Download Electro-analytical Techniques

Electro-analytical Techniques
Voltammetry
Answer sheet
01. Ideally polarized electrodes
An electrode is called “ideal polarizable" if no electrode reactions can occur within a fairly
wide electrode potential range. Consequently, the electrode behaves like a capacitor and only
capacitive current (no faradaic current) is flowing upon a change of potential. Many
electrodes can behave as an ideal polarized electrode but only within an electrode potential
range called the double-layer range. Ideally polarized electrode is also called "completelypolarizable electrode" and "totally-polarized electrode”.
Eg. Pt electrode, pyrolytic graphite electrode, etc.
Ideally non polarized electrode
An electrode that is practically not polarizable is called as “Ideally non polarized electrode”.
That is, the potential of the electrode will not change from its equilibrium potential with the
application of even a large current density. The reason for this behavior is that the electrode
reaction is extremely fast (has an almost infinite exchange current density). This is also
called "ideal depolarized electrode".
eg. Silver/silver-chloride electrode, calomel electrode
Reference electrode
Reference electrode is an electrode that has a well known and stable equilibrium electrode
potential. It is used as a reference point against which the potential of other electrodes
(typically that of the working electrode or measuring electrode) can be measured in an
electrochemical cell. In principle it can be any electrode fulfilling the above requirements. In
practice, there are a few commonly-used electrode assemblies that have an electrode
potential independent of the electrolyte used in the cell. Some examples for common
reference electrodes are, the silver/silver-chloride electrode, calomel electrode, and hydrogen
electrode.
Working electrode
The working electrode is the electrode in an electrochemical system on which the reaction of
interest is occurring. Depending on whether the reaction on the electrode is a reduction or an
oxidation, the working electrode can be referred to as either cathodic or anodic. Working
electrode must be electrochemically inert (i.e., does not generate a current in response to an
applied potential) over a wide potential range (the potential window). The choice of material
depends upon the potential window required, as well as the rate of electron transfer.
Commonly used working electrode materials for voltammetry include platinum, gold,
mercury, glassy carbon, pyrolytic graphite, etc.
2). Define the following parameters encountered in voltammetry.
i.
Residual Current (i r)
Residual Current in polorography is the small charging current observed in the
absence of a reactive species.
There are two sources for the residual current.
(i)
Discharge of impurities present in solution. The reduction of
trace impurities that are almost inevitably present in the blank
solution; contributors here include small amounts of dissolved
Oxygen, heavy-metal ions from the distilled water, and impurities
present in the salt used as the supporting electrolyte.
(ii)
Charging current / non-faradic current.
Charging current resulting from a flow of electron that
charge the Hg droplets with respect to the solution; this current
may be either negative or positive.
A non-faradic current is a charging current that
produces charged double layer across electrode / solution
interfaces. An example of a non-faradic current is the current that
charges a condenser.
ii.
Limiting Current (il)
The constant current beyond the steep rise is called the limiting current. It is
limited by the rate at which the reactant can be brought to the surface of electrode
by mass- transport processes.
iii.
Diffusion Current (i d)
Diffusion current is the limiting current observed in polarography when the
magnitude of the current is limited only by the rate of diffusion of the reactant to
the electrode surface.
iv.
Half Wave Potential (E 1/2)
Half wave potential is the middle point voltage of the sharp rise. For a given
species in a given medium has a unique E1/2.
id
E1/2
ir
il
3). Advantages using Dropping Mercury Electrode
ii. DME is high sensitive & the measurements are reproducible.
iii. This is very important due to high over voltage of liquid mercury to hydrogen,
which enables the determination of heavy metals (zinc, nickel and cobalt, etc.) and
other species with high negative half wave potentials.
iv. The actual potential window is extending.
v. Can take the continuous measurement and can remote the measurement.
vi. This is very suitable for field.
vii. It is a liquid at a room temperature.
viii. It forms amalgams with many heavy metals.
ix. DME contains renewable Surface.
Disadvantages using Dropping Mercury Electrode
i. Hg is high toxic.
ii. Handling of drop system is complicated.
iii. Due to the oxidation of Hg, cannot be used such electrode at potentials that are
more positive.
04. The three methods of transport of ions:
i. Diffusion
ii. Migration
iii. Convection
Natural
Mechanical
i. Diffusion
Diffusion is the movement of a charged species under the influence of a
concentration gradient. The diffusion is directly proportional to concentration
difference.
ii. Migration
Movement of a charged species under the influence of an electric field is
called migration. Migration causes anions to be attracted to the positive
electrode and cations to the negative electrode.
iii. Convection
Convection is the transport of ions or molecules through a solution as a result
of stirring, vibration, or temperature gradients.
5). Describe what is meant by concentration polarization.
Polarization is an electrode phenomenon that may affect either or both of the
electrodes in a cell. Concentration polarization occurs when reactant species do not
arrive at the surface of the electrode or product species do not leave the surface of
the electrode fast enough to maintain the desired current. Concentration
polarization sets in when the effects of diffusion, migration and convection are
insufficient to transport a reactant to or from an electrode surface at a rate that
produces a current of the magnitude. Concentration polarization requires applied
potentials that are more negative than theoretical to maintain a given current in an
electrolytic cell.
6). In voltammetry ideally the ion transportation should be carried out through diffusion
process. But due to electrostatic interaction between analyte ions and the oppositely
charged mercury droplets the analyte ions can be transported through the migration
process. This could be avoided by adding an inert (supporting) electrolyte.
Eg :- HCl in Cd2+ analysis
Here the negative charge of the Hg drop will be shielded by positively charged H +
ions.
7). Dissolved oxygen is electro active at negative potentials so the following reaction
occurs rapidly.
O2 + 2H+ + 2e
O2 + 2H2O + 2e
H2O2
(Acidic medium)
H2O2 + 2OH- (Neutral or alkaline)
With the H2O2 formed in this way also being electro active, i.e
H2O2 + 2e
2OH-
(Alkaline)
This being a four electrode process the associated non-faradic current can be
large.
So dissolved O2 should be removed from analyte solution.
Removal of dissolved Oxygen
The best way of removing O2 is to gently bubble gaseous nitrogen or Argon
through the solution.
11) Let unknown concentration be X
X α 10.5 µA…….. (1)
(5 ×10 + X) α 14.0 µA……...(2)
-4
(1)/(2) =
5 ×10-3 + 10X
0.00125
14X =
X =
Concentration of the unknown = 0.00125 mol dm-3
12.
i.
[ Cu2+] / mM
id / µA
0.039
0.256
0.078
0.52
0.159
1.058
0.489
3.06
Unknown
1.22
The graph of id Vs CU2+ concentration
Molarity of unknown = 0.19 mM
ii.
2+
[ Cd ] / mM
id / µA
0.99
6.37
1.97
13.00
3.83
25.00
8.43
55.80
Unknown
20.00
The graph of id Vs Cd2+ concentration
Molarity of unknown = 3.00 mM
(13) Unknown concentration = C μg/ l
14
12
Signal / s
10
8
y = 8.4x + 4.4667
R2 = 0.9998
6
4
2
-0.6
0
-0.1
0.4
0.9
Concentration of Pb2+
C = 0.53 μg / l
1.4