Demodulation polarography with triangularly modulated polarizing voltage
✍ Scribed by Wennemar A. Brocke
- Book ID
- 104148854
- Publisher
- Elsevier Science
- Year
- 1971
- Weight
- 126 KB
- Volume
- 33
- Category
- Article
- ISSN
- 0022-0728
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✦ Synopsis
Recently the sensitivity of the new second-order technique "demodulation polarography ''1 has been demonstrated to be higher by one order of magnitude than that of conventional a.c. polarography at almost equal complexity of apparatus. This advantage results from the higher ratio of faradaic to capacitive current as an inherent property of some polarographic methods based on the non-linearity of the faradaic impedance. Considering that the undesired capacitive current rather than random noise contributed by the cell or the amplifying device connected to it determines the sensitivity of demodulation polarography, a suitable method able to suppress coherent noise should be adapted for sensitivity improvement. In a recent paper 2 a new a.c. polarographic technique has been proposed. By applying a triangular a.c. voltage across the electrode interface the capacitive current can be transformed into a d.c. signal by a special measuring device which also separates it from the alternating faradaic current. This elimination method needs no phase alignment* so that there is no complication of measurement.
In this note it is proposed to adapt the capacitive current suppressing method 2 to demodulation polarography t . For this purpose the polarizing voltage Uc is modulated by a triangular signal rather than by a sinusoidal one corresponding to the triangular voltage superimposed on the d.c. potential in the mentioned type of a.c. polarography 2 . If the waveforms of the faradaic demodulation current iFD and the capacitive demodulation current iCD then coincide with those of the faradaic current i F and the capacitive current i C respectively the elimination method can successfully be combined with demodulation polarography.
E.g. for the reversible electrode process the computation of the waveforms proceeds as follows (Notation is taken from ref. 1). Equation (6) in ref. 1 has to be replaced by the triangularly modulated wave UD = t~D [1 + (8m/Tr 2) (sin Wmt --3 -2 sin 3~Omt + 5 -2 sin 5Wrnt -...) sin wot ] (1) Then eqns. (21) and (22) in ref. 1 become *Compare the a.c. polarographic technique using an ampfitude-modulated sinusoidal voltage 3.