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

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12.4

-1 -0.8 -0.6 -0.4 -0.2 0Potential (V)

Cu

rre

nt

(A

)

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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.

Men+

Me

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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

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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.

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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.

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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)

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Voltammetric equipment

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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

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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

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

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Apparatus for voltammetric analyses

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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

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7

-1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0

Potential (V)

Cu

rre

nt

(A

)

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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.

Detection of Zinc, Cadmium anf Lead on Dental Amalgam Electrode

15

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45

-1.2 -1 -0.8 -0.6 -0.4

Potential (V)

Cu

rre

nt

(A

)

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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.

Lead detected on silver electrode

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10

-800 -600 -400 -200 0 200

Potential (mV)

Cu

rre

nt

(A

)

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Soilwatch Project

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