Ch 23 Voltammetry Flashcards
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
When current proportional to analyte
concentration is monitored at a fixed potential, the technique is called
amperometry
are based on
measuring current as a function of the
potential applied to a small electrode.
Voltammetric methods
is voltammetry in that the working electrode is the unique dropping mercury electrode.
Polarography
What are the most common excitation signals used in voltammetry
linear scan
differential pulse
square wave
triangular
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.
supporting electrolyte
is the electrode at which the analyte is oxidized
or reduced.
working electrode
is often a coil of platinum wire or a pool of mercury.
counter electrode(CE)
The cell is made up of three electrodes immersed
in a solution containing the analyte namely
Supporting electrode
Working electrode
Counter electrode
many metal ions are reversibly reduced to
amalgams at the surface of a _______ electrode, which can be used in Large negative potentials
mercury electrode
Metals which are soluble in
mercury form liquid alloys
known as
amalgams
working electrodes
with surface areas smaller than a few
square millimeters were called
microelectrodes
micrometer-sized
electrodes were sometimes called
ultramicroelectrodes
is an ∫-shaped ( sigmoidal shape)
wave that appears in current-voltage
plots in voltammetry
voltammetric wave
is the current plateau that is observed
at the top of the voltammetric wave.
limiting current
occurs when
the current is equal to one half of the
limiting value
half-wave potential
is a type
of voltammetry in which the analyte
solution is kept in continuous motion.
Hydrodynamic voltammetry
during an electrolysis, reactant is carried to the surface of an electrode by three mechanisms:
migration
convection
diffusion
during an electrolysis, reactant is carried to the surface of an electrode under the influence of an electric field,
Migration
during an electrolysis, reactant is carried to the surface of an electrode resulting from stirring or vibration,
Convection
during an electrolysis, reactant is carried to the surface of an electrode due to concentration differences between the film of liquid at the electrode surface and the bulk of the solution.
diffusion
What is the formula of the potential between the working electrode and the reference electrode (Eappl)
Eappl = E0 - 0.0592/ n log cP/cA - Eref
Formula of current
i = nFAD (‘c/ ‘x)
liquid flow patterns as an effect of stirring that occurs at low flow velocities and has smooth and regular motion,
Laminar flow
liquid flow patterns as an effect of stirring that happens at
high velocities and has irregular, fluctuating motion
Turbulent flow
What is the Flow patterns and regions
of interest near the working electrode in
hydrodynamic voltammetry.
Nernst diffusion layer of stagnant solution > Laminar flow region > Turbulent flow region
a stagnant layer where the concentrations of reactant and product vary as a function of
distance from the electrode surface and that there are concentration gradients
Nernst diffusion layer
Region where the convection
maintains the concentration of A at its original value and the concentration of P at a
very small level.
turbulent and laminar flow regions
Thickness of Nerst Diffusion layer
ranges from 10^-2 to
10^-3 cm, depending on the efficiency of the stirring and the viscosity of the liquid.
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
The most important uses of hydrodynamic voltammetry include
(1) detection and
determination of chemical species as they exit from chromatographic columns or
flow-injection apparatus; (2) routine determination of oxygen and certain species
of biochemical interest, such as glucose, lactose, and sucrose; (3) detection of end
points in coulometric and volumetric titrations; and (4) 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
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 sensor
can be based on detecting hydrogen
peroxide, oxygen, or H1,
depending on the analyte and
enzyme.
Enzyme-based sensors
A plot of current versus the angular velocity of the disk in radians per second (ω^1/2) is known as a _________, and deviations from the linear relationship
often indicate kinetic limitations on the electron transfer process.
Levich plot
RDE detection with a mercury-film electrode is sometimes referred to as
pseudopolarography
was the first type of voltammetry to be discovered and
used. It is essentially no convection or migration, and a dropping mercury electrode
(DME) is used as the working electrode.
Linear-scan polarography
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
The diffusion current is ___________ to analyte concentration in the bulk of solution
directly proportional
Two sources of residual current
- the reduction of trace impurities
- charging, or capacitive
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. Does not accompanied by oxidation/reduction process.
nonfaradaic current
electrons are repelled from the surface toward the bulk of mercury, and a negative current is the result. The mercury surface
is uncharged, and the charging current is zero. This potential is called the
potential
of zero charge
The direction of the initial
scan may be either negative, as shown, or positive, depending on the composition of
the sample. A scan in the direction of more positive potentials /opposite direction is called a
reverse scan
The voltage extrema at which reversal takes place are called
switching potential
the current response of a small stationary electrode in an unstirred solution is excited by a triangular voltage waveform
cyclic voltammetry (CV),
The direction of the initial
scan may be either negative, as shown, or positive, depending on the composition of
the sample. A scan in the direction of more negative potentials is termed a
forward
scan,
two most important pulse techniques
differential-pulse voltammetry
square-wave voltammetry
detection limits with differential-pulse
voltammetry are two to three orders of magnitude lower than those for classical
voltammetry and lie in the range of
10^-7
to 10^-8 M.
is a type of pulse voltammetry that offers the advantage of great speed and high sensitivity. An entire voltammogram is obtained in less than
10 ms, which has been used with hanging mercury drop electrodes and with other electrodes
Square-wave voltammetry
The following organic functional
groups produce voltammetric
waves:
1. Carbonyl groups
2. Certain carboxylic acids
3. Most peroxides and epoxides
4. Nitro, nitroso, amine oxide,
and azo groups
5. Most organic halogen groups
6. Carbon/carbon double bonds
7. Hydroquinones and mercaptans.
the analyte is deposited by reduction and
then analyzed by oxidation from the
small volume mercury film or drop. 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
In anodic stripping methods
the analyte is electrolyzed into a small volume of mercury by oxidation and then
stripped by reduction. The working
electrode behaves as an anode during the deposition step and as a cathode during
stripping
In cathodic stripping methods,
The dimensions of this electrodes are typically smaller than about 20 mm and may be
as small as a 30 nm in diameter and 2 mm in length. The electrochemical behavior of
these tiny electrodes is significantly different from classical electrodes and appears to offer advantages in certain analytical applications. Such electrodes are often called
microscopic electrodes, or microelectrodes
The current in a cell containing a dropping mercury electrode undergoes periodic fluctuations corresponding in frequency to the drop rate
Polarographic currents
The following organic functional groups produce voltametric waves
Carbonyl groups
Carboxylic acids
Peroxide and epoxide
Nitro nitroso amine oxide and azo groups
Organic halogen groups
Carbon carbon double bonds
Hydroquinones and mercaptans
