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2. Chronopotentiometry or Chronoamperometry

Goal: decide what ADC to use in building a general electroanalytical instrument that can do either chronopotentiometry or chronoamperometry. While we could also look at the needed DAC and analog circuits to control current or potential, that is NOT part of this problem.

Analysis of the problem: In chronopotentiometry, one monitors the potential on an electrode through which current is constant (possibly including i=0). In chronoamperometry, one monitors the current at an electrode which is maintained at a constant potential vs. a reference potential. The electrochemical standard potential ranges from + 3.03 V (for 1/2 F2 + e- + H+ ↔ HF) to
- 2.923 V (for Cs+ + e- ↔ Cs). Currents may range from ~ 1 pA to ~ 1 A, 12 orders of magnitude.

Questions and choices:

  1. To measure potential in chronopotentiometry, would you choose a unipolar or bipolar ADC? What potential range would you choose (look on the web to find an example of an ADC that has this range)? How many bits would it have?
  2. To measure current in chronocoulometry, would you choose a unipolar or bipolar ADC?
  3. Converting current to voltage will be essential. What voltage output range would you choose? If you use a current-to-voltage converter with a 1 MΩ resistor so that output is 1V/μA, what is the highest current that can be digitized?
  4. An operational amplifier typically has noise of 10 nV Hz-1/2. If this is the dominant noise source, what is the fastest 16 bit ADC you need to carry out the measurements you chose in question 3? Is this a sensible answer?
  5. Is it always the case that amplifier noise constrains ADC resolution for chronoamperometry? If not, list other possible limiting factors.
  6. Finally, if you want to perform chronoamperometry with data collection at 100 kHz (Nyquist frequency 50 kHz), choose an ADC resolution that is appropriate and a converter type that is appropriate (again, look on the web to choose an ADC that meets your specifications).

Go to next page for answers.



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