Transcript Voltammetry1_hardware

MEASUREMENTS OF HEAVY METALS IN FLUIDS
WITH VOLTAMMETRIC METHODS
Øyvind Mikkelsen and Knut H. Schrøder
Norwegian University of Science and technology
Department of Chemistry
N - 7491 TRONDHEIM, NORWAY
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Voltammetry
• In voltammetry the
information about
the analyte is
achieved by measure
the current as a
function of a applied
potential.
• The current is
proportional to the
concentration.
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Cross section of the dental amalgam (or somewhat related material)
electrode.
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Working electrode
• Metal ions are
deposited on the
electrode surface.
Me
Men+
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Voltammetric cell
+
| |
(a)
(R)
/\/\/\/\/\/\/\/\/\/\/\/\
(d)
•
Counter
electrode
•
Working
electrode
•
Reference
electrode
(c)

(b)
A

Section of elctrode system.

Counter electrode
V
Working electrode
Reference electrode
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Antimony (Sb) in water samples and biological matrices
Bismuth (Bi) in water samples and biological matrices
Cadmium (Cd) in water samples and effluents
Chromium (Cr) in environmental samples and biological matrices
Cobalt (Co) in water samples and effluents
Copper (Cu) in environmental and water samples
Iron (Fe) in water samples
Lead (Pb) in water samples and effluents
Mercury (Hg) in water samples and biological matrices
Molybdenum (Mo) in water samples and biological matrices
Nickel (Ni) in water samples and biological matrices
Platinum (Pt) in environmental samples and biological matrices
Thallium (Tl) in water samples and effluents
Tungsten (W) in surface water samples
Uranium (U) in surface water samples
500 ppt
500 ppt
50 ppt
25 ppt
50 ppt
50 ppt
200 ppt
50 ppt
100 ppt
50 ppt
50 ppt
0.1 ppt
50 ppt
200 ppt
25 ppt
* 1 ppt = 1 part per trillion = 1 ng/kg
KILDE: http://metrohm.ch/products/va.html
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Signal Source & Measuring Unit (SMU)
- Control UNIT, Microcontroller unit that controls the card in
the SMU (voltammetric analyzer). Communicates with a
external PC via a RS232 interface.
- Signal Source UNIT, this unit consists of two electronic
boards; Signal Source ControlER Board and the
D/A Converting Unit. Digital numeric values of the voltage
signal to the cell are generated by the microcontroller and
placed in this unit. When a scan is started the values is cloked
out from the D/A converter as analog voltage steps.
- Converter/Measure UNIT, consists of one electronic board ref.
as the A/D converting unit. This board receives the current
feedback from the Ampl. Int. UNIT as a voltage signal from a
I/E converter, and converts the signal from the cell to digital
words, which the processor can read and place into a buffer for
transmission to the PC.
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Ampl. Int. UNIT
Amplifier / Interface UNIT for both the applied and the
received signal, to/from the spesific cell in use. Takes the
asserted potential from the SMU and puts it to the cell.
It also converts the current from the cell to a voltage wich
can be converted to digital values in the voltammetric
analyzer and send it to a PC.
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Signal Source & Measuring Unit
Control
UNIT
Signal
Source
UNIT
Convert/
Measure
UNIT
Sync
Ampl.
Int.
UNIT
Recording, analysing,
presentation and
documentation
UNIT (PC)
Ext. rec.
connect.
Ext.
trig.
Cntrl.
Cell
UNIT
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Voltammetric equipment
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Operational amplifier potentiostat
(OP AMP)
Linear-scan
voltage
generator
Signal source

A
+
Counter electrode
B

+
Referance electrode
Cell I
Potensiostatic control circuit
Working electrode

Eout
C
Recorder
+
Current-to-voltage converter
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Some typical signalmodulations in
voltammetry
Linear scan
Differential pulse
scan
Cyclic scan
Square wave
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Typical controlled parameters in voltammetri
C
B
D
A
A = 520 ms, measurement after 480 ms
B = 70 mV
C = 90 ms, measurement after 70 ms
D = 65 mV
Increase in staircase step; 5 mV.
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Apparatus for voltammetric analyses
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Software for setting/loading parameters to control unit
LabVIEW
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Background of dental amalgam electrode in acetatbuffer pH=4,6.
Scan from -1.2 to -0.2 V
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Zinc, cadmium and lead (25, 50 and 75 ppb) detected by DPASV on
dental amalgam electrode. All scan in NH4Ac (non-buffered) pH=6,4
also containing nickel and copper and gallium (75 ppb). Deposition
time 60 s at -1.2V. Scan from -1.2 to -0.4V, pulse height
50 mV.
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Plot of a typical scan at silver electrode before and after lead addition.
Scan performed by DPASV using a sine oscillating working silver
electrode. Deposition time 60 s, scan rate 15mV s-1, pulse height 40
mV.
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Soilwatch Project
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