VA Theory

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V

oltammetry

Voltammetry

An Introduction in Theory

Metrohm Ltd.

CH-9100 Herisau Switzerland

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V

V

oltammetry

Overview

What is Voltammetry?

Electrode Types Voltammetric Techniques

Measurement Modes

Calibration Techniques Advantages of Voltammetry

Summary

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V

oltammetry

Voltammetry

Voltammetry = Volt-Am(pero)-Metry

Voltage ramp applied to electrode

Current measured

I = f(U)

Method first described 1922 by Heyrovsky

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oltammetry

VoltageSource

(Potentiostat)

polarographic

cell

746

I

U

Instrumentation in voltammetry

Me+ + e- Me0

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oltammetry

+ -

Pb2+

Pb2+

Pb

2+

Pb2+

Pb2+Pb2+

Pb2+

Pb2+ Pb2+

Pb2+

Pb2+

Pb2+

Pb2+

Pb2+

Pb2+

Pb2+

Pb0Pb2+ +2e-

Principle of voltammetry

Pb ions are reduced and dissolved in mercury

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oltammetry

I

U

qualitative information

U

quantitative information


resulting curve

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oltammetry U

I

Pb

Cd

time

Reduction to metal

(amalgam formation)


potential increased

current measured

metal ions are

reduced and dissolvedin mercury.

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olta

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Electrodes

3

1 Working Electrode

1 Reference Electrode

1 Auxiliary Electrode

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Reference electrode

provides stable reference potential

Ag/AgCl/KCl 3 mol/L

Double junction system with exchangeableelectrolyte

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Auxiliary electrode

Current flows between working and

auxiliary electrode

Two types available:

Platinum (Pt) Glassy Carbon (GC)

Depending on application !

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Working Electrodes

MMEMul ti M ode Electrode

RDERotating Disc Electrode

GC, Au, Ag

Pt, UT

SMDEDME

HMDE

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MME

Multi Mode

Electrode

mercury

needle

tapper

capillary

Hg drop

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Multi Mode Electrode

SMDE

Stable surface

HMDE

One drop !

Needle

Capillary

Tapper

Hg drop

DME

Droplife

New Drop

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ppmLow ppm ppb

ppt

DME SMDE HMDE & RDE

Traces

Choice of the working electrode

Depends on the concentration of the analyte

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Which working electrode ?

Standard electrode: MME

Organics: DME or SMDE Metal Traces: HMDE

Special applications: RDE

Gold electrode: Hg, As

UT* with Hg film: Cd, Pb, Cu

UT* Hg free: Cr, W, Ni

* UT: Ultra Trace Graphite Electrode

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Which working electrode ?

Potential ranges of different electrode types

Glassy Carbon

Gold

Platinum

0.20-0.2-0.4-0.6-0.8-1.0-1.2 0.60.4Potential (V)

Mercury

10.8 1.2

H

+

+ e

-

H

Oxidation of electrode

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Voltammetric Techniques

Polarography

uses the dropping mercury electrode (DME,

SMDE)

(Str ipp ing) Vol tammetry uses a stationary electrode (HMDE, RDE)

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U U

I I

Pb

Cd

Pb

Cd

Electrolysis

time time

Reduction to metal(amalgam formation)

Reduction to metal(amalgam formation) Oxidation to ion(determination)

Voltammetric Techniques

Polarography Stripping Voltammetry

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CSV

Voltammetric Analysis

Anod ic Str ipp ing Vol tammetry (ASV)

2 steps

1. deposition (metal reduction)

Cd2+ Cd0(Hg)

2. determination (Stripping)

Cd0(Hg) Cd2+ 2 steps

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Voltammetric Analysis

Adso rpt ive Str ipp ing Vol tammetry (AdSV)

1. deposition (adsorption)

Ni2+(DMG-)2 Ni2+(DMG-)2

ads

2. determination (Stripping)

Ni2+(DMG-)2ads Ni2+ + 2 DMGred

DMG: Dimethylglyoxime, complexing agent

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PSA

Stripping Voltammetry

Propert ies

2 step measurement

electrochemical deposition (enrichment)

determination (stripping step)

high sensitivity

for trace analysis (down to ppt range) mainly for metal analysis.

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Measurement Modes

= modulation of potential ramp

Purpose:

Increasing sensitvity

decreasing background signals

Increasing selectivity

separation of faradayic and capacitive current

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Direct Current: DC

potential ramp resulting curve

wave height

half wave potentialdrop time

time

U

voltage step

time

U

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Differential Pulse: DP

potential modulation resulting curve

U

I

peak height

peak potential

drop time

voltage step

pulse

amplitude

time

U

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Modes

DC and DP

I

I

U

U

qualitative information

U



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Electrolyte in VA

Correct electrolyte is most important :

seeApplication Bulletins

Purpose of the electrolyte:

Increases the conductivity

Adjusts the pH Complexes the analyte

Increase selectivity (masking of interfering

elements)

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Na tartrate

NH3/NH4Cl

NH4Ac/HAc

Mn

Mn

Zn

Zn

Zn

Co

Co

Co

Ni

Ni

Cd

Cd

Cd

Pb

Pb

Pb Cu

Cu

Cu

Hg

Hg

Hg

0.20-0.2-0.4-0.6-0.8-1.0-1.2-1.4-1.6 0.60.4

Hg

Hg

Hg

Potential (V)

Electrolyte in VA

Peak potentials of different metals

depending on the electrolyte

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Concentration evaluation

Peak evaluation

Calibration standard addition

external calibration (calibration curve)

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Peak evaluation

Peak

maximum

Basepoints

Baseline

Peak

height

Peak area

U

I

basepoints baseline

height

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Calibration Curve

Voltammograms of standards Calibration Curve

peak

height

concc1 c2 c3

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Sample with Cal. Curve

Voltammograms of sample Calibration Curve

peak

height

conc

c sample

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Sum

Standard Addition

sample

additions

Voltammograms Standard Addition Curve

peakheight

concAdd1 Add2-cS

E

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Calibration Techniques

External Calibration

Measure standard solutions Calculate calibration curve

measure sample

Standard Addition

measure sample

measure additions to sample solution

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Why Voltammetry?

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Advantages of Voltammetry

No problem with high salt concentrations

Speciation possible: e.g. Cr3+ - Cr6+

free / complexed metal

Low detection limits: sub-ppb

Capital outlay: Low price

Low running costs: only N2

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No extensive laboratory infrastructure

Versatility: inorganics and organicsAnions

Official methods (DIN,ISO,USP, ASTM,

etc.)

Alternative and complementary method to

AAS/ICP


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Simultaneous Determination of Metals:

up to 4 metals in one run

Automation: 80 Samples plus all buffers

and standards

Calibration in every run: Variable

Standard Addition if required.

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Summary

Most important points

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What you should know about VA !

VA: Voltam[pero]metry

I = f(U)

Electrodes:

AE: Pt, GC

RE: Ag/AgCl/KCl 3 mol/L

WE: MME (DME, SMDE, HMDE) or RDE

Measurement Modes:

DC, DP, AC1/AC2, SQW, CV, PSA

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Methods

Polarography: "Liquid" electrode (DME, SMDE)

Stripping Voltammetry (HMDE, RDE):

Anodic Stripping Voltammetry

Adsorptive Cathodic Stripping Voltammetry

Electrolyte (buffer) Allows signal enhancement

Increase selectivity

Allows masking of elements

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Concentration Evaluation:

Peak evaluation

Calibration

Standard Addition Technique

Calibration Curve Technique


Documents

VA Theory