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Normal-Pulse Polarography (NPP)

Introduction

Pulse polarographic techniques are voltammetric measurements which are variants of the polarographic measurement which try to minimize the background capacitive contribution to the current by eliminating the continuously varying potential ramp, and replacing it with a series of potential steps of short duration. In Normal-pulse polarography (NPP), each potential step begins at the same value (a potential at which no faradaic electrochemistry occurs), and the amplitude of each subsequent step increases in small increments. When the Hg drop is dislodged from the capillary (by a drop knocker at accurately timed intervals), the potential is returned to the initial value in preparation for a new step.

For this experiment, the polarogram is obtained by plotting the measured current vs. the potential to which the step occurs. As a result, the current is not followed during Hg drop growth, and normal pulse polarogram has the typical shape of a sigmoid. By using discrete potential steps at the end of the drop lifetime (usually during the last 50-100 ms of the drop life which is typically 2-4 s), the experiment has a constant potential applied to an electrode with nearly constant surface area. After the initial potential step, the capacitive current decays exponentially while the faradaic current decays as the square root of time. The diffusion current is measured just before the drop is dislodged, allowing excellent discrimination against the background capacitive current. In many respects, this experiment is like conducting a series of chronoamperometry experiments in sequence on the same analyte solution. The normal pulse polarography method increases the analytical sensitivity by 1 - 3 orders of magnitude (limits of detection 10-7 to 10-8 M, relative to normal dc polarography.


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