Gamry Reference 600+ - Leakage Currents and Input Impedance

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cell. If you are measuring a paint film with a 100 pF capacitance, 5 pF of shunt capacitance is a very
significant error.
Shunt resistance in the cell arises because of imperfect insulators. No material is a perfect insulator (infinite
resistance). Even PTFE, which is one of the best insulators known, has a bulk resistivity of about 10
12
Ω·m.
Worse yet, surface contamination often lowers the effective resistivity of good insulators. Water films can
be a real problem, especially on glass.
Shunt capacitance and resistance also occur in the potentiostat itself. The Reference 600+ Potentiostat
mode specifications in Appendix A contain equivalent values for the potentiostat’s R
shunt
and C
shunt
. These
values can be measured by an impedance measurement with no cell.
In most cases, the cell’s shunt resistance and capacitance errors are larger than those from the potentiostat.
Hints for System and Cell Design
The following hints may prove helpful.
Faraday Shield
A Faraday shield surrounding your cell is mandatory for very low-level measurements. It reduces both
current noise picked up directly on the working electrode and voltage noise picked up by the reference
electrode.
A Faraday shield is a conductive enclosure that surrounds the cell. The shield can be constructed from
sheet metal, fine-mesh wire screen, or even conductive paint on plastic. It must be continuous and
completely surround the cell. Don’t forget the areas above and below the cell. All parts of the shield must
be electrically connected. You will need an opening in the shield large enough to allow a cell cable to
enter the shield.
The shield must be electrically connected to the Reference 600+’s floating-ground terminal.
An additional connection of both the shield and the Reference 600+ floating ground to an earth ground
may also prove helpful.
Avoid External Noise Sources
Try to keep your system away from electrical noise sources. Some of the worst are:
• Fluorescent lights
• Motors
• Radio transmitters
• Computers and computer monitors
Try to avoid AC-powered or computerized apparatus within your Faraday shield.
Only connect the Reference 600+ ground to earth ground if all conductive cell
components are well-isolated from earth ground. A glass cell is usually well isolated. An autoclave is
generally not well isolated.