Chapter 21: Voltammetry Flashcards
When the current is proportional to analyte concentration is monitored at a fixed potential,
amperometry
refers to a group of electroanalytical methods in which we acquire information about the analyte by measuring current in an electrochemical cell as a function of applied potential.
voltammetry
TRUE or FALSE
coulometry, measures are taken to minimize or compensate for the effects of concentration polarization. Furthermore, in voltammetry, there is minimal consumption of analyte, but in coulometry essentially all the analyte is converted to another state
TRUE
is voltammetry at the dropping mercury electrode
polarogrpahy
are based on measuring current as a function
of the potential applied to a small electrode.
voltammetric methods
is a salt added in excess to the analyte
solution. Most commonly, it is
an alkali metal salt that does not
react at the working electrode at
the potentials being used. The salt
reduces the effects of migration and
lowers the resistance of the solution.
supporting electrolyte
is the electrode at which the analyte is
oxidized or reduced and whose potential versus a reference electrode is varied linearly with time
working electrode
The potential between the working electrode and
the __________is controlled, which _____ has a potential that remains constant throughout the experiment
reference electrode
Electrolysis current passes between
the working electrode and a
counter electrode
take a variety of shapes and forms.
Often, they are small flat disks of a conductor that are press fitted into a rod of an inert material, such as Teflon or Kel-F that has imbedded in it a wire contact
working electrode
have been widely used in voltammetry for several reasons. One is the relatively large negative potential range and many metal ions are reversibly reduced to amalgams at the surface of a mercury electrode, simplifying the chemistry
mercury working electrodes
The simplest is a MERCURY FILM ELECTRODE formed by electrodeposition of the metal onto a disk electrode
hanging mercury drop electrode (HMDE)
Metals that are soluble in mercury form liquid alloys known as
amalgams
Historically, working electrodes with surface areas smaller than a few square millimeters were called _________. Such electrodes consist of small diameter metal wires or fibers (5 to 100 mm) sealed within tempered glass bodies. The flattened end of the microelectrode is polished to a mirror finish, which can be maintained using alumina and/or diamond polish
microelectrodes
micrometer-sized electrodes were sometimes called
ultramicroelectrodes
are plotted with positive current in the top hemisphere and negative currents in the bottom
voltammograms
is an ∫-shaped wave that appears in current-voltage plots in voltammetry.
voltammetric wave
The_____________ in voltammetry is the current plateau that is observed at the top of the voltammetric wave. It occurs because the surface
concentration of the analyte falls to zero. At this point, the mass transfer rate is its maximum value
limiting current
occurs when the current is equal to one half
of the limiting value.
half-wave potental, E1/2
is a type of voltammetry in which the
analyte solution is kept in continuous
motion
hydrodynamic voltammetry
are sometimes useful for identification
of the components of a solution.
half-wave potentials
can be achieved rapidly when either the analyte
solution or the working electrode is in continuous and reproducible motion
Reproducible limiting currents
Mass-transport processes include
diffusion
migration
convection
occurs at low flow velocities and has smooth and regular motion,
laminar flow
happens at high
velocities and has irregular, fluctuating motion,
turbulent flow
two types of flow depending on the average flow velocity
laminar flow
turbulent flow
is an identifier for the redox couple and is closely related to the standard reduction potential.
half-wave potential
In a ___________, either the forward or the
reverse reaction is so slow as to be completely negligible.
totally irreversible system
In a _______, the reaction in
one direction is much slower than
the other, although not totally
insignificant.
partially reversible system
is a process in which dissolved gases are swept out of a solvent by bubbling an inert gas, such as nitrogen, argon, or helium, through the solution.
sparging
forst step of amperometric procedures
removal of oxygen
it interferes in determining other species
complete voltammetric cells and are better referred to as
sensors
What sensor refers to the determination of dissolved oxygen in a variety of aqueous environments, such as seawater, blood, sewage, effluents from chemical plants, and soils, is of tremendous importance to industry, biomedical and environmental research, and clinical medicine.
oxygen sensors
is widely used in clinical laboratories for the
determination of dissolved O2 in blood and other body fluids
clark oxygen sensors
TRUE or FALSE
Voltammetric sensors are used for H2O2 and O2, while a potentiometric pH electrode is used for H1.
TRUE
can be used to estimate the equivalence point of titrations if at least one of the participants or products of the reaction involved is oxidized
or reduced at a working electrode.
hydrodyamic electrode
is a modified rotating disk electrode that is useful
for studying electrode reactions; it has little use in analysis
rotating ring-disk electrode
RDE detection with a mercury film electrode is sometimes referred to as
pseudopolarography
Polarographic currents are controlled by
diffusion only not convection
was the first type of voltammetry to be discovered and used. It differs from hydrodynamic voltammetry in two significant way
1. no convection or migration
2. dropping mercury electrode
linear scan polarography
The _______ in polarography is the small current
observed in the absence of an electroactive species
residual current
is the limiting current observed in polarography
when the current is limited only by the rate of diffusion to the dropping mercury electrode surface.
diffusion current
TRUE or FALSE
The diffusion current in polarography is proportional to the concentration of analyte.
TRUE
polarographic limiting currents are usually termed
diffusion currents id
The ____________ in an electrochemical cell is the current that results from an oxidation/
reduction process
faradaic current
is a charging current that results because the mercury drop is expanding and must be charged to the electrode potential
nonfaradaic current
two sources of current
reduction of trace impurities
charging/ capacitive/ current
the current response of a small stationary electrode in an unstirred solution is excited by a triangular voltage waveform
cyclic voltammetry
The voltage extrema at which reversal takes place are called
switching potentials
a scan in the direction of more negative potentials is termed a __________, while one in the opposite direction is called a _________
forward scan
reverse scan
is an electroanalytical technique used to study the electrochemical behavior of species in solution
pulse voltammetry
yield peaks that are convenient for qualitative identification of analytes based on the peak
potential, Epeak
derivative voltammograms
is a type of pulse voltammetry that offers the advantage of great speed and high sensitivity.
square wave voltammetry
TRUE or FALSE
Pulse methods have largely replaced classical voltammetry because of their greater sensitivity, convenience, and selectivity
TRUE
the working electrode behaves as a cathode during the deposition step and as an anode during the stripping step, with the analyte being
oxidized back to its original form.
anodic stripping methods
the working electrode behaves as an anode during the deposition step and as a cathode during stripping.
cathodic stripping method
TRUE or FALSE
A major advantage of stripping analysis is the
capability for electrochemically preconcentrating the analyte prior to the measurement step.
TRUE
are based on measuring current as a function
of the potential applied to a small electrode.
voltammetric methods
TRUE or FALSE
In voltammetry, the current that develops in an electrochemical cell is measured under
conditions of complete concentration polarization
TRUE
are made at currents that approach zero and where polarization is absent
potentiometric measurements
voltammetry comprises three electrodes such as ________ and a nonreactive electrolyte called ______
working electrode
reference electrode
counter electrode
supporting electrolyte
is often a coil of platinum wire or a pool of mercury, which electrolysis current passes between the working electrode and this electrode
counter electrode
are caused by large currents hat develo due to oxidation of the water to give molecular oxygen
positive potential limitations
arise from the reduction of water to produce hydrogen
negtaive limits
commercially available ______________for voltammetry (or amperometry) in flowing streams. The block is made of polyether (ether ketone) (PEEK) and is available in several formats with different size electrodes (3 mm and 6 mm; see the blue area in the center of the figure) and various arrays (dual 3 mm and quad 2 mm)
sandwich type microelectrode
is used in nearly all early polarographic experiments. It consists of roughly 10 cm of a
fine capillary tubing (inside diameter 5 0.05 mm) through which mercury is forced
by a mercury head of perhaps 50 cm.
dropping mercury electrode (DME)
important uses of hydrodynamic voltammetry
- detection and determination of chemical species as they exit in chromatographic columns
- routine determination of oxygen and certain species of biochemical interest
- detection of end points in coulometric and volumetric titrations
- fundamental studies of electrochemical processes
is widely used for detection and determination of oxidizable or reducible compounds or ions that have been separated by liquid chromatography
or that are produced by flow-injection methods
hydrodynamic voltammetry
what sensor refers to a membrane in this case is more complex and consists of three layers. The outer layer is a polycarbonate film that is permeable to glucose but impermeable to proteins and other constituents of blood. The middle layer is an immobilized enzyme; glucose oxidase in this example. The inner layer is a cellulose acetate membrane, which is permeable
to small molecules, such as hydrogen peroxide
enzyme-based sensors
sensors can be
based on detecting hydrogen
peroxide, oxygen, or H1,
depending on the analyte and
enzyme
enzyme based
____ are used for H2O2 and O2, while
a _______________ is used for H+.
voltammetric sensors
potentiometric pH electrode
