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Bulk electrolysis

    '''Bulk electrolysis''' is also known as also known as ''potentiostatic coulometry'' or ''controlled potential coulometry''Bard AJ; Faulkner LR '''Electrochemical Methods: Fundamentals and Applications''' New York:: John Wiley & Sons 2nd Edition '''2000'''Skoog DA; West DM; Holler FJ '''Fundamentals of Analytical Chemistry''' New York:: Saunders College Publishing 5th Edition '''1988''' The experiment is a form of coulometry in which generally employees a three electrode system controlled by a potentiostat In the experiment the a 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

    Fundamental relationships

    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 the product of the electrolysis can be isolated This is most convent when the product is neutral and can be isolated from the electrolyte solution through extraction or when the products plates out one the electrode or precipitates in another fashion Even if the product can not be isolated other analytical techniques can be preformed on the solution including NMR EPR UV-Vis FTIR among others 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 containing the auxiliary and working electrode The other contains the reference electrode Strictly the reference electrode does not require a separate cell A Quasi-Reference Electrode such as a silver/silver chloride electrode can be exposed directly to the analyte In such situations there is concern that the analyte and trace redox products may interact with the reference As a result it is commonly sequestered in its own cell as many of the the more complex reference electrodes The more complex references such as standard 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 preformed in a three part cell in which the auxiliary electrode and reference electrode each has it own cell which connects to the cell containing the working electrode This is intended to isolate the redox events taking place at the auxiliary electrode During a bulk electrolysis the analyte undergoes a redox event at the working electrode If the system was open than it would be possible for the product of that reaction to defuse back to the auxiliary electrode and undergo the inverse redox reaction In addition to maintain the proper voltage at the working electrode the auxiliary electrode will can experience extreme potentials often oxidizing or reducing the solvent or electrolyte to balance the current In voltammetry experiments the currents are small and it is not a problem to decompose a small amount solvent or electrolyte In contrast a bulk electrolysis involves a significantly greater amount of current which would decompose an significant amount of the solution/electrolyte probably boiling it in the process To mitigate this challenge the auxiliary cell will often contain a sociometric or greater amount of sacrificial reductant (ferricene) 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 the working electrode This is in efforts to prevent “hot spots” Hot spots are a 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

    Rates and kinetics

    The rate of such reactions/experiments is not determined by the concentration of the solution but rather the mass transfer of the substrate in the solution to the electrode surface Rates will increase when the volume of the solution is decreased the solution is stirred more rapidly or the area of the working electrode is increased Since mass transfer is so important the solution is stirred during a bulk electrolysis However this technique is generally not considered a hydrodynamic technique since a laminar flow of solution against the electrode is neither the objective or outcome of the siring
    Bulk electrolysis is occasionally cited in the literature as means to study electrochemical reaction rates However it is generally a poor method to study rates since the rate of bulk electrolysis is generally governed by the specific cells ability to preform mass transfer Rates slower than this mass transfer bottleneck are rarely of interest

    Efficiency and thermodynamics

    Electrocatalytic analyzes will often mention the current efficiency of a given process based on a bulk electrolysis For example if one molecule of hydrogen results form ever two electrons inserted into an acidic solution than the current efficiency would be 100% This indicates that the electrons did not ended up preforming some other reaction For example the oxidation of water will often result in oxygen as well as hydrogen peroxide with each product displaying a specific current efficiency for a given experimental arrangement
    Nor is current efficiency the same as thermodynamic efficiency since it never address the how much energy is in the electrons added or removed The voltage efficiency determined by the reactions overpotential is more directly related to the thermodynamics of the electrochemical reaction In fact the extent to which a reaction goes to completion is related to how much greater the applied potential is than the reduction potential of interest In the case where multiple reduction potentials are of interest it is often difficult to set an electrolysis potential a "safe" distance (such as 200 mV) past a redox event The result is incomplete conversion of the substrate or else conversion of some of the substrate to the more reduced form This factor must be considered when analyzing the current passed and when attempting to do further analysis/isolation/experiments with the substrate solution

    References