Transcript Chapters 22, 23, 24 & 25

Chapters 22, 23, 24 & 25
Electroanalytical Chemistry
Electroanalytical Chemistry
Electroanalytical Chemistry...
It encompasses a group of quantitative
analytical methods that are based upon
the electrical properties of a solution of
the analyte when it is made part of an
electrochemical cell.
Why Electroanalytical
Chemistry?
• Electroanalytical methods have certain advantages over
other analytical methods. Electrochemical analysis allows
for the determination of different oxidation states of an
element in a solution, not just the total concentration of the
element.
• Electroanalytical techniques are capable of producing
exceptionally low detection limits and an abundance of
characterization information including chemical kinetics
information. The other important advantage of this method
is its low cost.
Galvanic Electrochemical
Cell with Salt Bridge
Electroanalytical techniques are capable
of producing exceptionally low detection
limits and a wealth of characterization
information describing electrochemically
addressable systems. Such information
includes stoichiometry and rate of
interfacial charge transfer, rate of mass
transfer, extent of adsorption or
chemisorption, and rates and equilibrium
constants for chemical reactions.
History
Polarography was first discovered by a
czechoslovavian chemist by the name of
Heyrovsky in 1920. He won the Nobel
prize for it in 1959. He proposed that the
current recording generated by a
oxidation or reduction in a cell as the A.P.
is continuously increased:
Oxidizing Agent + ne  Reduction
Oxygen Probe
A.P. – 650 mV Ag|AgCl
Reactions:
O2 + 2H2O + 4e-  4OH-
4Ag + 4Cl-  4AgCl + 4e-
Hydrogen Probe
A.P. +650 mV
Reactions:
H2O2  O2 +2H+ +2e2Ag+ + 2e-  2Ag
Daniell Cell
This cell is based on the overall reaction
[Cu(OH2)6]2+(aq) + Zn --> Cu + [Zn(OH2)6]2+(aq)
and functions by dissolution of Zn from the anode
and deposition of Cu at the cathode. It is therefore
very simply represented as
Zn | [Zn(OH2)6]2+(aq) || [Cu(OH2)6]2+(aq) | Cu
or just as
Zn | Zn(II)(aq) || Cu(II)(aq) | Cu
Galvanic Cells
• A galvanic cell consists
of at least two half cells,
a reduction cell and an
oxidation cell. Chemical
reactions in the two half
cells provide the energy
for the galvanic cell
operations. The
reactions always run
spontaneously in the
direction that produced
a positive cell potential
Voltaic Cells
• A voltaic cell is an
electrochemical cell that
external electrical current
flow can be created using any
two different metals since
metals differ in their tendency
to lose electrons. Zinc more
readily electrons than copper,
so placing zinc and copper
metal in solutions of their
salts can cause electrons to
flow through an external wire
which leads from the zinc to
the copper. The following is a
diagram of a voltaic cell.
Cathodes and Anodes
Reactions at the Anode
• Some examples are:
Cu(s) <=>
Cu2+ + 2eFe2+ <=> Fe3+ + eH2(g) <=> 2H+ + 2eAg(s) + Cl- <=> AgCl(s) + e-
Reactions at Cathodes
• Electrons supplied by external circuit via an
inert electrode (platinum or gold)
• Some examples are:
Cu2+ + 2e- <=> Cu(s)
Fe3+ + e- <=> Fe2+
2H+ + 2e- <=> H2(g)
AgCl(s) + e- <=> Ag(s) + Cl-
Cells Without Liquid Junctions
• Liquid junction - the interface between 2
different electrolytic solutions
• Cell can contain more than one
• A small Junction Potential arises at these
interfaces
• Sometimes it is possible to prepare cells that
share a
• Common electrolyte to avoid this problem
Concentration Cells
• A concentration cell is an
electrochemical cell in which
the electrode couple at both
electrodes is the same but the
concentrations of substances
at the two electrodes may
differ. The potential
difference across a
concentration cell can be
calculated using the Nernst
equation.
Calculation of Cell Potential
from Electrode Potentials
• Nernst Equation: The cell potential for a
voltaic cell under standard conditions can
be calculated from the standard electrode
potentials. But real voltaic cells will
typically differ from the standard
conditions. The Nernst equation relates the
cell potential to its standard cell potential.
Cell Potential
•
One implication is that the cell
potential will be reduced from the
standard value if the concentration of
Zn2+(aq) is greater than that of
Cu2+(aq) at the standard
temperature. An excess concentration
of Cu2+(aq) will give a higher voltage.
The graph at right shows the increase
in cell voltage with increasing
concentration of the cation. Note that
the horizontal axis is logarithmic, and
that the straight line variation of the
voltage represents an logarithmic
variation with Q. Note that the cell
potential is equal to the standard
value if the concentrations are equal
even if they are not equal to the
standard value of 1M, since the
logarithm gives the value zero.
The Nernst Equation
R = gas constant
T = temperature in
Kelvins
Q = thermodynamic
reaction quotient
F = Faraday's constant
n = number of electrons
transferred
Currents in Electrochemical Cells
• Ohms law is usually obeyed:
E=IR
where E is the potential difference in volts
responsible for the movement of the ions, I
is the current in amps, and R is the
resistance in ohms of the electrolyte to the
current
Summary of Important Types of
Polarography
LCEC...
LCEC:
The Ilkavic Equation:
The diffusion current, id, is in the plateau region
of a polarogram and, thus, is independent of
potential
id = 706n C D1/2 m2/3 t1/6
Where: id = max value of the diffusion current in the life of
the drop
n = # of electrons involved
C = Cons. Substitution, mmoles/L
D = diffusion coefficient of the ion (cm2/s)
m = mass of Hg in mg/s
t = time between drops
References:
http://www.anachem.umu.se/jumpstation.htm
http://userwww.service.emory.edu/~kmurray/mslist.html
http://www.anachem.umu.se/jumpstation.htm
http://www.acs.org
http://www.chemcenter/org
http://www.sciencemag.org
http://www.kerouac.pharm.uky.edu/asrg/wave/wavehp.html