The ultimate goal of digital simulation of cyclic voltammetry is to be able to generate a simulated cyclic voltammogram that matches an experimental cyclic voltammogram. Although DigiSim® can be used for this purpose, it is important to note that considerable input from the user is required. The user must provide the mechanism and initial estimates of the parameter values, and, once a good match between the simulated and experimental data has been obtained, the user must also determine whether the optimized parameter values are reasonable; that is, are they consistent with previously measured values?
It is important to note that there are two functions that are used to examine experimental data files in DigiSim. The Import function (in the File menu) is used to display experimental data files in DigiSim (e.g., for visual comparison with simulations). The Select Files function (also in the File menu) is used to read experimental data files into DigiSim to be used in the Fitting routine (which optimizes the values of user-selected parameters to give the best fit of the simulated data to the experimental data).
Data from BASi (.txt), EG&G (.dat), and Cypress (.dat) electrochemistry software can be read by DigiSim. In addition, there is the DigiSim text format, with the extension .use. Experimental data from other sources can be converted to any of the above formats and read into DigiSim, subject to three constraints:
One advantage of using the .use format is that additional information about the experiment (e.g., the electrode surface area, concentrations of redox species) can be included in the header, and these values can be read by DigiSim. Existing data files (either simulated (.cvs) or experimental) can be converted to the .use format using the Export function in the File menu.
One method for optimizing parameter values is to vary the parameters manually. The procedure for this method is as follows:
The above method can be automated to a certain degree using the Fitting routine, which optimizes selected parameters. This routine also allows more than one experimental voltammogram to be fitted simultaneously. The procedure described below first generates and saves two simulated data files (a - c), and then uses these files as model files to illustrate how the Fitting routine is used (d - f). When fitting experimental files, only steps d - f should be used.
There are three points to note:
i) DigiSim will read the scan rate and concentrations from the experimental data files (provided that they have been included in the header, as is the case for the.use files). However, one non-zero concentration value must be entered.
ii) The parameters to be optimized are selected by clicking the box to the left of that parameter. In this example, Eo and ks are to be optimized. When a box is checked, a Fitting Range dialog box will appear; click Select.
iii) The optimized simulations are saved as .cvs files, and the optimized parameter set is saved as optimal.cvs (note that ONE parameter set is used to fit ALL the selected experimental data files).
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