Procedure

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Connect the cell (3- electrodes) to potentiostat (see instructor for direction or manual for potentiostat use)
Place the cell on top of the magnetic stirrer and put the stirring bar, the auxiliary Pt and the Ag/AgCl reference electrode in place.
Polish the glassy carbon electrode under light pressure with an alumina slurry on a micro-cloth polishing pad using a figure 8 motion. Five to 10 complete figure 8 tracings should be sufficient. Rinse the electrode immediately with pure water and remove any excess water by touching an edge to a Kimwipe™ or clean adsorbent paper. Fit the electrode into the cell. Further discussion on the activation of glassy carbon electrode may be found in the technical notes of this manual.
Add the 100 ppb Pb sample in the cell and replace the cover with the electrodes.
Set the applied potential, Ei, at –900 mV, the final potential, E_f, to = 0.00 mV and the sensitivity to 1 μA/V.
Turn the magnetic stirrer ON, set the timer for 2 minutes and then switch the potentiostat to the ON position so that –900 mV is applied to the glassy carbon electrode. Turn the stirrer off/on with an inline switch and leave the speed controller set to the same setting to maintain the same stirring rate from experiment to experiment.
If the current overload light turns on, decrease the sensitivity.
After 90 seconds, turn the stirrer off so that the solution quiets.
When the timer goes off after 2 minutes, push the scan button ON so that the potential is scanned from Ei of –900 mV to 0.00 mV. A scan rate of 500 mV/sec is recommended. A higher scan rate lessens the possibility of error due to oxygen in the solution oxidizing the deposited metal, like Pb, on the electrode.
Remove the Hg film by stepping the potential to +400 mV for 30 seconds. Do not re-polish the electrode between runs.
Repeat the experiment with the background solution and then with all three standard Pb solutions, running duplicates for each concentration. Always remove the Hg film between runs. Next, run duplicates on the unknown water sample.
Plot the peak heights vs. concentration for the blank and standards and then determine the amount of Pb in the tap water from the calibration plot (noting where the peak height of the sample lies on the plot of the standard solutions).
Do a linear regression analysis to confirm the ppb lead in the unknown water sample (run at least duplicates on the standards and the sample if time permits).
Note: If you are using a computer-controlled potentiostat like the CS-1200, the procedure for the analysis of lead is the same as that described above. With this instrument, you can and should scan at rates as high as 1 or 2 V/s. The timing and potentials can be set with the computer menu but please remember to turn the stirrer off at the preset time of 2 minutes.