Instructor’s Manual
Heavy Metal Analysis by Anodic Stripping Voltammetry
This document provides suggested answers to the questions posed during the learning module
Heavy Metal Analysis by Anodic Stripping Voltammetry.
Q1. How would you prepare the solid samples obtained from Lake Nakuru for an ASV analysis?
In what physical form should they be for ASV analysis? What are the expected concentrations in
your samples from Lake Nakuru?
The metals must ultimately be in solution for analysis. Recall the introduction to the overall
Lake Nakuru module which discusses sampling and sample preparation. In the Nelson 1998
paper, the samples were solid (filtered sediments and suspended solids). To get them into
solution phase for analysis, they were acid digested – a similar procedure is performed in
the reference Locatelli 1999. The metal concentrations in the sediments were in the ppm
(µg/g) range (Table 3 of Nelson 1998). After sample digestion the samples were diluted 20
to 100-fold for analysis).
Q2. What is the most common type (i.e. material used) of working electrode used in ASV? You
should be able to find this information in the material referenced in the Theory section of this
document.
Mercury is the most common metal used as a working electrode in ASV. Specifically, the
hanging mercury drop electrode (HMDE) is popular, along with a mercury film electrode.
Other working electrode materials include Au, Ag, Pt and C. A glassy carbon electrode with
a Hg-film is also commonly used.
Q3. What must happen to the metal at the electrode surface to form the neutral species?
At the electrode surface the metal cation is reduced to its elemental form.
e.g. Mn+ + ne-  M(s)
Q4. Look up the reduction potentials of the following metals of interest (Cu2+, Pb2+ and Zn2+).
You can find a table of reduction potentials in the back of most analytical textbooks. You can
also find one in the link here (Appendix 13, p. 1107 from the online textbook Analytical
Chemistry 2.0). Would you be able to discriminate between these metals based on their
reduction potentials?
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Table I. Reduction Potentials of Metals of Interest
Reduction reaction
E0
Cu2+ + 2e-  Cu(s)
0.3419 V
Pb2+ + 2e-  Pb(s)
-0.126 V
Zn2+ + 2e-  Zn(s)
-0.7618
From the potentials listed in Table I (taken from Analytical Chemistry 2.0), it appears that
the three metals can be resolved (no potential overlap). Keep in mind that E0 is a
thermodynamic parameter and this is merely a prediction.
Q5. What potential (versus SHE) would be necessary to simultaneously pre-concentrate zinc
and copper?
The potential would need to be more negative than the E0 for Zn2+.
Q6. Using the symbol M2+, write the appropriate redox reaction for the reduction of a divalent
metal cation at a mercury electrode. In addition to the applied potential, what other experimental
variable is used to “drive” the metal cation to the electrode?
M2+ + 2e-  M(s)
The cation is also driven to the electrode via convection (i.e. stirring).
Q7. In the space below, sketch out the resulting voltammogram from the experiment outlined in
Figure 1.
The resulting voltammogram is shown in the figure below (Figure 1). The ASV
voltammogram in Figure 1 is peak shaped. The maximum or peak current, ip, is centered
at a peak potential, Ep, which is metal specific.
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Figure 1. Potential versus time waveform during an ASV experiment (top). The resulting
analytical signal is the current generated during the anodic “stripping” step. The plot of current
versus potential is called a voltammogram.
Q8. What potential values would you choose as the start and end of the potential ramp for an
analysis of zinc?
In a typical experiment the potential would be scanned between -1.1 V and a value 300
– 500 mV greater than the E0 for zinc.
Q9. Summarize the quantitative and qualitative nature of the ASV experiment.
The peak current is proportional to analyte concentration and provides the quantitative
information. The peak potential value provides information about the identity of the metal
ion. For an example of real collected data, refer to Figure 30 in the Anodic Stripping
Voltammetry section of the Analytical Electrochemistry eLearning module.
Q10. The Lake Nakuru samples will most likely contain measurable concentrations of copper,
lead and zinc. Describe how multiple metals can be analyzed in a single ASV experiment. Draw
what the voltammogram would look like. What starting and ending potentials would you use?
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If the values of the redox potentials of the metals are sufficiently different, the peaks
corresponding to the oxidation of each metal should be resolved, as shown in the
general diagram below for the analysis of three metals.
Figure 2. Potential-time waveform during an anodic stripping experiment on a mixture of three
metals (top). The metals’ reduction potentials are represented by E1, E2, and E3. Resulting
voltammogram from the analysis of the mixture (bottom). In this example, each peak
corresponds to a different metal.
Q11. How could the experimental conditions be modified to increase the analyte signal during
an analysis? Hint: think about what variables will increase the amount of metal deposited and
look at the equation for ip. (This equation can be found in the ASV section of the Analytical
Electrochemistry module).
You could increase the Hg drop size (electrode area, A), increase the stirring rate,
increase the deposition time, just don’t say to increase the concentration!
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Q12. Are the levels observed for each of these four metals in sediments and suspended solids
amenable to analysis by ASV? Note that the Nelson paper does not prepare samples for ASV
analysis. In your answer, consider the sample preparation steps described by Locatelli 1999,
which is described in the next section.
The values reported for sediment and suspended solids (g/g) appear to be in the ppm
concentration range, however, students must consider the sample preparation steps.
These are outlined in the Locatelli paper and quantified in the corresponding Excel
sheet. Briefly, 1.0 g of dried solid is digested and diluted to 100.0 mL. These samples
are diluted by a factor of 20 prior to analysis. The final expected concentrations are
expected to be in the ppb range, which can be measured by ASV. The numbers can be
played with in the corresponding Excel sheet. Average values measured would range
from 11 – 73.5 ppb, correlating to 22 – 147 g/g in the solid samples.
Data Analysis – Sediment Samples
Q13. Recently, sediment samples were collected from Lake Nakuru and analyzed by ASV in
the manner described above. Given the sample data in Table 4, calculate the current
concentration levels of the heavy metals (Cu, Pb and Zn) in the sediment samples obtained
from Lake Nakuru. What do the data tell you about the heavy metal concentrations in the
sediments at Lake Nakuru? Have the concentrations increased or decreased since the 1996
sampling?
Table 4: Sample Data for Copper, Lead and Zinc Determination in Sediment Samples in
Lake Nakuru
Sample
number
ip (µA)
Sample
Cu
1
2
Sample + 100 µL spike
Pb
Zn
Cu
Pb
Zn
0.0985 0.1011 0.2134 0.2000
0.3045
0.3152
0.0996 0.1008 0.2159 0.2015
0.3026
0.3165
3
0.1000 0.1205 0.2200 0.2099 0.3225 0.3251
Note: All samples analyzed were a 10.00 mL aliquot. The 100.0 µL standard spike contained a
target concentration of 5.000 ppm of each metal of interest. The exact concentration (depending
on standard mass weighed out) can be determined on the corresponding Excel spreadsheet.
The data can be entered into the provided spreadsheet. Average calculated values and
the reported 1996 values (ing/g) are tabulated below (for Cu, Pb, Zn and Cd) in
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sediment samples and suspended solids for comparison. It appears as if the values
for all the measured metals have increased (except for Zn in suspended solids). Using
the mean and standard deviation data, students could perform statistical tests to
determine if some of the “closer” data is “significantly different” (e.g. t-test for comparison
of means).
Answer Table I. Metal Concentrations in Lake Nakuru Sediments and Suspended Solids
Samples in 1996 (Nelson 1998) and in Present Day
____________________________________________________________________________
Concentration in
Concentration in
dry sediments (g/g)
suspended solids (g/g)
__________________________________________________________
Trace metal
1996
Average
Present
Average
1996
Average
Present
Average
Chromium
67
77.2
8.3
19.6
Copper
24
93.1
19
25.5
Lead
22
52.1
11.7
19.6
Zinc
147
204.0
74
67.8
____________________________________________________________________________
Data Analysis – Suspended Solids
Q14. Recently, suspended solid samples were collected from Lake Nakuru and analyzed by
ASV in the manner described above. Given the sample data in Table 5, calculate the current
concentration levels of the heavy metals (Cu, Pb and Zn) in suspended solids in Lake Nakuru.
What do the data tell you about the heavy metal concentrations in the suspended solids at Lake
Nakuru? Have the concentrations increased or decreased since the 1996 sampling?
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Table 5: Sample Data for Copper, Lead and Zinc Determination in Suspended Solid
Samples in Lake Nakuru
Sample
number
ip (µA)
Sample
Cu
1
2
Sample + 100 µL spike
Pb
Zn
Cu
Pb
Zn
0.0999 0.1511 0.1985 0.4697
0.9513
0.4545
0.1001 0.1623 0.1752 0.4898
0.9253
0.4326
3
0.0959 0.1506 0.1863 0.4800 0.9222 0.4863
Note: All samples analyzed were a 10.00 mL aliquot. The 100.0 µL standard spike contained a
target concentration of 5.000 ppm of each metal of interest. The exact concentration (depending
on standard mass weighed out) can be determined on the corresponding Excel spreadsheet.
See answer Table I above and corresponding spreadsheet.
Chromium Measurement
Q15. Let’s see if you are paying attention . . . During the accumulation step in adsorptive
stripping voltammetry, does a redox reaction occur?
No redox reaction occurs during the accumulation step during adsorptive stripping voltammetry.
Q16. Recently, sediment and suspended solid samples were collected from Lake Nakuru
and analyzed by ASV in the manner described above. Given the sample data in Tables 7 and
8, calculate the current concentration levels of chromium in the sediment samples obtained
from Lake Nakuru. What do the data tell you about chromium concentration in the sediments
and suspended solids in Lake Nakuru? Has the concentration increased or decreased since the
1996 sampling?
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Table 7: Sample Data for Chromium Determination in Sediment Samples in Lake Nakuru
Sample
number
1
2
ip (µA)
Sample
Sample + 100 µL spike
Cr
Cr
0.1111
0.2522
0.1212
0.2855
3
0.1359
0.2989
Note: All samples analyzed were a 10.00 mL aliquot. The 100.0 µL standard spike contained a
target concentration of 5.000 ppm of Cr(III). The exact concentration (depending on standard
mass weighed out) can be determined on the corresponding Excel spreadsheet.
Table 8: Sample Data for Chromium Determination in Suspended Solid Samples in Lake
Nakuru
Sample
number
1
2
ip (µA)
Sample
Sample + 100 µL spike
Cr
Cr
0.1015
0.8986
0.1985
0.9563
3
0.1574
0.9254
Note: All samples analyzed were a 10.00 mL aliquot. The 100.0 µL standard spike contained a
target concentration of 5.000 ppm of Cr(III). The exact concentration (depending on standard
mass weighed out) can be determined on the corresponding Excel spreadsheet.
See the answer to Q14 and corresponding answer table.
Q17. If your sample contains metal ions that cannot be measured by ASV, what other
techniques would you suggest trying.
A few suggestions include: Other metals can be measured by atomic spectroscopic
techniques or other electrochemical methods, such as potentiometry with an ion-specific
electrode. The proper complexing agent may allow determination of metal ions normally
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invisible to spectroscopic analyisis to be detected using molecular absorbance or
fluorescence methods.
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