Electrochemistry for

analytical purposes

Examples for water analysis

Dr Riikka Lahtinen

Electrochemistry

Based on RedOx-reactions:

Reduction: receive electron(s)

Oxidation: give away electron(s)

Electrochemistry is

study of heterogeneous redox-reactions where

the oxidation and reduction reactions take place

at the surface of the representative electrodes.

Gibbs free energy and

potential

Potential is the inherent capacity for coming into being.

Electric potential: at a point, the work required to bring a

charge from infinity to that point in the electric field divided

by the charge. SI derived unit of electric potential is volt

V=J C-1.

Cu2++2e- Cu

Reduction of copper(II)

And

-> there is a direct relationship between the Gibbs free

energy of transfer and the potential E.

QnF

RTEE o ln

i

iiaQ

missä ,

QRTGG o ln

nFEG

Conductivity

17.2.2014 5

Conductivity as a measure for water quality

Ohm’s law: E=RI

Conductivity= R-1

Unit: siemens per cm

(S/cm)

Non-specific:

measures total amount

of ions.

Potentiometry

17.2.2014 7

A simple electrochemical

system

Solution II Solution I

Ecell Electrode

where

oxidation

takes place:

ANODE

separator

-will let ions through

Something to measure potential

Minimun reguirements for an electrochemical measurement: two electrodes in ionic

contact and a device (potentiometer) to measure the potential connected to the electrodes.

Electrode

where

reduction

takes place:

CATHODE

Electrochemical cell in equilibrium

[H+]=1 M

Fe3+

Fe2+

H2 Ecell

Platinum

Gold

porous membrane ANODE CATHODE

Cathode: Fe3++ e- →Fe2+

Anode : ½ H2 → H+ + e-

Total reaction: ½ H2 +Fe3+→ H+ + Fe2+

It is a virtual equilibrium

The potentiometer has very high impedance

-> no (or practically no) current goes through the system

-> a ”real” equilibrium can not be reached.

Cu | Pt | H+,H2 || Fe3+,Fe2+ | Au |Cu’

Galvani potential

Galvani potential is an electric potential difference

between points in the bulk of two phases. It is

measurable only when the two phases have identical

composition (e.g. two copper wires). It is the difference

of inner electric potentials in two phases, .

Equilibrium potential of the cell

Cu | Pt | H+,H2 || Fe3+,Fe2+ | Au |Cu’

a b s e g a’

In equilibrium, there will be following phase equilibriums:

phase a –phase b, phase b –phase s, phase s –phase e, phase e –

phase g ja phase g –phase a’.

When two phases are in equilibrium, their electrochemical

potentials are equal. Electrochemical potential is defined

by: Fziii

~

where ( )is the chemical potential,

zi the charge of i and the Galvani potential. i

i

o

i aRT ln

Phase equilibria (I):

ab ee

~~

phase a –phase b

sbs 2

~

2

1~~HeH

phase b –phase s the platinum electrode)

e

phase s –phase e

The liquid-liquid contact (porous membrane, salt bridge),

here it is assumed that the Galvani potential is equal on

both sides:

Phase equilibria (II):

ege 23~~~

FeeFe

phase e –phase g (the gold electrode)

phase g –phase a’ ag ee

~~

And using the definition of electrochemical potential:

abab ee

F

H

H

e

o

H

oH

a

aRTF

2/1

2

2ln

2

1)( bbs

2

3

23 ln)(

Fe

Fe

e

o

Fe

o

Fe a

aRTF geg

gaaga ee

F ''

The Nernst equation

HFe

HFeo

H

oH

o

Fe

o

Fe aa

aaRTF

2

23

223

2/1

' ln2

1)( aa

HFe

HFeo

Cellaa

aa

F

RTEE

2

2

3

2/1

ln

Combining the equations:

Which can be presented in the more familiar form as

as the Nernst equation

Standard potential of the cell

In Nernst equation, the Eo is the value of the cell potential

when all activities of the reacting species are 1.

HFe

HFeo

Cellaa

aa

F

RTEE

2

2

3

2/1

ln

o

HH

o

FeFe

o EEE2

23 //

oH

o

H

o

HHFE

22 2

1/

o

Fe

o

Fe

o

FeFeFE 3223 /

Standard potential of the cell can be diveded into two

Pt | H+,H2

where

Fe3+,Fe2+ | Au

corresponding

corresponding

The standard potential of the cell gives the work

necessary to transfer one electron between the

gold electrode and the redox-couple (Fe3+/Fe2+)

in solution, on a scale defined by the left-side

electrode at standard conditions.

nF

G

FE

ooH

o

H

o

Fe

o

Feo

223

2

1

Standard potential of the cell

Combining the half-reactions gives

Standard Hydrogen Electrode (SHE)

Reference electrode: the ”left-hand side

electrode” of the cell; the electrode against

which the potential is measured. o

HH

o

FeFe

o EEE2

23 //

SHE

Standard potential of SHE

It has been postulated, that the potential of SHE in all

temperatures at standard conditions is zero volts.

EoSHE≡ 0 V

Now the potentials of other half-cells can be determined by

measuring against SHE. These values are usually tabulated

as standard reduction potentials (the electrochemical

series). The tendencies of different species in relation to

each other can be compared based on these tables.

Example

Why is FeCl3 used to dissolve copper in the process of

making electrical circuits?

Solution: Standard reduction potentials:

• Fe3+ + e- -> Fe2+ Eo=0.77 V

• Cu2+ + 2e- -> Cu Eo=0.34 V

Let’s combine these to give a corresponding reaction:

2Fe3+ + 2e- -> 2Fe2+ Eo=0.77 V

Cu -> Cu2+ + 2e- Eo=-0.34 V

2Fe3+ + Cu ->2Fe2+ + Cu2+ Eotot=0.43 V

83kJ

C

J0.43

mol

C964852mol oo nFEG

Spontaneous reaction

Potential of silver/silver chloride

electrode

ClHo

H

so

AgAg

ClH

Hso

AgAg

aaF

RT

p

f

F

RTK

F

RTE

aa

aK

F

RTEE

lnlnln

ln

2/1

/

2/1

/

2

2

V0.222V0.577V0.799ln// s

o

AgAg

oAgAgCl K

F

RTEE

Ag++ e- →Ag

½ H2 → H+ + e-

ClHc

oAgAgCl cc

F

RTEE

HCllnlim 0/Experimentally:

Silver wire is coated (by

electrolysis) with silver

chloride, and immersed in a

chloride solution.

Half-cell reactions: 1 M

Important to note!

The potential value you measure is dependent on the

reference electrode you choose for the measurement!

Before you start to compare any potential values, it has to

be made sure, that they are presented on the same

potential scale.

Eocell=0,77 V Eo

cell=0,57 V

Absolute (vacuum) potential

scale

The standard potential of a redox-couple on the

absolute scale will determined from a

measurement with SHE:

FFE

o

H

o

H

S

M

esolutionmetalabso

oxred

22

1

,

/

a

Where aH+o is the real chemical potential of H+ and H2

o is

the standard chemical potential of gaseous H2.

From thermodynamic data:

H+(g)+e-(g) → H(g) Go= -13.613 eV

H(g) → ½ H2(g) Go= -2.107 eV

H+(g) → H+(aq) aH+o = -11.276 eV

Absolute potential scale

V44.4)107.2613.13(276.11,

/ 2

abso

HHE

V44.4,/Re

,/Re SHEo

Oxdabso

Oxd EE

The standard potential of a redox-couple on the

absolute scale can be calculated from potential

values on SHE scale :

Some references give this value as 4.6 V (or eV).

17.2.2014 25

•first ISE was the glass electrode

•contains as the membrane a

specific oxonium ion binding glass

•thefore the potential over this

membrane is dependent on

the pH

out

in

a

a

F

RTtconsE lntan

•requires calibration

Inner reference

electrode Ag/AgCl-

electrode

Ion selective electrodes:

Glass electrode

17.2.2014 26

•liquid membrane electrodes

•enzyme electrodes

•solid membrane electrodes (fluoride selective electrode

containing a LaF3-membrane below)

Reference-

electrode

Ag/AgCl-

electrode

Ion selective electrodes

17.2.2014 27

•calibration with known concentrations

•by calculations e.g. pH-meter (calibration with

buffer solutions)

•graphic calibration curve

•measure the potential E of the unknown sample

•possible error sources?

QnF

RTEE o ln

Measurement with ISE’s

17.2.2014 28

Potentiometry: measure

potential between galvanic

couples. Calibration gives

concentration.

Instrumental method:

measure a physical quantity that

is dependent on amount of

substance orconcentration

is

measuring system determination steps

1. reference electrode

2. indicator electrode

3. something to measure

potential e.g. voltmeter

measured

quantity

connection

between

measured

quantity and

concentration

Nernst:

QnF

RTEE o ln

Calibrate: calculate or calibration

curve

Measure: measure potential E in the

solutions, get c through

calibration

potential

E

Voltammetry/

amperometry

17.2.2014 29

The idea behind

amperometric sensors

Simple: the electrochemical current is directly

proportional to the concentration of the

electroactive species.

17.2.2014 30

Clark type oxygen sensors

17.2.2014 31

Operation voltage ca.

0,8 V

Measure CURRENT

which is directly

proportional to oxygen

concentration

Needs calibration

-no oxygen and

saturated oxygen

Ref: Falck Current

Separations 16(1) (1997) 19

The problem with current flowing

through an electrode

Amperometric methods measure current as a function of

potential.

Current flows- > a problem

(E=RI):polarisation of an

electrode i.e. the potential of the

electrode no longer corresponds

to the equilibrium value.

E

Polarisable and unpolarisable

electrodes

Ideally polarisable electrode: the

change in voltage results in no

change in current E

j

E

j

Blue: ideally polarisable electrode

Red: ideally unpolarisable electrode

Ideally unpolarisable electrode:

no change in potential when

current goes through the electrode

Ideal reference electrode would be an ideally unpolarisable

electrode, silver-silver chloride is quite unpolarisable.

The real life solution: three-electrode system

Three-electrode system

Vin Vref

Vout

RE

CE WE

Basic idea: to divide

reference electrode in two

parts: one to measure

potential and one to pass

current.

Working electrode (WE): the electrode where our

reaction of interest takes place. Potentiostat controls the

potential of the electrode and measures the current.

Reference electrode (RE): the electrode against which

the potential is measured.

Counter electrode (CE): the electrode through which the

current is passing

The ”old”

reference

electrode

divided

into two

Positioning the electrodes in a

three-electrode system (I)

Reference electrode and

working electrode should

be as close to each other

as possible because of iR-

drop.

If there is uncompensated

iR-drop in the system, it will

change the E values ( E=RI)

Often it is difficult to place the reference very close to the

working electrode, the solution to this problem is the use of

a Luggin capillary.

Solution -> finite conductivity-> resistance -> iR-drop.

Positioning the electrodes in a

three-electrode system (II)

The counter electrode should be

ideally symmetrically positioned

in relation to the working

electrode

Cell for the three-electrode measurements.

The solution for electrochemical

measurement

Our electroactive species of interest

Solvent to dissolve the electroactive species

Excess of inert electrolyte

Electrolyte:

compound that

produces ions i.e.

conductivity

Solvent:

Has to dissolve the desired

compound -> water is not

always possible

Has to have high enough

dielectric constant so that

ions will exist in the solutions

-> ions mean conductivity

Supporting electrolyte

Base electrolyte

Inert electrolyte

”Supports” i.e. carries the current

in the system

Ensures that the solution phase

has enough conductivity to reduce

iR-drop.

The need for a supporting electrolyte

iiii

iii cvcDRT

FzcDJ

Transport in electrolyte solution (Nernst-Planck equation):

where J is the flux of ion i, Di is

the diffusion coefficient of i, is

the velocity of the solvent. diffusion

migration convection

In usual systems, there is no convection.

The importance of migration is dependent on the transport number of

the species. Transport number states the proportion of the current an

ion carries in a solution. When inert electrolyte is added to the system

in excess (e.g. 100 times more than the electroactive species) the

transport number of the electroactive species is close to zero and the

migration term can be neglected.

-> analysis of the results is easier

The electrodes- WE

Working electrode

• Often some inert metal like platinum or gold, we

generally want it just to accept or give electrons and not

react with the solution in any other way

• Other choices would be other inert but very conductive

materials like glassy carbon, graphite or even ITO

• It is important, that the electrode can be repetitively

cleaned

– If you want to measure the quantity of the current, the

area of the electrode has to be known since current is

directly proportional to area

– The roughness of the electrode (which might come

from the cleaning procedure) affects the area

The electrodes- CE

Counter (auxiliary) electrode

• Inert and very conductive material

• Area should be much larger compared to the working

electrode

-> we do not want the reaction at this electrode (which

reaction, is usually not known and is not interesting) be

the rate determining reaction=smallest current in the

system

• typically platinum coil

• graphite rod

The current in electrochemical measurements is a direct measure of the reaction

rate. Electrochemistry is the only method which measures reaction rates directly.

The electrodes- RE

Reference electrodes

Stable potential through out the measurement

• Saturated calomel electrode (SCE) is one, especially in the

past, widely used electrode

• Eo(saturated KCl, 25 oC)= 0,2444 V vs SHE

• Eo(3,5 M KCl, 25 oC)= 0,250 V vs SHE

Note: the electrode potential is

dependent also on the chloride ion

concentration and possible junction

potentials of the system.

Remember to check the potential against e.g. a

commercial silver-silver chloride electrode after

preparing the electrode, and every now and

then (the potential difference between two identical electrodes

in the same solution should be zero. The concentration of

the chloride solutions should be the same in that case)

The electrodes- RE

Silver-silver chloride (Ag/AgCl)-electrode is the most

commonly used reference electrode now-a-days

• Easy to prepare

• Can be made small

• Eo(saturated KCl, 25 oC)= 0,199 V vs SHE

• Eo(3,5 M KCl, 25 oC)= 0,205 V vs SHE

How about organic

solvents?

•Generally, Ag/AgCl- or calomel electrodes can be used

•Problems might arise also from the leaking of chloride

ions or water into the the measuring solution

-> very water sensitive organic molecules cannot be

measured with these electrodes

•Another problem is related to solubility: the salts used in

these electrodes might not be very soluble in organic

solvents -> there might be some precipitation formation on

the electrode diminishing its area or precipitating on the

frit.

Reference electrodes for organic

solvents

• Silver-silver ion electrode

– Silver wire in silver ion (AgNO3)containing solution in organic

solvent (e.g. acetonitrile, THF, DMSO)

• Pseudo-reference electrodes

– A metal wire immersed in the studied solution (platinum or silver)

– Provide a constant potential, which is unknown and dependent

on the composition of the solution

– An internal standard should be used to tie the measured

potentials to the SHE scale

• IUPAC recommends the use of Ferrocene/Ferrocinium

couple

• In practice, after the measurement of the electroactive

species has been carried out, Ferrocene is added to the

system and the its oxidation potential is measured. The

assumption is, that this potential is the same in different

systems (Eo(water)= 0,40 V, Eo(MeCN)= 0,69 V, Eo(DMF)=

0,72 V, Eo(DMSO)= 0,68 V all vs. SHE)

Definition of signs for potential and

current

Positive potential means that the working electrode

potential is made more positive compared to the

reference electrode, i.e. it will be easier for it to accept

electrons.

By definition, the anodic current resulting from the oxidation

of some species in solution is positive.

Therefore, cathodic current, the reduction current, is

negative.

Note: Zero applied potential does not mean, that the potential of the working

electrode would be zero, and the current may or may not be zero at zero

applied potential.

Be careful when reading literature, not everybody respects these definitions...

Reactions at electrodes

Irreversible reaction: the mass transfer of the reactants and products is

rapid compared to the electron transfer reaction. Note, this has nothing

to do with the chemical reversibility of the reaction.

Reversible reaction: the electron transfer is rapid compared to the mass

transfer rate. The reaction is diffusion- or mass transfer limited.

Quasi-reversible: electron transfer and mass transfer take place in

comparable time scales.

Three steps:

1. Flux of reactants towards the electrode (diffusion)

2. Interfacial electron transfer reaction

3. Flux of the products from the surface towards the

solution (diffusion)

Steady-state current-potential curve

Current as a function of potential

is recorded.

Diffusion limited current:

Half-wave potential:

O

Ro

redOxD

D

nF

RTEE ln'

/2/1

RRanodicDnFAcI

lim,

Formal potential

If the ratio of the diffusion coefficients of the reduced and oxidiced species

are the same, the half-wave potential will give the formal potential of the

redox species. And if the solution is dilute, we can assume that the formal

potential and standard redox potential are close to each other.

Cyclic voltammetry

In cyclic voltammetry,

the potential is changed

from some initial value

Ei to some other value

Es and back while the

current is being

measured.

The current as a

function of potential is

presented.

t = 0 t = ts

Ei

Es

kulmakerroin = v slope= scan rate

Cyclic voltammogram

Cyclic voltammogram of a reversible system, this could be e.g. Ferrocene

Anodic peak current

Cathodic peak current

E1/2

(anodic) peak potential Ep Half peak potential Ep/2

These are values that should be defined from the measured

cyclic voltammograms.

Cyclic voltammetry- the parameters

OO 0,4463.0 DvRT

nFnFcj p

Peak current

nnF

RTEEp /0.2809.12/12/

Relation of half peak potential to half-wave potential (n is the

number of electrons transfered):

Ep = 59/n mV at 25 ºC for a reversible reaction (this is

the criteria for a reversible reaction)

Example:Cyclic voltammetry

in environmental Pb analysis

17.2.2014 51

Reference: Yang et al. Anal. Chem. (2013)

• WE: carbon fibre microelectrode (7 m

radius)

• RE: silver/silver chloride

• Fast scan cyclic voltammetry (scan rate 400

V s-1)

• Pb2+ adsorbs on carbon fibre -

>preconcentration

-> improved limit of detection (2 ppm)

Tested for real storm water samples. Idea is to

be able to do fast in situ measurements with no

sample preparation.

Differential Pulse Voltammetry

Same measuring system as in cyclic voltammetry. A

series of regular voltage pulses superimposed on the

potential linear sweep or stair steps. The current is

measured immediately before each potential change,

and the current difference is plotted as a function of

potential. By sampling the current just before the

potential is changed, the effect of the charging current

can be decreased.

The determined values:

peak potentials for anodic

and cathodic peaks

In organic solvents,

an internal standard used.

17.2.2014 53

Example: pesticides in water with

carbon paste electrode (CPE)

• Analyte: neonicotinoid

• WE: tricresyl phosphate based CPE

• RE: SCE

• CE: platinum

• Britton-Robinson buffer (pH 7) as supporting

electrolyte

• Analysed river water and commercial pesticide Actara

Example: pesticides in

water- DPV response

17.2.2014 54

Reference: Papp et al. J. Serb. Chem. Soc. 75(5)

(2010) 681.

17.2.2014 55

Voltammetry: control potential

and measure current between

two electrodes. Usually

calibration gives

concentration.

Instrumental method

is

measuring system determination steps

1. WE

2. RE (and CE for three-

electrode mode)

3. Potentiostat to control

potential and measure

current.

measured

quantity

connection

between

measured

quantity and

concentration

Directly

proportional

Current I

Calibrate: calculate or calibration

curve

Measure: Measure (limiting) current,

get c through calibration