Electroanalysis

measure the variation of an electrical parameter (potential, current, charge, conductivity) and relate this to a chemical parameter (the analyte concentration)

Selectivity: by choice of operating parameters (potential, current etc…) and/or the electrode material

applications– environmental analyses– quality control– biomedical analyses etc

Fundamentals

Redox reactions

neMMMneM

OxddOx

naanaa )()(

2121

:examples

reductant oxidant

Re Re

Electrochemical Cells

galvanic:spontaneous chemical reactions to produce

electrical energy (ΔG = -nFE, negative)

• applications: batteries, potentiometry (pH, ISE)

electrolytic:utilisation of energy (ex: applied V) to force a chemical rxn

to take place (ΔG +)

• applications: coulometry, voltammetry

Galvanic Cells- +

CathodeAnode

Cd(s) --> Cd 2+(aq) +2e -

Cd

e-

AgSalt bridge

Cl - K+

Cd2+

AgNO3(aq)

NO3-

V

CdCl2

2Ag +(aq) + 2e - -->2Ag(s)

line notation (shorthand)| interface between two phases. || salt bridge

Cd(s) | CdCl2(aq, M) || AgNO3(aq, M) | Ag(s)

Half-Reactions

(aq)Cd+2Ag(s)Cd(s)+(aq)2Ag :net

_____________________________

2e+(aq)Cd(s) Cd :

2Ag(s)2e+(aq)2Ag :

+2+

-+2

-+

anode

cathode

Ecell= Ecathode (+) - Eanode(-)

Standard Potentials

To predict the reactivity of oxidants or reductants we need to measure the potential of each half-reaction.impossible!!....for every oxidation we have a reduction reaction

• Define a standard half-cell of potential = 0V against which all other half-cell reduction potentials are measured (with the std half-cell attached to the negative terminal of the potentiometer). Each component in these standard cells having unit activity (pressure, conc.)

Standard Hydrogen Electrode

- +e-

AgSalt bridge

AAg+=1

V=+0.799V

AH+=1

AH2=1

Pt(s) | H2(g, A=1) | H+(aq, A=1) || Ag+(ag, A=1) |_________________________|

NHEH+(aq, A=1) + e- 1/2H2(g, A=1) E0=0 V

Electrochemical Series

Reduction half-reactions oxidant reducing agent E0 (V)

stronger oxidant F2(g) + 2e- 2F- 2.890 Ce4+ + e- Ce3+ 1.720 Ag+ + e- Ag(s) 0.799 Fe3+ + e- Fe2+ 0.771 O2 + 2H+ + 2e- H2O2 0.695 Cu2+ + 2e- Cu(s) 0.339 2H+ + 2e- H2(g) 0.000 Cd2+ + 2e- Cd(s) -0.402 Zn2+ + 2e- Zn(s) -0.762 K+ + e- K(s) -2.936 Li+ +e- Li(s) -3.040 stronger reducer

Nernst Equationfor a half-rxn

aOx + ne- bRed

R= gas constant T= temperature in Kelvinn= no. of electrons in half-rxnF= Faraday constant (96485 C/mol)A= activity (= 1 for a pure solid, liquid or solvent and expressed in mol/L for solutes and in bar for gases)

)ln( Re0

a

b

Ox

d

A

A

nF

RTEE

Nernst Equation

Converting ln to log10 (x 2,303) and at 25oC (298.15K)

)log(0592.0 Re0

a

b

Ox

d

A

A

nEE

Potentiometry

the measure of the cell potential to yield chemical information (conc., activity, charge)

Measure difference in potential between two electrodes:

reference electrode (E constant)indicator electrode (signal α analyte)

Reference electrodes

Ag/AgCl:Ag(s) | AgCl (s) | Cl-(aq) || .....

- +

Ag/AgClSalt bridge

KCl

Pt

Fe2+, Fe3+

- +

Ag

Soln. aq. satdin KCl + AgCl

Pt

Fe2+, Fe3+AgCl + KCl

AgCl

Porous glass

AgCl(s) + e - <=> Ag(s) + Cl -

E0=0.222V

Fe3+ + e - <=> Fe2+

E0=0.771VE(KCl sat.)=0.197V

Reference Electrodes

SCE:

Pt(s) | Hg(l) | Hg2Cl2 (l) | KCl(aq., sat.) ||.....

Hg(l)

Soln. sat. in KCl

Pt

KCl

Hg, Hg2Cl2 et KCl

Porous glass

E0=0.268V

E(KCl sat.)=0.241VGlass wool

Hg2Cl2 + 2e - <=> 2Hg(l) + 2Cl -

Indicator Electrodes

• Inert:Pt, Au, Carbon. Don’t participate in the reaction.

example: SCE || Fe3+, Fe2+(aq) | Pt(s)

• Certain metallic electrodes: detect their ions(Hg, Cu, Zn, Cd, Ag)example SCE || Ag+(aq) | Ag(s)Ag+ + e- Ag(s) E0

+= 0.799VHg2Cl2 + 2e- 2Hg(l) + 2Cl- E0

-= 0.241V

E = 0.799 + 0.05916 log [Ag+] - 0.241 V

Ion selective electrodes (ISEs)

A difference in the activity of an ion on either side of a selective membrane results in a thermodynamic potential difference being

created across that membrane

C a 2 + C a 2 + 0 . 0 1 M C a 2 +

0 . 0 2 M C l -

0 . 1 M C a 2 +

0 . 2 M C l -

( 0 . 1 + ) M C a 2 + ( 0 . 1 - ) M C a 2 +

0 . 0 2 M C l - 0 . 2 M C l -

+

+

+

+

-

-

-

-

Calcium selective molecular recognition ligand

ISEs

25C) (@log0592.0

ln

ln

2

1

2

1

2

1

A

A

nA

A

nF

RTE

nFEA

ARTG

Combination glass pH Electrode

Ag

Soln. aq. satdin KCl + AgCl

AgCl(s) + KCl(s)

AgCl porousglass

+ -

0.1M HCl inAgCl sat.

Combination glass pH Electrode

int ref

int+H

analyte

ext ref

Ag(s)|AgCl(s)|(aq)-Clint),(aq,Hext)(aq,H||(aq)-Cl|AgCl(s)|Ag(s)

β~ 1, constant = asymmetric potential (2 sides are not identical)

)log()0592.0(constantint

H

H

A

AE ext

Other ISEs

• by changing the composition of the glass, ISE selective for different ions can be fabricated

• By replacing the glass with a perm-selective barrier incorporating a selective binding agent (ion-exchanger, host, doped crystal) ISEs for different ions can be fabricated

Voltammetry

The measurement of variations in current produced by variations of the potential applied to a working electrode

polarography:• Heyrovsky (1922): first voltammetry experiments using a

dropping mercury working electrode

In voltammetry, once the applied potential is sufficiently negative, electron transfer occurs between the electrode and the electroactive species: Cu2+ + 2e → Cu(Hg)

• Hg liquid metal (surface can be renewed)

The polarogrampoints a to b

I = E/Rpoints b to c

electron transfer to the electroactive species.

I(reduction) depends on the no. of moecules

reduced/s: this rises as a function of Epoints c to d

when E is sufficiently negative, every molecule that reaches the electrode

surface is reduced.

Reactions that can be studied using voltammetry

amalgam-forming metallics;reducible metallic ions e.g. Fe(III) → Fe(II);reducible anions e.g. chromate, iodate...reduction of molecular oxidants e.g. NO2, O2, H2O2...reduction of organics, e.g. ketones, quinones, aldehydes, peroxides...

Glucose Monitoring and Diabetes

Diabetes is a serious disease, and, with its complications, is the fourth leading cause of death by disease in the

United States. Its causes are unknown, and there is no cure.

Testing: A Crucial ToolBlood tests, done by pricking the finger for a drop of blood, are recommended by most doctors because they give the

exact amount of blood sugar at any given moment.There are an estimated 14 million diabetics in the U.S.

It is recommended to test blood glucose levels at least 4 times daily.

Market of 56 millions disposable tests per day!!

Voltammetric Blood Glucose Monitors

An example of a test is the use of the immobilised enzyme glucose oxidase, which releases electrons on interaction with glucose. A sensor of this type could detect the level of glucose in the blood.

E le c tro ns

R ed u ced M ed ia to rO x id is ed M ed ia to r

E N ZY M E

G lu co s eG lu co n o lac to n e

Glucose Biosensors

Glucose + O2 Gluconic acid + H2O2 GOD

Glucose + mediator(ox) Gluconic acid + mediator(red) GOD