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