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212
CH Instruments Model 900D Series
A+B=C+D
(general reaction)
Mechanism Select
There are 10 pre-defined mechanisms covering the most commonly encountered reactions. The first item
is "User Input". Only Model 630 and Model 660 will accept the user defined mechanism.
Some mechanisms not listed can still be obtained from the pre-defined mechanisms. For instance,
EEC, ECC, and CEC mechanisms can be obtained from the ECEC mechanism by setting the
appropriate kinetic parameters to null. If you define the heterogeneous electron transfer rate ko to zero,
the corresponding electron transfer step will have no effect. If you define the forward and reverse rate
constants of a chemical reaction to zero, the corresponding chemical reaction will have no effect.
Clicking on a pre-defined reaction mechanism will automatically fill out the Mechanism Edit box. If "User
Input" is selected, the Mechanism Edit box will be empty.
System under study
A diffusive or adsorptive system can be simulated. For diffusive systems, planar diffusion is assumed.
For adsorptive systems, it is assumed that the adsorption obeys the Langmuir isotherm, and both
oxidized form and reduced form are strongly adsorbed.
Dimensionless current
When this box is checked, the program will calculate the dimensionless current, which is useful for
comparison with other theoretical predictions. When this box is not checked, current will be calculated
according to the concentration, electrode area, and time scale of the experiment.
Initial Concentration at the equilibrium
When this box is checked, the program will first calculate equilibrium concentrations and then use the
computed values as the initial conditions for the simulation. The equilibrium state is calculated according
to the kinetic parameters and input concentrations. When this box is unchecked, the program uses
input concentrations as initial condition.
Display concentration profile during run
When this box is checked, the program will display the concentration profile along with the
voltammogram during the simulation. This is very helpful to understand the reaction mechanism and is
ideal for teaching purposes.
For voltammograms, the current axis scale is determined by the sensitivity scale selected under the
Parameters command. If the current axis scale is too high, the voltammogram will appear as a flat line. If
the current axis scale is too low, the data points will be scattered everywhere. However, the post-run
data display will always autoscale the voltammogram to maximize readability. The sensitivity scale can
be adjusted for the next simulation run according to the post-run display.
For concentration profiles, relative concentrations and relative distances are used. The total
concentration of all involved species is set to unity by default, and all concentrations during simulation
are scaled relative to this value. The unit distance is set to 6*sqrt(Dt) by default, where D is the diffusion
coefficient and t the total time involved in the given experiment. To change the scales for concentration
and distance from these default values, see the two sections below.
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