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Bulk Electrolysis and Controlled Potential Electrolysis


Electrochemical techniques can be divided into two types: those that attempt to change the composition of the bulk solution and those that are limited to the diffusion layer. The first class of techniques, also called bulk electrolysis (or coulometry), is intended for preparative-scale reactions. For bulk electrochemical techniques, electrochemical cells must possess a large ratio of electrode area (A) to solution volume (V) and effective mass transfer, where convection and cell design play a very crucial role [1-2].

The second class of techniques is electroanalytical techniques such as cyclic voltammetry (CV) that typically employ small area-to-volume (A/V) conditions and do not change the bulk concentrations.

Occasionally, bulk electrolysis is referred to as electrolysis. It is the aim of bulk electrolytic experiments to generate a quantitative conversion by electricity, such that the amount of substrate consumed is directly proportional to the total consumed charge (measured in Coulombs). As such, bulk electrolysis is also known as coulometry. By definition and operation, bulk electrolysis is an electroanalytical technique that drastically affects the bulk concentration, as opposed to most other electroanalytical techniques in which the amount of substance consumed or produced is insignificant.

Bulk electrolysis has the following features: (a) massive counter and working electrodes, typically a hundred times larger than regular electrodes as those used in CV, (b) high currents in the order of milliamperes as opposed to microamperes or even nanoamperes in non-bulk electrolytic processes, and (c) stirring solution (usually obtained by convection, which is one of the mass transfer modes).

In terms of current and potential electrical parameters, there are four modes for bulk electrolytic reactions: three electrode controlled potential electrolysis, three electrode constant current electrolysis, two electrode controlled potential electrolysis, and two electrode constant current electrolysis.

Aside: coulometry vs. electrolysis

Coulometry, and electrolysis uses the same cell setup. It is possible to perform electrolysis with or without measuring how much charge is transformed, and coulometry determines how much charge is transferred during an electrolysis reaction, which is proportional to the amount of matter that is transformed. Consequently, coulometry and electrolysis can both be used interchangeably [4].

Basic Theory


How does bulk electrolysis work?

Bulk electrolytic reactions can either be carried out directly at the electrode surface or indirectly through the use of a mediator. In direct electrolysis, electrons are transferred between a substrate molecule and an electrode, while in indirect or mediated electrolysis, there is electron transfer between a redox mediator (sometimes also known as catalyst) and an electrode whereby the electrochemically-generated mediator undergoes a chemical reaction with a substrate to produce the product.

In order to be effective, a mediator should be stable in various oxidation states for a sufficient period of time. Meanwhile, the overpotentials of heterogeneous electron transfer from the electrode from/to the mediator and the kinetic barriers involved in the homogeneous electron transfer from/to the start materials must be low [3].

Bulk electrolysis can be performed in various ways. However, the fundamental principle of all bulk electrolytic processes is the same, with all bulk electrolytic processes employing and/or obeying Ohm's Law.

Bulk electrolysis typically requires a great deal of current flow. Redox reactions occur at the working electrode of a bulk electrolytic cell that includes a counter (or auxiliary) electrode, a reference electrode, and a working electrode. While a significantly high current flows through the working electrode, an enormously great potential is being maintained at the counter electrode, thus balancing the current. Within the bulk electrolytic cell setup, this current results in either the oxidation or reduction of the solvent or electrolyte contained within the cell.

The counter electrode and working electrode of a bulk electrolytic setup are sufficiently separated to prevent the by-products produced at the counter electrode from reaching the working electrode. In other words, the counter electrode and the working electrode (even though being of similar surface areas) are neatly kept in two separate cell compartments in a bulk electrolytic setup.

It is worth noting that bulk electrolysis is typically characterised by 100% current efficiency and a massive quantitative change in the oxidation state.

Modes


Controlled potential electrolysis

In this electrolysis mode, the working electrode is maintained at a constant potential to allow the quantitative reduction or oxidation of the analyte without also concurrently reducing or oxidising other species' in the solution. The current passing through the cell shows a direct proportional relationship to the concentration of the analyte, decreases as the reactants are depleted, and is negligible by the end of the reaction. Electricity is typically measured by electronic integrators. The area under the curve of a plot of current versus time shows how many coulombs were used.

Controlled potential electrolysis with two-electrode system

Constant or controlled potential electrolysis (CPE) or potentiostatic electrolysis with two-electrode systems is an electrolytic mode that makes use of a potentiostat and two electrodes, namely the working electrode and the counter or auxiliary electrode. The reference electrode has no influence whatsoever on the reaction in this mode.

In this electrolytic technique, there is maintenance of a constant cell potential, which enables the monitoring of the cell current throughout the entire reaction process. The working electrode and counter electrode potentials vary with time, as does the ohmic drop. Additionally, this operation mode does not allow individual adjustment of the working electrode. The setup for this technique is relatively inexpensive as a two-electrode system allows reactions to be powered by simple electric batteries or even portable power sources [4].

A good example of a reaction carried out using two-electrode CPE is the aminoxyl-catalysed electrochemical diazidation of alkenes [5].

Controlled potential electrolysis with three electrode system

Constant or controlled potential electrolysis (CPE) or potentiostatic electrolysis with three-electrode systems is an electrolytic mode that employs a potentiostat and three electrodes: the working electrode, the reference electrode, and the counter or auxiliary electrode.

In this electrolytic technique, the potential of the working electrode is kept constant or controlled (Ewe) between the working electrode and reference electrode. Furthermore, the current (I) flowing between the working electrode and counter electrode is continuously monitored throughout the entire experiment [4].

The cell potential value (Ecell) or compliance voltage of a potentiostat is a measure of the applied potential difference between the working electrode and counter electrode. Considering the potential needed for the counter electrode reaction, this cell potential differs from the potential difference between the working electrode and reference electrode (Ewe - Eref). Moreover, the current that flows through the solution between the working electrode and counter electrode suffers from high solution resistance, thereby resulting in Ohmic drop potential based on Ohm's Law.

Three-electrode CPE is best suited for electrolytic processes that involve selective oxidation of substrates or avoidance of the overoxidation of the products. The use of CPE with three electrodes is advantageous in electrocatalytic oxidation, where the substrate has a slightly higher redox potential than the catalyst [4].

An example of a CPE direct electrolytic reaction is the oxidation of vitamin E [6] while an example of a CPE mediated electrolysis reaction is alcohol oxidation catalysed by an aminoxyl radical under basic conditions [7].

Summarily, in CPE with three-electrode systems, the Ewe is fixed versus the reference electrode, whereas the Ewe - Eref value is held constant.

Constant current electrolysis

In this electrolytic modality, the current is held constant by an amperostat until an indicator signals completion of the analytical reaction.

Constant current electrolysis with a two-electrode system

Constant or controlled current electrolysis (CCE) or amperostatic/galvanostatic electrolysis with two-electrode systems is an electrolytic mode that makes use of an amperostat, a working electrode, and a counter or auxiliary electrode. In this electrolytic technique, a constant cell current is maintained, which enables the cell potential to be monitored throughout the entire reaction process.

It is not possible to determine the working electrode potential in this setup because both electrodes' potentials vary during the reaction. Due to the simplicity of the two-electrode setup combined with the fact that a potentiostat is not necessary, this method is the most widely adopted by most organic chemists for electrosynthesis. A cheap and easily available power supply that utilises direct current (DC), such as DC power sources routinely used for portable electronic devices, can be utilised for two-electrode CCE [4].

An interesting example of a reaction driven using two-electrode CCE is that involved in the synthesis of Alliacol A via the anodic cyclization-Friedel Crafts alkylation mechanism [8].

Constant current electrolysis with a three-electrode system

Constant or controlled current electrolysis (CCE) or amperostatic/galvanostatic electrolysis with three electrode systems is an electrolytic mode that makes use of a potentiostat and three electrodes.

In this electrolytic technique, a constant cell current is maintained between the working electrode and counter electrode with constant monitoring of the working electrode potential throughout the reaction course. The potentiostat changes the cell potential (V), including the potentials of both the working electrode and counter electrode in order to maintain a constant current.

Monitoring the potential of the working electrode is done with reference to the reference electrode, but the reference electrode has no influence whatsoever on the control function. As a result, this mode of electrolysis can also be referred to as constant current electrolysis with potentiometric analysis.

The calculation of the quantity of electricity consumed can be obtained using the relation, Q = I.t (Equation 5), where Q = electricity/charge consumed, I = current in amperes, and t = time for electrolysis to happen in seconds. With a constant current over time, the analysis is expedited. Hence, the typical analysis time for controlled current electrolysis is less than 10 minutes, compared with ~30-60 minutes for controlled potential electrolysis. Moreover, since there is a direct relationship between the total charge, current, and time, there is no need to integrate the current-time curve, which is a great advantage of this method.

Examples of direct- and mediated-CCE electrolytic reactions are the oxidation of carbamate and NHPI-catalysed electrochemical iodination of toluene derivatives [9].

On the basis of instrumentation, two-electrode systems are much simpler and less power demanding than three-electrode systems, thus, affording accelerated electrolysis.

  

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References


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Contributing Authors


  • Stephen O Aderinto
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