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The Open Lead Curve is the instrumental
limit of the impedance measurement for your EIS System. If the
impedance of your sample is in the range of the Open Lead Curve, your EIS
instrument cannot make an accurate measurement.
This is a conceptually simple
measurement. However, you will need a bit of care and considerable time to make the
measurement properly.
You must use a Faraday Cage when
making this measurement. The ends of the cell leads on most potentiostats are not
shielded, so they must be enclosed in the Faraday Cage to prevent noise pick-up.
A Faraday Cage is a conductive
enclosure that surrounds the cell. The Faraday Cage can be constructed from sheet metal, fine
mesh wire screen, or even conductive plastic. It must be continuous and completely
surround the cell. Don't forget the areas above and below the cell. All parts of the
Cage must be electrically connected.
The Faraday Cage must be electrically connected to the potentiostat’s ground
lead.
Place the ends of the cell leads in the Faraday Cage. The leads must be well isolated from the walls of the cage. Teflon or air
are the only recommended insulators.
Connect the Reference and Counter
electrode leads together. This provides a feedback path for the potentiostat.
If your system includes both a Working and Working Sense lead, connect them together. Make sure that the
Working lead is
well separated from the Counter and Reference leads and the wall of the Faraday
Cage. A
separation of at least 5 cm is recommended.
Set up an EIS measurement from your
highest available frequency to a frequency of 5 mHz or lower. Use the excitation amplitude
that you expect to use with "real" cells. We recommend that you choose a DC
voltage of zero versus the reference electrode.
Be careful, the open circuit voltage
of the open leads is not defined. On some systems. you may have to add a resistor to the
cell to measure the open circuit voltage (Working and Working Sense on
one side of the resistor, Reference and Counter on the other side), pause the test, remove the resistor, and then
proceed to measure the open lead spectrum.
Run the experiment in the normal
manner. Try to avoid movement near the Faraday Cage while recording the curve. Even slight
movement of the cell leads can cause large apparent responses.
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