Anodic Stripping Voltammetry

Purpose: This experiment is designed to introduce anodic stripping voltammetry (ASV), an electrochemical method for trace analyses of metals. Metal ions in solution are first reduced to metallic form and concentrated as mercury amalgam in a mercury film electrode. After concentration, they are re-oxidized into solution ("stripped") from the electrode. Any metal that forms a stable amalgam with mercury can be analyzed. The pre-concentration step permits analysis of very low levels of metal ions. The subsequent analysis step can be done in a number of ways; the linear sweep (DC) voltammetric method is employed here. The method is used to analyze trace levels of metals in a variety of environmental samples. Quantitation is acheived via the method of standard additions.

The ASV method is an example of an ultra-sensitive analysis, and as such will test your skills at careful, quantitative manipulations to the utmost. The most important principle to keep in mind as you do the experiment is that, since it is an ultra-sensitive analysis, all reagents and equipment must be "ultrapure" and you must also be "ultracareful". It is very easy in procedures such as this for contamination to occur at levels comparable to the analyte levels in the sample. All glassware must be carefully cleaned and protected from subsequent contamination. All reagents used must be "ultrapure" so that they do not add significant levels of the analyte to the sample.

References: J. Wang, Stripping Analysis, VCH Publishers (1985), J. Ass. Offic. Anal. Chem. (FDA method) 56(2), 483 (1973)

Equipment: PARC Model 384B Polarographic Analyzer, Digital Plotter, Glassy Carbon working electrode (substrate for Hg film), AgCl/KCl reference electrode, Pt counter electrode, magnetic stirrer, micropipettor.

Reagents:

• Ultrapure acids (HNO₃ and HOAc) for sample digestion; concentrations as needed (6M HNO₃ and 4% HOAc.
• 0.0100M Pb(NO₃)₂ in 4% acetic acid.
• 0.100M Hg(NO₃)₂ in 4% acetic acid.
• Nitrogen gas, oxygen-free.

Sample: One of any environmental sample such as water, milk, pottery, soil, orchard leaves, eggshells, animal feed, tuna, etc.

Procedure

Glassware: Common laboratory glassware will leach metal ions into solution, often in concentrations that significantly interfere with ultratrace analyses of samples. To minimize this problem, all glassware used in this experiment must first be allowed to soak in 6M ultrapure nitric acid for at least one hour (overnight is preferable, longer than that is fine too), then rinsed directly with deionized, distilled water. Do not rinse with tap water first; this will only introduce metal ion contaminants from the tap water into the glassware. The magnetic stirring bar should also be acid washed in this way. Use the smallest number of pieces and use small volume glassware to conserve the very expensive ultrapure acids.

Samples: Sample preparation will depend on the type of sample chosen. You will need enough sample to provide a minimum final sample volume of 150 mL (three aliquots of 50 mL each). Make up slightly more than this to allow for transfer losses.

• Water samples can be run with only filtering to remove any particulates.
• Milk should be acidified to a final level of about 4% acetic acid, centrifuged, and the supernatant carefully decanted or filtered before analysis.
• Pottery samples should be washed in detergent, thoroughly rinsed with distilled, deionized water, then soaked (tightly covered with plastic wrap, e.g. Saran wrap) in 4% acetic acid for at least 24 hours. Note the total volume of acetic acid used.
• Less tractable samples such as eggshells, tuna, or leaves can be run in two ways: (i) To analyze for "free" lead, soak a weighed amount in a measured volume of 4% acetic acid for at least 24 hours, then filter out particulate matter. (ii) To analyze for total lead, perform an acid digestion in nitric acid. You may also compare lead levels found using both methods for extra credit.

Electrode: In this experiment, a variation on the mercury film electrode is used where the Hg film is created in situ during the actual preconcentration step. The analyte solution is spiked with Hg²⁺, and the mercury reduced and co-deposited with the analyte, as an analyte-Hg metallic amalgam, onto a glassy carbon electrode substrate. The Hg film is then removed. The Hg metal is oxidized to Hg²⁺ during the analysis step. In this way, the Hg film is freshly plated onto the electrode substrate for each run. For good results, the glassy carbon substrate electrode must be very smooth. Visually inspect the glassy carbon electrode. It should have a smooth, mirror-like finish. If it appears dull on any part of its surface, it must be polished. Consult your instructor for the correct polishing procedure. The electrode is quite rugged and should not require this very often if handled with reasonable care. When changing solutions, carefully pull the carbon electrode out of solution and wipe off the deposited mercury with a clean laboratory tissue. Dispose of the Hg-contaminated tissues in the container provided. Rinse the carbon electrode with distilled, deionized water, and carefully wipe the surface with a lint-free tissue or cloth.

Data Collection: For all solutions described below:

(i) place a magnetic stirring bar in the cell.

(ii) Using the micropipettor, add 10 µL of 0.100M Hg(NO₃)₂ solution.

(iii) Gently bubble nitrogen through the solution (purge) for 4 minutes and maintain a nitrogen blanket over the solution throughout the experiment.

(iv) Turn on the stirrer, then begin deposition by setting the Selector switch to External Cell. Stir the solution during the deposition time, then turn off the stirrer and wait 30 seconds for the solution to become quiescent before starting the stripping (analysis) scan.

(v) Use the following voltages: Deposition (Initial) voltage is ­0.80 V; Stripping voltages, scan in + direction from -0.80 to a final voltage of 0.00 V. Adjust the current sensitivity as needed to obtain reasonably sized, peaks that remain on scale.

Solutions:

• Pipet 50 mL unknown sample into the analysis cell. Do the following series of ASV experiments using linear sweep (DC) voltammetry for detection, redepositing from the same solution each time:
  
  (i) Use deposition times of 0.5, 1, 2, 5, 10, and 15 minutes, using the same scan rate of 1 mV/s for each.
  
  (ii) Use scan rates of 0.5, 1, 2, 5, and 10 mV/s, using a deposition time of 120 s for each. Choose an optimum scan rate and deposition time and use these values for all the subsequent runs. You may see more than one peak, as more than one metal may be present. Compare with the peak in the lead analysis step to identify the one due to lead.

• Pipette 50 mL of 4% acetic acid into a clean sample cell. Spike with 10 µL [Pb²⁺] standard solution. Add 10 µL Hg solution, deposit, and scan the solution using DC mode. Obtain a rough estimate of the [Pb²⁺] concentration in the unknown by comparing the magnitude of the peak current with that obtained in the standardization step for equivalent conditions of deposition time and scan rate.

• Prepare a few mL of lead solution from the standard solution, (analytically) diluting as required such that the final [Pb²⁺] is roughly the same as the [Pb²⁺] in the unknown. The final concentration should be precisely known, but need only agree with the unknown lead concentration to within a factor of 2-5 or so.

• Starting with a new 50 mL aliquot of the unknown sample, add 10 µL Hg solution, deaerate, and analyze the sample using linear sweep (DC) voltammetry. Use the micropipettor to spike the sample with 10 µL of the lead solution prepared in the standardization step. Reanalyze. Repeat the lead spiking and analysis 3-4 more times.

• Turn off the N₂ tank and the instrument.

Important Points

1. The instrument is quite susceptible to external shorts and open circuits. Be sure that the zero switch is depressed and that the selector switch is OFF before lowering the cell and preparing for the next scan.
   
2. Discard the used solutions into the labeled waste bottles provided. Rinse the sample cells thoroughly with distilled water.
   
3. Samples containing large amounts of various metals, or lead at very low concentrations, will be more difficult to analyze. If you experience difficulties in getting good lead peaks, try this alternate method:
   
   i) Add the Hg to the cell but not to the sample or lead standards. Deposit the Hg with stirring for 15 min. at -0.7 V, then scan to -0.1V and hold there for 2 minutes to remove co-deposited impurities.
   
   ii) Set the Selector to Off, add 10 µL standards and/or sample (you may have to preconcentrate the sample), then follow the same deposition and stripping procedures, except use a Final voltage of -0.250 V instead of 0 V.

Report

1. Plot the magnitude of the peak potential vs. scan rate and versus deposition time. Indicate the optimum values used and explain the observed dependence. The determination of the peak current for a single run requires that the background current be known. This can be extrapolated or a blank sample can be run and compared to the analyte. Background subtraction is easiest using computer aided analysis.
   
2. Plot peak current versus standard [Pb²⁺] added. Use a linear regression routine in a spread-sheet program (e.g. Quattro) to find the best-fit straight line through the data, extrapolate to zero current, and determine the unknown concentration as the x-axis intercept. Report the lead concentration in the unknown solution and the lead level in the sample as ppm (w/w) and the limits of precision for the method.
   
3. Why is the lead concentration determined by the method of standard additions instead of running several different standards and using a calibration plot?