WheeStat User's Manual
What this page is about:
The WheeStat is a potentiostat, which is an instrument used to study electron transfer reactions between species in solution and electrodes. This instrument can be used to teach electrochemistry or to quantify the concentration of some analyte (such as a metal ion) in water. There is a PublicLab wiki page that discusses potentiostats here.
Other pages on the WheeStat:
The history of the project was first described here. Discussions of how to build the WheeStat hardware are described here and here. While the former note discusses how to build your own pcb, the latter tells how to order the pcb from OSH Park. Some early discussion on the software and graphic user interface are here and here. A discussion of making low cost electrodes is here. Use of the WheeStat and low cost electrodes in monitoring airborne hydrogen sulfide is here.
To run the WheeStat, you need to install the appropriate drivers on your computer and have the application file for the GUI installed.
Drivers: The WheeStat is based on the Stellaris or Tiva microcontroller development board from Texas Instruments. The first thing you will need to do is download and install drivers from Texas Instruments. Drivers for the In-Circuit-Debug-Interface (ICDI) can be downloaded from this web page found here. At the same web page is a link to instructions for installing the drivers on windows machines. I recommend you print out the instructions and have them handy when installing the drivers. GUI Application Files You should be able to get this software from our GitHub page (here. You will need the application file (.exe) and the folders containing the source code, the libraries, and the data files. These will all need to be in one folder on your computer.
While the files described above can be used to run the WheeStat, some users may wish to change the functionality of the microcontroller or GUI. In addition, if you are building the potentiostat yourself, you will need to install firmware on your microcontroller board. Microcontroller Code: The firmware was written using the program Energia, which can be downloaded from www.energia.nu. You will need the most recent version of the WheeStat source code and the altSPI library. As of this writing, the most recent version is WheeStat5_4a. You should be able to get this software from our GitHub page (here Let me know if you have trouble with this. I am a little slow on updating software.). Once you download the program, you will need to have the altSPI library (altSPI.h and altSPI.cpp files) in a folder called "libraries" inside your sketchbook folder. GUI Code: The Graphic user interface (GUI) was written using a program called Processing that can be downloaded from processing.org. Source code for the GUI can be downloaded from the same GitHub page described above (here). As of this writing, the most current version was WheeStat5_4.
Graphic User Interface:
The following video shows the setup of the GUI and some of it's functionality. It shows the expected behavior of the instrument, tested using a resistor of known value. The demonstration shows that Ohm's Law can be used to predict the current / voltage behavior of the voltage ramp experiment.
The WheeStat is programmed to run a number of experiments that are useful for a number of tasks. Below is a quick discussion of what each does and what it is used for.
Instrument Testing and Calibration: The first experiment in the dropdown list is called "ramp". It is a quick experiment that used to test how well the instrument is functioning. The voltage / time profile is shown in Figure 2, below and the experiment is demonstrated in the video demonstrating the GUI, above.
Experiments for Teaching Electrochemistry: 1. Chronoamperometry: . In single step chronoamperometry, the potential of the working electrode is stepped from its initial value to a final value and the current is sampled as a function of time. Typically, the initial voltage is established where the compound in question is stable and no current passes. The voltage is then stepped to a potential where the compound will either accept or lose an electron to give a second compound. To illustrate, we consider electrochemical response of the hexacyanoferrate ion (FeCN63-). In this experiment, the initial voltage was set to +600 mV, where the iron(III) compound is stable. At time = 0, the voltage was stepped to +100 mV. At this voltage, the iron(II) form of the compound is the stable species and the following half reaction occurs at the working electrode:
FeCN63- + e- <==> FeCN64- (1) Data from such an experiment employing the iron(III) compound, FeCN63- is presented in Figure 3.
The electrical current passed to the electrode is presented in Figure 3 as a positive current that decays with time. To understand current response using other voltammetric techniques, it is necessary to understand the origin and nature of this decay. Just after the potential step, there is a great excess of the iron(III) compound close to the working electrode, giving rise to a large current. As time evolves, the amount of iron(III) available for reaction diminishes and subsequently, the electrical current decays. The current / time profile in chronoamperometry experiments is governed by the rates at which compounds reach the working electrode. In an unstirred electrolyte solution, the current is determined by the compound diffuses. For a planar electrode under these conditions the current is described as a function of time by the Cottrell equation: I(t) = nFA(D/0.5C/(t)0.5 (2) Where I(t) is the observed current, n is the number of electrons transferred, F is the Faraday constant, A is the area of the electrode, D is the diffusion coefficient of the compound and t is the time. Note that all the terms in Equation 2 are constant with the exception of t. Thus, the current is expected to be proportional to time raised to the -1/2 power. Analysis of data (Presented in Figure 1) shows that this was the case.
1B. Double step chronoamperometry. Figure 2A shows the time / voltage profile for a double potential step experiment. In this experiment, the voltage is stepped from the initial to the second
Normal pulse voltammetry: 1C. Normal pulse voltammetry. For our discussion of chronoamperometry, we assumed that the iron(III) form of the compound was the only species stable at the initial voltage and the iron(II) form was the only one stable at the second voltage. At intermediate voltages, however, both oxidation states exist at equilibrium, with their ratio determined by the Nernst equation: E = E° + RT/nF ln([Fe3+]/[Fe2+]) (3) Where E is the applied voltage, E° is the standard reduction potential of the couple, and R is the ideal gas constant. Thus, Equation 3 tells us that the difference between the applied potential and the standard reduction potential of the couple determines the position of the equilibrium mixture of oxidized and reduced species. The relationship between the difference in potentials and the percentage of each species expected is presented in Figure _. If we start a single step chronoamperometry experiment at a voltage where only the iron(III) form is stable and step to a voltage where both the forms coexist, we expect the shape of the current profile to be similar to that in the above case, but at a lower total current, as the electrical current only passes to achieve the equilibrium condition, and not to total reduction of reactant. The normal pulse experiment illustrates how the chosen step potential affects the current profile. In this experiment, a series of voltage steps are generated at incrementing potentials, as shown in Figure 2A. Unlike the case for chronoamperometry, however, the current in normal pulse voltammetry is only sampled once per step, at fixed time after the step is initiated. The current sample times are illustrated as the red circles in Figure 3A. Results of this experiment are plotted as the measured current versus the voltage at which the current was measured. Figure 3B shows a screen shot of normal pulse data obtained using the same solution as that from Figures 1 and 2. Figure 3C shows an analysis of the data from figure 3B. In this figure, original data are presented as open diamonds and the model is shown as a solid blue line. The data were treated by assuming (1) a linear baseline drift (caused by uncompensated solution resistance and double layer capacitance), (2) that the solution conditions present at the start of the experiment were re-established by returning to the initial voltage for one second between voltage steps, (3) that the iron complex was the only electroactive species in solution, and (4) that Nernstian conditions were established and maintained for the duration of the pulses. As shown in Figure 2C, there was significant deviation of the data from the predicted behavior, as the transition from oxidized to reduced iron complex was not as steep as predicted by Equation 3.
Differential pulse voltammetry Potential sweep methods: Cyclic voltammetry
The following video demonstrates some of the experiments that the WheeStat is programmed to do: