lecture 9, potentiometgnfgry, pakinaz - copy

7/24/2019 Lecture 9, Potentiometgnfgry, Pakinaz - Copy

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Electrochemical Methods of AnalysisElectrochemical Methods of Anal

ysis

Electrochemical methods depend onElectrochemical methods depend onmeasuring ofmeasuring of electrical quantityelectrical quantitywhich iswhich is

in directin direct relation torelation to concentration ofconcentration of

substance analysed.substance analysed.

Advantages of electrochemical methodAdvanta

ges of electrochemical method::

1-1-Short time needed for analysis.Short time needed for analysis.

2-2-t is suitable for determination oft is suitable for determination of

colored or turbid solution.colored or turbid solution.


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Electrochemical Methods of AnalysisElectrochemical Methods of Ana

lysis

When a piece of metal is immersed in aWhen a piece of metal is immersed in asolution of its own ions:solution of its own ions:

a potential differencea potential differencewill be created atwill be created atinterface of metal and solution.interface of metal and solution.

Magnitude of potential difference is theMagnitude of potential difference is themeasure of the tendency of an element or ionmeasure of the tendency of an element or ionto undergo an oxidation or reduction,to undergo an oxidation or reduction,

i.e. to lose or gain electrons.i.e. to lose or gain electrons.


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Electrochemical Methods of AnalysisElectrochemical Methods of A

nalysis

The elemention,The elemention, M and MM and M!"!"combinationcombination is ais ahalf cellhalf cell,

,

TheThe reaction isreaction is a half reactiona half reaction,,

TheThe immersed metal isimmersed metal is an electrodean electrode,,

and theand the

##otentialotential due to thedue to the reaction at the interface ofreaction at the interface ofelectrode and the solutionelectrode and the solutionisis the electrodethe electrode

potential

potential..


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An electrode, which in the figureAn electrode, which in the figure

are the plates of $inc %&n' andare the plates of $inc %&n' and

copper %(u'.copper %(u'.

An electrolyte,An electrolyte,

which in the figurewhich in the figure

are a)ueousare a)ueoussolutions of $incsolutions of $inc

sulfate %&n*+sulfate %&n*+' and' and

copper%--' sulfatecopper%--' sulfate%(u*+%(u*+'.'.


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Electrochemical Methods of AnalysisElectrochemical Methods of Analysis

Two systems are in use for the expression of anTwo systems are in use for the expression of an

electrode potential.electrode potential.

+xidation potentials in which product of the+xidation potentials in which product of thehalf cell is in thehalf cell is in the oxidi$ed stateoxidi$ed state

M / MM / M!"!" " !e" !e

metal ionmetal ion

! 0eduction potential in which product is in the! 0eduction potential in which product is in thereduced formreduced form

MM!"!" " !e" !e // M M


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Electrochemical cells:Electrochemical cells:

An electrochemical cell consists of twoAn electrochemical cell consists of two

conductors called electrodes each of whichconductors called electrodes each of whichis immersed in an electrolyte solution.is immersed in an electrolyte solution.


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(athodes and Anodes(athodes and Anodes::

!!athodeathode is the electrode at which a reductionis the electrode at which a reductionreaction occurs.reaction occurs.

AAnodenodeis the electrode at which an o"idation ta#esis the electrode at which an o"idation ta#esplace.place.

E"amples of typical cathodic reactions includeE"amples of typical cathodic reactions include::

Ag$ $Ag$ $

ee

Ag%Ag%

ss&&

'e'e($($ $$ ee 'e'e2$2$

)*)*((-- $ 1+,$ 1+,$$ $$ 1e1e ),),$$ $ (,$ (,22**


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ypical anode reaction includesypical anode reaction includes::

!u%!u%ss& !u& !u2$2$$ 2e-$ 2e-

2!l2!l-- !l !l22%%gg& $ 2e-& $ 2e-

'e'e2 $2 $ 'e 'e( $( $$ e-$ e-


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Types of electrochemical cellsTypes of electrochemical cells

Electrochemical cells are eitherElectrochemical cells are either

// 0alvanic0alvanic

oror //electrolytic.electrolytic.

0alvanic or voltaic0alvanic or voltaic cells cells store electrical energystore electrical energy..

An electrolytic cellAn electrolytic cellin contrastin contrast requires an e"ternalrequires an e"ternal

source of electrical energysource of electrical energyfor operation.%consumefor operation.%consume

electricity&electricity&

'or both galvanic and electrolytic cells'or both galvanic and electrolytic cellseductioneductionalways ta#es placealways ta#es place at the cathodeat the cathode

33 *"idation*"idationalways ta#es placealways ta#es place at the anodeat the anode..


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0alvanic cell0alvanic cell Electrolytic cellElectrolytic cell

- !hemical reaction occurs to -Electrical energy is used- !hemical reaction occurs to -Electrical energy is used

produce electrical energy to forceproduce electrical energy to force

% store energy% store energy& non-spontaneous chemical& non-spontaneous chemicalreactionreaction

occur.occur.

..

- 4irection of current is- 4irection of current isreversed.reversed.

0alvanic cell %A&: !hemical reaction occurs to produce

electrical

energy Electrolytic cell %5&:Electrolytic cell %5&: electrical energy is used to force )onelectrical energy is used to force )on

spontaneous chemical reaction occurspontaneous chemical reaction occur -4irection of the current is reverse-4irection of the current is reverse -he reaction at the electrodes are reversed-he reaction at the electrodes are reversed


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Electrolytic cellElectrolytic cell 0alvanic cell0alvanic cell

# t ti t


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#otentiometry#otentiometry

6otentiometry is an electrochemical method of6otentiometry is an electrochemical method of

analysis based on the use of potentialanalysis based on the use of potentialmeasurements for determining concentration.measurements for determining concentration. 6rinciple6rinciple: solution containing an analyte of: solution containing an analyte of

un#nown concentration is determined by dippingun#nown concentration is determined by dippinga suitable sensor %indicator electrode& in itsa suitable sensor %indicator electrode& in its

solution.solution. %indicator electrode constitute an electric half-cell&%indicator electrode constitute an electric half-cell&

.. 6otentia6otential of the indicator electrode isl of the indicator electrode is measuredmeasured

relative torelative to reference electrodereference electrode%second half-cell&%second half-cell&possessing constant potential.possessing constant potential.

6otential6otential of an indicator electrode is measured asof an indicator electrode is measured asaa functionfunctionofof volume of titrant added.volume of titrant added.


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#otentiometric cell :#otentiometric cell :

To ma2e a potential measurementTo ma2e a potential measurement,,

//A complete cell consisting of two halfcells must be setA complete cell consisting of two halfcells must be setup.up.

//+ne halfcell usually consists of+ne halfcell usually consists of::

Test solution Test solution !An electrode whose!An electrode whose potential is determined by thepotential is determined by the analyteanalytewe wish to measure.we wish to measure. This electrode isThis electrode is the indicator electrodethe indicator electrode..

//+ther halfcell is any arbitrary halfcell+ther halfcell is any arbitrary halfcellwhose potentialwhose potentialis not dependent on the analyte.is not dependent on the analyte.

This halfcell electrode is calledThis halfcell electrode is called the reference lectrodethe reference lectrode..-ts-ts potential is constant,potential is constant,


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#otentiometric cell :#otentiometric cell :

Measured cell potential reflects theMeasured cell potential reflects the

indicator electrode potential relati3e toindicator electrode potential relati3e to

that of reference electrode.that of reference electrode.

*ince*ince reference electrode potential isreference electrode potential is

constantconstant,,

any changes in potential of the cell isany changes in potential of the cell is

attibuted to the indicator electrode andattibuted to the indicator electrode andtherefore to a change in analytetherefore to a change in analyteconcentration.concentration.


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Typical cell for potentiometric analysis :Typical cell for potentiometric analysis :

0eference electrode0eference electrode 44 salt bridgesalt bridge 44 analyte solution 4 indicatoranalyte solution 4 indicator electrodeelectrode

5alf 6 cell #re3ent the component immersed in5alf 6 cell #re3ent the component immersed inaa

%2nown electrode of analyte soln. solution of%2nown electrode of analyte soln. solution of #otential' That is from mixing with analyte#otential' That is from mixing with analyte

independent of those of reference electrode % E depends onindependent of those of reference electrode % E depends on

(oncentration. conc. of(oncentration. conc. ofanalyte'analyte'

of the analyte or anyof the analyte or any other ions in theother ions in the

solution. %Treated as anode'solution. %Treated as anode'


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The areThe are two basic ways a celltwo basic ways a cellmay be setmay be set

up, either without or with a salt bridgeup, either without or with a salt bridge

(ells without li)uid 7unction(ells without li)uid 7unction

!(ells with li)uid 7unction!(ells with li)uid 7unction


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8i)uid 7unction potential8i)uid 7unction potential

8i)uid 7unction potential8i)uid 7unction potential::

is a potential that de3elopedis a potential that de3eloped across the interfaceacross the interfacebetween two solutions that differ in their electrolytebetween two solutions that differ in their electrolytecomposition .composition .

The 7unction potential is minimi$ed by usingThe 7unction potential is minimi$ed by using saltsaltbridgebridgein which electrolyte is )uite concentrated andin which electrolyte is )uite concentrated andcation and anion ha3e e)ual mobilities.cation and anion ha3e e)ual mobilities.

99""

and (land (l

ions represent such a case and salt bridgeions represent such a case and salt bridgeof saturated a)ueous 9(l is widely used.of saturated a)ueous 9(l is widely used.


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*alt bridge*alt bridgeis included tois included to completecompleteelectrical circuitelectrical circuitbut withoutbut withoutintroducing any more bits of metalintroducing any more bits of metal

into the system into the system pre3ent mixing of thepre3ent mixing of thetwo solutions;.two solutions;.

Electrolyte in the salt bridge isElectrolyte in the salt bridge ischosen so that itchosen so that it doesn


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Salt bridges allow the flow ofSalt bridges allow the flow of chargecharge

between thebetween the o"idation and reductiono"idation and reduction

vesselsvesselswhile #eeping them separated.while #eeping them separated.


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The salt bridgeThe salt bridge

//he salt bridge is included to complete thehe salt bridge is included to complete the

electrical circuit but without introducing any moreelectrical circuit but without introducing any morebits of metal into the system.bits of metal into the system.

//t is 7ust a glass tube filled with an electrolyte li#et is 7ust a glass tube filled with an electrolyte li#epotassium nitrate solution.potassium nitrate solution.

//he ends are 8stoppered8 by bits of cotton wool.he ends are 8stoppered8 by bits of cotton wool.his stops too much mi"ing of the contents of thehis stops too much mi"ing of the contents of thesalt bridge with the contents of the two bea#ers.salt bridge with the contents of the two bea#ers.

//he electrolyte in the salt bridge is chosen sohe electrolyte in the salt bridge is chosen sothat it doesn9t react with the contents of eitherthat it doesn9t react with the contents of eitherbea#er.bea#er.


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Types of Electrodes =ApplicationsTypes of Electrodes =Applications

1-1- eference electrodeseference electrodes2-2- ndicator electrodesndicator electrodes

Applications and titration curve %normalApplications and titration curve %normal

first and second titration curves&first and second titration curves&


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0eference electrodes0eference electrodes

0eference electrodes0eference electrodes::

!ompletes the!ompletes thecell butcell but- doesnt respond to %react with& analyte.- doesnt respond to %react with& analyte.

- is separated from test solution by- is separated from test solution bya salt bridgea salt bridge..

Ideal reference electrode has a potential that isIdeal reference electrode has apotential that is

Accurately #nownAccurately #nown 'i"ed'i"ed !onstant!onstant

and completely nsensitive to analyteand completely nsensitive to analytesolution.solution.


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Example of reference electrodeExample of reference electrode::

.(alomel electrodes:.(alomel electrodes:

A calomel electrode can be representedA calomel electrode can be representedschematically asschematically as

,g ; ,g,g ; ,g22!l!l2%2%satd&

Electrode reaction in calomel half-cells isElectrode reaction in calomel half-cells is

,g,g22!l!l22%s& $2e-%s& $2e- // 2,g%l& $2!l2,g%l& $2!l --%aq&%aq&

,ere the reference electrode is based on the,ere the reference electrode is based on the

reduction ofreduction of ,g,g22!l!l22to ,g in an aqueous solutionto ,g in an aqueous solutionsaturated with

>arge e"cess of arge e"cess of

potential.potential.


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-ndicator electrodes :-ndicator electrodes :

?is one that?is one that measure analyte concentration@measure analyte concentration@An ideal indicator electrodeAn ideal indicator electrodeshouldshould

respond rapidly and reproducibly to changesrespond rapidly and reproducibly to changes

in concentration of an analyte ionin concentration of an analyte ion

ndicator electrodes are either:ndicator electrodes are either:

metallicmetallicoror membranemembrane::


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ndicator electrodes are either:ndicator electrodes are either:

1-1-

=etallic electrodes=etallic electrodes

a-a- Electrodes of the first kindElectrodes of the first kind

b-b-Electrodes of the second kindElectrodes of the second kind

cc-- Inert redox electrodeInert redox electrode

or 2-or 2- =embrane electrodes=embrane electrodes

a- 0lass electrodea- 0lass electrode

b- >iquid membrane electrode.b- >iquid membrane electrode.


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- Metallic indicator electrodes:- Metallic indicator electrodes:

They are classified intoThey are classified into:: A-A- Electrodes of the first kindElectrodes of the first kind

B-B-Electrodes of the second kindElectrodes of the second kind

C-C-Inert redox electrodeInert redox electrode



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A- Electrodes of the first kind:A- Electrodes of the first kind:

(cations sensitive electrode)(cations sensitive electrode) BBAn electrode of the first #ind consists of aAn electrode of the first #ind consists of a

pure metal electrode that is in directpure metal electrode that is in direct

equilibrium with its cations in the solution.equilibrium with its cations in the solution.

//A single reaction is involved.A single reaction is involved.

'or e"ample equilibrium between a metal'or e"ample equilibrium between a metalC and its cation CC and its cation C n$n$is:is:

CC n$n$%%aqaq& $& $ n en e-- DD C% C%s)s)


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A- Electrodes of the first kind:A- Electrodes of the first kind:

'or e"ample:'or e"ample:

!upper immersed in !upper salt %!uS*!upper immersed in !upper salt %!uS*&& ,g immersed in ,g!l,g immersed in ,g!l2.2.

// 6b immersed in 6bS*6b immersed in 6bS*

// Ag immersed in Ag)*Ag immersed in Ag)*((ElectrodeElectrode potentialpotential depends ondepends on concentration ofconcentration of

its own ions.its own ions.

Ag electrode potential is responsive to any changeAg electrode potential is responsive to any changeof the concn. of Ag $ of the concn. of Ag $

becomingbecoming more positive upon addition of ?Agmore positive upon addition of ?Ag$$@@

andand more negative when ?Agmore negative when ?Ag$@$@decreasesdecreases


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Electrode of the first #ind is not suitable forElectrode of the first #ind is not suitable for

the determination of an anion in its half cell.the determination of an anion in its half cell.

nsteadnstead Electrodes of the second tyeElectrodes of the second tyeisis

used as for exa!leused as for exa!le

"ilver-silver chloride electrode"ilver-silver chloride electrode


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B- Electrodes of the second tyeB- Electrodes of the second tye::

"ilver-silver chloride electrode"ilver-silver chloride electrode

An electrode ofAn electrode of silversilverwire iswire is coatedcoated withwithanan insoluble saltinsoluble salt%%Ag!lAg!l&of the metal %Ag&&of the metal %Ag&

and its anion %!l-& and isand its anion %!l-& and is immersedimmersedininaa

solution of the same anionsolution of the same anion %


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=etals not only serves as indicator electrodes for=etals not only serves as indicator electrodes for

their own cations %Ag$&their own cations %Ag$&

but alsobut also responds to the activities of anions %!l-&responds to the activities of anions %!l-&

that forms sparingly soluble precipitate or stablethat forms sparingly soluble precipitate or stablecomple" with such cations.comple" with such cations.

6otential of silver electrode for e"ample6otential of silver electrode for e"ample

correlates reproducibly with the activity of !lcorrelates reproducibly with the activity of !l --in ain asolution saturated with Ag!l.solution saturated with Ag!l.

,ere electrode reaction can be written as:,ere electrode reaction can be written as:

Ag!l%Ag!l%ss& $ e-& $ e- // Ag% Ag%ss& $ !l& $ !l--%%aqaq&&


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6otential of silver electrode for e"ample6otential of silver electrode for e"ample

correlates reproducibly with the activity ofcorrelates reproducibly with the activity of

!l!l--in a solution saturated with Ag!l.in a solution saturated with Ag!l.,ere electrode reaction can be written as:,ere electrode reaction can be written as:

Ag!l%Ag!l%ss& $ e-& $ e- // Ag% Ag%ss& $ !l& $ !l--%%aqaq&&

f we measure the !lf we measure the !l-- anionanionB AnyB Any increase in !lincrease in !l-- decreases the potentialdecreases the potential

B AnyB Any decrease in !ldecrease in !l-- increases the potential increases the potential


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!- nert redo" electrode:!- nert redo" electrode:

%edo" sensitive electrodes&%edo" sensitive electrodes&

E"ample platinum electrodeE"ample platinum electrode: it is usually: it is usuallyused to detectused to detect changes in concentrationchanges in concentrationof aof a

sample ofsample of o"idiedo"idiedandand reduced ionsreduced ionsas aas a

sample of !esample of !e$$;!e;!e($($ or sample of 'eor sample of 'e($($;'e;'e2$2$


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-- Membrane electrodes-- Membrane electrodes::

Example:Example:

>lass electrode>lass electrode

The most con3enient method for determining p5 .The most con3enient method for determining p5 .

>lass electrode is composed of:>lass electrode is composed of:

a sil3ersil3er chloride electrodea sil3ersil3er chloride electrodeas an inneras an innerreference electrode immersed inreference electrode immersed in a solution ofa solution of

2nown 5"; %?.M 5(l'2nown 5"; %?.M 5(l'

This solution isThis solution is enclosed in a thin glass membraneenclosed in a thin glass membraneo3er which potential is de3eloped since it iso3er which potential is de3eloped since it is

highly sensiti3e to 5"highly sensiti3e to 5"


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-t in3ol3es measurement of potential that-t in3ol3es measurement of potential that

de3elops across a thin glass membranede3elops across a thin glass membrane

which separates two solutions withwhich separates two solutions with

different 5different 5""conc.conc.

>l l t d


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>lass electrode>lass electrode

>lass electrode>lass electrode: The electrode is made of special glass highly: The electrode is made of special glass highly sensiti3e to 5sensiti3e to 5"";,;, As thin walled bulbAs thin walled bulb sealed to the bottom of ansealed to the bottom of an

ordinary glass tube.ordinary glass tube.

-nside is-nside is ?. @ 5(l?. @ 5(landand

immersed in which a sil3er wire coated with Ag(l.immersed in which a sil3er wire coated with Ag(l.

Thus comprisesThus comprises anan inner Ag4Ag(l referenceinner Ag4Ag(l reference electrodeelectrode..

or determination of p5or determination of p5

>lass electrode is immersed in un2nown p5 solution>lass electrode is immersed in un2nown p5 solution

and is coupled toand is coupled to saturated calomel electrode %externalsaturated calomel electrode %externalreference electrode'.reference electrode'.

#otential that de3eloped across the thin glass#otential that de3eloped across the thin glass

membrane which separates two solutions of differentmembrane which separates two solutions of different55""conc. will indicate un2nown 5"; and so p5 isconc. will indicate un2nown 5"; and so p5 isdetermined.determined.


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-- Membrane electrodes:-- Membrane electrodes:


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Applications of potentiometryApplications of potentiometry

Applications of potentiometryApplications of potentiometry

1.4irect potentiometric meausurements1.4irect potentiometric meausurements

2.ndirect potentiometric titrations2.ndirect potentiometric titrations

#otentiometric titrations:#otentiometric titrations:


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#otentiometric titrations:#otentiometric titrations:

-n a potentiometric titration-n a potentiometric titration potentiapotentiall of anof anindicator electrode is measuredindicator electrode is measured as a functionas a function

of 3olume of titrant added.of 3olume of titrant added.

Titration cur3e is obtainedTitration cur3e is obtainedby plotting e.m.f.by plotting e.m.f.readings againstreadings against 3olume of the titrating3olume of the titrating

solution.solution. E)ui3alence pointE)ui3alence pointof reaction will be detectedof reaction will be detected

by a sudden change in potentialby a sudden change in potential

Buring titration analyte solution must beBuring titration analyte solution must bethoroughlythoroughly stirredstirred


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Titration cur3e in potentiometryTitration cur3e in potentiometry

@ormal plot %sigmoid @ormal plot %sigmoid

shape'shape'


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@ormal plot %sigmoid shape' @ormal plot %sigmoid shape'

)ormal plot:)ormal plot:

A direct plot of potential %E in millivolts&A direct plot of potential %E in millivolts&

versus volume of titrantversus volume of titrant

!.irst deri3ati3e plot:!.irst deri3ati3e plot:


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pp

he rate of change of potential with additionhe rate of change of potential with addition

of titrant is ma"imum at the end point.of titrant is ma"imum at the end point.


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C.*econd deri3ati3e plot

C.*econd deri3ati3e plot

2E; F2

=athematically second

derivative of a titrationcurve should pass through

ero at the equivalence

point.

lecture 9, potentiometgnfgry, pakinaz - copy

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