Bulk Electrolysis: Electrogravimetry and Coulometry Flashcards
are related methods that are based on electrolysis carried out long enough to ensure complete oxidation or reduction of the analyte to a product of known composition.
electrogravimetry and coulometry
is to determine the amount of analyte present by converting it to a product that is weighed as a deposit on one of the electrodes
electrogravimetry
determine the amount of analyte by measuring the quantity of electrical charge needed for complete conversion to a product
coulometry
two additional phenomena that must be considered when current is present, which because of these phenomena, potentials larger than the thermodynamic potential are needed to operate an electrolytic cell
IR drop and polarization
also known as the IR drop, which describes the effect of this resistance on the magnitude of the current in the cell
Ohmic potential
The product of the resistance R of a cell in ohms 1V2 and the current I in amperes (A) is
called the
ohmic potential or IR drop
is current that is always in one direction, that is,
unidirectional, dc
direct current
is the rate of charge flow in a circuit or solution
current
direction always reverses unidirectionally, ac
alternating current
Usually try to minimize the IR drop in the cell by having a very small cell resistance (high ionic strength) or by using a special
three-electrode cell
is the departure/ 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 and the actual cell potential at a given level of current
overvoltage
Cells that exhibit nonlinear behavior at higher currents exhibit polarization, and the degree of polarization is given by an __________. it also refers to the potential difference between the theoretical cell potential and the actual cell potential at a given level of current
overvoltage or overpotential
is an electrode phenomenon that may affect either or both of the electrodes in a cell.
polarization
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, which electron transfer between a reactive species in a solution and an electrode can take place only from the interfacial region located at the adjacent to the surface of the electrode
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
Reactants are transported to the surface of an electrode by three mechanisms
diffusion
migration
convection
When there is a concentration difference between two regions of a solution, ions or molecules move from the more concentrated region to the more dilute.
diffusion
TRUE or FALSE
The rate of diffusion is directly proportional to the concentration difference.
TRUE
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
The electrostatic process by which ions move under the influence of an electric field is called ___________. primarily, this process causes the mass transfer in the bulk of the solution in a cell.
migration
migration of analyte species can be minimized by having a high concentration of an inert electrode called a
supporting electrolyte
It refers to the transport of ions or molecules through a solution as a result of stirring, vibration, or temperature gradients, which reactants can also be transferred to or from an electrode by mechanical means.
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
is the transport of ions or molecules through a solution as a result of stirring, vibration, or
temperature gradients.
convection
the magnitude of the current is limited by the rate of one or both of the electrode reactions, that is, the rate of electron transfer between the reactants and the electrodes
kinetic polarization
The current in a kinetically
polarized cell is governed by the
rate of electron transfer rather
than the rate of mass transfer
is most pronounced for electrode processes that yield gaseous products because the kinetics of the gas evolution process are complicated and often slow and can be negligible for deposition or dissolution of metals
kinetic polarization
is the current per unit surface area of the electrode
current density
is most commonly encountered when the reactant or product in an electrochemical cell is gas
kinetic polarization
2 general types of electrogravimetric methods
potential of the working electrode
potentiostatic method
maintains the working
electrode potential at a constant value
relative to a reference electrode
potentiostat
The electrolysis current passes
between the working electrode and
a ____________. The ________ has no effect on the reaction at the working electrode
counter electrode
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
is the amount
of charge required to produce
0.00111800 g of silver metal from
silver ions.
coulomb
the quantity of electrical charge required to convert a sample of an analyte quantitatively to a different oxidation state is measured
coulometric methods
are usually rapid and do not require that the product of the electrochemical reaction be a weighable solid.
coulometric methods
is the basis of the other electrical quantities, current, voltage, and power
electrical charge
charge of electron or proton
1.6022x10^-19 C
Two methods have been developed that are based on measuring the quantity of charge
controlled-potential (potentiostatic) coulometry
controlled-current coulometry
often called coulometric titrimetry
controlled-current coulometry
are similar to other titrimetric methods in that analyses are based on measuring the combining capacity of the analyte with a standard reagent
coulometric titrations
is the change brought about by
1 mol of electrons
one equivalent of chemical change
In ________, 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.
controlled-potential coulometry
is automated and equipped with a computer or an electronic current integrator that gives the charge in coulombs necessary to complete the reaction
potentiostat
have been used to determine more than
55 elements in inorganic compounds and consists of an electrolysis cell, potentiostat, and a device for determining the charge consumed by the analyte
controlled-potential couloometric methods
has been used in the nuclear energy field for the relatively interference-free determination of uranium and plutonium.
Controlled-potential coulometry
re performed with a constant-current source, sometimes called a 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
coulometric titrations
Constant-current generators are
sometimes called
galvanostats
requires no preliminary calibration against chemical standards because the functional relationship between the quantity measured and the analyte concentration can be derived from theory and atomic mass data
electrogravimetry
the electrical potential difference, is the potential energy that results from the separation of charges.
voltage
is the movement of material, such as ions, from one location to another
mass transfer
factors that influence polarization
- electrode size, shape, and composition
- composition of the electrolyte solution
- temperature and stirring rate
- current level
- physical state of the species participating in the cell
TRUE or FALSE
The rate at which ions migrate to or away from an electrode surface generally increases as the electrode potential increases. This charge movement constitutes a current, which also increases with potential.
TRUE
TRUE or FALSE
Migration causes anions to be attracted to the positive electrode and cations to the negative electrode
TRUE
experimental varibales that influence the degree of concentration polarization are
- reactant concentration
- tota electrolyte concentration
- mechanical agitation
- electrode size
is the electrode at which the analytical reaction
occurs.
working electrode
the potential of the working electrode is
maintained at a constant level versus
a reference electrode, such as a SCE
potentiostatic method
what cells use tall-form beakers are often
used, and solutions are usually mechanically stirred to minimize concentration polarization.
The anode is often rotated to act as a mechanical stirrer
electrolysis cells
is particularly useful for removing easily reduced elements as a preliminary step in an analysis. For
example, copper, nickel, cobalt, silver, and cadmium are separated at this electrode from such ions as aluminum, titanium, the alkali metals, sulfates, and phosphates
mercury cathode
is a powerful tool for separating and determining
metallic species having standard potentials that differ by only a few tenths of a volt.
keywords: tartrate ion to complex the tin (IV)
controlled potential electrolysis
Faraday’s constant
96 485 C/mol e-
applications of coulometric titrations
neutralization titration
precipitation and complex-formation reactions
redox titration
neutralization titration is less susceptible to carbonate error, which is avoided through boiling the CP2 or by bubbling what insert gas
nitrogen
