voltammetry basics

1. Voltammetry

2. Voltammetry and Polarograph
• Voltammogram—The plot of the
electrode current as a function of
potential.
• Electrochemistry techniques based
on current (i) measurement as
function of voltage (Eappl)
• Voltammetry—Usually when the
working electrode is solid, e.g.,
Pt, Au, GC.
• Polarograph—A special term
used for the voltammetry carried
out with a (liquid) MURCURY
electrode.

3. “Polarographic curves”--Voltammograms
 Typical polarographic curves
(dependence of current I on the
voltage E
applied to the electrodes; lower
curve - the supporting solution of
ammonium chloride and hydroxide
containing small amounts of
cadmium, zinc and manganese,
upper curve - the same after
addition of
small amount of thallium

4. Electrochemical Cell
• Working electrode:
place where redox
occurs, surface area
few mm2 to limit
current flow.
• Reference electrode:
constant potential
reference
• Counter (Auxiliary)
electrode: inert
material, plays no part
in redox but completes
circuit
•Supporting electrolyte:
alkali metal salt does not react with electrodes
but can reduce the effect of
migration and lower the resistance of the
solution.

5. 2-Electrode vs.
3-Electrode Cell
 • 2-electrode cell is OK in potentiometry--
very small i
• Now in voltammetry, measuring (big) i vs.
applied E, but
(1) Potential drops when current is taken
from electrode
due to solution resistance (iR drop): The
actual EWE is
smaller than E
Appl (vs ERef. E)
(2) Large i passes the ref. electrode
instability of the
reference potential (not constant)

6. Advantages of
3- over 2-
electrode Cell
System
 3-electrode system:
1. Provides great flexibility in location of the
reference and the working electrodes and
minimizes the effect of solution iR drop.
2. Virtually has no current passing through
the reference electrode.

7. Potentiostat
Voltage (E) source that drives the cell
• Supplies whatever E needed between WE
and CE to maintain
specific E between WE and Ref. electrode
• Very high impedance (so that i passes
though the ref. electrode is
minimized)

8. Working
Electrodes
• Mercury electrodes (liquid)—dropping mercury
electrode, hanging mercury drop electrode…
• Solid electrodes: mm in diameters, Pt, Au, GC.
• Micro(Ultramicro) electrodes: mm in diameter: Pt,
Au, carbon fiber.
• Solid/liquid electrode: Mercury film electrodes,
carbon paste electrode.
• Chemically modified electrodes
• ITO electrode (Transparent glass coated with InSnO2
• Screen printing electrodes

9. Working Electrodes
Dropping Mercury
Electrode
(DME)
Hanging Mercury
Drop Electrode
(HMDE)
Solid disk
electrode
microelectrode
Screen
printing
electrode

10. Electrode material vs.
Potential Window
 Potential window varies with
material/solution due to overpotentials

11. Potential Excitations vs
Voltammogram at a Solid Electrode
Linear
Sweep/San
Voltammetry
(LSV)
Cyclic
Voltammetry
(CV)

12. Potential Excitations vs
Voltammogram at a Solid Electrode
Potential
Step
Voltammetry
Differential
Pulse
Voltammetry
(DPV))

13. Potential Excitations vs
Voltammogram at a Solid Electrode
Staircase
Voltammetry
Square Wave
Voltammetry (SWV)

14. Mass Transfer/Transport
Migration Diffusion Convection

15. Hydrodynamic Voltammetry
 Voltammetry in which analyte solution is
kept in continuous motion.
 Two ways: Stirring the solution, and
rotating the electrode.
electrode Rotator

16. Flow patterns and regions of
interest near the working electrode
in hydrodynamic voltammetry

17. Dropping
Mercury
Electrode (DME):
 Dropping mercury electrode (DME) is a
working electrode arrangement for polarography
in which mercury continuously drops from a
reservoir through a capillary tube (internal
diameter 0.03 - 0.05 mm) into the solution.
 The optimum interval between drops for most
analyses is between 1 and 5 s.
 The unique advantage to the use of the DME is
that the constant renewal of the electrode
surface, exposed to the test solution, eliminates
the effects of electrode poisoning.

18. Construction:
 The assembly consists of a mercury
reservoir.
 It consists of fine capillary having bore
size ranged from 20-50 µ and 10-15 cm
long.
 The capillary is connected to mercury
reservoir by rubber tubing.
 A small glass electrolysis cell in which
the unknown solution is placed.
 The height of the mercury reservoir is
adjusted such that drop time is 1-5
seconds.

19. Working
 Dropping mercury electrode (DME) is a
polarizable electrode and can act as both
anode and cathode.
 The pool of mercury acts as counter
electrode, i.e., anode if DME is cathode
or cathode if DME is anode.
 The counter electrode is a non-
polarizable electrode.
 To the analyte solution, electrolyte like
KCl is added i.e., 50-100 times of
sample
 concentration.
 Pure nitrogen or hydrogen gas is
bubbled through the solution, to expel
(remove) out oxygen.

20. Working
 Eg: If the analyte solution contains
cadmium ions, then cadmium ions are
discharged at cathode.
 Cd2+ + 2e- → Cd
 Then, gradually increasing voltage is
applied to the polarographic cell and
current is recorded.
 Graph is plotted between voltage
applied and current. This graph is called
Polarograph, and the apparatus is known
as Polarogram.
 The diffusion current produced is
directly proportional to concentration of
analyte and
 this is used in quantitative analysis.
 The half wave potential is characteristic
of every compound, and this is used in
 qualitative analysis.

21. Advantages:
 1. Surface area is reproducible.
 2. Electrode can be renewed and thus
eliminates poisoning effect.
 3. Mercury forms amalgam (solid
solutions) with many metals.
 4. The surface area can be calculated
from the weight of the drop

22. Disadvantages:
 1. Capillary is very small and thus can
be easily blocked.
 2. Mercury is very toxic.
 3. Surface area of each drop of mercury
is never constant.
 4. It cannot be used at higher positive
potential due to oxidation of mercury.

23. 1.)
Potentiometric
Methods:
 Introduction: based on measurements of the
potential of electrochemical cells in the absence
of appreciable currents (i . 0)
 2.) Basic Components:
 a) reference electrode: gives reference for
potential measurement
 b) indicator electrode: where species of interest
is measured
 c) salt bridge
 d) potential measuring device

24. Principle of
electrochemical
sensor:
 Electrochemical sensors are managed based on
the dissemination into the sensor of the gas of
interest, bringing about the yield of an electrical
sign relative to the convergence of the gas. In
return, the diffused gas is oxidized or reduced.at
the detecting electrode.
 A normal electrochemical sensor comprises of a
detecting cathode (or working anode), and a
counter terminal isolated by a slender layer of
electrolyte.

25. Electrochemical
Sensors are divided
into several types:
1.Potentiometric
(measure voltage)
2.Amperometric
(measure current)
3.Conductometric (measure
conductivity)

26. 1.Potentiometric
sensor:
 A potentiometric sensor is a kind of
electrochemical sensor that can be utilized to
compute the insightful grouping of certain
logical gas or arrangement segments. Such
sensors measure an anode's electrical likely
when there is no flow present. The impact of
focus on the equilibrium of redox responses
happening at the anode electrolyte interface of
an electrochemical cell is used by potentiometric
sensors

27. Principle:
 The signal is resolved between the working
cathode and the reference terminal as the likely
contrast (voltage).
 The capability of the working cathode must be
reliant on the analyte focus in the gas or
arrangement stage. To give a given reference
potential, a reference cathode is required.

28. Types of
Potentiometric
sensor:
 Film based particle specific terminals (ISE),
 screen-printed anodes,
 particle particular field impact semiconductors
(ISFET),
 strong state gadgets and synthetically adjusted
cathodes (utilizing, for instance , metal oxides or
electrodeposited polymers as delicate layers) are
the fundamental sorts of potentiometric sensors.

29. 2. Amperometric
sensor:
 Amperometric sensors measure the current
response between the reference and working
terminals.
 Amperometric estimations are made as
electroanalytical procedures by recording the
current stream in the cell at a solitary applied
potential

30. Principle:
 The amperometric sensor standard depends on
the estimation of the current delivered by an
anode surface enzymatic or bio affinity response
with a steady working potential comparable to
the reference terminal.

31. 3. Conductometric
sensor:
 Conductometric sensors are utilized to figure the
propensity of a medium or electrolyte answer for
permit the section of electrical flow between the
working cathode and counter anode or reference
terminal.
 The sub-sets of impedimetric sensors are
conductometric strategies. The strategy of
conductometric detecting is utilized to dissect
capacitance changes.

32. Advantages of
electrochemical
sensors:
 1. It could be interesting to a particular gas or
fume in the scope of parts per million. The level
of selectivity relies upon the kind of sensor, the
sensor is worked to distinguish, the objective
gas, and the gas focus.
 2. Straight performance, low necessities for
force and great goal.
 3. Remarkable repeatability and accuracy. The
sensor can give an exact perusing of a repeatable
objective gas once adjusted to a known fixation.
 4. Doesn't get harmed by other gases. The
presence of other surrounding fumes would not
abbreviate or restrict the sensor 's life.
 5. Less expensive than most different advances
for gas detection. Electrochemical sensors are
practical, not normal for infrared and PID
innovations.

33. Limitations of
electrochemical
sensors:
 1. Limited or confined scope of temperatures. They
are temperature touchy, so the sensors are regularly
temperature remunerated inside. It is more secure
to keep the temperature of the example as
consistent as could be expected under the
circumstances.
 2. Short or confined rack life. Depending on the
gas to be distinguished and the climate in which it
is utilized, an electrochemical sensor regularly has
a timeframe of realistic usability of a half year to
one year.
 3. Cross-affectability of different gases. In spite of
the fact that this is an advantage, it tends to be a
disadvantage too. A few sensors are fit for
meddling with different gases. To know about
conceivable bogus readings, it is critical to
understand what gases can cause impedance with
your sensor.
 4. The more prominent the objective gas
presentation, the more limited the existence span. A
one-to three-year future is typically characterized.
Low mugginess and high temperatures can make
the electrolytes of the sensors dry out. The
electrolyte is likewise depleted by presentation to
target gas or cross-affectability gases.