Circuit potentiostatic

Fig. 5.17 A four-electrode potentiostatic circuit (voltage clamp). and R2 are reference electrodes with Luggin capillaries (arrows) attached as close as possible to the membrane or ITIES (dashed line), B, and B2 are auxiliary electrodes, P and P2 potentio-stats, G programmed voltage generator and Z recorder... 

The impedance can be measured in two ways. Figure 5.23 shows an impedance bridge adapted for measuring the electrode impedance in a potentiostatic circuit. This device yields results that can be evaluated up to a frequency of 30 kHz. It is also useful for measuring the differential capacity of the electrode (Section 4.4). A phase-sensitive detector provides better results and yields (mostly automatically) the current amplitude and the phase angle directly without compensation. 

A. Frey, M. Jenkner, M. Schienle, C. Paulus and B. Holtzapfl, Design of an Integrated Potentiostat Circuit for CMOS Bio Sensor Chip, IEEE, 2003, pp. 9-12. 

The most convenient and reliable electrical biasing method for use with a hydrated SPE cell has been shown to be a three electrode potentiostatic circuit which maintains the sensing electrode at a predetermined potential vs. a stable reference (1 >3.>j0e The most reversible reference is a Pt/Hp, H+, static or dynamic. In practical instruments, however, good accuracy and convenience are achieved using a large surface area platinoid metal black/air (Op). All work reported in this study utilized the air reference which has a potential of approximately +1.05 V vs. a standard hydrogen electrode (SHE). For convenience, all potentials reported are vs. the SHE ... 

Potentiostatic Circuit. The electrical circuit used for breadboard testing of three-electrode sensor cells is shown in Figure 2. Amplifier U1 sensed the voltage between the reference and... 

Figure 2. Schematic of simple potentiostatic circuit for sensor tests...

Overcoming Solution Resistance with Stability and Grace in Potentiostatic Circuits... 

There are several other methods of achieving stability in potentiostatic circuits. A capacitor may be added between the counter and reference electrodes to reduce phase shift in the critical frequency region. Some caution must be exercised since a low-resistance reference electrode then becomes the counterelectrode at high frequencies. A particularly interesting method is known as input lead-lag compensation a series RC is connected between the input terminals of the control amplifier, and a second resistor is connected between the noninverting input and common. This form of compensation has minimum effect on the slew rate of the control amplifier. Further details can be found in the book by Stout and Kaufman listed in the bibliography. 

It is reasonable to ask at this point Are there other approaches to reach stability with grace in true potentiostatic circuits The answer is indeed affirmative, but unfortunately with qualifications. One technique is discontinuous control of cell potential. It is not a new approach it was, in fact, the method used in the very first electronic potentiostat by Hickling in 1942. The principle is quite simple Current pulses are applied to the counterelectrode so that the desired potential is maintained between reference and working electrode. Since the potential can be measured between pulses, there is no iR drop. Cell potential is not steady it depends on the sensitivity of the comparator circuit and the rate at which current demand can be met. 

Fig. 12.63. In a potentiostatic circuit, an imposed potential difference V drives the current from the auxiliary electrode to the test electrode until the potential difference V becomes equal to V, so that the total input into the potentiostat falls to zero.

The specimens were mounted on stubs and attached to a rotary drive within the electrochemical vessel and turned upside down to contact the electrolyte surface with their circular front plane. The electrolyte level was arranged such that the specimens were not wetted around the edge (hanging meniscus). Short passivation times of less than 1 second were finished by disconnection of the counter electrode with a relay, thus opening the potentiostatic circuit. Alternatively, the potential was pulsed to a value where no further reaction occurs. 

Fig. 7.7. Potentiostat circuit for control of working electrode potential. All resistances are equal, except / D which is variable.

Ref [i] Roe DK (1996) Overcoming solution resistance with stability and grace in potentiostatic circuits. In Kissinger PT, Heineman WR (eds) Laboratory techniques in analytical chemistry, 2nd edn. Marcel Dekker, New York, pp 195-235... 

Potentiostatic circuit — The potentiostatic circuit consists of the electrochemical cell with three electrodes (-> working electrode WE, reference electrode RE, and -> counter electrode CE) and a special electronic amplifier, the potentiostat. 

Refs. [i] Myland JC, Oldham KB (2000) Anal Chem 72 3972 [ii] Oldham KB, Stevens NPC (2000) Anal Chem 72 541 [iii] Roe DK (1996) Overcoming solution resistance with stability and grace in potentiostatic circuits. In Kissinger PT, Heineman WR (eds) Laboratory techniques in electroanalytical chemistry, 2nd edn. Marcel Dekker, New York, chap 7 [iv] Oldham KB, Myland JC (1994) Fundamentals of electrochemical science, 1st edn. Academic Press, San Diego, p 397... 

Fig. 5 Electrical circuitry for polarography/voltammetry. (A) A simple two-electrode system. (B) Illustrates a modern three-electrode system incorporating a potentiostat circuit.

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