Bulk Electrolysis: Electrogravimetry and Coulometry

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Chapter 22

• Bulk Electrolysis Electrogravimetry and
  Coulometry

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22A The effect of current on cell potential When
there is a net current in an electrochemical
cell, the measured potential across the two
electrodes is no longer simply the difference
between the two electrode potentials as
calculated from the Nernst equation. Two
additional phenomena, IR drop and polarization,
must be considered. The following electrolytic
cell for the determination of Cadmium (II) in HCl
solutions can be considered AgAgCl(s),
Cl-(0.2M), Cd2(0.005M)Cd The right-hand
electrode is the working electrode and operates
as a cathode. The left-hand electrode or the
reference electrode is a silver/silver chloride
electrode whose electrode potential remains
nearly constant during the analysis.
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Figure 22-1 An electrolytic cell for determining
Cd2. (a) Current 5 0.00 mA. (b) Schematic of
cell in (a) with internal resistance of cell
represented by a 15.0-V resistor and Eapplied
increased to give a current of 2.00 mA.
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Ohmic Potential IR Drop Ohms law describes the
effect of this resistance on the magnitude of the
current in the cell. The product of the
resistance R of a cell in ohms (V) and
the current I in amperes (A) is called the ohmic
potential or the IR drop of the cell. In order
to generate a current of I amperes in this cell,
a potential that is IR volts more negative than
the thermodynamic cell potential must be
applied. Eapplied Ecell IR
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Polarization Effects Figure 22-2 Experimental
current/voltage curve for operation of the cell.

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The term polarization refers to the deviation of
the electrode potential from the value predicted
by the Nernst equation on the passage of current.
Cells that exhibit nonlinear behavior at higher
currents exhibit polarization, and the degree of
polarization is given by an overvoltage, or
overpotential, which is symbolized by
?. Eapplied Ecell IR ? Factors that
influence polarization are (1) electrode size,
shape, and composition (2) composition of the
electrolyte solution (3) temperature and
stirring rate (4) current level and (5)
physical state of the species participating in
the cell reaction.
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Polarization phenomena can be divided into two
categories concentration polarization and
kinetic polarization. Concentration polarization
occurs because of the finite rate of mass
transfer from the solution to the electrode
surface. It 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.
Reactants are transported to the surface of an
electrode by three mechanisms diffusion,
migration, and convection. Products are removed
from electrode surfaces in the same ways.
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Figure 22-3 Pictorial diagram (a) and
concentration versus distance plot (b) showing
concentration changes at the surface of a cadmium
electrode.
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Figure 22-4 Current-potential curve for
electrolysis showing the linear or ohmic region,
the onset of polarization, and the limiting
current plateau. In the limiting current region,
the electrode is said to be completely polarized
since its potential can be changed widely without
affecting the current.
rate of diffusion to cathode surface k(Cd2
- Cd20)
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Convection is the transport of ions or molecules
through a solution as a result of stirring,
vibration, or temperature gradients.
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22 B The selectivity of electrolytic methods
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• 22C Electrogravimetric methods
• They are of two general types
• the potential of the working electrode is
  uncontrolled, and the applied cell potential
• is held at a more or less constant level that
  provides a large enough current to complete
• the electrolysis in a reasonable length of time.
• (2) the controlled-potential or potentiostatic
  method the potential of the working electrode is
  maintained at a constant level versus a reference
  electrode, such as a SCE.

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Figure 22-6 Apparatus for electrodeposition of
metals without cathode-potential control.
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Figure 22-7 (a) Current. (b) IR drop and cathode
potential change during electrolytic deposition
of copper at a constant applied cell potential.
The current (a) and IR drop (b) decrease steadily
with time.
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Figure 22-9 Changes in cell potential (A) and
current (B) during a controlled-potential
deposition of copper. The cathode is maintained
at 20.36 V (versus SCE) throughout the experiment.
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Figure 22-10 A mercury cathode for the
electrolytic removal of metal ions from solution.
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22 D Coulometric methods In coulometric methods,
the quantity of electrical charge required to
convert a sample of an analyte quantitatively to
a different oxidation state is measured. The
proportionality constant between the quantity
measured and the analyte mass is calculated from
accurately known physical constants. Electrical
charge is the basis of the other electrical
quantities, current, voltage, and power. The
charge on an electron (and proton) is defined as
1.6022 ? 10-19 coulombs (C).
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A rate of charge flow equal to one coulomb per
second is the definition of one ampere (A) of
current. The charge Q that results from a
constant current of I amperes operated for t
seconds is Q It For a variable current i,
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The faraday (F ) is the quantity of charge that
corresponds to one mole or 6.022 ? 1023
electrons. Each electron has a charge of 1.6022
? 10-19 C, hence, the faraday equals 96,485
C. Faradays law relates the number of moles of
the analyte nA to the charge Q
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Two methods have been developed that are based on
measuring the quantity of charge 1.
controlled-potential (potentiostatic) coulometry
and 2. controlled-current coulometry, or
coulometric titrimetry. 1. In
controlled-potential coulometry, the potential of
the working electrode is maintained at a constant
level such that only the analyte is responsible
for conducting charge across the
electrode/solution interface. The charge
required to convert the analyte to its reaction
product is then determined by recording and
integrating the current-versus-time curve during
the electrolysis. The instrumentation for
potentiostatic coulometry consists of an
electrolysis cell, a potentiostat, and a device
for determining the charge consumed by the
analyte.
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Figure 22-11 Electrolysis cells for
potentiostatic coulometry. Working electrode (a)
platinum-gauze, (b) mercury-pool.
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Coulometric titrations are performed with a
constant-current source (galvanostat), which
senses decreases in current in a cell and
responds by increasing the potential applied to
the cell until the current is restored to its
original level.
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Figure 22-14 A typical coulometric titration
cell.
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Figure 22-15 A cell for the external coulometric
generation of acid and base.
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• Applications of Coulometric Titrations
• Neutralization titrations
• 2. Precipitation and Complex-formation reactions

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3. Oxidation/reduction titrations