LecturenotesSquarewavevoltammetry

LecturenotesSquarewavevoltammetry - SQUARE—WAVE...

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Unformatted text preview: SQUARE—WAVE VOLTAMMETRY In reviewing the techniques of tast polarography, normal pulse polarography. and differential pulse polarography, one realizes that each technique utilizes a series 9f individual mercury drops issuing from a capillary at precisely timed intervals. Another form of polamgraphy is called square—wave voltammetry, and one is immediately struck by the fact that the entire set of measurements is done so rapidly that it can be done with a single mercury drop. Key relerences pertaining to square—wave voltammetry are as follows: (1) L. Ramalay and M. S. Krause, Jr._, Anal. Chen-1., 41. 1362 (1.969). (2) L. Ramaiay and M. S. Krause= Jr., Anal. Chem, 41, 1365 (1969). (3) J. H. Christie, J. A. Turner, and R. A. Ostetyoung. Anal. Chem, 49, 1899 (197?). (4) .l. A. Turner. .1. H. Christie, M. VLIkOViC, and R. A. Osteryoung- Anal. Chem. 49, 1904 (1977). (5) R. Samuelsson, J. J. O’Dea, and J. Oste1young,Amn’. Chem, 52, 2215 (1980}. (6) J. J. O’Dea, J. Osteryoung, and R. A. Osteryoung, Anal. Chem. 53, 695 (1981). (7) .l. Osteryoung and .1. J. O‘Dea, in li‘fecrroamdyrfcal Chemistry, A. J. Bard, ed.= Volume 14, Marcel Dekker, lne., New York, 1980. Shown in the figure below is the excitation wavefomi for modern square—waye voltammetry; the baseline potential increases by Air.” for each full cycle of the square wave whose half-height is Es“. and whose period is r, and the current is measured at the end of each half cycle (times I and 2 in the figure). This waveform is applied to a single drop from a polarographic capillary; the time interval TL. is designed to allow the drop to grow to a predetermined size (usually to almost its full size just before it detaches from the tip of the capillary). ' POTENTIAL TIME Figure l. Excitation signal for square-wave voltammetry. [Adapted [rom Reference 7.] Typical results are shown in the figure below; the current component labeled A is the current for the forward half cycle (time 1), component B is due to the reverse half cycle (time 2'), and component C is the difference (curve A - curve B). In appearance, curve C. resembles that seen in differential pulse polarography; the detection limit for square—wave voltammetry is between 10"0 and 10"9 M. |5 c IO 0 -s zoo o -200 ~4oo nlE—EW) mV Figure 2. Calculated square-wave voltammograms for reversible electron transfer: (A) forward current; (B) reverse current; (C) net current, dimensionless units. [Adapted from Reference 9.] Reference 7 above provides a comprehensive review of this technique, along with predictions for the response formally types of complicated reaction schemes. October. 2010 ...
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LecturenotesSquarewavevoltammetry - SQUARE—WAVE...

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