Sargent Polarograph Calibration Procedure
polarcal.doc 06/24/98
1. Special Equipment
Apparatus: Sargent-Welch Model 4001 Polarograph; magnetic
stirrer and stirring bar; ring stand; 3 mini-clamps with
insulated claws.
Electrodes:
Indicator electrode (I): Wax- impregnated graphite rod, ~ 6", with unsharpened cylindrical tip;
Reference electrode (R): commercial calomel or Ag/AgCl.
Counter Electrode (C): Spectroscopic graphite rod, ~ 6".
Electrode Polishing: The blunt tip of the WIGE must be polished
between each run by rubbing it with a circular motion on a flat
piece of filter paper.
Figure 1. Electrochemical cell set-up.
2. Procedures
2.1. Solution Preparation (As Needed).
(a) Electrolyte Solution.
Weigh out 3.746 grams of potassium chloride (F.Wt.=74.55) in a clean 100-ml beaker, and dissolve with distilled water. Transfer to a clean 500 mL volumetric flask. Dilute to volume.
(b) Potassium ferrocyanide Sample.
Weigh out about 0.0280 grams of potassium ferrocyanide trihydrate (F.Wt.= 422.41) in a clean 100-ml beaker, and record the exact weight (4 sig. figs.). Dissolve with a portion of the KCl solution prepared in 2.1.a; transfer to a clean 250 mL volumetric flask; dilute to volume with the KCl solution.
2.2. Instrument Initial Settings (before measurements)
Chart: 10 cm/min
Operating mode: Off
Sensitivity: 4.0 mAmp full scale
Scan direction: "+" position
Scan rate: 2.0 V/min.
Initial Voltage: -0.400 V (±.005V)
Final Voltage: +1.200 V (±.005V)
Technique: DC Normal
2.3. Measurement Procedures
(a) Pipet out 50.00 mL of the potassium ferrocyanide solution into a clean 100 mL beaker (with a stirrer bar. Place the cell on the stirrer.
(b) Mount the three electrodes (Indicator, I; Reference, R; Counter, C) on a ring stand using the three mini-clamps. (Be sure that the clamps are insulated electrically from the graphite electrodes; slip plastic tubing over the electrodes in the clamped region, if necessary.)
(c) Position the ring stand, clamps and electrodes so that the 3 electrodes submerged to a depth of about 1/2" below the solution surface in the 100 mL sample beaker.
(d) Connect each electrode to the Polarograph using the three individual cables marked "I", "R", and "C".
(e) Turn the Polarograph power switch to "ON". Adjust initial settings as indicated in section 2 above. Turn power switch to "PEN". Adjust the pen displacement knob so that the recorder pen is on the first major chartpaper line on the left hand side. Mark this line as "zero current". Return power switch to "ON". Turn stirrer "ON".
(f) Switch the operating mode switch from "OFF" to "AUTOSCAN"; move the meter switch to "SCAN V." position; change the power switch to the "PEN" position; adjust the sensitivity control to an appropriate level ( about 4 µAmp full scale); turn off the stirrer.
(g) When the recorder pen stops drifting (~30 sec.), turn the power switch to "RECORD". This starts the recorder chart in motion. Place the pen on the paper. When the pen crosses a major x-axis division, momentarily depress the scan control to "START". This action begins the scan of the Indicator electrode potential in a positive (+) direction, and a sharp vertical pen mark is made on the chart to indicate the start of the voltage sweep.
(h) When the voltage sweep reaches the pre-set "FINAL VOLTAGE"(+1.200V.), the Indicator electrode voltage automatically returns to the "Initial Voltage" (-.400 V.). This event can be detected by observing the meter indicating the cell voltage; also, a sharp change in the current level indicated by the recorder pen will accompany the end of the voltage sweep.
(i) When the end of the voltage sweep is observed (see (h)), turn the operating switch to "OFF"; turn the power switch to "ON". When the pen moves back, repeat 2.3.f - h, twice.
(j) Set the operating mode and power switches to "OFF"; detach and rinse the electrodes; turn off the stirring motor, and clean the beaker.
(k) Add 50 mL of potassium chloride solution to a clean 100 mL beaker. Repeat 2.3.b - i, obtaining three background results.
(l) Repeat 2.3.j.
3. Calculations
Record data and calculations in your Lab Notebook. Record data summaries in the Polarograph Calibration Record Book (section 4)
(a) Calculate the average peak current, iP, standard deviation, sP, average peak potential, EP, standard deviation, sE, for ferrocyanide voltammograms. Each peak current is calculated from the vertical location of the peak (a) on the chart paper (# divisions x sensitivity/division); peak potential is obtained from the horizontal location of the peak [# cms. from the start x scan rate x (1/chart speed)]. (See Figure 2.)
Figure 2. Typical current-voltage curve for linear sweep voltammetry.
(b) Calculate the average background current and standard deviation, sB, for the electrolyte voltammogram. Each background current is calculated from the magnitude of the signal (b) on the chart paper (# divisions x sensitivity/division) at the same potential, EP, (horizontal position) as the peak current (iP) is observed for the ferrocyanide voltammograms.
(c) Calculate the average net peak current, iP(Net), (a - b), and the pooled standard deviation sC, where: sC2 = sB2 + sP2
4. Calibration Report
Record in the Polarograph Calibration Record Book
4.1 Date, Time, Operator
4.2 Instrument I. D. #
4.3 Stock Solution Preparation Data; Ferrocyanide concentration.
4.4 Results Summary:
(a) Instrument settings used
(b) Peak potentials, peak currents, blank currents
(c) Averages and standard deviations of items (b)
(d) Average net peak current
(e) Pooled standard deviation for net peak current
sC2 = [(n1-1)s12 + (n2-2)s22]
(n1 + n2 - 2)
Click here to view and/or print the Table for this section
Click here to view and/or print the Sample Table for this section
