POLAROGRAPHY/ VOLTAMMETRY
 Introduction
 Instrumentation, common techniques
 Direct Current (DC) polarography
 Mercury electrodes (DME, SMDE, HMDE)
 Polarographic currents
 Tast polarography
 Ilkovič equation
 Half-wave potential, limited diffusion current
 Logarithmic analysis
 Current maxima
 Brdička reaction
 Analytical applications
POLAROGRAPHY/ VOLTAMMETRY
 Linear Sweep Voltammetry (LSV)
 Cyclic Voltammetry (CV)
 Normal Pulse Polarography (NPP) or
Voltammetry (NPV)
 Differential Pulse Polarography (DPP) or
Voltammetry (DPV)
 Square Wave (SW) Polarography or Voltammetry
 Alternating Current (AC) Polarography or
Voltammetry
 Elimination Polarography (EP)
 Elimination Voltammetry with Linear Scan (EVLS)
Jaroslav Heyrovský
* Dec. 20, 1890, Prague, Bohemia, Austro-Hungarian Empire [now Czech Rep.]
† March 27, 1967, Prague, Czechoslovakia
Jaroslav Heyrovský was an inventor of
the polarographic method, father of
electroanalytical chemistry, recipient of
the Nobel Prize (1959). His contribution
to electroanalytical chemistry can not be
overestimated. All voltammetry methods
used now in electroanalytical chemistry
originate from polarography developed by
him.
Introduction
Picture of the first polarograph designed by J. Heyrovský and
M. Shikata (1924). This instrument is saved in the museum of J.
Heyrovský Institute of Physical Chemistry.
Introduction
Introduction
 Polarography is an voltammetric measurement whose response is
determined by combined diffusion/convection mass transport.
Polarography is a specific type of measurement that falls into the
general category of linear-sweep voltammetry where the electrode
potential is altered in a linear fashion from the initial potential Ei
to the final potential E. As a linear sweep method controlled by
convection/diffusion mass transport, the current vs. potential
response of a polarographic experiment has the typical sigmoidal
shape. What makes polarography different from other linear
sweep voltammetry measurements is that polarography makes use
of the dropping mercury electrode (DME).
 A measure of current as a function of potential when the working
electrode is a dropping mercury (or other liquid conductor)
electrode and unstirred solutions are used.
I = f (E) E = Ei  v t
E – electrode potential, v – scan rate, t - time, I - current
also LSV - Linear Sweep Voltammetry or CV – Cyclic Voltammetry
v = dE/dt
Instrumentation, common techniques
Two-electrode set
Three-electrode set
Instrumentation, common techniques
Instrumentation, common techniques
EcoTribo Polarograph
Polaro Sensors - Eco Trend
Prague, Czech Republic
Instrumentation, common techniques
Electrochemical
analyzer
AUTOLAB
Autolab
Ecochemie
Utrecht
The Netherlands
VA-Stand 663
Metrohm
Zurich
Switzerland
Autolab 20
Autolab 30
Mercury electrodes (DME, SMDE, HMDE)
Despite its toxicity, the metallic mercury has been used as an
electrode material for decades and is the original material for
polarography.
Dropping Mercury Electrode - DME
Static Mercury Drop Electrode - SMDE
Hanging Mercury Drop Electrode - HMDE
 its liquid state at ambient temperature, renewable surface
 high purity material availability,
 high conductivity,
 inertness chemically at low potentials (because of its high
overvoltage potential for hydrogen evolution)
 formation of amalgams with numerous metals
 microelectrodes (Hg drop diameter smaller than a millimeter).
Instrumentation, common techniques
Instrumentation, common techniques
Potential windows for different electrodes
Direct Current (DC) polarography
Apply Linear Potential with Time Observed Current Changes with Applied Potential
Apply
Potential
E << Eo
Direct Current (DC) polarography
a) ½ wave potential (E½) characteristic of Mn+ (E)
b) height of either average current maxima (i avg)
or top current max (i max) is ~ analyte concentration
c) size of i max is governed by
rate of growth of DME > drop time (t, sec)
rate of mercury flow (m, mg/s)
diffusion coefficient of analyte (D, cm2/s)
number of electrons in process (n)
analyte concentration (c, mol/ml)
Mn+ + ne- +Hg = M(Hg) amalgam
Ilkovič equation
(id)max = 0.706 n D1/2 m2/3 t1/6 c
(id)avg = 0.607 n D1/2 m2/3 t1/6 c
Half-wave potential, limited diffusion current
Residual currentHalf-wave potential
i max
i avg
Limiting diffusion current
Typical polarographic curves - polarographic spectrum
• One problem with data detection DC polarography is that current varies over
lifetime of drop, giving variation on curve
• One simple way to avoid this is to sample only current at particular time of drop life.
Near end of drop = current sampled polarography
Sample i at same
time interval
Easier to determine iavg
Tast polarography
-0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4
i (A)
Potential applied on the working electrode is usually swept over (i.e. scan)
a pre-defined range of applied potential
0.001 M Cd2+ in 0.1 M KNO3 supporting electrolyte
V vs SCE
Working electrode is
no yet capable of
reducing Cd2+ 
only small residual
current flow through
the electrode
Electrode become more and more
reducing and capable of reducing Cd2+
Cd2+ + 2e- Cd
Current starts to be registered at the
electrode
Current at the working
electrode continue to rise as
the electrode become more
reducing and more Cd2+
around the electrode are being
reduced. Diffusion of Cd2+
does not limit the current yet
All Cd2+ around the electrode has
already been reduced. Current at the
electrode becomes limited by the
diffusion rate of Cd2+ from the bulk
solution to the electrode. Thus, current
stops rising and levels off at a plateau
id
E½
Base line
of residual
current
Logarithmic analysis
Select about four or five readings on each side of the half-wave potential from
the graph of current versus voltage in order to calculate the values of the
logarithmic term in the above equation. Indicate on the graph:
1) value of the slope,
2) calculated value of n,
3) half-wave potential E1/2
4) reversibility.
Plot log (Id-I)/I against potential (E) from your polarogram data
log [(Id-I)/I] = n(E - E1/2)/ 0.0591
E = E1/2 + 0.0591/n (log [(Id-I)/I])
when the value of the logarithmic term becomes zero, the above equation becomes E = E1/2
Polarographic Wave Equation
n the number of electrons involved in reduction
ii
i
EE
n
d 
 log)(
058.0
21
Current is just measure of rate at which species can be brought to electrode surface
Three transport mechanisms:
(i) migration – movement of ions through solution by electrostatic attraction to
charged electrode
(ii) convection – mechanical motion of the solution as a result of stirring or flow
(iii) diffusion – motion of a species caused by a concentration gradient
Polarographic currents
Two methods
Stirred - hydrodynamic voltammetry
Unstirred - polarography (DME)
Polarographic currents
Diffusion current: f(c), f(h1/2), f(t1/6)
Capacity, charging current: f(h), f(E), f(t -1/3), SE,
ECM
Kinetic or catalytic current : f(h), f(c)(I. order) ,
f(t 2/3), f(E), SE (pH), f(T),
preceeding, proceednig,following reactions
catalytic currents (Brdička reation)
Adsorption: adsorption isotherms…………..I = f(c)
( c……concentration of analyt) ( h ……height of resorvoir) ( t……time) ( T……temperature)
A plot of the current vs. potential in a polarography experiment shows
the current oscillations corresponding to the drops of Hg falling from
the capillary. If one connected the maximum current of each drop, a
sigmoidal shape would result. The limiting current (the plateau on the
sigmoid), called the diffusion current because diffusion is the
principal contribution to the flux of electroactive material at this point
of the Hg drop life, is related to analyte concentration by the
Ilkovic equation
Id, = 0.708 n D1/2 m2/3 t1/6 c
Where D is the diffusion coefficient of the analyte in the medium
(cm2/s), n is the number of electrons transferred per mole of analyte, m is
the mass flow rate of Hg through the capillary (mg/sec), and t is the drop
lifetime in s, and c is analyte concentration in mol/cm3.
Ilkovič equation
Direct Current (DC) polarography
Ilkovič equation
Current maxima
Analytical applications
cathodic
anodic
Polarographic "Brdicka reaction" of blood sera of different patients with following
diagnoses: 1) status febrilis 2) tumor hepatis susp 3) ca. ventriculi susp 4) normal
serum 5) cirrhosis hepatic 6) atherosclerosis.
Brdička reaction
Heyrovsks's second assistant, Dr. Rudolf Brdicka, discovered a sensitive catalytic
hydrogen-evolution reaction of proteins: in buffer solutions of pH about 9,
containing ions of cobalt, proteins yield a prominent catalytic "double-wave"; this
polarographic reaction was used in many countries over several decades as a
diagnostic tool in treatment of cancer
VOLTAMMETRY
DPV
LSV
CV
ACV
NPV
VOLTAMMETRY (LSV, CV)
A) Comparison of Voltammetry to Other Electrochemical Methods
1) Voltammetry: electrochemical method in which information about an analyte is
obtained by measuring current (i) as a function of applied potential
- only a small amount of sample (analyte) is used
Instrumentation – Three electrodes in solution
containing analyte
Working electrode: microelectrode whose
potential is varied with time
Reference electrode: potential remains constant
(Ag/AgCl electrode or calomel)
Counter electrode: Hg or Pt that completes
circuit, conducts e- from signal source through
solution to the working electrode
Supporting electrolyte: excess of nonreactive
electrolyte (alkali metal) to conduct current
2) Differences from Other Electrochemical Methods
a) Potentiometry: measure potential of sample or system at or near zero current.
Voltammetry : measure current as a change in potential
b) Coulometry: use up all of analyte in process of measurement at fixed current or potential
Voltammetry: use only small amount of analyte while vary potential
B) Theory of Voltammetry
Excitation Source: potential set by instrument (working electrode)
- establishes concentration of Reduced and Oxidized Species at electrode
based on Nernst Equation:
Eelectrode = E0 - log
0.0592
n
(aR)r(aS)s …
(aP)p(aQ)q …
- reaction at the surface of the electrode
-current generated at electrode by this process is proportional to concentration at
surface, which in turn is equal to the bulk concentration
For a planar electrode:
dCA
dx
measured current (i) = nFADA( )
n = number of electrons in ½ cell reaction, F = Faraday’s constant, A = electrode area (cm2),
D = diffusion coefficient (cm2/s) of A (oxidant)
dCA
dx
= slope of curve between CMox,bulk and CMox,s
Apply
Potential
Fe3++ e- = Fe2+
As time increases, push banding further and further out.
Results in a decrease in current with time until reach point where convection of analyte takes
over and diffusion no longer a rate-limiting process.
chrono
chrono
Voltammetric analysis
Analyte selectivity is provided by the applied potential on the working electrode.
 Electroactive species in the sample solution are drawn towards the working electrode
where a half-cell redox reaction takes place.
 Another corresponding half-cell redox reaction will also take place at the counter
electrode to complete the electron flow.
 The resultant current flowing through the electrochemical cell reflects the activity (i.e.
 concentration) of the electroactive species involved
Pt working
electrode at -1.0
V vs SCE
SCE
Ag counter
electrode at
0.0 V
xM of PbCl2
0.1M KCl
Pb2+ + 2e- Pb EO = -0.13 V vs. NHE
K+ + e- K EO = -2.93 V vs. NHE
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+ Pb2+K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+ K+
K+
K+
K+
K+
K+
K+
K+
K+
-1.0 V vs SCEPb2+ + 2e- Pb
K+
K+
K+
K+K+ K+
Layers of K+ build up around the electrode stop the
migration of Pb2+ via coulombic attraction
Concentration gradient created
between the surrounding of the
electrode and the bulk solution
Pb2+ migrate to the
electrode via diffusion
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+ Pb2+K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+ K+
K+
K+
K+
K+
K+
K+
K+
K+
-1.0 V vs SCEPb2+ + 2e- Pb
K+
K+
K+
K+K+ K+
Layers of K+ build up around the electrode stop the
migration of Pb2+ via coulombic attraction
Concentration gradient created
between the surrounding of the
electrode and the bulk solution
Pb2+ migrate to
the electrode
via diffusion
1) Method used to look at mechanisms of redox
reactions in solution
2) Looks at i vs. E response of small, stationary
electrode in unstirred solution using triangular
waveform for excitation
Cyclic voltammogram
Cyclic Voltammetry (CV) v = dE/dt
Randles – Sevcik equation
2/102/1235
10.69.2 vcDAnI Oxp 
Delahay equation
Ip (A ) A (cm2), D (cm2.s-1), v (V.s-1), cox ( mol.cm-3)
2/102/1215
)(10.99.2 vcDAnnI Oxap 
CV Pt electrodes
1 - desorption of hydrogen
2 - Pt surface without hydrogen
3 - oxidation Pt (oxides)
4 – oxidation of solvent (H2O)
5 – reduction of oxides at Pt surface
6 – reduction and adsorption of hydrogen
7 – adsorption of hydrogen
Start at E >> E0 Mox + ne- = Mred
- in forward scan, as E approaches E0 get current due to
Mox + ne- = Mred
< driven by Nernst equation
 concentrations made to meet
Nernst equation at surface
< eventually reach i max
< solution not stirred, so d grows with time and
see decrease in i max
- in reverse scan see less current as potential increases
until reduction no longer occurs
< then reverse reaction takes place (if
reversible reaction)
< important parameters
 Epc – cathodic peak potential
 Epa – anodic peak potential
 ipc – cathodic peak current
 ipa – anodic peak potential
< ipc / ipa
< d(Epa – Epc) = 0.0592/n,
where n = number of electrons in reaction
< E0 = midpoint of Epa  Epc
    nEE cpap 0565.0 nEE pp 0565.02 
Pulse Polarography (Voltammetry)
Normal pulse (NPP, NPV) Differential pulse (DPP,DPV)
m
NPP
t
cnFAD
I

21

Cottrell equation
tm time after application of the
pulse where the current is sampled











 1
121
m
DPP
t
cnFAD
I
 2/)(/(exp ERTnf 
frenquencyf
amplitudepulseE
...
...
Instead of linear change in Eappl with time use step changes (pulses in Eappl) with time
Advantages of DPP (DPV)
- can detect peak maxima differing by as much as 0.044 – 0.05 V
- 0.2V peak separatioin for normal polarography
- can do more elements per run
- decrease limits of detection by 100-1000x compared to normal polarography
Pulse Polarography (Voltammetry)
- S1 before pulse S2 at end of pulse
- plot i vs. E (i = ES2 – ES1)
- peak height ~ concentration
- for reversible reaction, peak potential -> standard potential
-derivative-type polarogram
Measure two
potentials at
each cycle
Differential Pulse Polarography (Voltammetry)
Improving by
stripping mode
nF
RT
W
52.3
2/1 
The width of the peak (at half-height)
Square Wave (SW) Polarography or Voltammetry
Large amplitude differential technique in which a waveform composed of a symmetric square
wave, superimposed on a base staircase potential, is applied to the working electrode. The
current is sampled twice during each SW cycle. Once at the end of the forward pulse (t1) and
once at the end of the reverse pulse (t2).
difference between the two measurements = f (staircase potential)
Progress is made by trial and failure.
(William Ramsay)
A problem solved is dead.
(Frederick Soddy
Work, finish, publish !
(Michael Faraday)
A man must either resolve to bring out nothing new or to
become a slave to defend it.
(Isaac Newton)