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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. Mikkelsen 2000. Voltammetry. - PowerPoint PPT Presentation
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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
Mikkelsen 2000
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.
Voltammetric detection of Cadmium
4.4
5.4
6.4
7.4
8.4
9.4
10.4
11.4
12.4
-1 -0.8 -0.6 -0.4 -0.2 0Potential (V)
Cu
rre
nt
(A
)
Mikkelsen 2000
Cross section of the dental amalgam (or somewhat related material) electrode.
Mikkelsen 2000
Working electrode
• Metal ions are deposited on the electrode surface.
Men+
Me
Mikkelsen 2000
Voltammetric cell - + | |
(c) (R) (a) /\/\/\/\/\/\/\/\/\/\/\/\ (b) (d)
Section of elctrode system.
Counter electrode
Working electrode
Reference electrode
V
A
• Counter electrode
• Working electrode
• Reference electrode
Mikkelsen 2000
Mikkelsen 2000
Antimony (Sb) in water samples and biological matrices 500 pptBismuth (Bi) in water samples and biological matrices 500 pptCadmium (Cd) in water samples and effluents 50 pptChromium (Cr) in environmental samples and biological matrices 25 pptCobalt (Co) in water samples and effluents 50 pptCopper (Cu) in environmental and water samples 50 pptIron (Fe) in water samples 200 pptLead (Pb) in water samples and effluents 50 pptMercury (Hg) in water samples and biological matrices 100 pptMolybdenum (Mo) in water samples and biological matrices 50 pptNickel (Ni) in water samples and biological matrices 50 pptPlatinum (Pt) in environmental samples and biological matrices 0.1 pptThallium (Tl) in water samples and effluents 50 pptTungsten (W) in surface water samples 200 pptUranium (U) in surface water samples 25 ppt
* 1 ppt = 1 part per trillion = 1 ng/kg
KILDE: http://metrohm.ch/products/va.html Mikkelsen 2000
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.
Mikkelsen 2000
Mikkelsen 2000
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.
Mikkelsen 2000
Signal Source & Measuring Unit
Control UNIT
Signal Source UNIT
Convert/Measure
UNIT
Sync
Ampl.Int.
UNIT
Ext. rec.connect.
Cell UNIT
Ext. trig.
Cntrl.
Recording, analysing, presentation and documentation
UNIT (PC)
Mikkelsen 2000
Voltammetric equipment
Mikkelsen 2000
Linear-scanvoltage
generator A
+
B+
C
+Recorder
Current-to-voltage converter
Potensiostatic control circuit
Referance electrode
Counter electrode
Working electrode
Signal source
Eout
Operational amplifier potentiostat(OP AMP)
Cell I
Mikkelsen 2000
Some typical signalmodulations in voltammetry
Linear scan Differential pulse scan
Cyclic scan Square wave
Mikkelsen 2000
Typical controlled parameters in voltammetri
A = 520 ms, measurement after 480 msB = 70 mVC = 90 ms, measurement after 70 msD = 65 mV
Increase in staircase step; 5 mV.
C
A
B D
Mikkelsen 2000
Apparatus for voltammetric analyses
Mikkelsen 2000
Software for setting/loading parameters to control unit LabVIEW
Background of dental amalgam electrode in acetatbuffer pH=4,6. Scan from -1.2 to -0.2 V
Background signal of Dental Amalgam Electrode
0
1
2
3
4
5
6
7
-1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0
Potential (V)
Cu
rre
nt
(A
)
Mikkelsen 2000
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.
Detection of Zinc, Cadmium anf Lead on Dental Amalgam Electrode
15
20
25
30
35
40
45
-1.2 -1 -0.8 -0.6 -0.4
Potential (V)
Cu
rre
nt
(A
)
Mikkelsen 2000
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.
Lead detected on silver electrode
0
1
2
3
4
5
6
7
8
9
10
-800 -600 -400 -200 0 200
Potential (mV)
Cu
rre
nt
(A
)
Mikkelsen 2000
Soilwatch Project
Mikkelsen 2000
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