How Fast Can My Potentiostat Scan?

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Several times I have been asked a question about whether potentiostat Model XYZZY can scan at xyz V/s.  Rarely is the answer in the data sheet for the instrument. Often, however, enough information is given to assess the limits. The path I follow  to arrive at an answer is outlined here.


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An important fact about potentiostatic circuits is that the CELL is part of the circuit! It is in the feedback loop of the 'control amplifier,' and its characteristics will influence the response of the potentiostat-cell system! Any exact derivation of a potentiostat's response must include a complete description of the cell. Although this is rarely known, we can make a few sweeping generalizations to arrive at some guidelines for potentiostat response. It is important, however, that we not take these answers as absolutes. They are only approximations.

The question "How fast can I scan" is really two questions. The first asks whether the potentiostat can faithfully deliver a voltage ramp or triangle wave to the cell. The second asks whether the current can be faithfully recorded.  
 

To find the answer to the first question, we must look at the data sheet for the 'control amplifier' section, if there is one. If there is not, look for the 'potentiostat' section. The specification of 'slew rate' tells how fast the counter electrode can move to try to control the working-reference potential. Although you might think that this establishes an upper scan rate, it is only part of the answer. Depending on the cell, the counter electrode may have to move two volts, or even more, in order to change the working-reference voltage by just one volt. This is often the case for poorly conducting solutions (low ionic strength solutions or low dielectric constant non-aqueous solvents), where the 'extra' voltage is used to overcome the resistance of the solution. In the very best case, however, the counter electrode will 'slew' only one volt for each volt change in the working electrode potential.

Sometimes you will not find a slew rate specification, but you may find a specification for 'bandwidth' or 'frequency response.'  You may estimate the slew rate by multiplying the bandwidth or frequency response value (in Hz) by about 3 to get the slew rate in V/s. ( Why? ) A potentiostat bandwidth of 100 kHz would correspond to a slew rate of about 300,000 V/s. 

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