Gamry Reference 600+, Руководство оператора

Страница: 66 / 87

60
current are DC and can be tuned out in impedance measurements. As a general rule, the DC leakage
should not exceed the measured AC signal by more than a factor of 10.
The Reference 600+ uses an input amplifier with an input current of around 1 pA. Other circuit
components may also contribute leakage currents. You therefore cannot make absolute current
measurements of very low pA currents with the Reference 600+. In practice, the input current is
approximately constant, so current differences or AC current levels of less than one pA can often be
measured.
Voltage Noise and DC Measurements
Often the current signal measured by a potentiostat shows noise that is not the fault of the current-
measurement circuits. This is especially true when you are making DC measurements. The cause of the
current noise is noise in the voltage applied to the cell.
Assume that you have a working electrode with a capacitance of 40 µF. This could be represented by a 1
cm
2
polished bare metal immersed in an electrolyte solution. A rough estimate of the capacitance of the
electrical double-layer formed by a metal/electrolyte interface is 20 µF/cm 2 . The area is the microscopic
area of the surface, which is larger than the macroscopic geometric area, because even a polished surface
is rough. The impedance of this 40
 F electrode, assuming ideal capacitive behavior, is given by:
Z = 1/ j

C
At 60 Hz, the impedance magnitude is about 66

.
Apply an ideal DC potential across this ideal capacitor and you get no DC current.
Unfortunately, all potentiostats have noise in the applied voltage. This noise comes from the instrument
itself and from external sources. In many cases, the predominant noise frequency is the AC power-line
(mains) frequency.
Assume a realistic noise voltage,
V
n
, of 10 µV (this is lower than the noise-level of most commercial
potentiostats). Further, assume that this noise voltage is at the North American power-line (mains)
frequency of 60 Hz. The noise voltage creates a current across the cell capacitance:
I
=
V
n
/
Z
 10 × 10
– 6
/66  150 nA
This rather large noise current prevents accurate DC current measurement in the low nA or pA ranges.
In an EIS measurement, you apply an AC excitation voltage that is much bigger than the typical noise
voltage, so this is not a significant problem.
Shunt Resistance and Capacitance
Non-ideal shunt resistance and capacitance arise in both the cell and the potentiostat. Both can cause
significant measurement errors.
Parallel metal surfaces form a capacitor. The capacitance rises as the metal ’s area increases and as the
separation distance between the metals decreases.
Wire and electrode placement have a large effect on shunt capacitance. If the clip leads connecting to the
working and reference electrodes are close together, they can form a significant shunt capacitor. Values of
1 to 10 pF are common. This shunt capacitance cannot be distinguished from
“r eal ” capacitance in the