(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) Obtain three replicate voltammograms for the potassium ferrocyanide solution [Follow the procedures in Section 3, Measurement Procedures, for three replicate runs.] Then go to the next step.

(f) Turn OFF the Potentiostat and remove sample beaker.

(g) Add 50 mL of potassium chloride solution to a clean 100 mL beaker, repeat steps 2(b) and 2(c). Then obtain three replicate voltammograms for this BLANK solution. [Follow the procedures in Section 3, Measurement Procedures, for three replicate runs.] Then return to the next step.

(h) Turn OFF the Potentiostat and remove sample beaker. Rinse electrodes, and place in beaker containing distilled water. Leave computer on for next user.

[3] Measurement Procedures.

If the Potentiostat is OFF, turn it ON (Black push-button switch, lower left corner).

(a) When you are ready to run the experiment, 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 3.b:

NOTE: There are some actions to be taken by the operator during the experiment. The item in BOLD LETTERS below are to be performed by the operator. The experiment can be terminated at any time (in case of a setup error) 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 -0.400 V. for 60 sec. [Cond. time]. (This conditions the indicator electrode by removing oxidized materials from the surface.) Turn on the magnetic stirring during this time.

(2) keeps the voltage at -0.400 V. for another 30 sec. [Equil. time]. (This is the start potential for the voltage sweep.) Turn the stirrer OFF during this period [Equil. time], so the solution becomes still for 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.)

Figure 2. Oxidation voltammograms. A. After blank correction. B. Without blank correction. Voltage (and time) increase to left; oxidation current is down.

 

(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 2.

(d) Return to Section 2.

 

[4] Data Treatment.

When all sample and blank runs have been completed, the following DATA TREATMENT can be applied to saved files. (Refer to Figure 2 which shows voltammograms, displayed right-to-left, oxidation current down.)

(a) Obtain Net Current Voltammogram. Click on: Main; File; Get Data; "Click Here" (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 first file saved from step 2(e)). Select Numeric; File Math; "-"; "Click Here" (in the screen block indicating where data files are found). Then select the first file you saved previously as your sample BLANK (obtained in 2(g)) from your diskette in Drive A. This will subtract the blank voltammogram from the sample voltammogram, giving the Net Current voltammogram.

 

(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 and select two points on the flat part of the baseline through which the computer will draw a straight line. First move the cursor to the left side of the flat part of the baseline; click the right mouse button; then move the cursor to the right side of the flat part of the baseline; click the right mouse button. The computer will display a straight line fit to the two selected points. The Net Peak Current is measured with respect to this line (4(c)). NOTE: The baseline should be horizontal, and nearly zero. A significant slope indicates the blank correction may not be valid, and it may be necessary to repeat the voltammograms.

(c) Find the Peak Potential & 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 (Fig. 4A), or the region where the current stops changing rapidly (Fig. 4B). Observe the y-value displayed on the upper part of the screen when you have the cursor on the apparent peak. Record this value and the x-value; repeat this measurement three times. Record the average y-value as the Net Peak Current, i_p(Net), and the average x-value as the Peak Potential. (Press the "ESC" key to go to previous screen.)

(d) Repeat steps 4(a) to 4(c) for each voltammogram-blank combination. Record the Peak Potentials and Net Peak Currents for each of these. Each Net Current Voltammogram can be saved as a separate file, using the procedure of [3.c].

(e) Any voltammogram displayed on the screen 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.

 

5. Calibration Report

Record in the EG&G/PAR 263A Calibration Record Book

5.1 Date, Time, Operator

5.2 Instrument I. D. (EG&G/PAR 263A-413)

5.3 Stock Solution Preparation Data; Ferrocyanide concentration; Conditions

[Note: electrode Area for unsharpened electrode is 0.18 cm²]

5.4 Results Summary:

(a) Procedure used (PARCAL.SET)

(b) Peak potentials, peak currents, ratio: Ip(net)/([Ferrocyanide][Area])

(c) Averages and standard deviations of items (b)

 

 

[6] 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 says "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.) If the cell status is not displayed at all, 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.

(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 PARCAL.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.

 

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