10-60
10.9
Miscellaneous Techniques (BE,
ECM, HR, IMP)
10.9.1
Bulk Electrolysis with Coulometry (BE)
The principle behind the Bulk Electrolysis (
BE
) experiment is very simple. If only
the oxidized species is initially present, then the potential is set at a value sufficiently
negative to cause rapid reduction and is maintained at this value until only the
reduced species is present in solution. The total charge passed during the
BE
experiment (Q) is related to the number of electrons transferred per molecule (n) and
the number of moles of the oxidized species initially present (N) through Faraday's
law:
Q = nFN
where F is Faraday's constant (96500 C/mole). Therefore, if one of n or N is known,
the other can be calculated. Hence,
BE
has both analytical and synthetic applications.
The cell required for
BE
is significantly different to that required for voltammetry
experiments (in which only a very small fraction of the electroactive molecule of
interest is electrolyzed). The rate of electrolysis is enhanced by using a working
electrode with a large surface area (e.g., platinum gauze, reticulated vitreous carbon
or a mercury pool) and an auxiliary electrode with a large surface area (e.g., platinum
coil or gauze); in addition, the solution is stirred to increase the rate of mass transport
to and from the working electrode. The auxiliary electrode must be isolated from the
working electrode to prevent species that are electrogenerated at the auxiliary
electrode from interfering with electrolysis at the working electrode. However, care
must be taken when choosing the material used to isolate the auxiliary electrode from
the working electrode, since high resistance material may affect the efficiency of the
electrolysis.
Before a
BE
experiment can be run, the potential must be chosen. For a reduction, the
ideal potential is ca. 200 mV more negative than the redox potential (measured by
e.g.,
Cyclic Voltammetry) so that the rate of electrolysis is controlled by the rate of mass
transport to the working electrode. However, it is not always possible to use a
potential too far removed from the redox potential due to electrolysis of other
electroactive materials (e.g., electrolyte, solvent or other components of the solution
mixture).
When the
BE
experiment is running, a software generated clock on the PC screen
indicates the experiment run time. At each
Sample
Interval
, the current passed
during that interval and the total charge passed to that time are displayed on the
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