Digisim® Tutorials

4. CV -the Movie™

There are two possible output displays in DigiSim®. The first is the basic current vs. potential curve, which is generated using Simulation in the Run menu. The other is the dynamic display of the concentration profiles (i.e., the concentration as a function of the distance from the electrode surface) of all the species involved in the proposed mechanism as they change during the potential scan. This is known as CV - the MovieTM.

 

CV - the Movie frame


Figure 1. A 'frame' from CV - the Movie.

 

A typical CV - the Movie frame is shown in Fl. The y axis represents concentration, and the x axis represents distance from the electrode surface (which lies at x = 0). The most important parameter that affects CV - the Movie is the rate of expansion of the exponential grid that is used in the calculations. This rate of expansion is controlled by the Expanding grid factor (in the Model Parameters dialog box). The 'frame' shown in Fl was run using the default value of 0.5. This should be compared with the 'frame' obtained when using a value of 0.2 (F2).

 

CV - the Movie frame


Figure 2. A 'frame' from CV - the Movie with Expanding grid factor = 0.2.

 

It is apparent that decreasing the value of the Expanding grid factor makes the concentration profiles smoother, especially at the tail. However, it also decreases the speed of the Movie, but the size of the decrease is typically insignificant on most Pentium PCs. Indeed, the default speed of CV - the Movie is generally too fast. However, it can be slowed down by adjusting the Movie Speed in the Movie-Display section of the Preferences dialog box in the View menu (F3). The Set Breakpoint function available in this dialog box enables the user to stop the Movie at a preset potential (e.g., to illustrate the Nernst equation).

 

Preferences dialog box


Figure 3. Movie-Display section of the Preferences dialog box.

 

The Zoom function in the View menu (or on the Tool bar)can be used to expand both axes (F4). This is particularly useful when the reaction layer (i.e., the layer of solution next to the electrode surface in which the chemical reactions occur) is thin, or when there are large differences between the concentrations of the species involved in the reaction. The expanded axes are used when restarting the Movie unless the Unzoom function has been performed.

 

Expanded CV - the Movie frame


Figure 4. An expanded 'frame' from CV - the Movie (from F2).

 

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Copyright 1995-2000, BASi® E'CHEM Electrochemical Products Group

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