismethod.
e.g.SO
4
-2
canbedeterminedbyadding
excessofBa
+2
toprecipitateasBaSO
4
.
Theprecipitateisfilteredandwashed.
TheexcessBa
+2
inthefiltrateisthen
titratedwithEDTA.
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19. Methods of Increasing
Selectivity in Complexometric
Titrations
This in turn increases the applicability of
complexometric titrations based on EDTA.
Some of the common methods to increase
selectivity are as follows are as follows.
• Use of masking and demasking agents
• pH control
• Classical separation
• Solvent extraction
• Kinetic masking
19

20. Methods of Increasing
Selectivity in
Complexometric Titrations
Use of masking
and
demaskingagents
pH control
Classical
separation
Kinetic
masking
20

21. MaskingandDemasking
Maskingmaybedefinedastheprocessin
whichasubstancewithoutthephysical
separationofitoritsreactionproductsis
sotransformedthatitdoesnotparticipate
inareaction.Insimplewordsitsreactionis
masked.
Inoneofthewaysofmasking,the
maskingagentactseitherbyprecipitation
orbyformationofcomplexesthataremore
stablethantheinterferingion-EDTA
complex. 21

22. Ex. Of Masking and demasking:
AmixtureofZnandMgcanbedetermined
bytreatingthemixturewithKCNwhich
wouldformacomplexwithZnionandthe
magnesiumionscanbetitratedwithEDTA.
ThemaskedZnionscanbelibratedor
demaskedbytreatingwithaldehydessuch
asformaldehydeandtitratedwithEDTA.
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23. Determination of three metal at time
by masking and demasking
1.InthefirststepanexcessofstandardEDTAisaddedtothe
mixtureandtheremainingEDTAisbacktitratedwithastandard
solutionofMg2+ionsusingsolochromeblackasindicator.This
providesthesumoftheconcentrationsofallthethreemetals
present.
2.Inthesecondstepaportionofthemixtureistreatedwithan
excessofKCNsoastomasktheZnandCuionsintermsoftheir
cyanidecomplexes.OntitrationwegettheamountofMgonly.
3.Inthenextphaseanexcessofchloralhydrate(ora3:1
solutionofformaldehydeandaceticacid)isaddedtothetitrated
solution.ThisliberatestheZn2+fromthecyanidecomplex.The
solutionisnowtitrateduntiltheindicatorturnsblue.Thisgivesthe
amountofZnonly.
4.Knowingtheamountsofmagnesiumandzinc,theamountof
coppercanbedeterminedbysubtractingtheamountsofMgand
copperfromthetotalamountofthemetalionsobtainedinstep1.
23

24. Determination of three metal at
time by masking and
demasking
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Zn, Cu, Mg
+EDTA Excess
and back
titration =
Total amount
KCN added
Zn, Cu masked
as cyanide
Mg titrated
with EDTA
Formaldehyde is
added
Zn is free and
titrated against
EDTA
Amount of
Cu= Amount
of Mg -
Amount of Zn

25. pHcontrol
Thismethodisbaseduponthedifferencesinstabilityof
thecomplexesformedbetweenthemetalionsandthe
chelatingagent.Asyouhavelearntearlier,theformation
ofametalchelateisdependentonthepHofthereaction
medium.Inweaklyacidsolution,thechelatesofmany
metalssuchasalkalineearthmetalsarecompletely
dissociated,whereaschelatesofBi,Fe3+orCrare
readilyformedatthispH.Thus,inacidicsolution,Bican
beeffectivelytitratedwithachelatingagentinthe
presenceofalkalineearthmetals.Amixtureofbismuth
andleadionscanbesuccessfullytitratedbyfirsttitrating
thebismuthatpH2withxylenolorangeasindicator,and
thenaddinghexaminetoraisethepHtoabout5,and
titratingthelead.
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26. Classicalseparation
Theseareattemptedonlybeappliedif
theyarenottedious;furtheronlythose
precipitatesmaybeusedfor
separationsinwhich,afterbeingre-
dissolved,thecationscanbe
determinedcomplexometrically.Some
oftheexamplesareCaC2O4,nickel
dimethylglyoximate,andCuSCN.
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27. Solvent extraction
Solvent extraction may sometimes be
employed for selectivity. In this method a
metal ion in the mixture can be converted into
a complex that can be extracted by a suitable
solvent and then determined by EDTA. For
example, Zinc can be separated from copper
and lead by adding excess of ammonium
thiocyanatesolution and extracting the
resulting zinc thiocyanatewith 4-
methylpentan-2-one (isobutyl methyl ketone);
the extract is diluted with water and the zinc
content determined with EDTA solution.
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28. Kineticmasking
Thisisaspecialcaseinwhichthe
complexationofmetalionistooslowtobe
effective.Inotherwords,themetaliondoes
notformacomplexduetoitskinetic
inertness.Forexample,thereactionof
chromium(III)withEDTAisquiteslow.Itis,
therefore,possibletotitrateothermetalions
whichreactrapidlywithoutinterferencefrom
Cr(III).Determinationofiron(III)and
chromium(III)inamixtureisatypical
example.

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29. Thank you
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