Overview
Electrochemistry is a field of chemistry that relates electrical and chemical effects.  Indeed, the term “electrochemistry” is defined as “chemical changes caused by the passage of an electric current” (1).  The term “Research Electrochemistry” as used here describes techniques and applications in electroanalytical chemistry for fundamental studies that includes a large area of quantitative methods.  These are:

• Potentiometric methods
• Coulometric methods
• Voltammetry methods
• Polarography

Potentiometric Methods

The Nernst equation relates potential to concentration of electroactive species:

Indicator Electrodes

There are 3 types of indicator electrodes:

• Class I (primary) electrode: metal immersed in solution containing the ion/molecule of interest:
   

Most common of these are Pt, carbon or Au electrodes

• Class II (secondary) electrode: an electrode in direct contact with an anion with which its ion forms a precipitate or a stable complex ion so that the potential response measures the inactive species:

  

  Here, the determination of the chloride ion is facilitated by the formation of a stable complex or precipitate such as AgCl. An example of this is the silver-silver chloride reference electrode.

• Class III (tertiary) electrodes: an electrode in direct contact with a cation with which its ion forms a precipitate or a stable complex ion so that the potential response measures the inactive species, for example a Hg electrode in the presence of EDTA, with Ca-EDTA added:

  
  

  Y = EDTA anion

  Here, the Hg electrode has become a Class III electrode for the detection of Ca+ ion.

Coulometric Methods

These methods involve the measuring of charge (in coulombs) needed to convert the analyte under investigation to a different oxidation state.  Coulometry offers the advantage that the proportionality constant between the measured amount of charge (coulombs) and the weight of the analyte can be derived (remember Faraday’s Law of Electrolysis).  Coulometric methods can be as accurate as gravimetric or volumetric techniques, and moreover, they are usually faster and more convenient!

There are two types of coulometric methods, potentiostatic and amperostatic. Potentiostatic coulometry maintains the potential of the working electrode at a constant so that K0026 Coulometry Cell Kit oxidation or reduction occurs of the ion or species of interest and can be quantifiably measured, without involvement of other components in the solution. For example, arsenic can be determined coulometrically by the electrolytic oxidation of arsenous acid (H3AsO3) to arsenic acid (H3AsO4) at a platinum electrode.

Voltammetry

This technique is used in many methods where information about the analyte is determined from the measurement of current as a function of applied potential under polarization of the indicator or working electrode. This is in contrast to potentiometric methods that rely on measurements with currents that approach zero and polarization is absent.  Voltammetry is different from coulometry in that the effects of polarization is minimized or compensated for at the surface of the electrode. The field of voltammetry developed from polarography, which was discovered by Heyrovsky in the early 1920’s. Polarography is different from other types of voltammetry in that the working electrode is a dropping mercury electrode (DME).

Jaroslav Heyrovsky (1890-1967)

Voltammetry is widely used by inorganic, physical and biological chemists for non-analytical measurements such as fundamental           studies of oxidation-reduction processes in various electrolytes,         adsorption processes on surfaces, and electron-transfer mechanisms at chemically modified electrode surfaces and pharmaceutical applications. In voltammetry, the potential (excitation signal) is varied to yield a current response from the electrode; these excitation signals are applied as waveforms and the 4 most common excitation signals are: linear, differential pulse, square wave and triangular (cyclic).

Generally, the electrodes used in voltammetry are either millielectrodes, having surface areas of less than a few square millimeters or microelectrodes, having surface areas of less than a few square micrometers.

Another area of voltammetry is termed hydrodynamic voltammetry, which can be performed in many ways. One method involves stirring of the solution while it is contact with a fixed electrode. An alternate method is to rotate the electrode at a constant speed in the electrolyte. Still another way is to flow the electrolyte past a microelectrode mounted in a flow tube.  This last technique is widely used for electrochemical detection of analytes coming off of a chromatography column.

Linear scan polarography differs from hydrodynamic voltammetry in that: (1) convection is avoided and (2) a hanging or dropping mercury electrode is used as the working electrode.  Limiting currents are therefore controlled by diffusion alone rather than by diffusion and convection; because of this, the limiting currents are usually smaller in magnitude by one or more orders of magnitude. While linear-scan polarography offers the above advantages over other voltammetric techniques, pulse techniques such as differential pulse polarography and square wave polarography offer decreased measurement times, and increased sensitivity.  While the excitation signals are essentially the same as described for pulsed voltammetric techniques, the terms for the procedures are different: differential and square wave voltammetry.

References

(1)  “Electrochemical Methods: Fundamentals and Applications.” A.J. Bard and L. R.
Faulkner, 2nd Edition, John Wiley and Sons, NY (2001).
(2) “Principles of Instrumental Analysis.” D.A. Skoog and J.J. Leary, 4th edition,
Saunders College Publishing, TX (1992).
(3) J.B. Flato, Anal. Chem. 44:75A (1972).
(4) “Reference Electrodes: Theory and Practice.” D.J.G. Ives and G.J. Janz, eds. , Academic Press, NY (1961). Reprinted by NACE International, TX (1996).