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Experiment

  • Page ID
    60889
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    Equipment

    For a real cell where the time constant of the cell is fairly long (i.e., several milliseconds) an analog potentiostat with an inexpensive digital oscilloscope for data acquisition with a printer readout is recommended. With a computer-controlled potentiostat, fast data acquisition with readout is part of the instrument.

    • An analog or computerized potentiostat with appropriate data acquisition capability is needed. Please see your laboratory instructor about the potentiostat and accompanying manual.
    • Test circuit
      • 100 Ω and 10,000 Ω resistors
      • 2 μF capacitors (not electrolytes)
    • Small volume (< 5 ml) cell and electrodes
      • 3 – 4 mm diameter glassy carbon or Pt planar tip working electrode
      • Ag/AgCl reference electrode
      • Pt auxiliary electrode
      • Small volume cell
      • Electrode polishing kit
    • Pipettes and other laboratory glassware and supplies

    Chemical Solutions:

    • 100 ml of 5 mM potassium ferricyanide in 0.1 M KNO3
    • 100 ml of 0.1 M KNO3 solution
    • 10 ml of 5 mM ferrocene carboxylic acid (FCA) in pH 7 buffer (provided by instructor)
    • Pure water for dilutions
      Note: The concentrations of ferricyanide and FCA should be known to 3 significant figures.

    Procedure

    1. Test of RC circuit –
      1. Connect in series a 100 Ω resistor, a 10,000 Ω resistor and a 2 μF capacitor. Use a solid-state capacitor, not an electrolytic one.
      2. The AE connector from the potentiostat is attached to the 100 Ω resistor, the RE between the two resistors, and WE to the end of the capacitor:

        \[\begin{align}
        \ce{AE > — [R,\: 100\, Ω] &\textrm{——} [R,\: 10\, KΩ] \textrm{——} [C,\: 2\, μF]→WE} \tag{5}\\
        &\:\:↑\\
        &\:\:\ce{RE}
        \end{align}\]

        The 100 Ω resistor helps to stabilize the potentiostat at short rise times (< 20 μS).

      3. Please obtain directions for the operation of the potentiostat from the instructor.
      4. Set the potential and current measurement parameters on the potentiostat to the following values:

        Ei = 0.00 V Current scale: 50 μA

        E1 = 0.500 V Filter: < 100 μS

        E2 = 0.00 V

        Operate the potentiostat in the chronoamperometry mode

        If the time duration of E1 can be set on the potentiostat, a 100 – 1,000 mS time duration is sufficient, depending on the RC time constant. Exact timing is not critical so that you can start/stop the experiment within 1 or 2 seconds.

        The instantaneous current is given by E1/R = 50 μA. The current decreases exponentially.

    2. Determine the value of the diffusion coefficient, D
      1. With a 3 mm diameter disk electrode (either GC or Pt), fill the cell with a 0.1 M KNO3 solution.
      2. Step the potential from 0.0 V to +0.500 V for 200 mS and then step back to 0.0 V. Repeat the experiment and record the i-t profiles.
      3. Repeat with a 5 mM ferricyanide in 0.1 M KNO3 solution. Step the potential from an initial 600 mV to 0.0 V for 300 mS and then return the potential to 600 mV. Wait 5 minutes and then repeat the experiment. Save and record the i-t profiles.

        Repeat the procedure with a 5 mM ferrocene carboxylic acid solution in a pH 7 buffer solution, stepping the potential from 0.0 V to 600 mV for 300 mS and then back to 0.0 V. Wait 5 minutes and then repeat the experiment. Save and record the i-t profiles.

        Please note that the potential is stepped from 600 mV to 0 V for ferricyanide, a reduction, whereas the potential is stepped from 0 V to 600 mV for ferrocene carboxylic acid, an oxidation.


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