Potentiostat

A potentiostat is a control and measuring device that, in an electrolytic cell, keeps the potential of the working electrode at a constant level with respect to the reference electrode. It consists of an electric circuit which controls the potential across the cell by sensing changes in its resistance, varying accordingly the current supplied to the system: a higher resistance will result in a decreased current, while a lower resistance will result in an increased current, in order to keep the voltage constant. It is a simple application of Ohm's law:$\{R\}\; =\; \{E\; over\; I\}$ As a result, the variable system resistance and the controlled current are inversely proportional:$I\_o=\{E\_c\; over\; R\_v\}$:*"$I\_o$" is the output electrical current of the potentiostat:*"$E\_c$" is the voltage that is kept constant:*"$R\_v$" is the electrical resistance that varies.

Basic principles

Since 1942, when Hickling built the first threeelectrodes potentiostat,cite significant progress has beendone as to improve the potentiostat capabilities.Hickling had the genius idea to automaticallycontrol the cell potential by the means of a thirdelectrode: the reference electrode. His principlehas remained the same until now.At a glance, a potentiostat measures the potentialdifference between the working and the referenceelectrode, applies the current through the counterelectrode and measures the current as an $i$ $R$ voltage dropover a series resistor ($R\_\{\; extrm\; m\}$ in Fig. 1).

The control amplifier CA is responsible to keep thevoltage between the reference and the workingelectrode as close as possible to the voltage of theinput source $E\_\{\; extrm\; i\}$. It adjusts its output toautomatically control the cell current so that thisequality condition is satisfied. To understand howit works we have to write down some equationsvery well known by electronics engineers.

Before going forward with maths note that from anelectrical point of view the electrochemical cell andthe current measuring resistor $R\_\{\; extrm\; m\}$ can be regardedas two impedances (Fig. 2). $Z\_1$ includes $R\_\{\; extrm\; m\}$ inseries with the interfacial impedance of thecounter electrode and the solution resistancebetween the counter and the reference. $Z\_2$represents the interfacial impedance of theworking electrode in series with the solutionresistance between the working and the referenceelectrodes.

The role of the control amplifier is to amplify thepotential difference between the positive (or noninverting)input and the negative (or inverting)input. This can be translated mathematically intothe following equation:

:$E\_\{\; extrm\{out=A,(E^+-E^-)=A,(E\_\{\; extrm\; i\}-E\_\{\; extrm\; r\})$. (1)

where $A$ is the amplification factor of the CA.At this point we should make the assumption thatno or only insignificant current is flowing throughthe reference electrode. This corresponds to thereal situation since the reference electrode isconnected to a high impedance electrometer.Thus the cell current can be written in two ways:

:$I\_\{\; extrm\; c\}=frac\{E\_\{\; extrm\{out\}\{Z\_1+Z\_2\}$, (2)

and

:$I\_\{\; extrm\; c\}=frac\{E\_\{\; extrm\; r\{Z\_2\}$. (3)

Combining Eqs. (2) and (3) yields Eq. (4):

: (4)

where is the fraction of the output voltage of thecontrol amplifier returned to its negative input;namely the feedback factor:

:.

Combining Eqs. (1) and (4) yields Eq. (6):

:. (6)

When the quantity $A$ becomes very large withrespect to one, Eq. (6) reduces to Eq. (7),which is one of the negative feedback equations:

:$E\_\{\; extrm\; i\}=E\_\{\; extrm\; r\}$. (7)

Eq. (7) proves that the control amplifier worksto keep the voltage between the reference and theworking close to the input source voltage.

In a potentiostatic coulometry experiment, the cathode is usually a platinum electrode with a large surface area or a mercury pool, although other particularly reactive electrodes can be used in rare circumstances such as Lithium counter electrodes in Li-ion battery studies.

In impedance measurements, generally a second equipment is attached to the potentiostat such as a function generator or frequency response analyzer. Some cases impedance measurement capability is built in to the potentiostat so that no need of a second equipment.

The functionality of a potentiostat can be extended by virtual instrumentation: in virtual instruments the potentiostat is used as an actuator.

ee also

*Amperostat
*Galvanostat

References

*|url=http://chemeducator.org/sbibs/s0011001/spapers/1110023dr.htm|format=|accessdate=2008-10-06
*citation | last = Staicopoulos | first = D. N. | year = 1961 | title = High Current Electronic Potentiostat | journal = Review of Scientific Instruments | volume = 32 | pages = 176 | doi = 10.1063/1.1717304

External links

*Genady Ragoisha (webmaster), " [http://www.abc.chemistry.bsu.by/vi/ potentiodynamic electrochemical impedance spectroscopy (PDEIS)] ", Physico-Chemical Research Institute, Belarusian State University. A description of the use of a potentiostat in virtual instrumentation for electrochemical experiments.
*Pierre R. Roberge (Webmaster) " [http://www.corrosion-doctors.org/Instrumentation/potentiostat.htm Potentiostat] ", corrosion-doctors.org [http://www.corrosion-doctors.org/Principles/Glossary.htm Electrochemistry Dictionary] .
* [http://www.bio-logic.info/potentiostat/notes/Application%20note%204.pdf Potentiostat stability mystery explained]