Appendix 8.
Measurment Procedures.
Determination of Vitamin C in Fruit Drink.
Sargent-Welch Model 4001 Polarograph.
wp.app8sarg 09/09/98
(1) Instrument Settings
(Refer to these for Part 2-e)
Chart: 10 cm/min
Operating mode: (variable)
Scan direction: "+" position
Scan rate: 2.0 V./min.
Initial voltage: -0.400V ±.003
Final voltage: +1.200V ±.003
Technique: Normal
Figure 1.
Voltammetric cell configuration
(2) Measurements
(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)
(d) Connect each electrode to the Polarograph using the three individual cables marked "I", "R", and "C".
(e) First Sample Aliquot: Turn the Power Switch to "ON". Adjust initial settings as indicated in section 1 above. Turn Power Switch to "Pen"; Operating Mode to "Off". 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". Turn Power Switch to "ON". [Go to 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 MEASUREMENT PROCEDURE. Then return to next step.]
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(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 MEASUREMENT PROCEDURE. Then return to next step.]
(h) Second Sample Aliquot: Repeat (a) to (g) for a second 50.00 mL aliquot of the sample solution.
(i) Shut Down Procedure: Set the Operating Mode and Power switches to "OFF"; rinse the electrodes; replace your beaker with a beaker of distilled water.
MEASUREMENT PROCEDURE:
mAmp full scale).(M.1) Turn the Meter switch to the "INITIAL" position; Change the Initial Voltage to -1.500 V; change the Operating Mode to "AUTOSCAN"; and wait 2 minutes.
(M.2) Change Operating Mode to "OFF"; change the Initial Voltage back to -0.400 V.
(M.3) 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 8
(M.4) After ~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 (+) direction, and a sharp vertical mark is made on the chart, indicating the start of the voltage sweep.
(M.5) When the voltage sweep reaches the pre-set "FINAL VOLTAGE" (+1.200 V.), the
Indicator electrode voltage automatically returns to the "Initial Voltage"
(-0.400V.). This event can be detected by observing the meter which indicates the cell voltage; also, a sharp change in the current level indicated by the recorder pen will accompany the end of the voltage sweep.
(M.6) When the end of the voltage sweep is observed, turn the Operating
Mode switch to "OFF"; turn Power Switch to "ON". [Return to next step in Main Procedure.]
(3) Current Measurements.
The Net oxidation current is obtained by subtracting the background current, iB, from the total current (see Figure 2). The optimum point to make this measurement is at the peak maximum, or, if no maximum is observed, just beyond the point where the current appears to have completed a step excursion. Typically, but not always, this is at about +0.740 V. (~5.2 cm from the sweep start on the chartpaper). Because the background current (iB) is not known, it must be estimated by extrapolating the current observed before the oxidation of ascorbic acid (see lower dashed line for iB in Figure 2).
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Figure 1. Net current measurement with Sargent Polarograph.
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