Lecture Date: April 10 th , 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 (mm 2 to um 2 ) 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:

E cell



E right



E left



IR

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: Waveform generator Potentiostat ← e e → Counter electrode Working electrode Computer Current-to voltage converter E applied Reference electrode (i = 0) Cell See Fig. 29.13 in Stroebel and Heineman, Chemical Instrumentation, A Systematic Approach 3 rd Ed. Wiley 1989.

Instrumentation for Voltammetry

 Sweep generators, potentiostats, cells, and data acquistion/computers make up most systems Basic voltammetry system suitable for undergraduate laboratory work From

www.edaq.com/er461.html

Cyclic voltammetry cell with a hanging mercury drop electrode From

www.indiana.edu/~echem/cells.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: Nernst Plot Half-wave potential E 1/2 A +

n

e  P A +

n

e  P  The half-wave potential E 1/2 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 Limiting current Remember, E is scanned linearly to higher values as a function of time in linear sweep voltammetry

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 O + 2H + + 2e  Different waves are obtained depending on the starting sample O quinone Cathodic wave OH OH hydroquinone  Both reduction and oxidation waves are seen in a mixture Anodic wave Diagram from Stroebel and Heineman, Chemical Instrumentation, A Systematic Approach 3 rd Ed. Wiley 1989.

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 O 2 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

O 2

At the platinum cathode: O 2 + 2H 2 O + 4e 4OH At the Ag/AgCl anode: Ag + Cl AgCl + e -

dissolved O 2 analyte solution O 2 O 2 electrolyte

O 2 (O 2 permeable membrane crosses via diffusion) i d = 4 F P m A P(O 2 )/b i d measured current F - Faraday's constant P m - permeability of O 2 A - electrode area P(O 2 ) - oxygen concentration b - thickness of the membrane platinum electrode (-0.6 volts)

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 Cd => Cd 2+ + 2e Cu => Cu 2+ + 2e The currents observed for Cd and Cu are proportional to the concentration of each metal in solution. See pages 748 of the text for a similar figure.

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 irreversible reduction (a, E pa -0.61 V) reversible reduction oxidation (b, E pa -0.08 V, c, E pc = = 0.0 V) (c) (a) (b) M. Sbai, et al. Sensors and Actuators B 124 (2007) 368 –375.

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

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.