72
eDAQ Potentiostats
idealised curve. The Curve Fit analysis extension and accompanying
documentation can be downloaded free from the eDAQ web site,
www.eDAQ.com. The extension file should be put into the Chart
Extensions folder, inside the Chart folder on your hard disk.
It is unlikely that the current signal will decay to exactly zero (due to
convection currents in the vessel causing non-ideal diffusion) so a small
non-zero constant,
b
, is added. Also the equation is very sensitive to the
exact instant when
t
is zero which is a problem if you have selected
data very close to zero time (in the Curve Fitting window, it is usually
best to use the ‘t=0 at start of block’ option as shown in
).
In this case, to get a well behaved equation when the data selected
start at very close to zero time, initially add a small time correction,
c
.
This allows for the errors in the estimation of time zero, and the fact that
the Potentiostat has a finite bandwidth (and so the current signal will
take a small amount of time to register a true value after a sudden
change in potential).
Thus a better form of the equation to be fitted is:
This must be entered as a New Equation in the Curve Fit extension the
very first time you use it,
.
Because of the discontinuity that occurs when the denominator is zero,
first try fixing
c
to a small positive value, and iterating for better values
of
a
and
b
. Some trial and error will be involved while you make
guesses that are suitable for the initial values to iterate. After the better
estimates for
a
and
b
have been determined, and iterated,
c
can be
unlocked so that it refines during the next iteration,
Your data selection should not include points very close to zero time
(the Potentiostat will require some time to settle), nor points taken at
very long times when convection aided diffusion will produce non-ideal
behaviour. The exact time period that is suitable will depend on the
size of the electrode, and some trial and error to determine the time
interval to give best results will probably be required.
After the file is saved, the Curve Fit Cottrell equation will be available
whenever you open Chart data file on the same computer or even when
you transfer the data file to another computer on which Chart software
i
a
t c
+
--------------
b
+
=
Summary of Contents for Picostat
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