The potentiostat is an electronic circuit commonly used in electrochemistry. It is often (but not always) needed for the proper operation of electrochemical sensors.
Figure 1. Bank of four potentiostats retrofitted into an "electronic nose" instrument. The four electrochemical sensors at at bottom right, mounted in the manifold.
In electrochemical reactions, you generally either want to know, or to control, the potential across the double layer at the working electrode (WE). Directly measuring the potential of the electolyte side of the double layer is difficult. Most of the time, it is sufficient to measure the potential of the electrolyte using a well-behaved electrode called a reference electrode (RE). In order for the reference electrode to be well-behaved, that is, to maintain a constant potential over all conditions, no current should flow through it. For that reason, a third electrode is usually added, whose sole purpose is to conduct current into or out of the cell. This current has to exactly balance the current generated at the working electrode. The third electrode is called the counter electrode (CE). A cell of this kind is called a three-electrode cell.
Figure 2. Schematic symbol of a three-electrode cell, using conventional symbols.
The equivalent circuit of the electrochemical cell is shown below. The current generator responds to the analyte chemical reacting at the WE. The capacitor represents the double layer capacitance, which can be very large for some gas sensors. The resistor at the counter electrode represents the solution resistance between the counter electrode and the current source. There is another resistor in series with the reference electrode, although the reference electrode does not generally conduct current.
Figure 3. Equivalent circuit of the three-electrode cell. The WE capacitance often dominates.
With this background, it follows that a circuit that manages a cell of this kind, including a three-electrode electrochemical sensor, has to have special properties:
* It has to maintain a fixed potential between the WE and RE
* It is bipolar and will operate regardless of whether the current flows to or from the WE
* It must measure the current from the WE, delivering a usable signal to an output terminal.
Now we can look at the circuit for the potentiostat. There are two operational amplifiers in this simple circuit, with distinct functions. U1 is the current-measuring circuit. How does it do this?
(If you are not familiar with the operation of operational amplifiers, or op amps, read here.)
Recall that no current flows in or out of the inputs of an operational amplifier. Yet the working electrode is connected directly to the inverting input. This means that the current has to come through the feedback resistor Rf. Suppose, for example, a 1.0 microamp current was flowing into the op amp from the working electrode. If the feedback resistor is 1.0 megohm, then a voltage of +1.0 V would have to exist on the output of the op amp (E = I.R = 1.0 microamp x 1.0 megohm = 1.0 V). If the output voltage rises too high, the positive-going potential feeds back to the inverting input and forces the voltage back down; the reverse happens if the output falls too low. The output voltage is therefore exactly proportional to the current; the proportionality factor is the value of the feedback resistor R1 in ohms. Importantly, the inverting input is held very close to ground by the operation of the amplifier, so the working electrode appears to be connected to ground.
Figure 4. Basic potentiostat circuit.
It is important to remember that if the potentiostat power is turned off, this active forcing of the ground no longer occurs. Sensors may therefore require some time to come to equilibrium after being powered down for a time. High-surface area electrodes, such as those in a carbon monoxide sensor, may require 24 hours or more to stabilize.
The other op amp is responsible for the actual potentiostatic action. Let?s suppose, for example, that a potential of +0.100 V is applied to the bias input. In order for the output voltage of the op amp to be within a reasonable output range, therefore, the inverting input of the op amp has to be very close to the input bias voltage, +0.100. The inverting input is connected directly to the reference electrode, but draws essentially no current from it. The op amp delivers a voltage to the counter electrode that is sufficient to keep the electrolyte (and the reference electrode) at the preset bias voltage. Suppose that the reference electrode is driven to a voltage that is higher than the bias input. The inverting input is greater than the bias input, so the op amp output will decrease. The decreased voltage drives less current into the cell, and the electrolyte voltage falls. If the reference electrode voltage falls, the op amp raises the voltage of the electrolyte.
Imagine, now, that an oxidizable compound like carbon monoxide reacts as the working electrode. Electrons are deposited on the electrode, which tends to force the inverting input of U1 negative. A positive voltage appears at the output of U1, proportional to the oxidation taking place on the WE.
Meanwhile, the oxidation depletes the sensor of electrons, causing the electrolyte potential to increase. This forces the output of the op amp negative, driving electrons into the electrolyte through the CE. At equilibrium, the current into the cell through the CE is equal to the WE current, so no net charge can develop in the cell.