All electrochemical cells require at least two electrodes, since the potential of a given electrode can only be measured relative to another electrode, the potential of which must be constant (a reference electrode). In potentiometric measurements (such as measurement of pH), there is no current through the cell, and these two electrodes are sufficient. (It should be noted that many pH and ion-selective electrodes used in potentiometric measurements are combination electrodes — both electrodes are contained within the same body.) However, in a cyclic voltammetry experiment, an external potential is applied to the cell, and the current response is measured. Precise control of the external applied potential is required, but this is generally not possible with a two-electrode system, due to the potential drop across the cell due to the solution resistance (potential drop (E) = current (i) x solution resistance (R)) and polarization of the counter electrode required to complete the current-measuring circuit. Better potential control is achieved using a potentiostat and a 3-electrode system, in which the potential of one electrode (the working electrode) is controlled relative to the reference electrode, and the current passes between the working electrode and the third electrode (the auxiliary electrode).
A working electrode acts as a source or sink of electrons for exchange with molecules in the interfacial region (the solution adjacent to the electrode surface), and must be an electronic conductor. It must also be electrochemically inert (i.e., does not generate a current in response to an applied potential) over a wide potential range (the potential window). Commonly used working electrode materials for cyclic voltammetry include platinum, gold, mercury, and glassy carbon. Other materials (e.g., semiconductors and other metals) are also used, for more specific applications. The choice of material depends upon the potential window required (e.g., mercury can only be used for negative potentials, due to oxidation of mercury at more positive potentials), as well as the rate of electron transfer (slow electron transfer kinetics can affect the reversibility of redox behavior of the system under study). The rate of electron transfer can vary considerably from one material to another, even for the same analyte, due to, for example, catalytic interactions between the analyte and active species on the electrode surface.
Glassy carbon is an amorphous form of carbon, whereas pyrolytic graphite has a more ordered structure, with distinct planes – the basal plane and the edge plane. The edge plane is considerably more conductive than the basal plane. Glassy carbon is mechanically more durable than pyrolytic graphite.
If material adsorbs to the surface of a working electrode, then the current response will degrade and the electrode surface needs to be cleaned. Such adsorption occurs more readily for some analytes than for others, and hence the required cleaning frequency varies. In many cases, the only cleaning required is light polishing with a fine material such as 1 mm diamond, or 0.05 mm alumina. A few drops of polish are placed on a polishing pad (brown Texmet for alumina, and white nylon for diamond), and the electrode is held vertically and the polish rubbed on in a figure-eight pattern for a period of 30 seconds to a few minutes, depending upon the condition of the electrode surface. After polishing, the electrode surface is rinsed thoroughly with water (for alumina) or methanol (for diamond), and allowed to air dry. (Electrodes polished with alumina may also need to be sonicated in distilled water for a few minutes to remove any residual alumina particles. The choice of polish depends upon the analyte and the electrode; use the polishing method that gives the best results (i.e., reproducible current response) for a given system. More pronounced surface defects, such as a scratch, may need to be polished with a coarser polish. Once the defect has been removed, the electrode must then be polished with successively finer polishes to obtain a mirror-like surface. Electrochemical cleaning (applying large anodic or cathodic potentials to the electrode) has also been shown to be effective in some instances.
Mercury drop electrodes have three major advantages over solid electrode materials such as platinum and glassy carbon:
a) a more reproducible surface
b) more negative potentials can be attained in aqueous systems
c) amalgamation with heavy metals (e.g., lead and cadmium)
Therefore, mercury drop electrodes are used for determination of trace metals with stripping voltammetry (where reproducibility is critical) and measurements at negative potentials in aqueous systems. A mercury drop electrode consists of a mercury drop at the end of the capillary. The other end of the capillary is attached to a reservoir of mercury, and control of the flow of mercury from the reservoir is controlled by a valve. The simplest mercury electrode is the Dropping Mercury Electrode (DME), for which the valve is held open throughout the experiment. The mercury drop is therefore dynamic, growing to a certain size before falling off the capillary under its own weight. (The drop can also be displaced at set time intervals using a drop knocker). An alternative mercury electrode is the Static Mercury Drop Electrode (SMDE), for which the valve is held open for a set length of time. The size of the mercury drop generated is constant once the valve is closed. The drop is displaced using a drop knocker. In the Controlled Growth Mercury Electrode (CGME), which is available only from BASi®, the drop is grown incrementally, using a user-defined series of valve openings. The timing of the valve openings and drop knocks for the SMDE and CGME, and their coordination with changes in the applied potential and the current measurement require microprocessor control. An electrochemical experiment can use one mercury drop (a Hanging Mercury Drop Electrode — HMDE) (e.g., stripping experiments) or a series of mercury drops coordinated with potential pulses (e.g., pulse polarographic experiments).
Mercury film electrodes consist of a thin film of mercury deposited on an electrode surface (typically glassy carbon) by reduction of a mercury(II) salt in solution. It can be difficult to obtain a reproducible film, and this can affect the reproducibility of the results, particularly when compared to the reproducibility obtained using a mercury drop electrode. However, the surface area/volume ratio is larger for the mercury film electrode, and this electrode is more stable, which allows a faster stirring rate to be used in the deposition step. Both these factors decrease the deposition time required for the mercury film electrode. In addition, the resolution for adjacent peaks is better for the mercury film electrodes, due to sharper peaks.
The standard BASi® working electrode for voltammetry is a disk with a diameter of 1.6 - 3 mm. Decreasing the size of the electrode to micron dimensions (microelectrodes) decreases the iR drop at the electrode, decreases the electrode capacitance (which allows a faster scan rate to be used for cyclic voltammetry), and changes the diffusion to the electrode surface from linear to radial.
The major requirement for a reference electrode is that the potential does not change with time. Since the passage of current through an electrode can alter the potential, such effects are minimized for the reference electrode in the three electrode system by a) having a high input impedance for the reference electrode (thereby decreasing the current passing through the reference electrode to negligible levels) and b) using a non-polarizable electrode as the reference electrode (i.e., the passage of small currents does not alter the potential).
These reference electrodes are similar, and consist of a redox reaction between a sparingly soluble chloride and the metallic element in an aqueous chloride solution. They can be used interchangeably, BUT it is extremely important to specify which is used, since their potentials are different (e.g., the potential of the BASi® silver/silver chloride reference electrodes is -35 mV relative to the saturated calomel electrode). Since potential values are relative to the reference electrode, failure to specify the reference electrode makes any quoted potential values meaningless.
The salt solution required for a reference electrode must be separated from the analyte solution by a frit that allows ionic conduction between the two solutions, but does not allow appreciable contamination of the analyte solution by the reference electrode solution (or vice versa). In the BASi® electrodes, this frit is made of either a ceramic material (RE-4 and RE-6 electrodes for aqueous solutions) or of porous CoralPor(TM) (RE-5 or RE-5B for either aqueous or non-aqueous solutions). Typically, the solutions separated by the frit do not contain the same ions, and the different rates of diffusion across the frit by the different ions gives rise to a potential across the frit - the junction potential. This is a further contribution to the potential between the working and reference electrodes. Since the junction potential is different for solutions of different ionic compositions, strictly speaking, redox potentials measured in different solutions (e.g., different organic solvents) cannot be compared directly, and an internal standard is required.
The potential of a reference electrode varies with temperature (typically 0.5 - 1.0 mV/oC). Therefore, precise measurement of redox potentials requires the use of a constant temperature bath for the cell. The potentials of the silver/silver chloride and calomel reference electrode are also affected by the concentration of chloride in the electrode solution, which must therefore be maintained at a constant value by proper storage.
Since the potential of a chloride-containing reference electrode is sensitive to chloride concentration, the electrode must be stored with the frit immersed in a solution identical in composition and concentration to the reference electrode solution (e.g., 3 M sodium chloride for the BASi® silver/silver chloride reference electrode). Since this solution can corrode the electrode connectors, the electrodes must be stored in an appropriate storage vial (MR-5275) that protects the connectors from the solution. When BASi® reference electrodes are shipped, the frit is covered with yellow plastic to maintain electrode integrity during shipping. This plastic should be carefully removed upon receipt of the electrodes, which should then be stored in the appropriate solution. During shipping, air bubbles can become lodged at the inside of the CoralPor(TM) tip. These must be dislodged (by flicking the end of the electrode) before the electrode can be used, otherwise artifacts (e.g., excessive noise) may appear in the experimental data.
Aqueous reference electrodes can be used in non-aqueous solutions in many instances, but problems can arise. First, junction potentials can be quite large for non-aqueous solutions, so comparison of redox potentials between aqueous and non-aqueous solutions (and between different non-aqueous solutions) requires an internal standard. Second, salts from the electrolyte solutions can precipitate in the frit, leading to increased noise in the current response. For example, if a perchlorate salt is used in the analyte solution, and a potassium solution is used in the reference electrode, potassium perchlorate can precipitate in the frit. This problem is decreased in BASi® reference electrodes by using sodium chloride in silver/silver chloride reference electrode, since sodium perchlorate is more soluble than potassium perchlorate. Third, since water and chloride ions can diffuse through the frit into the analyte solution (albeit slowly), aqueous reference electrodes are not suitable for water and chloride sensitive analytes.
If contamination by water from aqueous electrodes is a problem, there are a number of alternatives. The simplest is to use a salt bridge (MF-2030) containing the anhydrous electrolyte to separate the aqueous reference electrode from the analyte solution. Other alternatives include using a non-aqueous reference electrode or a pseudo-reference electrode. The BASi® non-aqueous reference electrode (MF-2062) requires user assembly, and consists of a silver wire immersed in a solution containing silver nitrate (0.001 - 0.01 M) dissolved in a solution of an appropriate electrolyte. Ideally, this electrolyte is the same as that used for the analyte (to eliminate junction potentials), but not all organic solvents are suitable. (Acetonitrile, DMSO, methanol, ethanol, and THF are suitable, whereas DMF and chlorinated solvents are not.) If the analyte electrolyte is not suitable, an acetonitrile-based electrolyte can generally be used. The potential of the non-aqueous reference electrode depends on the solvent, the electrolyte, and the concentrations of silver nitrate and salt. Since the potential of a non-aqueous reference electrode can vary among different electrodes, redox potentials measured using such a reference electrode should be quoted relative to an internal reference compound (e.g., ferrocene). A pseudo-reference electrode is simply a platinum or silver wire immersed in the analyte solution. This has the advantage that there can be no contamination of the analyte, but the disadvantage is that the reference potential is unknown, as it is dependent on the composition of the analyte solution. Therefore, redox potentials measured using a pseudo-reference electrode should again be quoted relative to an internal reference compound such as ferrocene.
The auxiliary electrode is typically a platinum wire that provides a surface for a redox reaction to balance the one occurring at the surface of the working electrode, and does not need special care, such as polishing. To support the current generated at the working electrode, the surface area of the auxiliary electrode must be equal to, or larger than, that of the working electrode. Three auxiliary electrodes are available from BASi: two are straight platinum wires for use with stationary solution voltammetry experiments, and the other (MW-1033) is a longer platinum coil used for experiments that generate larger currents, such as rotating disk voltammetry and bulk electrolysis. One of the platinum wire electrodes (MW-4130) should be used with C-1 Cell Stands, the VC-2 Cell (MF-1052), the MicroVolume Voltammetry Cell (MF-1065), and the C-2 Low Volume Cell (MF-2040), whereas the other (MW-1032) should be used with the C-2 and C-3 Cell Stand.
During any electrochemical experiment, a redox reaction occurs at the surface of the auxiliary electrode (to balance the redox reaction at the surface of the working electrode), and the products of this reaction can diffuse to the working electrode and interfere with the redox reaction occurring at that site. However, in electroanalytical experiments such as cyclic voltammetry, the time scale of the experiment is too short for this diffusion to be able to cause significant interference, so there is no need to place the auxiliary electrode in a separate compartment. However, electrosynthetic (bulk electrolysis) experiments are typically much longer than electroanalytical experiments, so separation of the auxiliary electrode is required (see, e.g., the BASi® bulk electrolysis cell (MF-1056)).