Introduction to Electroanalytical Chemistry

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• Introduction to Electroanalytical Chemistry
• Liquid Junction potentials
• Occur whenever two electrolytes of different
  composition are in contact
• A potential develops across the interface
• The potential results from the production of a
  charge separation due to unequal distributions
  of cation and anion concentrations on either side
  of the interface
• The unequal distribution results from variation
  in the mobilities of different ions and
  different diffusion rates caused by concentration
  differences
• Example the interface between 1 M HCl and 0.01 M
  HCl

H is transported by diffusion many times
faster than Cl- Both move from a region of
higher concentration to a region of lower
concentration The ions reach a state of
equilibrium when the transport of H is stopped
by the attraction of H for negative charge
produced by the excess Cl- ELJ 10s of mV
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• Introduction to Electroanalytical Chemistry
• Liquid Junction potentials
• Salt bridge is a device that reduces the
  magnitude if ELJ if properly constructed
• Place a concentrated solution between the two
  electrolytes in contact
• The effectiveness improves by increasing the
  concentration of the salt in the salt bridge and
  as the mobilities of the cation and anion in the
  salt bridge approach each other
• Saturated KCl which is about 4 M at room
  temperature is often used since the mobilities
  of K and Cl- differ by only 4
• ELJ for the KCl salt bridge is only a few mV
• The IR drop produced when current is drawn
  through a cell is the 3rd source of potential
• The IR drop occurs because work is required to
  drive the charge carriers, which are ions,
  through the electrolyte solution
• The potential produced is EIRIR, where I is the
  current and R is the internal resistance of the
  liquid electrolyte

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• Introduction to Electroanalytical Chemistry
• The IR drop
• Example consider the galvanic cell CdCdSO4(1
  M)CuSO4(1 M)Cu
• Ethermo0.74 V
• R 4?
• Ohms Law
• If 0.04 A is drawn IR0.04 4 0.16 V
• The cell potential would be Ethermo - IR 0.74 -
  0.16 0.58V
• Figure. Plot of current vs. potential for a
  galvanic cell in which the internal resistance
  is 4? and Ethermo is 0.74 V

EcellEcath-Eanode- IR - ELJ
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• Introduction to Electroanalytical Chemistry
• The IR drop
• Example consider the electrolytic cell
  CuCuSO4(1 M)CdSO4(1 M)Cd
• Ethermo-0.74 V
• R 4?
• Ohms Law
• Assume ELJ 0 V
• If 0.04 A is drawn IR0.04 4 0.16 V
• The cell potential would be Ethermo - IR -0.74
  - 0.16 -0.90 V
• Figure. Plot of current vs. potential for an
  electrolytic cell in which the internal
  resistance is 4? and Ethermo is -0.74 V

EcellEcath-Eanode- IR - ELJ
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• Introduction to Electroanalytical Chemistry
• Polarization is a fourth contribution to cell
  potential
• Polarization produces deviations from Ohms law
  when high cell currents are involved
• In either a galvanic or electrolytic cell the
  current becomes a nonlinear function of
  potential
• Complete polarization occurs when current becomes
  independent of potential, i. e., current -
  potential curve become horizontal

Figure. Plot of current vs. potential for an
electrolytic cell showing polarization at
currents gt 0.04 A
Figure. Plot of current vs. potential for a
galvanic cell showing polarization at currents gt
0.04 A
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• Introduction to Electroanalytical Chemistry
• Polarization is an electrode phenomenon
• Could be at the cathode or anode or both
  electrodes
• Phenomena responsible for polarization
• Size and shape of electrode
• Composition of electrolyte
• Agitation of solution
• Temperature
• Magnitude of the current
• Physical states of reactants and products
• Composition of Electrode
• Two kinds of polarization concentration
  polarization and
• kinetic polarization
• Concentration polarization is produced when the
  rate of mass transport of an electroactive
  species is limited in some way

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• Introduction to Electroanalytical Chemistry
• Concentration polarization
• Example
• Consider a cell whose reaction is complete and
  reversible in the thin layer of solution near
  the electrode surface
• Cu2 2e- Cu
• The Nernst equation shows how the concentration
  of electroactive species is related to the
  applied potential
• If Ecath is changed, Cu2 must change
  instantaneously to meet the requirements of the
  Nernst equation
• In order to conduct current, Cu2 must be moved
  from the bulk of solution to replace the Cu2
  removed from solution
• The rate of mass transport of Cu2 must match the
  current
• If

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• Introduction to Electroanalytical Chemistry
• Concentration polarization
• If the rate of mass transport cannot match the
  current requirement, concentration polarization
  will occur
• Mass transport in solution occurs by
• Diffusion
• Electrostatic attraction or repulsion of ions for
  charged electrodes
• Mechanical or convective transport
• Diffusion is characterized by Ficks law
• The flux is the rate of mass transport in
• D is the diffusion coefficient in
• is the concentration gradient or
  change in concentration with distance in the
  neighborhood of the surface of the electrode in
• d is the thickness of the difussion gradiant
  and c0 is the concentration at the surface of
  the electrode
• The negative sign indicates flux is in the
  opposite direction to the concentration
  gradient, that is, mass transport is from a
  region of high concentration to low
  concentration

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• Introduction to Electroanalytical Chemistry
• Diffusion
• If a concentration difference exists between the
  surface of the electrode and the bulk of
  solution, mass transport will be established
• Toward the electrode if cbulk is gt c0
• Away from the electrode if c0 is gt cbulk
• and the rate of transport will be directly
  proportional to cbulk - c0
• C0 is determined by Eapplied and can be
  calculated from the Nernst equation
• Electrostatic forces and mass transport
• Electrodes have polarity either the same sign or
  opposite sign as electroactive ions
• Therefore, electroactive ions can be attracted to
  or repelled by the electrode
• The total electrolyte concentration affects this
  attraction
• As the concentration of inert electrolyte in
  solution increases, the attraction of an ion for
  an electrode approaches zero when the
  concentration of electroactive ion is a small
  fraction of ionic species present

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• Introduction to Electroanalytical Chemistry
• Mechanical transport
• Stirring, agitation or convection through
  temperature gradients move matter around a
  solution
• Such transport prevents the establishment of
  concentration polarization
• Upshot concentration polarization is an
  extremely important phenomenon in
  electrochemical cell behavior
• In some cases every effort is made to reduce
  concentration polarization
• In other cases, many analytical methods depend on
  seeing to it that concentration polarization is
  established