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Chemistry LibreTexts

Experiment

Equipment

  • An analog or computer-controlled potentiostat with appropriate data acquisition capability (ask you laboratory instructor about potentiostat and data recorder).
  • Electrochemical cell
  • 1 - 3mm diameter flat tipped glassy carbon electrode
  • Ag/AgCl reference electrode
  • Pt auxiliary electrode
  • Electrode polishing kit

Chemical Solutions

  • 100 ml of 10 mM AA in 0.10 M phosphate buffer at pH 2 containing 0.5 g/L Na2EDTA to prevent the oxidation of AA. It is recommended that the solution be deaerated to remove oxygen prior to adding AA. Store solution in a refrigerator when not in use. It is important to use high purity water and clean glassware. Record the concentration to three significant figures.
  • 250 ml of stock 0.1 M phosphate buffer at pH 2.
  • 50 ml of 2.0 mM AA in pH 2 phosphate buffer by dilution of stock solutions 1 and 2. Keep tightly sealed when not in use. Prepare daily from stock.
  • 50 ml test sample by diluting 1:20 commercial fruit drink with the pH 2 phosphate buffer. Use immediately.

Procedure

  1. Recommend the use of a 3.0 ml cell, with a 1.0 mm or 3.0 mm disk GC electrode, an Ag/AgCl reference and a Pt auxiliary electrode. It is recommended that rubber gloves be used to handle the cell and electrodes.
  2. Run #1: Fill the cell so that the electrodes are immersed in the 2.0 mM AA solution. Connect the electrodes to the potentiostat. All scans will be from 0.0 V to +0.6 V and back to 0.0 V at scan rates of 20 mV/s, 100 mV/s and 200 mV/s. Use the GC electrode without polishing (this will serve as the “inactive” electrode). Wait 2 minutes between scans to allow the concentration at the electrode surface to equilibrate with the bulk solution.
  3. Run #2: Repeat after removing and polishing the GC electrode with 0.05 μm alumina on a clean glass plate and rinse with pure water prior to use. Please refer to “Activation of Glassy Carbon Electrode” that is available as a Technical Note. Add the scan rates of 200 mV/s and 1.0 V/s and set the number of scans to 2. If the CV wave at 100 mV/s does not exhibit a sharp peak, as seen in Trace C of Figure 2, it may be necessary to polish the electrode first on a fine grit 0000 paper followed by alumina.
  4. Set the scan rate to 100 mV/s and obtain CV of AA at concentrations of 0.50 mM, 1.0 mM, 2.0 mM and 4 mM in pH 2 phosphate buffer. Run a background CV of the buffer without AA at these scanning rates to correct for the peak heights.
  5. Run duplicates of the fruit juice sample and determine its concentration by comparing the peak height to a calibration plot of concentration versus peak height, as determined in step #3. If the CV peaks are not sharp and well defined, re-polish with alumina to activate the electrode.
  6. It is recommended that you save all CV data on the hard drive or memory card with file names or in a folder with your name for future reference. If data are saved as an ASCII file, transfer data to a Microsoft EXCEL program and re-plot the data files. Excel gives you options to make different formatted graphs.