Ch 22 Bulk Electrolysis: Electrogravimetry and Coulometry Flashcards
The two bulk electroanalytical methods
electrogravimetry, and
coulometry
In the bulk electroanalytical methods __________ is present
Current
Two additional phenomena, ____________, must be considered when current is present.
IR drop and polarization
is the rate of charge flow in
a circuit or solution. One ampere of
current is a charge flow rate of one
coulomb per second
Current
, the electrical potential
difference, is the potential energy that
results from the separation of charges.
Voltage
Ohm’s law:
E = IR, or I = E/R.
Ohmic Potential is the product of the resistance R of a cell in ohms (V) and the current I in amperes (A). Also called
IR Drop
is current that is
always in one direction, that is, unidirectional.
Direct current (dc)
The direction of____________ reverses periodically.
alternating
current (ac)
to minimize the IR drop in the cell by having a very small cell resistance (high ionic strength) _____is used
three-electrode cell
is the deviation of the
electrode potential from its theoretical
Nernst equation value on the passage
of current.
Polarization
is the potential difference between the theoretical
cell potential from Equation 22-2 and
the actual cell potential at a given level
of current.
Overvoltage
is the movement of
material, such as ions, from one
location to another
Mass transfer
Polarization phenomena can be divided into two categories:
concentration polarization and kinetic polarization.
occurs because of the finite rate of mass transfer from
the solution to the electrode surface
Concentration polarization
is the movement of a species
under the influence of a concentration
gradient. It is the process that causes
ions or molecules to move from a more
concentrated part of a solution to a
more dilute
Diffusion
Reactants are transported to the surface of an electrode by three mechanisms:
diffusion, migration, and convection
is the movement of ions
through a solution as a result of
electrostatic attraction between the
electrodes and the ions.
Migration
the rate of diffusion and the current become correspondingly larger until the
surface concentration falls to zero, and the maximum or __________ is reached
limiting current
Migration of analyte species can be
minimized by having a high concentration of an inert electrolyte, called a _____ present in the cell.
supporting
electrolyte,
is the transport of
ions or molecules through a solution
as a result of stirring, vibration, or
temperature gradients.
Convection
such as stirring or agitation, tends to decrease the thickness of the diffusion layer at the surface of an electrode and thus decrease concentration
polarization
Forced convection,
resulting from temperature or density differences also
contributes to the transport of molecules and ions to and from an electrode.
. Natural convection
The current in a kinetically
polarized cell is governed by the
rate of electron transfer rather
than the rate of mass transfer.
kinetic polarization
is the current per
unit surface area of the electrode
(A/cm2).
Current density
is most commonly encountered when the reactant
or product in an electrochemical
cell is gas
Kinetic polarization
the only practical way of achieving separation of species whose electrode potentials differ by a few tenths of a volt is to measure the cathode potential continuously against a reference electrode whose potential is known. The applied cell potential can then be adjusted to maintain the cathode potential at the desired level. An analysis performed in this way is called a
controlled-potential electrolysis.
the method where potential of the working electrode is
maintained at a constant level versus a
reference electrode, such as a SCE.
potentiostatic method,
is the electrode
at which the analytical reaction
occurs.
working electrode
The voltage applied to the cell is controlled by the variable
resistor, R.
A ____ is a species that is
easily reduced (or oxidized). It helps
maintain the potential of the working
electrode at a relatively small constant
value and prevents reactions that
would occur under more reducing
or oxidizing conditions.
depolarizer
A ________- maintains the working
electrode potential at a constant value
relative to a reference electrode.
potentiostat
is particularly useful for removing easily reduced elements such as copper, nickel, cobalt, silver, and cadmium are separated at this electrode
mercury cathode
is a potent tool for separating and determining
metallic species having standard potentials that differ by only a few tenths of a
volt.
Controlled-potential electrolysis
is the amount
of charge required to produce
0.00111800 g of silver metal from
silver ions.
coulomb (C)
The charge on an electron (and proton) is defined as .
1.6022 x 10^19
coulombs (C)
A rate of charge flow equal to one coulomb per second
one ampere (A) of current
Two methods often called the coulometric titrimetry
controlled-potential (potentiostatic) coulometry & controlled-current coulometry
is also called coulometric titrimetry.
Constant-current coulometry
One equivalent of chemical change
is the change brought about by
1 mol
of electrons
The two working electrode of Electrolysis cells for potentiostatic coulometry.
(a) platinum-gauze, (b) mercury-pool
Coulometric titrations are performed with a constant-current source, sometimes
called a
galvanostat
Constant-current generators are
sometimes called .
galvanostats
Detecting the End Point in Coulometric titrations,
Potentiometric or
amperometric end points are used in Karl Fischer titrators
The constant-current source for a coulometric titration is an electronic device capable of maintaining a current of
200 mA
Applications of Coulometric Titrations
Neutralization Titrations
Precipitation and Complex-Formation Reactions
Oxidation/Reduction Titrations
Applications of coulometric titrations
Neutralization titration
Precipitation and complex -formation reactions
Oxidation/reduction titration
