If the instrument/computer system is already ON and operational, proceed
to the next step. Otherwise, perform the operations in Section [4] first.
(a) Pipet 50 mL of the juice solution into a clean, dry 100 mL beaker.
(b) Mount the three electrodes (Indicator, I; Reference, R; Counter, C) on a ring
stand using the three mini-clamps. Polish the tip of the WIGE before
mounting. (The clamps must be insulated electrically from the electrodes;
slip plastic tubing over the electrodes or clamps, if necessary.)
(c) Position the ring stand, clamps and electrodes so that the 3 electrodes are
submerged to a depth of about 1/2" below the solution surface in the 100 mL sample beaker. (See Figure 1.)
Figure 1.
Voltammetric cell configuration.
(d) Connect each electrode to the Instrument using the three individual cables; white cable to the reference electrode; green cable to the wax-coated graphite (indicator) electrode; and the red cable to the plain graphite (counter) electrode.
(e) First Sample Aliquot: [Go to Section 2 (MEASUREMENT PROCEDURE). Then return to next step.]
(f) First Standard Addition: Pipet 1.00 mL of standard ascorbic acid solution into the beaker. Briefly stir (or swirl) the solution. [Discontinue stirring, and go to Section 2 (MEASUREMENT PROCEDURE). Then return to next step.]
(g) Second Standard Addition: Pipet 1.00 mL of standard ascorbic acid solution into the beaker. Briefly stir (or swirl) the solution. [Discontinue stirring, and go to Section 2 (MEASUREMENT PROCEDURE). Then return to next step.]
(h) Turn OFF Voltammetric Analyzer; disconnect the green cable from the Indicator electrode (WIGE); remove and polish the WIGE; then return the WIGE to the cell; re-connect the green cable.
A9.1
(i) Second Sample Aliquot: Repeat (a) to (h) for a second 50.00 mL aliquot of the sample solution.
(j) Shut Down Procedure: When finished collecting data and/or analyzing data, rinse the electrodes; replace your beaker with a beaker of distilled water. To exit the operating program, select Main; File; then Quit.
[2] Measurement Procedure.
If the Voltammetric Analyzer is OFF, turn it ON (Black push-button switch, lower left corner).
(a) When you are ready to run the experiment, insert your diskette in drive A; click on Setup, then Run from the computer screen. (If this is the first run after turning on the instrument, the computer asks if you want to run with the Dummy Cell; select NO.) The experiment begins, and the computer applies the control sequence described in section 2.b. NOTE: Items in BOLD LETTERS below are performed by the operator during the experiment. The experiment can be terminated at any by using the mouse to click on "STOP" from the display screen.
(b) The computer applies the following sequence to the sample cell:
(1) applies a voltage of -1.500 V. for 120 sec. (This conditions the indicator electrode by removing oxidized materials from the surface.) Turn on the magnetic stirring during this time [Cond. time].
(2) changes the voltage to -0.400 V. for 60 sec. (This is the starting potential for the analytical voltage sweep.) Keep the stirrer on during the first 30 seconds [Dept. time], tapping the Indicator electrode gently to remove bubbles from the surface. Turn the stirrer OFF during the last 30 seconds of this period [Equil. time], so the solution becomes still during the following procedure.
(3) applies a voltage sweep from -0.400 V. to +1.200 V. at a sweep rate of 33.3 mV/sec.; after this sweep, the cell voltage is returned to -0.400 V.
(4) during and after the voltage sweep, cell current is displayed vs voltage on the screen. Oxidation current is down; positive voltages are left. (See Figure 2.)
(c) To save the voltammogram data, insert a diskette in Drive A, click on File and Save Data; type the drive and file name (up to eight characters), with a three-character extension, "A:xxx.dat"; then press ENTER. (Use a file name that incorporates your name and a serial number, e.g, "FARADY1.DAT", "FARADY2.DAT", etc.) Make a record of the file name assigned to each data collection step from Section 1.
(d) Return to Section 1.
A9.2
[3] Data Treatment.
When all sample runs have been completed, the following DATA TREATMENT can be applied to the saved files. (Refer to Figure 2 which shows voltammograms, displayed right-to-left, oxidation current down.)
Figure 2. Oxidation voltammogram. Voltage (and time) increase to left; oxidation current is down.
(a) Obtain Net Current Voltammogram. Click on Main; File; Get Data. Type "A:*.*" in the screen block indicating where data files are found. From the Files Table displayed on the screen, click on your first data file (the file saved from step 1(e)).
(b) Draw the Baseline. With the Voltammogram displayed, click on Main; Graph; Curve; Draw Line. Observe that the cursor becomes a "+" displayed on the voltammogram. Use the mouse to move the cursor; select two points on the linear part of the baseline through which the computer will draw a straight line. Move the cursor to the left side of the linear part of the baseline; press Enter; then move the cursor to the right side of the linear part of the baseline; press Enter. The computer displays a linear fit to the selected points. The Net Peak Current is measured with respect to this line (3(c)).
(c) Find the Net Peak Current. Select Curve; Peak. The cursor "+" is again displayed on the voltammogram. When the cursor is moved, the (x, y) data values are displayed on screen. Move the cursor to the voltammogram peak region, or the region where the current stops changing rapidly (Fig. 2). Observe the y-value displayed on the upper part of the screen with the cursor on the apparent peak. Record this value; repeat this measurement three times. Record the average as the Net Peak Current, ip(Net) for this sample. (This is iaa referred to in the Data Analysis section of the Experiment writeup (p. 38)). Press the "ESC" key to go to previous screen.
(d) Repeat steps 3(a) to 3(c) for each voltammogram obtained with the two standard additions. Record the Net Peak Currents for each of these (ia2 and ia3 referred to in Section 4). Each Net Current Voltammogram can be saved as a separate file, using the procedure of [2.c].
A9.3
(e) Any voltammogram displayed can be printed out by using the command sequence: Main; File; Print/Plot; Graph. Any saved voltammogram file can be printed out by using the command sequence: Main; File; Get Data; [type in file disk:name, "A:xxxx.dat"]; Graph; Print; Plot.
[4] Instrument Start-up Procedures
(a) Turn on the computer and potentiostat. The front panel display of the potentiostat should say "Cell-type = Dummy". If the cell status is not displayed, depress the SYSTEM button repeatedly until the cell status is displayed. If this does not appear, depress the NEXT or PREVIOUS buttons until the cell status is displayed. If the cells status is "Real", rotate the large silver dial until the display reads "Cell-type = Dummy". (This condition substitutes a simple resistor for the electrochemical cell, so that erroneous modifications of conditions will not damage the real cell components.)
(b) Use the computer mouse to select and double-click on the M270 Icon. The computer screen will display the "EG&G PAR" Logo. Depress any key to get to the next screen.
(c) Place your data diskette in the A Drive. From the screen menu, click on Setup; Get Setup. Select or type "*.*" to get the list of file names. From the file list, click on VCHICLSV.SET. This will display the operating conditions on the computer screen. (If this file is not on your diskette, the file can be obtained by typing "C:\m270\data\*.*" to get the list of file names after clicking on Get Setup.
[5] REPORT Procedure
(a) Compute the sample concentration values requested in the Report sheet. Use the net current values obtained for the various solutions following the Data Treatment section [3], and the Standard Addition Procedure (see Data Analysis section of the Experiment writeup (p. 38)).
(b) Prepare summary voltammetric graphs by overlaying plots of several voltammograms (see overlay procedure 5(d) below): (1) overlay of uncorrected voltammograms for sample 1; sample plus standard addition; sample plus second addition; include estimated baseline in plot. (2) same overlay plot for sample 2.
(c) Submit a completed Report Sheet; the labeled diskette containing data files; and labeled copies of the summary voltammetric graphs described in (b).
(d) Overlay procedure: Use the File; Get Data; commands. Then to overlay other curves, use the Overlay command to select other data files. Overlaid plots are color-coded on the screen; and each is given a label. Save overlaid plots with appropriate new file names; and record the file contents and labels in your Lab Notebook.
A9.4
