Nov 16, 2004Voltammetry

Lecture Date: April 10th, 2013

Voltammetry

Voltammetry techniques measure current as a function of applied potential under conditions that promote polarization of a working electrode

Polarography: Invented by J. Heyrovsky (Nobel Prize 1959). Differs from voltammetry in that it employs a dropping mercury electrode (DME) to continuously renew the electrode surface.

Amperometry: a current proportional to analyte concentration is monitored at a fixed potential

– In other words, voltammetry at a constant potential

DC Polarography

The first voltammetric technique (first instrument built in 1925)

DCP measures current flowing through the dropping mercury electrode (DME) as a function of applied potential

Under the influence of gravity (or other forces), mercury drops grow from the end of a fine glass capillary until they detach

If an electroactive species is capable of undergoing a redox process at the DME, then an S-shaped current-potential trace (a polarographic wave) is usually observed

www.drhuang.com/.../polar.doc_files/image008.gif

Current in Electrochemical Cells

Some electrochemical cells have significant currents

– Electricity within a cell is carried by ion motion

– When small currents are involved, E = IR holds

– R depends on the nature of the solution (next slide)

When current in a cell is large, the actual potential usually differs from that calculated at equilibrium using the Nernst equation

– This difference arises from polarization effects

– The difference usually reduces the voltage of a galvanic cell or increases the voltage consumed by an electrolytic cell

Polarization

Electrodes in cells are polarized over certain current/voltage ranges

– Electrodes are purposely kept small (mm2 to um2) in voltammetry to promote polarization

“Ideal” polarized electrode: current does not vary with potential

Ohmic Potential and the IR Drop

To create current in a cell, a driving voltage is needed to overcome the resistance of ions to move towards the anode and cathode

This force follows Ohm’s law, and is governed by the resistance of the cell:

IREEE leftrightcell

Electrodes

IR Drop(needed when current

is significant)

Overvoltage and Polarization Sources

Overvoltage (overpotential) the difference between the equilibrium potential and the actual potential; it develops because of polarization

– Net result is you must means must apply greater potential before redox chemistry occurs

Sources of polarization in cells:– Concentration polarization: rate of transport to electrode

is insufficient to maintain current

– Charge-transfer (kinetic) polarization: magnitude of current is limited by the rate of the electrode reaction(s) (the rate of electron transfer between the reactants and the electrodes)

– Other effects (e.g. adsorption/desorption)

Voltage-Time Signals in Voltammetry

A variable potential excitation signal is applied to the working electrode

Different voltammetric techniques use different waveforms

Many other waveforms are available (even FT techniques are in use)

Instrumentation for Voltammetry

Block diagram of a typical 3-electrode voltammeter:

PotentiostatWaveform generator

Current-to-voltage

converterComputer

Eapplied

Cell

See Fig. 29.13 in Stroebel and Heineman, Chemical Instrumentation, A Systematic Approach 3 rd Ed. Wiley 1989.

Counter electrode

Working electrode

Reference electrode(i = 0)

← e-

e- →

Instrumentation for Voltammetry

Cyclic voltammetry cell with a hanging mercury drop electrode

From www.indiana.edu/~echem/cells.html

Sweep generators, potentiostats, cells, and data acquistion/computers make up most systems

Basic voltammetry system suitable for undergraduate laboratory workFrom www.edaq.com/er461.html

Linear Sweep Voltammetry

Linear sweep voltammetry (LSV) is performed by applying a linear potential ramp in the same manner as DCP.

However, with LSV the potential scan rate is usually much faster than with DCP.

When the reduction potential of the analyte is approached, the current begins to flow.

– The current increases in response to the increasing potential.

– However, as the reduction proceeds, a diffusion layer is formed and the rate of the electrode reduction becomes diffusion limited. At this point the current slowly declines.

The result is the asymmetric peak-shaped I-E curve

The Linear Sweep Voltammogram

A linear sweep voltammogram for the following reduction of “A” into a product “P” is shown:

A + n e- P

The half-wave potential E1/2

is often used for qualitative analysis

– n can also be fitted

The limiting current is proportional to analyte concentration and is used for quantitative analysis

Half-wave potentialE1/2

A + n e- P

Limiting current

Remember, E is scanned linearly to higher values as a function of time in linear

sweep voltammetry

Nernst Plot

Hydrodynamic Voltammetry

Hydrodynamic voltammetry is performed with rapid stirring in a cell

– Electrogenerated species are rapidly swept away by the flow

Reactants are carried to electrodes by migration in a field, convection, and diffusion. Mixing takes over and dominates all of these processes.

– Most importantly, migration rate becomes independent of applied potential

Hydrodynamic Voltammograms

Example: the hydrodynamic voltammogram of quinone-hydroquinone

Different waves are obtained depending on the starting sample

Both reduction and oxidation waves are seen in a mixture

O

O

quinone hydroquinone

+ 2H+ + 2e

OH

OH

Diagram from Stroebel and Heineman, Chemical Instrumentation, A Systematic Approach 3 rd Ed. Wiley 1989.

Anodic wave

Cathodic wave

Oxygen Waves in Hydrodynamic Voltammetry

Oxygen waves occur in many voltammetric experiments

– Here, waves from two electrolytes (no sample!) are shown before and after sparging/degassing

Heavily used for analysis of O2 in many types of sample

– In some cases, the electrode can be dipped in the sample

– In others, a membrane is needed to protect the electrode (Clark sensor)

Diagram from Stroebel and Heineman, Chemical Instrumentation, A Systematic Approach 3 rd Ed. Wiley 1989.

The Clark Voltammetric Oxygen Sensor

Named after its generally recognized inventor (Leyland Clark, 1956), originally known as the "Oxygen Membrane Polarographic Detector“

It remains one of the most commonly used devices for measuring oxygen in the gas phase or, more commonly, dissolved in solution

The Clark oxygen sensor finds applications in wide areas:– Environmental Studies

– Sewage Treatment

– Fermentation Process

– Medicine

The Clark Voltammetric Oxygen Sensor

dissolvedO2

analyte solution

O2 permeable membrane(O2 crosses via diffusion)

platinum electrode

electrolyte

O2

O2

O2

O2 + 2H2O + 4e- 4OH-

At the platinum cathode:

At the Ag/AgCl anode:

Ag + Cl- AgCl + e-

(-0.6 volts)

id = 4 F Pm A P(O2)/b

id - measured current

F - Faraday's constant

Pm - permeability of O2

A - electrode area

P(O2) - oxygen concentration

b - thickness of the membrane

The Clark Voltammetric Oxygen Sensor

General design and modern miniaturized versions:

Hydrodynamic Voltammetry as an LC Detector

One form of electrochemical LC detector:

Classes of Chemicals Suitable for Electrochemical Detection:

Phenols, Aromatic Amines, Biogenic Amines, Polyamines, Sulfhydryls, Disulfides, Peroxides, Aromatic Nitro Compounds, Aliphatic Nitro Compounds, Thioureas, Amino Acids, Sugars, Carbohydrates, Polyalcohols, Phenothiazines, Oxidase Enzyme Substrates, Sulfites

Cyclic Voltammetry

Cyclic voltammetry (CV) is similar to linear sweep voltammetry except that the potential scans run from the starting potential to the end potential, then reverse from the end potential back to the starting potential

CV is one of the most widely used electroanalytical methods because of its ability to study and characterize redox systems from macroscopic scales down to nanoelectrodes

Cyclic Voltammetry

The waveform, and the resulting I-E curve:

The I-E curve contains a large amount of analytical information (see next slide)

Cyclic Voltammetry

CV for a simple system: hexacyanoferrate(III) and (II) ions

CV can rapidly generate a new oxidation state on a forward scan and determine its fate on the reverse scan

Advantages of CV– Controlled rates– Can determine

mechanisms and kinetics of redox reactions

P. T. Kissinger and W. H. Heineman, J. Chem. Ed. 1983, 60, 702.

Electrochemical Stripping Voltammetry

A two step process:(1) The analyte is deposited (accumulated) on the

working electrode from solution.

(2) The analyte is then stripped off of the electrode with observation of current by a voltammetric method.

The aim is to concentrate the analyte to obtain lower LOD and LOQ.

Anodic stripping: the working electrode behaves as a cathode during the deposition step, then behaves as an anode during the stripping step.

– Cathodic stripping (less common) is the opposite process.

See pages 748 of the text for more about electrochemical stripping techniques.

Electrochemical Stripping Voltammetry

-1.0 V

-0.6 V

-0.1 V

See pages 748 of the text for a similar figure.

Cd => Cd2+ + 2e-

Cu => Cu2+ + 2e-

The currents observed for Cd and Cu are proportional to the concentration of each metal in solution.

Electrochemical Stripping Voltammetry: Elemental Analysis

Elemental detection using a bismuth-modified carbon paste electrode

Three toxic elements (Cd, Pb, Tl) are easily detected at 200 ppb in this example.

Svancara, et al., Electroanalysis 18, 2006, 177-185.

Electrochemical Stripping Voltammetry: Molecular Analysis

An early example of stripping voltametry (polarography) using a hanging mercury drop electrode on the drug diazepam:

R. Kaldova, Analytica Chimica Acta, 162 (1984) 197—205.

Electrochemical Stripping Voltammetry: Molecular Analysis

Detection of the insecticide methyl parathion using stripping square-wave voltammetry with an electrode made from tetrasulfonated phtalocyanine (p-NiTSPc) electrodeposited on a carbon surface with a Nafion® sulfonated tetrafluoroethylene copolymer coating

M. Sbai, et al. Sensors and Actuators B 124 (2007) 368–375.

irreversible reduction (a, Epa -0.61 V)

reversible reduction-oxidation (b, Epa = -0.08 V, c, Epc = 0.0 V)

(a)(b)

(c)

CV and Spectroelectrochemistry (SEC)

CV and spectroscopy can be combined by using optically-transparent electrodes

This allows for analysis of the mechanisms involved in complex electrochemical reactions

Example: ferrocene oxidized to ferricinium on a forward CV sweep (ferricincium shows UV peaks at 252 and 285 nm), reduced back to ferrocene (fully reversible)

Y. Dai, G. M. Swain, M. D. Porter, J. Zak, “New horizons in spectroelectrochemical measurements: Optically transparent carbon electrodes,” Anal. Chem., 2008, 80, 14-27.

More Spectroelectrochemistry

A typical system (Gamry Interface 1000 and Agilent/Varian Cary 50 UV-Vis)

SECM and SECM-AFM

Scanning electrochemical microscopy (SECM) uses nanometer sized tips (electrodes) to probe surface phenomena

– Analyses are run in constant height mode or constant current mode

– Can be combined with AFM

The figures compare steady-state voltammograms of 1 mM ferrocenemethanol and 0.2 M NaCl obtained using a bulk system and using a SECM with a 36 nm polished Pt tip

Sun and Mirkin, Anal. Chem. 2006, 78, 6526-6534.

Bulk

SECM

SECM: Applications to Metal Corrosion SECM can be used to identify precursor sites for corrosion

in passive oxide films that protect metals

The metal substrate is biased with a voltage and the SECM tip detects the product of a reaction, providing an image of the reactive site.

Allows imaging of surface reactivity

Basame and White, Langmuir 1999, 15, 819-825.

SECM Instrumentation

Princeton Applied Research/Ametek VersaSCAN:

Reading Material

● Skoog, Holler and Crouch: Ch. 25

● Cazes: Chapter 17

● Optional reading:– C. Amatore and E. Maisonhaute, “When voltammetry reaches

nanoseconds”, Anal. Chem., 2005, 303A-311A.– Y. Dai, G. M. Swain, M. D. Porter, J. Zak, “New horizons in

spectroelectrochemical measurements: Optically transparent carbon electrodes,” Anal. Chem., 2008, 80, 14-27.

– A. J. Bard and L. R. Faulkner, “Electrochemical Methods”, 2nd Ed., Wiley, 2001.