Chem 3253 Electrochemistry April 14, 2004

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Chem 3253ElectrochemistryApril 14, 2004
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Four Electroanalytical Methods

• 1) Potentiometric
• 2) Voltammetric, Polarographic, Amperometric
• 3) Electrolysis (including electrogravimetric
  and coulometric)
• 4) Conductiometric
• We will consider the details of each of these
• separately.

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Polarography

• The Ilkovic equation id 607nD1/2m2/3t1/6C
• What is generally done is to keep all of the
• Variables constant and plot id vs. Conc.
• Alternately, the Standard Addition Method
• may also be used this is shown later for
• voltammetry.
•  

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Polarography

• Advantages
• Both qualitative (value of E1/2) and
  quantitative (id ) data obtained.
• Quantitative data to 10-5 M
• Applications to kinetic data such as the rate of
  electron transfer

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Polarography

• Disadvantages
• Samples must have O2 removed.
• Oxidation/reduction must occur in the potential
  window provided by the DME,
• -1.3 to 0.1 volts vs. SCE

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Other forms of polarography

• In classical polarography the applied voltage is
  a linear ramp imposed
• on the working electrode as shown in a).
  Modifications have been a
• Time stair step ramp as shown in b), or an small
  AC signal on the ramp.

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Voltammetry

• The data is very similar to polarography, the
  measurement of current vs. voltage at the working
  electrode. The equipment is shown in the
  following slide. Other electrodes are used,
  including graphite pictured in the next slide,
  and Pt or Au. The various electrodes allow for
  more anodic potentials as shown in the left
  margin on page 356. The voltammetrograms
  obtained are similar to polarogragrams, without
  the oscillations caused by the breaking mercury
  drop.

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Voltammery
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Voltammery
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Data for Oxidation of Ascorbic Acid
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Data for Oxidation of Ascorbic Acid
Ascorbic Acid in a 50.0 mL sample of orange juice
is determined by voltammetry using the standard
addition method.
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Data for Oxidation of Ascorbic Acid
Standard addition plot of data for the oxidation
of Ascorbic Acid.
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Cyclic Voltammetry

• Cyclic Voltammetry - applies a cathodic dc ramp
  voltage to
• the working electrode (generally a hanging drop
  mercury
• (HDME) or a spinning Pt electrode), then reverses
  the
• ramp and sweep back in anodic direction to the
  starting
• potential. A cyclic voltammetrogram is shown in
  the next
• slide.

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Cyclic Voltammetry
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Cyclic Voltammetry

• Advantages
• rapid analysis - can vary the sweep rate
• useful in study of the reversibility of the redox
  process.
• study the kinetics of redox process

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Stripping Analysis

• Stripping Analysis - consists of 2 steps
• pre-electrolysis, where the analyte is
  concentrated by electrolysis on the working
  electrode.
• 2) reverse electrolysis or stripping the amount
  of material deposited is obtained from the
  integral of the current-time curve, essentially
  coulometry
• sensitivity to 10-9 M separate peaks for each
  component.

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Stripping Analysis