Two types of simple potentiostat circuits


If you want to design a simple circuit where you program the voltage between two electrodes, viagra the Working Electrode and the Counter Electrode, cialis sale two solutions come to mind: a current follower (Fig 2) and using a sense resistor (Fig 3). The easier solution seems to be the current follower as you only need two op-amps. The idea here is that you buffer the input voltage to the WE side of the electrode-electrolyte-electrode cell and then keep the CE side at ground potential and track the current required to do so across Rf. If you combine figures 1 and 2 and assume that the WE is just an input voltage, illness you can see that the circuit is a high pass filter. As the frequency goes up, Cd (the electrode double layer capacitance) will have a lower and lower impedance and will result in a circuit whose gain is Rf/Ru (the uncompensated solution resistance). At the same time, as the frequency goes up, the output impedance of the op-amp also goes up effectively increasing Rf. Since the sweep rate used in cyclic voltammetry is slow, usually triangular wave at 50mV/s sweep, any high frequency noise sources can easily have higher gain than the signal we are looking for and give us poor recordings. Depending on the situation, this can lead to resonance which can totally corrupt the data.

On the other hand, the circuit in figure 3 can be used to avoid this problem. Since both WE and CE are driven with voltage followers, the feedback path for CE is low impedance and will be lower than Ru for reasonable frequencies giving us a stable system. The downside is that the applied potential is actually reduced due to the voltage drop across Rsense. We can calculate the applied potential by subtracting the measured potential across Rsense and be done with it, but this will not guarantee that our sweep rate is actually 50mV/s. This can become important when dealing with electrochemistry in sparse solutions or a medium with low ionic permeability. A solution to this problem would be to introduce a feedback system to vary WE or CE inputs according to the potential drop across Rsense, but that would just bring us back to the same feedback problem in the above paragraph.

Given our desire to only do cyclic voltammagrams in this situation, a good solution is to introduce a feedback capacitor in parallel with the feedback resistor in figure 2 to give the overall transfer function a low-pass shape. The justification for this is that if we scan, for example, from -0.6V to +0.6V (a reasonable value for iridium-oxide electrode v.s. silver/silver-chloride) at 50mV/s, we get a very low frequency of about 0.022Hz. The first harmonic past the base frequency is already 20dB below the base frequency power, so we can use the 22mHz to determine the impedance of our Cf and make sure it is high enough so that it doesn’t affect the effective Rf. I ended up using an Rf of 4.7KOhm and Cf of 100nF. Given these values and schematics, I was able to design a very simple potentiostat that had modest noise values and adequate performance for the job.

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  1. Pingback: μblog: engineering from the trenches » A simple, three electrode potentiostat design

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