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Determination Of Lead In Water With Anodic Stripping Voltammetry

Background

Lead in drinking water is a major global problem. The sources of the lead are from solder, which was used to join copper pipes, and from lead pipes used in older homes and buildings. Over time, lead can leach out and contaminate water. Acute lead poisoning in children can cause anorexia, vomiting, malaise and convulsions. Permanent brain damage can result. Chronic lead poisoning can cause weight loss, weakness and anemia.
The following laboratory procedure will enable you to measure lead at low concentrations in the water you drink.

Anodic Stripping Voltammetry (ASV)

In recent years there has been a revival of interest in anodic stripping voltammetry (ASV), which is one of the most sensitive analytical methods for heavy metals like lead. It can detect lead at levels as low as few parts per billion (ppb). An advantage of ASV is its low cost compared to many other methods.

ASV involves the preconcentration of lead into a Hg or Hg film electrode by electro-depositing it at a negative potential. After deposition, the potential is scanned anodically toward positive potentials to strip (oxidize) the lead from the electrode back into the solution. This stripping results in a current peak at the potential for oxidizing the lead. If more than one metal is deposited, multiple peaks are found on the anodic potential scan. In the experiment to follow, a "bare" glassy carbon electrode is used to eliminate any exposure to Hg, which is toxic if ingested.

Experimental

Equipment:

  • 66-CS1090 or 66-CS1200 computer-controlled potentiostat or 66-OMNI101 microprocessor controlled potentiostat with x-y or strip-chart recorder
  • Electrochemical cell fitted with a 1.5 mm diameter glassy carbon electrode (working electrode),
  • A Pt auxiliary (counter) electrode and an Ag/AgCl reference electrode
  • A small magnetic stirring bar inside the cell
  • A magnetic stirrer
Chemical solutions:
  • Prepare three separate solutions containing 100 ppb, 500 ppb and 1,000 ppb solutions of
    Pb(NO3)2 in 0.1 M KNO3 containing 50 mM HNO3 (use doubly distilled water for dilutions)
  • Dilute a sample of tap water diluted with an equal volume of 0.1 M KNO3 containing 50 mM HNO3

Procedure:

With the 66-Omni101-

    1. Connect an x-y recorder or strip-chart recorder to the 66-Omni101 (or computer with data acquisition software, ACQUIRE 101).
    2. Set the cell on a magnetic stirrer and put the auxiliary and reference electrodes in place
    3. Lightly polish the glassy carbon electrode with an alumina slurry on a microcloth polishing pad using a figure 8 motion. Rinse the electrode quickly with doubly distilled water, remove any excess water by touching an edge with Kimwipe and fit the electrode into the cell
    4. Add the 100 ppb Pb sample in the cell and replace the cover with the electrodes
    5. Set the applied potential, Ei, at -900 mV and final potential, Ef, to + 100 mV.
    6. Turn the magnetic stirrer ON, set the time for 2 minutes and then switch the potentiostat ON so that the -900 mV is applied to the glassy carbon electrode vs the Ag/AgCl reference
    7. If the current overload light turns on, decrease the sensitivity.
    8. After 1 minute and 30 seconds, turn the stirrer off so that the solution quiets
    9. When the timer goes off marking 2 minutes, push the scan button ON so that the potential is scanned from Ei of -900 mV to Efof +100 mV. A scan rate of 500 mV/sec is recommended to obtain a good peak height (a lower scan rate gives smaller peak heights and also allows possible interference from oxygen)
    10. Repeat the experiment with all three standard solutions and then run the tap water sample
    11. Plot the peak heights versus concentration for the standards and then determine the amount in the tap water from the calibration plot (where the peak height of the sample lies on plot for the standards)
    12. Do a linear regression analysis to confirm the ppb lead in the tap water sample (run at least duplicates on the standards and the sample if time permits)


If you use the 66-CS1090 or 66-CS1200 computer-controlled potentiostat the procedure for the analysis of lead is the same as that described above. You can use scan rates as high as 1 or 2 V/s with these instruments. The computer will do all the timing and potential setting/scanning but please remember to turn the stirrer off at the preset time of 2 minutes. A scan rate of 1.0 or 2.0 V/s is recommended.

Additional Experiments

  1. Instead of electrodepositing lead directly on the glassy carbon electrode, you can dissolve
    Hg(NO3)2 in the solutions so that Hg is co-deposited along with the Pb to make an Hg/Pb amalgam. If you do this, you may see peaks for copper and cadmium if present in the solution.
  2. You can use standard addition methods to calibrate for metals of interest. The solution pH may need adjustment to optimize results with some metals.
  3. ASV is an excellent method for heavy metal analysis and comparison of results to traditional wet chemical methods may be of interest to students.

Reference for ASV

Wang, Joseph, Anodic Stripping Voltammetry: An Instrumental Analysis Experiment, J. Chem. Ed. 60, December 1983, pages 1074-1075.

Additional lab experiments are being developed for the undergraduate teaching laboratory to introduce students to electroanalytical chemistry.


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