Chapter 6 electrochemical analysis

www.themegallery.com

Chapter 6Electrochemical Analysis


Anode reaction:

Red === Ox + ne -

Cathode reaction:

Ox + ne - === Red

6.1 Introduction

1. Oxidation – reduction reaction

Cell reaction expression

Anodesolution,(Ox)solution, (Red) Cathode

(6r-1)

(6r-2)


For example:

Zn ZnSO4,(xMol) CuSO4, (yMol) Cu

Anode: Zn Zn2+ + 2e-

Cathode: Cu2 + + 2e- Cu

(6r-3)

(6r-4)


2. Half-cell Potential

For half – cell reaction :

rAred + ne- pAOx

Nernst equation:

For a Cell:

Ecell = Ecathode - Eanode

If, Ecell > 0: Primary Cell Ecell < 0: Electrolyic Cell

r

red

p

ox

nFRTEE ln0

(6r-5)

(6-1)

(6-2)


3.The Types of Electrodes

1) A metal in Equilibrium with its ions

(Class electrodes)Ⅰ

Ag+ + e- Ag

AgF

RT

AgAg

AgAg

EE ln0

(6r-6)

(6-3)

www.themegallery.com2) A metal in equilibrium with a saturated solution of a slightly soluble salt(Class electrodes)Ⅱ

AgAgCl Cl -,(=1)

AgCl(s) + e- Ag + Cl –

Reference electrodes

Saturated calomel electrode (SCE)

HgHg2Cl2(s)Cl -,(sat’d KCL)Hg2Cl2(s) + 2e- 2Hg + 2Cl –(sat’d KCL)

(6r-7)

(6r-8)


3) A metal in equilibrium with tow slightly soluble salts with a common Anion(Class electrodes)Ⅲ

AgAg2S,CdSAg+,Cd2+,S2-,

Ag2S(s) 2Ag++S2-

CdS(s) Cd2++S2-

(6r-9)

(6r-10)


4. The departure of potential1) Liquid-junction potential HCl(0.1M) KCl(salt bridge, xM) KCl(0.1M)

When x>3.6 Eljp<1mV

2) Polarization

Efact ≠ENernst and Csurf ≠Cbolk 3) Over-voltage real potential start a reaction > equilibrium potential

4) Ohm drop

Ecell = Ecathode - Eanode + IR R: resistance of solution, I: current (6-4)


6.2 Potentiometry

1. Principle

LJrefindicell EEEE

red

ox

nfRT

indiindi EE ln0

oxnfRT

indiindi EE ln0

oxntcons

oxnfRT

LJrefindi

LjrefoxnfRT

indicell

E

EEE

EEEE

lg

ln

ln

0591.0tan

0

0

(6-5)

(6-6)

(6-7)

(6-8)


2. Ion selective Membrane Electrode Structure of ISE

Types Fig 6-1


(1) The Glass Electrode

Ag︱ Agcl(s) ︱ HCl(inner) ︱ glass ︱ H+(unknown solution)

pHK

K

kE

outer

fRT

glassinner

outer

0591.0

lg0591.0

ln

(6-9)

Fig 6-2


pHK

pHKEE

EEEE

LjSCE

Ljglassrefcell

0591.0'

0591.0

pHK

K

kE

outer

fRT

glassinner

outer

0591.0

lg0591.0

ln

Glass electrode︱ unknown solution ︱ SCE

0591.0

'KEpH cell

(6-10)

(6-11)

(6-12)


Selectivity of Glass electrode

H+G-+M+(sol) M+ G- + H+ (sol)

MH

kk: selectivity coefficient

)lg(0591.0 MHglass kKE

)lg(0591.0 imiHglass kKE

(6-13)

(6-14)

(6r-11)


(2) The Response Behavior of ISE

Nernst response and Detect limit

MnfRT

ISEISE EE ln0 (6-15)

Fig 6-3


Selectivity

Response time

)ln( /.

0 jA

jj

nnM

potMAAnf

RTISEA KEE

(6-16)

Fig 6-4


(1) The Prerequisite of Experiments Ion Intensity Buffer

3.Quantitative Analysis

ionion fc

)( ,totleioncFf

c

fcK

gKE

n

n

n

lgK'

)lg( 0.0591

0591.0

0591.0

(6-17)

(6-18)

(6-19)

f_activity coefficient

If Cion,T≈constant, f ≈constant.


pH Buffer

MZ+ + xOH- M(OH)x (z-x)+

H+ + OH- H2O Complex reagent

M Z+ + nL MLnZ+

n

Zn

nz

Zn

[L]

]ML[

L]][M[

]ML[

cx

K

(6-20)

(6r-12)

(6r-13)

(6r-14)


c

ccc

x

x

x

K

1

)[L]1(

]ML[n

Zn0

)[L]1(1 nK

(6-21)

(6-22)

cffcx 0

),,(0L0M

Kf cc (6-23)

(6-24)


(2)Standard calibration Methods

C0 / molL-1 10-3 3.16x10-4 10-4 3.16x10-5 10-5

lgc -3 -3.500 -4 -4.500 -5

standard concentration series

If =1:

E = K + s lgC0

Fig 6-5


(3)Standard Addition Methods

XcfSKSKE 1111 lglg

X

SS

Xs

ss

V

VC

VV

VCC

CCVV

VCVCC X

SX

SSXXSX

)(lg

lg

22

22

ccfSK

cfSKE

x

SXSX

(6-25)

(6-26)

(6-27)

(6-28)


X

XSX cf

ccfSEEE

11

221

)(lg

X

X

c

ccSE

)(lg

CC SE

X

)110(

assume: f1=f2 , 1=2 , S = 0.0591/n

(6-29)

(6-30)

(6-31)


6.3 Polarography

1. Introduction(1) Electrolytic cellCathode:

M+ + e- →M Hg(l) M∣ +(C)︱ SCE

Wkg: Working Electrode

Ref:Reference Electrode(SCE)


(2) Polarization M +(Bulk) → M +(Cathode)

Fig 6-7


2. The Dropping Mercury Electrode(DME)

(1) Structure of DME

Fig 6-8


(2)Electrolytic current and current density

Fig 6-9


3. Quantitative Analysis

(1) Ilkovic Equation

ctmDnid

6/13/22/1

0.607

id

m ____rate of mercury flow

D ____diffusion coefficient

ckid

____Average diffusion current

(6-32)

(6-33)


(2)The factor of affect diffusion current

Residual current

• Changing current

• Migrating current

• Maximum phenomenon

• Oxygen interference


4. Qualitative Analysis

Half wave potential

iii

nfRT

edcdEE )(

2/1. ln

2/1

2/1

ln02/1

sa

as

Df

DfnfRTEE

(6-34)

(6-35)

Chapter 6 electrochemical analysis

Technology

e ag

e cathode

e cathode e anode

e cathode e anode

primary cell e cell

e cu

cathode fig

n e p