Potentiostat

A potentiostat is a commonly used in electrochemistry device. In the simplest case, it acts as a precision DC voltage source, or as a voltmeter or ammeter. Compared with ordinary DC power sources has a potentiostat the following advantages:

• A desired voltage is also used in alternating current load - often fluctuate around current levels several decades, for example, from 100 mA to 1 uA - kept constant.
• The currents are measured in full scale precision.
• The voltage measurement in the potentiostat is practically dead; This also applies, if the potentiostat is used as voltmeter.
• It can also serve as ammeter, its internal resistance is zero: there is no voltage in the device, since the current is actively controlled.
• A significant difference from other DC power sources is the ability to use the device with three or more electrodes. Many measurements in electrochemistry is carried out with three electrodes: The potential of the electrode to be tested (working electrode) is measured relative to the reference electrode while the current flowing to the counter electrode.

• In addition to a constant voltage potentiostat can also perform voltage ramps usually, for example, for the cyclic voltammetry.

Some of the DC voltage potentiostat can be superimposed on an AC voltage with low amplitude, so that impedance spectroscopy is possible.

The main applications are for potentiostat electrochemical studies. These include not only the investigation of chemical reactions caused by the electrical current ( in the electrolysis ) and produce a current ( in batteries or fuel cells), but also the characterization of the electrode and electrolyte ( ionically conductive liquid or solid state ), for example in the analysis.

Operation

A potentiostat is an electronic control amplifier to which the potential, ie the voltage of one electrode relative to a reference point is adjusted to a desired value. For this purpose, the current to be regulated between the electrode ( working electrode ) and an auxiliary electrode (counter electrode) is controlled by the potentiostat so that the desired potential is reached. A third electrode ( reference electrode ), the potential of which is defined in the electrochemical series, here represents the reference point

Thus, the reference electrode maintains unchanged its potential, it is necessary that by themselves, no current flows; this is ensured by the potentiostat.

Another distinctive feature compared to other variable-gain amplifiers (eg PID - controllers) is that on the one hand a very high gain is needed to keep the deviation low: Demanded are accuracies of 1 mV or less. On the other hand, it should be very fast control, typical control time constants are 10 microseconds or less. The control path between the working electrode and the counter electrode is a complex structure consisting of resistors and capacitors, which can itself change rapidly over time. Must be able to compensate such potentiostat unknown loads without becoming unstable, ie to vibrate.

Practical implementation

Simple potentiostat can be produced comprising an operational amplifier, whose non-inverting input of which the desired potential is supplied as a voltage with respect to ground. To the inverting input of the potential of the reference electrode is fed. The working electrode is then connected to ground, the opposing electrode to the output of the operational amplifier. A reference electrode is connected to the inverting input. The amplifier controls the output current in such a way that the inverting input at the same voltage level as the non-inverting input, so that the desired potential is adjusted.

Embodiments

Potentiostat are commercially available in a variety of outfits. Most devices can now be controlled with a PC, some potentiostat even be used as a plug in the measurement computer. Many potentiostat have analog outputs which make it possible to record the current and voltage to a chart recorder or storage oscilloscope. The most important distinguishing features between the models are the available current and voltage ranges substantially. Typical current strengths in non-industrial applications range from a few nA to a few A. The potential control is usually in the range / - 10 V, a few devices can regulate potentials to about 50 volts. The counter electrode voltage is usual devices between / - 10 and / - 50 V, for poorly conducting electrolytes devices with control voltages above 100V are offered.

Technical differences exist both in the real potentiostatic control circuit as well as in the nature of the current measurement. Thus, e.g. either the working electrode or the reference electrode may be referenced to ground. If the working electrode with respect to ground, there are advantages of simple structure and high stability against vibrations. If the reference electrode with respect to ground, one can combine several working electrodes in a common vessel independently operate (Bi- potentiostat ).

The measurement of current is carried out in the simplest case, counter- electrode circuit. If you measure the current through a resistor in the working electrode circuit, an additional module ( differentiator ) has to correct the current measurement by the evoked potential errors. This type of current measurement is preferably used for devices that are designed to provide high currents. A variant of the current measurement is to use a zero -ohm ammeter in the working electrode circuit. Although this variant is technically complicated, but it can measure currents up to the area pA and below.