Experiment

Equipment

• An analog or computer-controlled potentiostat with appropriate data acquisition capability (please see laboratory instructor for directions on operation of potentiostat).
• Electrochemical cell and electrodes
  • Small volume cell
  • 1 – 3 mm glassy carbon or Pt flat tipped working electrode
  • Pt auxiliary electrode
  • Ag/AgCl reference electrode
  • Electrode polishing kit or the electrode maintenance kit

Chemical Solutions

1. Prepare 100 ml stock phosphate buffer solution at pH 7 containing 1 M KCl.
   pH 7 (0.25 M Na₂HPO₄ and 0.25 M NaH₂PO₄)
2. Prepare 100 ml stock phosphate buffer solutions at pH 2 containing 1 M KCl.
   pH 2 (1 M NaH₂PO₄ and 1 M H₃PO₄)
3. Prepare100 ml stock solution of 0.1 M APAP in 1 mM HClO₄ or HCl (calculate the exact concentration of the APAP to 3 significant figures from the amount of APAP weighed)

Procedure

1. It is important to clean the electrode surface prior to each run that involves a change in the solution. Polish the electrode (Pt or GC) with 1 micron or smaller particles of alumina on a clean flat surface, preferably glass. Use of light pressure on the electrode while tracing a figure 8 for 8-10 times is suggested. Rinse the electrode with pure water and sonicate (if available) for 1 minute in a beaker containing pure water. Wipe the edges of the electrode (never the electrodes surface) with clean tissue paper, like Kimwipe^TM and use immediately. Further discussion on cleaning and polishing GC is discussed in the Tech Note titled “The Care and Feeding of Electrodes.”
2. Prepare 100 ml of 2 mM APAP in pH 7 buffer (pipette 5.0 ml of solution A and 5.0 ml of solution C into a 100 ml volumetric flask and fill to mark with pure water).
3. Insert the working electrode (glassy carbon or Pt), the reference and the auxiliary electrode in the cell and fill the cell with 2 mM APAP solution until the electrodes are covered. Attach electrodes to the potentiostat cable.
4. Set scan rate at 100 mV/s and scan potential range of E_i (initial value) at -200 mV, Es (switching potential) at 1000 mV, and E_f (final) at –200 mV (see Figure 2).
5. See laboratory instructor for set-up and operation direction for either analog or computerized potentiostat.
6. Run duplicate scans, allowing 2-3 minutes between each scan to allow equilibration of the concentrations at the electrode surface with the bulk solution. Repeat at scan rates of 50, 200 and 500 mV/s. You may need to adjust the sensitivity range to see a reasonable current peak on the CV. [Note: the peak current, I_p, varies as a square root of the scan rate].
7. Run duplicate CVs of 2.0 mM APAP in pH 2 phosphate buffer at scan rates of 50, 100, 200 and 500 mV/s.
8. To determine the amount of APAP in your Tylenol sample, run duplicate CVs of APAP at the following concentrations: 10 mM, 5.0 mM, 2.0 mM, 1.0 mM and 0.2 mM at pH 2 with GC or Pt working electrode. Set the scan rate at 100 mV/s and the scan range of 400 mV to 1000 mV. The results will be used to make a calibration plot. Direction for preparing the standard solutions for calibration is as follows -

	Solution B	Solution C	Concentration of standard solution, 50 ml final volume
#1	5.0 ml	1.0 ml	2.0 mM
#2	5.0	2.5	5.0
#3	5.0	5.0	10
#4	5.0	5.0 ml of #3	1.0
#5	5.0	5.0 ml of #1	0.20

9. Obtain a sample Tylenol tablet from the lab supervisor, grind it into a powder with a mortar and pestle, and place 0.03 to 0.05 g of the powder into a 50 ml volumetric flask. Add 5.0 ml of solution B and dilute to mark with pure water. The tablet may contain some insoluble material that does not dissolve. Shake for 5 minutes to assure that the soluble portions dissolve. Run triplicate CVs of the Tylenol.

   Save all CV data on the hard drive or other storage medium with file names for future reference. If data are saved as an ASCII file, you can import data files to Microsoft EXCEL to have flexibility to select different formats for CV graphs.