potentiometry_1clinic

7/24/2019 potentiometry_1clinic

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Electrochemistry

By

Prof Salwa Rizk El-Shabouri


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References

-Analytical Chemistry An Introuction

Se!enth Eition

"ou#las A$Skoo#% "onal $&est% '$(amews-)o#el s *e+tbook of ,uantitati!e inor#anic

analysis

-Christian $"$ .Analytical Chemistry/-/0ner#rauate Instrumental Analysis/

(ames &$ Robinson


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Electroanalytical Chemistry...

It encompasses a group of quantitative

analytical methods that are based upon

the electrical properties of a solution of

the analyte when it is made part of an

electrochemical cell.


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Electro-analytical chemistry is the field of electrochemistry

that utilizes the relationship between chemical phenomenawhich involve charge transfer (e.g. redox reactions, ion

separation, etc.) and the electrical properties that

accompany these phenomena for some analytical

determination. This relationship is further broken down into

fields based on the type of measurement that is made.

Potentiometryinvolves the measurement of potential for

quantitative analysis, andelectrolytic electrochemicalphenomena involve the application of a potential or current

to drive a chemical phenomenon, resulting in some

measurable signal which may be used in an analytical

determination.

Electro-analytical Chemistry


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Why Electroanalytical

Chemistry?4 Electroanalytical methos ha!e certain a!anta#es o!er

other analytical methos$ Electrochemical analysis

allows for the etermination of ifferent o+iation states

of an element in a solution% not 5ust the total

concentration of the element$

4 Electroanalytical techni,ues are ca6able of 6roucin#

e+ce6tionally low etection limits an an abunance of

characterization information incluin# chemical kinetics

information$ *he other im6ortant a!anta#e of thismetho is its low cost$


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Terminology for edo! eactionsTerminology for edo! eactions

4 89I"A*I8:89I"A*I8:;loss;lossof electron increase in o+iation number>>

increase in o+y#enincrease in o+y#en$$

4 RE"0C*I8:RE"0C*I8:;;#ain#ainof electronof electronecrease in o+iation number> ecrease inecrease in o+iation number> ecrease in

o+y#en> increase in hyro#eno+y#en> increase in hyro#en$$

4 89I"I?I: AE:*89I"I?I: AE:*;electron acce6tor>;electron acce6tor>

s6ecies is reuce$s6ecies is reuce$

4 RE"0CI:RE"0CI:AE:*AE:*;electron;electrononor>onor>

s6ecies is o+iize$s6ecies is o+iize$

4 89I"A*I8:89I"A*I8:;loss;lossof electron increase in o+iation number>>

increase in o+y#enincrease in o+y#en$$

4 RE"0C*I8:RE"0C*I8:;;#ain#ainof electronof electronecrease in o+iation number> ecrease inecrease in o+iation number> ecrease in

o+y#en> increase in hyro#eno+y#en> increase in hyro#en$$

4 89I"I?I: AE:*89I"I?I: AE:*;electron acce6tor>;electron acce6tor>s6ecies is reuce$s6ecies is reuce$

4 RE"0CI:RE"0CI:AE:*AE:*;electron;electrononor>onor>

s6ecies is o+iize$s6ecies is o+iize$


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Currents in Electrochemical

Cells

4 8hms law is usually obeye

EIR

whereEis the 6otential ifference in !olts


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D

"alvanic Cells

-

CathoeAnoe

C


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8+iation 6otential K reuction

6otential

4 8+iation 6otential the 6rouct of the half reacti

in the o+iation state

4 ----- 2E

2e4 Reuction 6otential the 6rouct is in thereuce

form

4 E2 2e ----- 4 &here is the metal atom

4 E2is the metal ion

4 e is the electron


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Malf cell an electroe 6otentialN &hen a 6iece of metal is immerse in a solution of its

own ions% a 6otentialifferencewill be create at theinterface of the metal an the solution$ *he ma#nitue

of the 6otential ifference is the measure of thetenency of a chemical s6ecies either an element orion to uner#o an o+iation or reuction$

N *he element combination% an Ecombination is ahalf-cell$

N *he reaction is a half-reaction$N *he immerse metal is an electroeN *he 6otential ue to the reaction at the interface of

the electroe an the solution is the electroe6otential


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Electroe 6otential


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Stanar electroe 6otential EL

EL% of a half reaction is efine as its electroe 6otential when theacti!ities of the reactants an 6roucts are all unity$

'or the same cell % the ELfor the half-reaction

A# e ----- A#

Can be obtaine by measurin# Ecellwith the acti!ity of A#e,ual to1$LL$ Inthis case the cell can be re6resente schematically as

Pt%M2


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Stanar electroe 6otential% EL

*he sil!er electroe is 6ositi!e with res6ect to the stanarhyro#en electroe$ *here for the stanar electroe6otential is #i!en a 6ositi!e si#n% an we write

A#E e --- A# ELA#EOA# L$@JJ)

Incontrast to the sil!er electroe%the camium electroene#ati!e with res6ect to the stanar hyro#enelectroe$ Conse,uently% the stanar electroe 6otentialof the C2EOC cou6le is #i!en a ne#ati!e si#n% an

ELC

2E

OC -L$L )$$Because% the cell 6otential is ne#ati!

the s6ontaneous cell reaction is not the reaction aswritten% that is% o+iation on the left an reuction on theri#ht$ Rather the s6ontaneous reaction is in the o66ositeirection


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A zinc electroe immerse in a solution ha!in#azinc ion acti!ity of unity e!elo6e a 6otential of-L$@7 )when it is the ri#ht-han electroe6aire with a stanar hyro#en electroe in the

leftEL?n2EO?n -L$@7 )

*he stanar electroe 6otentials for the four half-cells 5ust escribe can be arran#e in the

followin# orerA#E F ME F C2E F ?n2E

L$@JJL$LLL-L$L-L$@7


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A ne#ati!e stanar reuction 6otential for a meta-ion 6air means

that uner stanar conition the metal will reuce the hyro#en

ion$ *he 6rouct of the reaction bein# the metal ion an

hyro#en #as

'or e+am6le the stanar reuction 6otential for the zinc ion-zinc

metal 6air as state by the e,uation an corres6onin# stanar

6otential

?n2 2 e ---- ?n EL - L$@7 )

Is ne#ati!e therefore% zinc ion will reuce hyro#en ion in solutionaccorin# to the reaction

?n 2 M ---- ?n2 M2

A reuction 6otential with a 6ositi!e !olta#e means that the metal

ion will be reuce by hyro#en #as to the corres6onin# metal

2A# M2 ---- 2 A# 2M2A# M2 ---- 2 A# 2M2A# M2 ---- 2 A# 2M


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%ydrogen-silver galvanic cell

- Ee-

A#Salt bri#e

AA#1

)L$@JJ)

AM1

AM21

Pt


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1@


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1D

Stanar Reuction Potentials

Reuction

6otentials for

manyelectroes

ha!e been

measure an

tabulate$


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Electrochemical &eries

Reduction half-reactions oxidant reducing agent E0(V)

stronger oxidant

F2(g) + 2e-2F- 2.890

Ce++ e-Ce!+ ".#20

$g++ e-$g(s) 0.#99

Fe!++ e-Fe2+ 0.##"

%2+ 2&++ 2e-&2%2 0.'9

Cu2++ 2e-Cu(s) 0.!!9

2&++ 2e-&2(g) 0.000

Cd2+

+ 2e-

Cd(s) -0.02 n2++ 2e-n(s) -0.#'2

*++ e-*(s) -2.9!'

i++e-i(s) -!.00

stronger reducer


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'!idi(ing and educing )gents

4 *he stron#est

o+iizers ha!e the

most 6ositi!ereuction 6otentials$

4 *he stron#est

reucers ha!e the

most ne#ati!ereuction 6otentials$


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Electrochemical Cells

galvanic

s6ontaneous chemical reactions to 6rouce

electrical ener#y

N a66lications batteries% 6otentiometry


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Types of electrochemical cells

Electrochemical cells are eitherElectrochemical cells are either

Qal!anical!anic

oror Qelectrolyticelectrolytic$$

al!anic or !oltaic% cells% cells store electrical ener#y$$

An electrolytic cell%in contrast%%in contrast% re,uires an e+ternal

source of electrical ener#yfor o6eration$


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4al!anic cell Electrolytic cell

- Chemical reaction occurs- Chemical reaction occurs to -Electrical ener#y is useto -Electrical ener#y is use

6rouce electrical ener#y to force6rouce electrical ener#y to force< store ener#y= non-s6ontaneous chemical= non-s6ontaneous chemical

reaction occur$reaction occur$

$$

- "irection of current is re!erse$- "irection of current is re!erse$

4 al!anic cell


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Electrolytic cellElectrolytic cell al!anic cellal!anic cell


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Electrolytic cell*

- electrical energy is used to force

nonspontaneous chemical reaction

occur.

-direction of the current is reversed

-the reaction at the electrodes are

reversed


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*he terms #al!anican electrolyticare escri6ti!e of thecell 6rocess not the cell com6osition$ E+am6le theautomobile battery is a #al!anic while .ischar#in#/ anbecomes an electrolytic cell urin# the .char#in#/6rocess

ost electrochemical cells contain metallic an electrolyticconuctors$

etallic conuctor the current is carrie by electronsthrou#h the metal with no chan#e in the com6osition ofthe conuctor$

Electrolytic conuctor conuctance by the flow of either6ositi!e or ne#ati!e ions an uner#oes a chan#e in thechemical com6osition simultaneously with the mo!ementof the irect current$


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If the electron flow fromthe metallic to

electrolytic conuctor the electroe is

calle the cathoe$

If the irection of electron flow is the re!erse

of this the electroe is the anoe$

In any at theelectrochemical cell o+iation

occurs atthe anoe an reuction occurs

cathoe$


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2D

+oltaic Cells

4 A ty6ical cell looks

like this$

4 *he o+iationoccursat theanoe$

4 *he reuction

occurs at the

cathoe$


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+oltaic Cells

4 *herefore% we use a

salt bri#e% usually a

0-sha6e tubethat

contains a saltsolution% to kee6 the

char#es balance$

N Cations mo!e towar

the cathoe$NAnions mo!etowar

the anoe$


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L

+oltaic Cells

8nce e!en one

electron flows from

the anoe to thecathoe% the

char#es in each

beaker woul not be

balance an theflow of electrons

woul sto6$


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&alt ,ridge

4 Conuctin# solution ofsoluble salt


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&alt ,ridge

It consist of a #el such as a#ar-a#ar in which an

inert electrolyte is issol!e such as GCl$ *here

is no 6otential at the interfaces of the salt bri#e

an the solutions$ *he rates of iffusion of the GE

an Cl- are close to each other$*he 5unction

6otential is minimize$

*he use of metallic wire as a bri#e between the

two solutions woul be unsatisfactory$ *hiswoul be e,ui!alent to introucin# aitional

half-cell into the system


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Cell otentials

4 'or the o+iation in this cell%

4 'or the reuction%

Ere L$@7 )

Ere L$ )


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4 'or !oltaic cell to continue to 6rouce

e+ternal electric$ ne#ati!e ions in solutionmo!e from ri#htleft to balance electron

flow in e+ternal circuit

N etal ions cannot mo!e between electroes

4 Porous membrane or salt bri#e 6ro!ies

selecti!e mo!ement of


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Voltaic cells can be described by a line notation based on the

following conventions.

Single vertical line indicates change in state or phase.Within a half-cell, the reactants are listed before the

products.

Activities of aqueous solutions are written inparentheses after the symbol for the ion or molecule.A double vertical line indicates a salt bridge or junction

potential between half-cells may be ignored

The line notation for the anode (oxidation) is writtenbefore the line notation for the cathode (reduction).

The line notation for a standard-state Daniell cell is written

as follows.

ine /otation 0or +oltaic Cellsine /otation 0or +oltaic Cells


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Zn | Zn2+(1.0 M) || Cu2+(1.0 M) | Cuanode

(oxidation)

cathode

(reduction)

Electrons flow from the anode to the cathode in a voltaic

cell. (They flow from the electrode at which they are givenoff to the electrode at which they are consumed.) Reading

from left to right, this line notation therefore corresponds to

the direction in which electrons flow.


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Calculation of Cell otential from

Electrode otentials

4 Nernst Equation:

4 *he cell 6otential for a !oltaic cell uner

stanar conitions can be calculate

from the stanar electroe 6otentials$

But real !oltaic cells will ty6ically iffer

from the stanar conitions$

4 *he :ernst e,uation relates the cell

6otential to its stanar cell 6otential$


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/ernst Equation

for a half-r+na8+ ne-bRe

R#as constant *tem6erature in Gel!innno$ of electrons in half-r+n

''araay constant


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:ernst E,uation

)log(

092.0 Re0

a

b

Ox

d

A

A

nEE


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:ernst E,uation

4 E 6otential of the half reaction

4 Eo stanar reuction 6otential of the

half reaction

4 n no$ of electron in half reaction

4 8$L3J1 substitutin# for the !alues of R

an ' an calculatin# * for 23o

C

The /ernst Equation 0or Complete


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The /ernst Equation 0or Complete

Cell

+ = EEE

Here E+and E-are the potentials of the half cells

connected to the positive and negative terminals ofpotentiometer respectively. Lets consider an

example.


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&tandard Cell otentials

*he cell 6otential at stanar conitions

can be foun throu#h this e,uation

Ecell Ere


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Cell otentials

4 'or the o+iation in this cell%

4 'or the reuction%

Ere L$@7 )

Ere L$ )


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Cell otentials

Ecell Ere


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Concentration Cells

4 :otice that the :ernst e,uation im6lies that a cell

coul be create that has the same substance at

both electroes$4 'or such a cell% woul be L% but Qwoul not$Ecell

4 *herefore% as lon# as the concentrations

are ifferent% Ewill not be L$


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Electrochemistry &1illsElectrochemistry &1ills

4 0nerstan how !oltaic cells work$4 Be able to calculate stanar reuction 6otentials

for !oltaic cells% #i!en the chemical reactions$

4 Be able to escribe a !oltaic cell usin# the line

notation an !isa !ersa$ Gnow which way

electrons flow an where the anoe an cathoe

are$

4 Gnow how to work with the :ernst E,$ to inclueconcentration e6enencies an calculate

e,uilibirum constants$


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Electromoti!e 'orce


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Electromotive 0orce #emf$

4 *he 6otential ifference between the

anoe an cathoe in a cell is calle the

electromoti!e force


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+oltaic Cells

4 In the cell% then%electrons lea!e the

anoe an flow

throu#h the wire to

the cathoe$

4 As the electrons

lea!e the anoe% the

cations formeissol!e into the

solution in the

anoe com6artment$


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3L

+oltaic Cells

4 As the electronsreach the cathoe%

cations in the

cathoe are

attracte to the nowne#ati!e cathoe$

4 *he electrons are

taken by the cation%an the neutral

metal is e6osite

on the cathoe$


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&tandard %ydrogen Electrode

4 *heir !alues are reference to a stanar

hyro#en electroe

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