Bulk electrolysis
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Bulk electrolysis is also known as potentiostatic coulometry or controlled potential coulometry.[1][2][3][4][5] The experiment is a form of coulometry which generally employs a three electrode system controlled by a potentiostat. In the experiment the working electrode is held at a constant potential (volts) and current (amps) is monitored over time (seconds). In a properly run experiment an analyte is quantitatively converted from its original oxidation state to a new oxidation state, either reduced or oxidized. As the substrate is consumed, the current also decreases, approaching zero when the conversion nears completion.
The results of a bulk electrolysis are visually displayed as the total coulombs passed (total electric charge) plotted against time in seconds, even though the experiment measures electric current (amps) over time. This is done to show that the experiment is approaching an expected total number of coulombs.
The sample mass, molecular mass, number of electrons in the electrode reaction, and number of electrons passed during the experiment are all related by Faraday's laws of electrolysis. It follows that, if three of the values are known, then the fourth can be calculated. The bulk electrolysis can also be useful for synthetic purposes if the product of the electrolysis can be isolated. This is most convenient when the product is neutral and can be isolated from the electrolyte solution through extraction or when the product plates out on the electrode or precipitates in another fashion. Even if the product can not be isolated, other analytical techniques can be performed on the solution including NMR, EPR, UV-Vis, FTIR, among other techniques depending on the specific situation. In specially designed cells the solution can be actively monitored during the experiment.
Cell design
In most three electrode experiments there are two isolated cells. One contains the auxiliary and working electrode, while the other contains the reference electrode. Strictly speaking, the reference electrode does not require a separate compartment. A Quasi-Reference Electrode such as a silver/silver chloride wire electrode can be exposed directly to the analyte solution. In such situations there is concern that the analyte and trace redox products may interact with the reference electrode and either render it useless or increase drift. As a result, even these simple references are commonly sequestered in their own cells. The more complex references such as standard hydrogen electrode, saturated calomel electrode, or silver chloride electrode(specific concentration) can not directly mix the analyte solution for fear the electrode will fall apart or interact/react with the analyte.
A bulk electrolysis is best performed in a three part cell in which both the auxiliary electrode and reference electrode have their own cell which connects to the cell containing the working electrode. This isolates the undesired redox events taking place at the auxiliary electrode. During bulk electrolysis, the analyte undergoes a redox event at the working electrode. If the system was open, then it would be possible for the product of that reaction to diffuse back to the auxiliary electrode and undergo the inverse redox reaction. In addition to maintaining the proper current at the working electrode, the auxiliary electrode will experience extreme potentials often oxidizing or reducing the solvent or electrolyte to balance the current. In voltammetry experiments, the currents (amps) are so small and it is not a problem to decompose a small amount of solvent or electrolyte. In contrast, a bulk electrolysis involves currents greater by several orders of magnitude. At the auxiliary electrode, this greater current would decompose a significant amount of the solution/electrolyte and probably boiling the solution in the process all in an effort to balance the current. To mitigate this challenge the auxiliary cell will often contain a stoichiometric or greater amount of sacrificial reductant (ferrocene) or sacrificial oxidant (ferrocenium) to balance the overall redox reaction.
For ideal performance the auxiliary electrode should be similar in surface area, as close as possible, and evenly spaced with the working electrode. This is in an effort to prevent "hot spots". Hot spots are the result of current following the path of least resistance. This means much of the redox chemistry will occur at the points at either end of the shortest path between the working and auxiliary electrode. Heating associated with the capacitances resistance of the solution can occur at the area around these points, actually boiling the solution. The bubbling resulting from this isolated boiling of the solution can be confused with gas evolution.