Chapter 24' Coulometry

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Chapter 24. Coulometry

• measurement of charge
• Charge is a function of analyte concentration,
  transport to electrode surface, electron
  transfer, among other factors.

Detector/ Transducer/ Sensor
Excitation Process
voltage
signal
Sample
Readout
View charge
Current is integrated to obtain charge. Charge is
transformed to voltage by electronics.
Voltage or current is applied to analyte
appreciable current flows.
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• constant potential (potentiostatic)
• hold potential constant, measure current,
  integrate over time to get charge.
• constant current (galvanostatic or amperostatic)
• hold current constant, electrolytically generate
  a reactant
• this reactant is the titrant in a coulometric
  titration.
• titrant reacts with analyte until analyte
  consumed. Measure length of time that current
  flowed.
• electrogravimetry - measurement of mass of solid
  product deposited on one electrode
• Q it Q charge (C) i current (A C/s)
• t time (s)

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Chapter 25. Voltammetry
I. Concept

• Current is a function of
• analyte concentration
• how fast analyte moves to electrode surface
• rate of electron transfer to sample
• voltage, time...

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II. Excitation process

• A. What happens when a voltage is applied to an
  electrode in solution containing a redox species?
• generic redox species O
• O e- --gt R E -0.500 V v. SCE
• Imagine that we have a Pt etrode in soln at an
  initial potential of 0.000 V v. SCE and we switch
  potential to -0.700 V.
• First

supporting electrolyte
O redox
solvent
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B. Events that happen

• 1. supporting electrolyte forms an electrical
  double layer

cation movement to etrode causes an initial spike
in current Formation of double layer is good
because it ensures that no electric field exists
across whole soln (requires 1001 conc ratio of
supporting elyteredox species).
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2. Electron transfer rxn
O is converted to R at etrode surface.
?R
Eapp -0.7
?R

A depletion region of O develops - a region in
which conc of O is zero.

• How does more O get to etrode surface?
• mass transport mechanisms

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C. Mass transport to the etrode

• 1. Migration - movement in response to electric
  field. We add supporting electrolyte to make
  analytes migration nearly zero. (fraction of
  current carried by analyte ? zero)
• 2. Convection
• stirring
• rotated disk etrode (RDE)
• Levich equation
• A etrode area
• w angular velocity (s-1) 2p rotation
  speed
• DO diffusion coefficient of O (cm2/s)
• n kinematic viscosity (cm2/s)
• CO bulk conc (mol/cm3) - (not surface conc)

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2. Convection (contd)

• concentration profile in solution - x distance
  from etrode surface

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conc at distance x relative to bulk conc
0
dimensionless distance from etrode surface
At what distance does depletion region end and
bulk region begin when D 1 x 10-5 cm2/s and
etrode spins at 31 rpm?
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2. Convection (contd)

• RRDE (rotated ring-disk etrode) - permits study
  of rxn mechanisms

disk O ne --gt R
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3. Diffusion

• In experiments relying upon diffusion, no
  convection is desired, soln is quiescent.
• Consider an etrode at which we step voltage
  beyond E of redox couple.
• Concentration profile in soln

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x distance from etrode surface
0
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3. Diffusion (contd)

• Very common technique cyclic voltammetry -
  voltage is varied linearly as a function of time.
• Conc profile hard to draw because both potential
  and current are varying with time.

Randles-Sevcik equation v scan rate
(V/s) Polarography - older technique using
dropping Hg etrode
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4. Microelectrodes

• Diffusion
• Linear (planar) vs. radial (spherical)
• microelectrodes rely on radial diffusion
• advantages include
• steady-state measurement (not dependent on
  time)
• less iR because less current
• less capacitance
• for a microdisk, iL 4nFrDOCO r etrode
  radius (cm)

etrode
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D. Voltage programs

• We can apply
• a single dc voltage for a desired period of time
  (chronoamperometry)
• a voltage that varies linearly with time (linear
  sweep voltammetry or cyclic voltammetry)
• a voltage that varies linearly with time but also
  has regular pulses to increase sensitivity or
  remove background signal (differential pulse
  voltammetry (DPV) and square wave voltammetry)
• a voltage that collects metals and then oxidizes
  them (anodic stripping voltammetry - ASV)

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D. Voltage programs revisited

• 1. DPV and square wave voltammetry
• 2. ASV
• metal cations reduced to metal, which dissolves
  in Hg drop
• Mn ne- --gt M(Hg)
• metal solid is then oxidized - highly
  concentrated in Hg
• M(Hg) --gt Mn ne-
• (concentrated)
• sensitive technique

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E. Solutions and electrodes

• 1. Solutions redox couple solvent
  supporting electrolyte
• supporting elyte salt that migrates and
  carries current, and doesnt do redox in your
  potential window of interest
• a wide potential window is desirable
• water - good for oxidations, not reductions
  except on Hg supporting elytes lots of salts
• nonaqueous solvents acetonitrile,
  dimethylformamide, etc.
• supporting electrolytes tetraalkylammonium BF4,
  PF6, ClO4
• Oxygen is fairly easily reduced - we remove it by
  deoxygenating with an inert gas (N2, Ar).

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2. Electrodes

• working etrode (WE) is where redox activity
  occurs
• auxiliary etrode (AE) catches current flow from
  WE
• reference etrode (RE) establishes potential of WE
• a. working etrode materials
• Pt, Au, C, semiconductors
• Hg - messy but good for reductions in water.
  Not good for oxidations.
• b. auxiliary etrodes similar materials, large
  in area
• c. reference etrodes real vs. quasi -
• real refs have an actual redox couple (e.g.
  Ag/AgCl)
• quasi refs (QRE) - a wire at which some (unknown)
  redox process occurs in soln. QREs OK if
  currents are needed but not potentials.

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III. Detection

• Current is measured and converted to voltage for
  readout.
• (gain or sensitivity is set using electronics,
  e.g. 10 mA/V)
• To minimize iR drop - three etrode potentiostat

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IV. Advantages and disadvantages

• Advantages
• sensitive (good for quantitation)
• instrumentation is relatively inexpensive
• Disadvantages
• not so good at identification
• not too selective (although this is changing)

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V. Some Applications

• A. Elucidation of rxn mechanism, or stability of
  a cmpd
• EC
• CE
• B. Chemically modified electrodes
• putting a layer on the electrode surface that
  only allows attachment of analyte of interest
• doing electrochemistry on attached analyte
• C. ECL (electrochemiluminescence)