Preliminary Evaluation of Coulostatic Analysis as a Rapid Scanning Voltammetric Technique
Reiss, John Joseph
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https://hdl.handle.net/2142/67265
Description
Title
Preliminary Evaluation of Coulostatic Analysis as a Rapid Scanning Voltammetric Technique
Author(s)
Reiss, John Joseph
Issue Date
1981
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Analytical
Language
eng
Abstract
Electrochemical monitors are in widespread use as HPLC detectors because of their sensitivity and selectivity. One desirable feature these detectors lack, however, is the ability to scan rapidly through a voltage region of interest. At present, the scan rates are limited by the influence of charging current. One way to circumvent this problem is to use a technique which is independent of charging current, such as coulostatic analysis.
In coulostatic analysis, a charge pulse is rapidly added to an electrode, increasing its potential. The electrode is then allowed to decay back to its equilibrium potential. The decay curve is collected and analyzed to determine the concentration of electroactive species present.
Typically, a voltage region is scanned using this technique, by applying a series of successively larger charge pulses to the cell. A plot is then made of observed decay rate vs. intercept voltage. An attempt was made to apply this type of analysis at an HMDE. Although charging current was not a factor, the scan rate was limited by the maximum rate at which pulses could be applied to the electrode. Scan rates higher than those available with conventional instrumentation were not attained. Factors which limited the scan rate are discussed.
As an alternative to building a voltammogram from a series of pulses, a single pulse can be allowed to decay through the entire voltage region of interest. The resulting long decays can then be analyzed. It was found that at an HMDE, single pulse voltammograms could be constructed which yielded both qualitative and quantitative information about Pb('2+) and Cd('2+) in solution. The useful concentration range for this type of analysis was limited to (TURN)10('-5) - 10('-3) M. At lower concentrations, the decay became too slow, and the desired voltage range ((DELTA)E 200 mV) was not covered in reasonable times. At the higher concentrations, the decay occurred too rapidly for the collection system used.
An attempt was made to apply these results to a CPE. The resulting decay curves did not follow any of the simple predicted models. An empirical relationship was found which allowed the desired qualitative and quantitative information to be extracted from the decays. A qualitative study was then made of the influence of flow rate on the single decays. The decays were found to be independent of flow rate, as long as the base potential was in a region where analyte did not react.
It can be concluded that the single pulse experiments show promise as a rapid scanning technique. Use of the system in a flowing stream should be quite straightforward.
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