1. Potential and Current Control

2. Control of Potential
The lowest cost method of potential control is to connect the sample to a low resistance electrode with a stable potential

3. Control of Potential
For a variable potential, a voltage source can be inserted between the two electrodes

4. Control of Potential
If the potential is monitored, it can be manually adjusted to the desired value

5. Control of Potential
However, the above techniques suffer from difficulties when trying to control potential in some systems (e.g. active-passive transitions)
For these situation the potentiostat provides near-ideal control characteristics

6. The Potentiostat - Practical

7. The Potentiostat - Practical
The Working Electrode is connected to 0 V of the power supply. It may be internally connected to mains earth, or, as here, a separate earth (or ground) terminal may be provided - this should be connected to the Working Electrode terminal if possible

8. The Potentiostat - Practical
The Reference Electrode input is connected to the inverting input of the amplifier. It is a high resistance input.

9. The Potentiostat - Practical
There may be a potential output that is derived from the difference between the reference electrode and working electrode terminals. It will have a low output impedance, and the reference electrode input will not be affected by loads connected to this output (it is said to be buffered)

10. The Potentiostat - Practical
The Output from the amplifier and the connection to the Counter Electrode may be separated on some potentiostats. This allows a resistor to be connected between the two terminals, with an internal buffer amplifier providing the same voltage at the current output terminals

11. The Potentiostat - Practical
An internal source of controlled voltage allows the control potential to be set.
External input terminals allow an external control voltage (e.g. from a sweep generator) to be used

12. The Potentiostat - Practical
Some potentiostats have an internal meter to monitor cell potential or current - beware of the current ranges, as the resistors are often damaged by excessive currents

13. The Potentiostat - Practical
Various forms of IR compensation allow for automatic or semi-automatic correction for the resistive potential difference between the tip of the Luggin probe and the specimen surface

14. The Potentiostat - Practical
The overload indicator (where fitted) provides a warning that the potentiostat is no longer able to control the potential, because too much current or too large a potential is required

15. Problems with Potentiostats - 1 Oscillation
The potentiostat relies on negative feedback to control the potential
However, delays in the feedback loop due to the charging of capacitances can shift the phase of an ac signal such that the feedback becomes positive
This causes oscillation, typically in the kiloherz region

16. Problems with Potentiostats - 1 Oscillation - a solution
A cure for this problem (and a method of diagnosis) is to fit a 1 F capacitor between the counter and reference electrode terminals
This produces a strong negative feedback at high frequencies that swamps the oscillation
Unfortunately, it also slows down the response of the potentiostat

17. Problems with Potentiostats - 2 Noise Pickup
The reference electrode input is a high impedance point, and is very sensitive to noise pickup (most commonly at mains frequency)
Mains frequency noise is rejected by standard digital multimeters, and is often overlooked
But what does it do to the electrochemistry?

18. Problems with Potentiostats - 2 Noise Pickup - solutions
Check for noise (or oscillation) with an oscilloscope connected between the counter electrode and the working electrode (not the reference electrode, as the connection of the oscilloscope may affect the behaviour, and the observed potential at the reference electrode will be held constant by the action of the potentiostat)

19. Problems with Potentiostats - 2 Noise Pickup - solutions
Screen the reference electrode with a conductor connected to ground (or the working electrode)
Screen the whole system by mounting it in a Faraday Cage (e.g. a metal box)

20. Problems with Potentiostats - 3 Electronic Noise
Some instruments may produce a significant level of noise - check by performing an experiment on a dummy cell made of electronic components, and with similar characteristics to the real cell

21. Problems with Potentiostats - 4 Overloading
If the control of the cell potential requires either a higher voltage or a higher current than the potentiostat can deliver, then the potential will not be controlled
Watch out for very constant current traces, as these are not usual for real systems
Check the overload indicator if one is fitted to the potentiostat

22. Problems with Potentiostats - 4 Overloading - solutions
If the current is limiting
use a smaller electrode
If the cell voltage is limiting
use a larger counter electrode
reduce the spacing between the counter and working electrodes
increase the conductivity of the solution (if possible)