Experiment
- Page ID
- 60889
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)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
- Test of RC circuit –
- 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.
- 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).
- Please obtain directions for the operation of the potentiostat from the instructor.
- 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.
- Determine the value of the diffusion coefficient, D
- With a 3 mm diameter disk electrode (either GC or Pt), fill the cell with a 0.1 M KNO3 solution.
- 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.
- 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.