Voltammetry

 ( Electroanalytical Methods ) 

There are three main shapes for voltammograms. The first shape is dependent on the diffusion layer. If the analyte is continuously stirred, the diffusion layer will be a constant width and produce a voltammogram that reaches a constant current. The graph takes this shape as the current increases from the background residual to reach the limiting current (i_l). If the mixture is not stirred, the width of the diffusion layer eventually increases. This can be observed by the maximum peak current (i_p), and is identified by the highest point on the graph. The third common shape for a voltammogram measures the sample for change in current rather than current applied. A maximum current is still observed, but represents the maximum change in current ($\backslash Delta$i_p).

$\{\backslash displaystyle\; j=j\_\{0\}\backslash cdot\; \backslash left\backslash \{\backslash exp\; \backslash left\{\backslash frac-\backslash exp\; \backslash left-\{\backslash frac\backslash right\backslash \}\}$

Where:

• $j$: electrode current density, A/m² (defined as j = I/S)
• $j\_0$: exchange current density, A/m²
• $E$: electrode potential, V
• $\{\backslash displaystyle\; E\_\{\backslash rm\; \{eq\}\}\}$: equilibrium potential, V
• $T$: absolute temperature, K
• $z$: number of electrons involved in the electrode reaction
• $F$: Faraday constant
• $R$: universal gas constant
• $\backslash alpha\_c$: so-called cathodic charge transfer coefficient, dimensionless
• $\backslash alpha\_a$: so-called anodic charge transfer coefficient, dimensionless
• $\backslash eta$: activation overpotential (defined as $$

At high , the Butler–Volmer equation simplifies to the Tafel equation. The Tafel equation relates the electrochemical currents to the overpotential exponentially, and is used to calculate the reaction rate. The overpotential is calculated at each electrode separately, and related to the voltammogram data to determine reaction rates. The Tafel equation for a single electrode is:

$\{\backslash displaystyle\; \backslash eta\; =\backslash pm\; A\backslash cdot\; \backslash log\; \_\{10\}\backslash left(\{\backslash frac\; \{i\}\{i\_\{0\}\}\}\backslash right)\}$

Where:

• the plus sign under the exponent refers to an anodic reaction, and a minus sign to a cathodic reaction
• $\backslash eta$: overpotential, V
• A: "Tafel slope", V
• $i$: current density, A/m²
• $i\_0$: "exchange current density", A/m².

As the redox species are oxidized and reduced at the electrodes, material accumulates at the electrode/electrolyte interface. Material accumulation creates a concentration gradient between the interface and the bulk solution. Fick's laws of diffusion is used to relate the diffusion of oxidized and reduced species to the faradaic current used to describe redox processes. Fick's law is most commonly written in terms of moles, and is as follows:

$\{\backslash displaystyle\; J=-D\{\backslash frac\; \{d\backslash varphi\; \}\{dx\}\}\}$

Where:

• J: diffusion flux (in amount of substance per unit area per unit time)
• D: diffusion coefficient or Mass diffusivity. (in area per unit time)
• φ: concentration (in amount of substance per unit volume)
• x: position (in length)

In 1942, the English electrochemist Archie Hickling (University of Leicester) built the first three electrodes potentiostat, which was an advancement for the field of electrochemistry. He used this potentiostat to control the voltage of an electrode. In the meantime, in the late 1940s, the American biophysicist Kenneth Stewart Cole invented an electronic circuit which he called a voltage clamp. The voltage clamp was used to analyze the ionic conduction in .

The 1960s and 1970s saw many advances in the theory, instrumentation, and the introduction of computer aided and controlled systems. Modern polarographic and voltammetric methods on mercury electrodes came about in three sections.

The first section includes the development of the mercury electrodes. The following electrodes were produced: dropping mercury electrode, mercury steaming electrode, hanging mercury drop electrode, static mercury drop electrode, mercury film electrode, mercury amalgam electrodes, mercury microelectrodes, chemically modified mercury electrodes, controlled growth mercury electrodes, and contractible mercury drop electrodes.

There was also an advancement of the measuring techniques used. These measuring techniques include: classical DC polarography, oscillopolarography, Kaloussek's switcher, AC polarography, tast polarography, normal pulse polarography, differential pulse polarography, square-wave voltammetry, cyclic voltammetry, anodic stripping voltammetry, convolution techniques, and elimination methods.

Lastly, there was also an advancement of preconcentration techniques that produced an increase in the sensitivity of the mercury electrodes. This came about through the development of anodic stripping voltammetry, cathodic stripping voltammetry and adsorptive stripping voltammetry.

These advancements improved sensitivity and created new analytical methods, which prompted the industry to respond with the production of cheaper potentiostat, electrodes, and cells that could be effectively used in routine analytical work.

• Current–voltage characteristic
• Neopolarogram

• Douglas A. Skoog (1995). 9780030059384, Harcourt Brace College Publishers. ISBN 9780030059384
• P. Zanello (2003). 9780854046614, Royal Society of Chemistry. ISBN 9780854046614

• http://new.ametek.com/content-manager/files/PAR/App%20Note%20E-4%20-%20Electrochemical%20Analysis%20Techniques1.pdf
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