BAS 100B/W - Impedance (IMP)

10-67
10.9.4 A.C. Impedance (IMP)
(N.B. The
IMP
technique requires connection of the BAS 100B/W with the BAS
Impedance Module)
In voltammetric techniques, the potential is varied over a specific range and the
current response is monitored. In any one experiment, the timescale remains constant
(e.g. the scan rate). In
IMP
experiments, the potential is held at a fixed value (e.g.,
the redox potential), and a small amplitude A.C. potential is applied. Within one IMP
experiment, a range of A.C. frequencies can be applied, so electrochemical and other
physical processes with differing timescales can be detected within one experiment.
The range of frequencies available on the BAS 100W is 0.1 mHz to 1 kHz (i.e., the
timescales that can be detected are between 10000 sec and 1 msec).
If the frequency range were measured one frequency at a time, the experimental time
would be very long. This can lead to problems with the system stability; that is, the
nature of the system may change significantly during the experiments. It is more
efficient to measure many frequencies simultaneously. On the BAS 100B/W, the
frequencies can be measured 1 or 2 decades at a time; since the quality of the data is
substantially worse when using 2 decades, measurement of 1 decade at a time is
generally recommended. Even when measuring 2 decades simultaneously, the time
required for the lowest frequency ranges may still be too long when compared to the
stability of the system.
The BAS 100B/W uses a Fourier method for both waveform generation and data
analysis. In the frequency domain, many frequencies of equal amplitude and random
phase are chosen such that the ratio of one frequency to the next is approximately the
same (Figure 10-43). This yields a relatively equally spaced frequency set in which
no frequency is a harmonic of another. The frequency components for one decade
are: 12, 15, 19, 23, 28, 35, 44, 55, 73 and 95 (ratio between adjacent frequencies is
1.25). Note that the power line frequencies of 50 and 60 Hz are avoided. This
frequency set is transformed into a digital representation of a time domain waveform,
which is applied to the cell via a D/A converter and analog filters. This procedure can
be repeated for many cycles to improve the signal to noise ratio. For high frequency
decades, the cycle time is relatively short, so many cycles can be run without
significantly increasing the experimental time. However, at lower frequencies,
increasing the number of cycles may not always be possible due to system instability.
The current response is monitored by a current transducer whose sensitivity range is
automatically adjusted for each frequency range examined. When each frequency
range is completed, the data is analyzed by a Fourier Transform algorithm.