'/
United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-85/063-Aug. 1985
vxEPA Project Summary
Microprocessor-Controlled
Anodic Stripping Vo I tarn meter
for Trace Metal Analysis in
Tap Water
R. G. Clem, F. W. Park, F. A. Kirsten, S. L Phillips, and E. P. Binnall
This report discusses the construc-
tion and use of a portable, micro-
processor-controlled anodic stripping
voltammeter for onsite, simultaneous
metal analysis of copper (Cu), lead (Pb),
and cadmium (Cd) in tap water. The in-
strumental system consists of a pro-
grammable controller, permits keying
in analytical parameters such as sparge
time and plating time, a rotating cell for
efficient oxygen removal and amalgam
formation, and a magnetic tape that
can be used for data storage. Analysis
time can be as short as 10 to 15 min.
The stripping analysis is based on a pre-
measurement step during which the
metals from a water sample are con-
centrated into a thin mercury film by
deposition from an acetate solution of
pH 4.5. The concentrated metals are
then electrochemically dissolved from
the film by applying a linearly increas-
ing anodic potential. Typical peak-
shaped curves are obtained. The
heights of these curves are related to
the concentration of metals in the
water by calibration data. Results of tap
water analysis showed 3 ± 1 (ig/L Pb,
22 ± 0.3 fig/L Cu, and less than 0.2 fig/L
Cd for a Berkeley, California, tap water.
For 10 samples of Seattle, Washington,
tap water analyses showed 1 to 1 |j.g/L
Cu and 1 to 2 |tg/L Pb. Recommenda-
tions are given for a next generation
instrument system.
This Project Summary was devel-
oped by EPA's Water Engineering Re-
search Laboratory, Cincinnati, OH, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Trace metals in public drinking water
supplies are of special concern because
of their potential long-term harmful ef-
fects when ingested by humans. Water
supply resources for municipalities con-
tain various types and concentrations of
dissolved metals. In untreated water,
these concentrations are generally sig-
nificantly lower than the EPA Interim
Primary Drinking Water Standards. But
chemicals used in water treatment may
contain traces of metals as impurities
and may introduce them to water at the
treatment plant. Another potential
source of metals is corrosion and disso-
lution of the metallic piping of tank inte-
riors through which the treated water
passes or is stored before ingestion.
Thus a need exists for measuring the
concentration of dissolved metals at the
user's tap, where the water is actually
ingested.
Although a number of methods are
available for measuring trace metals in
drinking water, anodic stripping voltam-
metry (ASV) was selected for this study
because it offers a number of advan-
tages, including (a) simultaneous analy-
ses of more than one metal, (b) sensitiv-
ity to concentrations of less than 1 M-g/L,
(c) portable instrumentation, (d) metal
speciation at ambient concentrations,
(e) easy automation, and (f) rapid analy-
ses at a relatively low cost.
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This report describes a portable,
microprocessor-controlled ASV system
that was constructed for the onsite anal-
ysis of Cu, Pb, and Cd in tap water (Fig-
ure 1). The instrumental parameters
such as a sparge and plating time were
keyed into the computer, and analyses
were then done automatically. The ASV
system was applied to the analysis of
one sample of Berkeley, California, tap
water and 10 samples of tap water from
Seattle, Washington. The report dis-
cusses calibration of the instrument
with known metal concentrations and
analyzes the results of the tap water
analyses. Selected information is given
on (1) the anodic stripping electro-
chemistry of Cd, Pb, and Cu; (2) the ASV
method using mercury thin films plated
onto graphite electrodes; and (3) the in-
strumentation electronics.
Anodic Stripping Voltammetry
at Mercury Film Electrodes
Anodic stripping is applied to the
analysis of ^g/L quantities of mercury-
soluble metals such as Cd and Pb using
thin films (4 to 100 jim) formed by the
deposition of mercury (Hg) onto carbon
electrodes. The Hg is deposited along
with metals (Cd, Pb, Cu) being analyzed.
The first step involves electroplating,
during which the metals being deter-
mined are concentrated on the order of
100-fold in 5 min within the Hg film. This
deposition step is not exhaustive (non-
stoichiometric); only 2% to 3% of the
total amount in the water sample is de-
posited. The applied potential is
stepped to a sufficiently negative value
so that the electro-deposition rate is lim-
ited only by the rate of mass transfer of
the dissolved metals from the bulk of
the solution to the electrode surface.
The solution is stirred by rotating the
cell to enhance mass transfer. The
plated metals establish a concentration
gradient within the Hg film whereby the
concentration of metal at the electrode-
solution interface is larger than that to-
ward the interior of the electrode.
After a set time (such as 5 to 10 min),
stirring is stopped and the solution be-
comes quiet. During this quiescent pe-
riod, the concentration gradient within
the Hg film is essentially eliminated be-
cause deposition is almost negligible,
and the plated metals diffuse through
the film from regions of high concentra-
tions to lower concentrations. A quies-
cent time of 60 sec was used here.
The third step is anodic stripping of
the metals from the amalgam by appli-
Figure 1. ASV instrument constructed by the Lawrence Berkeley Laboratory.
2
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cation of a linearly increasing positive
potential. At potentials near the Nerns-
tian value for a particular metal, the
metal begins to dissolve (strip), giving
rise to typically peak-shaped curves at
more positive potentials. A peak is ob-
tained for each metal, and thus a
charge-voltage curve can consist of
more than one peak. Usually this peak
height is measured and related to the
metal concentration in the solution by
means of calibration data. Peak areas
have also been measured.
Instrument System
Components
The ASV system constructed here
was designed to automate settings for
the analytical procedure, to acquire data
relating peak height to metal concentra-
tion in water samples, and to store data
from the analysis of tap water for subse-
quent calculations. The system consists
mainly of the following items: a rotating
cell containing the water sample, elec-
trodes, and inlets for the sparge gas; a
digital potentiostat for automating the
procedure; and a pen recorder or mag-
netic tape for data storage and calcula-
tion. The functions of these four major
components are described in the fol-
lowing sections and in Table 1.
Rotating Cell
A cylindrical rotating cell machined
from Lucite* was used to provide both
for quick oxygen removal during the
sparging step and for a high rate of
metal preconcentration during the
metal deposition. Only 60 sec is re-
quired for effective oxygen removal,
thereby reducing the time required for
analysis. The solution volume used was
generally 15 ml.
During oxygen sparging, the cell con-
taining the 15 mL of solution is rotated
automatically in a clockwise direction of
0.9 sec. A counterclockwise pulse is
then imparted for a duration of 0.1 sec.
Under these conditions, the dissolved
oxygen concentration is lowered suffi-
ciently so that the background current
resulting from oxygen reduction does
not interfere with the analysis proce-
dure.
Nitrogen or other inert gas such as
carbon dioxide is introduced through
the bottom of the electrode holder at a
rate of ~5 L/min to effect sparging. At
the conclusion of the 60-sec sparging
Table 1. Major Functions of the ASV System Components
Component Functions
Rotating cell
Digital potentiostat
Microcomputer controller
Recorder
Contains samples and electrodes, sparges, oxygen,
preconcentrates trace metals, and performs anodic
stripping of amalgams.
Applies and maintains plating potential, applies anodic
stripping ramp potential, and measures stripping
peak heights.
Starts and stops the following: cell rotation for sparg-
ing, cell rotation for plating, sparge and plating
times, anodic stripping ramp; also transmits peak
height data to cassette and strip chart recorders.
The strip chart records a background trace and records
peak height traces. The cassette provides fast-rate
data storage for later calculation.
'Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use.
period, the microprocessor stops cell
rotation, and the solution drains com-
pletely to the floor of the cell, because a
Lucite surface is not wetted by water.
The the agitation (stirring) period be-
gins, during which amalgam is accumu-
lated in the preconcentration step. A
low flow rate of nitrogen (e.g., 1 to
2 L/min) is maintained during the plat-
ing step to prevent infusion of air into
the cell and, thereby, to maintain a low
background current.
Efficient stirring is important for short
analysis times such as those used here.
Stirring is produced in the rotated cell
during plating by controlled reversals of
the direction of cell rotation every sec-
ond. The graphite working electrode is
placed on a radius of the cell and is po-
sitioned to face the cell wall. Turning the
electrode through 180° so that it faces
the center of the cell results in a marked
decrease in the rate of amalgam accu-
mulation because of a substantially
lowered stirring rate. Thus the elec-
trode, once positioned and locked into
place with the set screw, must not be
moved until the analysis is completed.
Digital Potentiostat
The digital potentiostat controls the
potential between the working elec-
trode and the reference electrode by in-
jecting and extracting pulses of charge.
In this manner, the feedback current is
digitized. Advantages of digitized cur-
rent include the capability for direct
readout on a scalar, direct computer
compatibility, and the elimination of a
need for analog to digital converters.
The current or integral of charge is mea-
sured by counting the number of charge
pulses over a specified period of time
with the scalar. A schematic of the digi-
tal potentiostat appears in Figure 2.
The circuit diagram in Figure 2 con-
sists of the electrochemical cell, opera-
tional amplifier, digitalizing logics, and
current pumps. The arrangement
shown is used to maintain a control po-
tential and to measure the current
change digitally during electroplating
and stripping. Two pumps exist in
series—one a 0.1 milliampere (ma) neg-
ative constant current supply, and the
other a positive, 0.6- to 1.0-ma ad-
justable and controllable constant cur-
rent supply. The cell (which is con-
nected between the two current
supplies to ground) provides an alter-
nate current path. The maximum posi-
tive or negative voltage drop across the
cell depends on the cell impedance and
the current being provided by the posi-
tive and negative source. All voltages
between these extremes are regulated
by the feedback loop, which consists of
the reference electrode, the voltage
comparator, the J-K flip-flop, the cur-
rent switch transistors Q3 and Q4, and
diode D2. When a pulse from the cur-
rent switch forward-biases D2, the neg-
ative 0.1 current supply is pulling the
cell voltage down. Since the positive
current supply can provide an average
of 0.3 to 0.5 ma, it will pull the cell volt-
age up when being pulsed. The positive
source actually is a 0.6- to 1.0-ma sup-
ply, which is normally sunk through
diode D2. Only when Q4 is turned off
will this supply provide current to the
cell.
The PULSE ON pulses, which come
from the J-K flip-flop, are also output
through Q of the same J-K to a pulse
counter whose counting period is
program-selected.
Voltage is maintained across the cell
by the use of one fixed 0.1-ma supply
and one controllable 1.0-ma supply.
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both connected to the cell auxiliary elec-
trode so that the sum of the currents
across the cell impedance will generate
the voltage called for by the systems
program. The feedback loop consists of
the reference electrode and an LM 310
(IC-4) unity gain buffer leading to a dif-
ferential amplifier whose output goes to
an LM 311 voltage comparator. When
the reference electrode is lower than the
voltage demanded by the program, a
current switch (2 each 2N 2369)
back-biases diode D3 (1N 4148) so that
current from the 1-ma constant current
source (IC-1 and 2N 2608) is directed to
the cell auxiliary electrode. To facilitate
relative current measurement, pulses
are injected into the cell. A 1-MHz clock
is input to a 7470 J-K flip-flop; when the
J-K is enabled by a high signal from the
LM 311 voltage comparator, its output
will be a 500-kHz square wave.
Microcomputer Controller
In addition to the applied potential,
control is also provided for the follow-
ing functions: time for each step, se-
quencing of each step, storage of the
ASV data, output of the data, and a vis-
ual indication of the stripping process
using a strip chart recorder. This control
is provided by a microcomputer based
on an Intel 8080A 8-bit microprocessor
central processing unit. Changes in test
parameter variables are entered by the
operator from a 16-button keyboard on
the front panel of the instrument. A flow
chart detailing the various controlled
functions appears in Figure 3.
The POWER ON or SYSTEM RESET
buttons return all parameters to the ini-
tial conditions programmed for the plat-
ing and stripping parameters. When ei-
ther of these buttons is pressed,
preprogrammed values from PROM
memories are set into the microproces-
sor. These are the following input com-
mands:
Sparge: 60 sec
Start: 1000 mV
Step: 5 mV
Plate: 420 sec
Delay: 1 msec
Count: 400 msec
Day: 0
Test No.: 0
Spike A: 0
Spike B: 0
The MONITOR light will come ON.
Pressing any key will show the value
programmed. For example, to see
"SPIKE", press SPIKE and A, or SPIKE
and B. To enter a value of 25 for
SPIKE B, press SPIKE, press B, press
ENTER, press 2, press 5, press ENTER
and the change is made. The MONITOR
light will go ON.
Data Handling
The output signal of analytical inter-
est from the ASV is the stripping peak
height as a function of applied potential.
For this instrument, the stripping peak
heights were mainly measured from the
tracing on either a strip chart recorder
or an X-Y recorder. Output to the strip
chart recorder is the digital-to-analog
(DAC) conversion times a scale factor of
the counts accumulated during each
5-mV step (or other selected step mag-
nitude) during the stripping cycle. The
scale factor (sensitivity) of the strip
chart recording can be chosen by the
operator as appropriate for a particular
+ 72
1 ma
J Q
>'- 2
K Q
500KHZ Bursts
_nnn_nnn_
Aux
Sirrent
A/itch
-12
Voltage
Comparator
0.1 ma
-12
Volt DAC
Figure 2.
Schematic for Anodic Stripping Voltammeter.
4
-------
(initialize'}
Power
On
•
Set Run
No. To 1
• . * , ,
Aux. Relay
Open
.
Cell Drive
MTR 'Off
*
Ref DAC To
EndMV
I
Set Test
Parameters
PS
T
Close
>- Open
Relay
Day XXXX
Test XXXX
Ref order LSE X
Spike A XXX 1
Sparge 060 Sec
Start -1000MV
Step 5 MV Fixed
End +350 MV Program
Plate 350 Sec Values
Delay IMS
Count 400 MS
Install Cell
With
Conditioned
Sample
r
(Push Test^)
Freon Valve
To High Flow
, •
Spin Cell
0.9 Sec On
0. 1 Sec Off
*
For 60 Sec
*
Freon Valve
To Low Flow
®
1
Oscillate Cell
»
Ref DAC
To Start MV
»
Aux Electrode
Relay 'On'
»
Plate For
Selected Time
»
Stop Cell
Oscillation
Strip Chart
Recorder 'On
Wait 5 Sec For
Cell Stability
Manually
Turn On
Freon
,
X
7
1
DAC Steps By
Selected MV
Reset &
Start CNT r-= — 7— -i
Cnunter «*„_«,.. DAC To
Holds For
Count
rlVo*"^
^H'MVs'
Stop DAC
Ramp
t
Open Aux
Electrode
Relay
*
Cell Drive
MTR 'Off
»
MAG Record
I// Parameters
And Data
»
Advance to
Next Run
[ Wait [
Enter Spike
'ype & Amour
*
Push Test
»
Stan TNT l"""<5 rKr "AM -.. .,.—.-..
stop c/v/ ^ Sfef) 1 | '"" . 1 Recorder
> Rerun Data
ToS/C
Recorder
tP.S.)
Operator adds spike of A or B solution. Type
and amount depending on peak observed on
t * chart recorder. Attempt is made to double
peak.
Repeat Of Run No. 1
Except Run 2 & 3
Plate Time is
Run 1 Time - 2
_, I
<
End Of Run 3
Completes Tes
Of A Sample
1
Advance Test
Number
»
Wait For
Next Test
t
Figure 3. Flow chart for Anodic Stripping Voltammeter.
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measurement. Data are also recorded
on a cartridge digital tape recorder,
where they are available for further off-
line processing.
Each water analysis is identified by
both test number and run numbers. A
complete analysis must have three se-
quential runs. These three runs are per-
formed on: (1) the water sample; (2) the
water sample with spike A added; (3)
the water sample with spikes A and B
added. Runs 2 and 3 establish the cali-
bration of the instrument for that run.
A sample of tap water from the
Lawrence Berkeley Laboratory was ob-
tained by running the water for 1 to
2 min then collecting 500 ml in a
polyethylene bottle that had been
cleaned and rinsed with the tap water.
Hydrochloric acid was added to pH 1,
and the solution was permitted to equi-
librate for about 48 hr. The sample was
then discarded, and an additional 500
mL was collected in the same manner.
After 48 hr, 15 mL was added to the ro-
tating cell, and ammonia-treated
sodium acetate was added drop-by-
drop to achieve pH 4.7. Then 25 jo-L of
0.0025 M mercury nitrate solution was
pipetted in, and background anodic
stripping curves were obtained. Known
pi concentrations of Cu, Pb, and Cd
standards were pipetted in, and strip-
ping curves were again obtained. The
peak heights were measured on an
X - Y recorder, and the concentrations
of the three metals in the tap water were
determined by the method of standard
additions. Any Cd in the water was be-
low the limits of detectability of the pro-
cedure used. Analysis by atomic ab-
sorption detected no Cd either. The
results obtained for Cu, Pb, and Cd were
22, 3, and <0.2 ppb, respectively. Re-
sults obtained for 10 Seattle water sam-
ples are shown in Table 2.
Recommendations
Based on the experience gained for
tap water analysis with the instrument
described in this report, a next-
generation ASV might include the fol-
lowing items: (1) A cell that incorpo-
rates self-cleaning capabilities. Carbon
electrodes with electro-deposited mer-
cury films age with use and give both
spurious peaks as well as increased
.background current during the stripping
step. The result is a loss in sensitivity
and possible erroneous analyses be-
cause of peaks shown for metals that
are otherwise not present in detectable
amounts. Currently, cleaning is done
Table 2. Analysis of Municipal Water From Seattle, Washington
Sample Designation
NE 24 105NE
7914 NE 26
9850 Belfair Rd, Standing
9850 Belfair Rd, Running
Taylor Creek Well 7/4
Taylor Creek Well 7/13
Jones Res. Running
Jones Res. Standing
Park Place Standing, 1st run
Park Place Standing, 2nd run
AA Values
Cu Pb
1000 ppb 10 ppb
680 < 10
950 < 10
510 <10
<6 <10
<6 <10
—
—
— —
—
ASV
Cu
—
550 ppb
1100
800
2.6
1.4
12.4
1800
WOO
1000
Values
Pb
—
1.3 ppb
N.D.
N.D.*
N.D.
N.D.
2.3
N.D.*
N.D.*
N.D.*
N.D. = Not detected.
N.D.* = Non detection likely due to solid phase amalgam formation.
manually by periodically rubbing the
surface gently with filter paper. (2)
Methods for removing large quantities
of dissolved Cu. Plated Cu interferes in
two ways: by forming intermetallic
compounds with Cd (which reduces the
anodic peak height for Cd) and by mask-
ing the Pb and Cd peaks. Large concen-
trations of dissolved Cu (estimated at
>800 |ig/L) interfered in this way with
measurement of Pb and Cd in the sam-
ples of Seattle water. Cu plates more
easily than either Pb or Cd, so one ap-
proach to this problem might be a pre-
analysis step involving deposition of Cu
with a dual-working electrode system.
(3) Methods for measuring the total con-
centration of dissolved metals. Metals
can form more than one species in natu-
ral waters; some of these species are
not measured by ASV. A pre-ASV step
such as ozonolysis might be effective in
destroying complexes between metals
and electro-inactive species. (4) Pro-
gramming of the ASV system so that
the readout will be in ^.g/L of the metals
being measured. Currently the output
signal is either in analog form on a strip
chart recorder or in digital form on mag-
netic tape. Both must be further pro-
cessed.
The full report was submitted in fulfill-
ment of Contract No. EPA-70-D-X0507
by the University of California under the
partial sponsorship of the U.S. Environ-
mental Protection Agency.
ft US GOVERNMENT PRINTING OFFICE. 1985 559-111/20638
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R. G. Clem, F. W. Park, F. A Kirsten, S. L Phillips, and E. P. Binnall are with
University of California, Berkeley, CA 94720.
Marvin C. Gardels is the EPA Project Officer (see below).
The complete report, entitled "Microprocessor-Controlled Anodic Stripping
Voltammeter for Trace Metal Analysis in Tap Water," (Order No. DE 85-002
781; Cost: $10.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S2-85/063
0000329 PS
U S ENVIR PROTECTION AGENCY
REGION 5 LIBRARY
230 S DEARBORN STREET
CHICAGO It 60*04
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