Normal pulse polarography NPP

The problem still remains of the capacitive current. When the voltage pulse is imposed a significant capacitive current will be required to produce this potential. However near the end of the drop lifetime the growth of the surface area of the drop has almost ceased, particularly relative to the short length of the pulse, and the electrode is almost stationary. As a glance at Fig. 3.2b will show, this results in the capacitive current decaying rapidly, much more rapidly than with the classical technique. The current measurement is therefore taken in an even shorter pulse (about 15 ms) near the end of the potential pulse once the capacitive current has decayed to a low steady value (Fig. 3.3a). 

The overall form of the applied voltage signal is of a series of potential pulses, one to each drop, rising in a linear ramp with a base potential maintained between the pulses. The current recorded at the end of each pulse is recorded and plotted against the potential of the pulse. 

The resultant current/potential curve is similar in form to the classical dc polarographic curve with equivalent halfwave potential and limiting current (Fig. 3.3b). In practice the curve is made up of very short flat segments and is of a clearer, cleaner , less noisy form. The height of the pulse polarographic wave, the analytical signal, is 

Thus by using more sophisticated electronics and the imposition of a more complicated potential signal a much simpler current/potential curve can be obtained with the interferences of depletion and the capacitive current effectively minimised. 

It is not eliminated, just greatly reduced. The drop continues to grow very slowly and even with a stationary electrode capacitive current never totally reaches zero. 

The technique was introduced by Barker and Gardner [48] and was originally applied to the dropping mercury electrode. Typical durations of the intervals and the pulses are 2—5 s and 5—100 ms, respectively. Application to a stationary electrode is equally possible, provided that the interval is long enough to restore the initial surface concentrations [21]. 

The mathematical description of the normal pulse polarogram is easily derived from either eqn. (33) or eqn. (38). For sufficiently large lt 2 values, eqn. (35b) holds and reversible behaviour is observed corresponding to 

Note that for f 0 the current attains its anodic limiting value independent of potential 

If f = 0, i.e. E = E, 2 (the reversible half-wave potential), rev equals the mean of the two limiting values 

At smaller values of lt 2, charge transfer will be concomitantly ratedetermining in accordance with the general expression for jF, which can be written in the form 

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In DPP, after application of the pulse, the potential returns to a continually increasing value, which eventually is sufficient to cause electrolysis during the nonpulse part of the experiment. Therefore, DPP does not have the advantage of restricted electrolysis times seen for normal-pulse polarography (NPP). Besides its application to trace analytical work, DPP can be advantageous because of the better resolution inherent in a peak-shaped output. The reaction of iron-sulfur protein site analogues [Fe S (SR)4] , with electrophiles is studied by DPP, where closely spaced reduction waves of the reactant and product are adequately resolved". The reduction of cobaltocene in the presence of phenol studied using DPP, allows quantitative measurement of the amount of cyclopentadienylcobalt cyclopentadiene produced in the electrolysis at the dme by" ... 

This limitation led to the development of normal pulse polarography (NPP) introduced by G.C. Barker, which minimized background responses (principally the capacitance cm-rent of the growing mercury drop) and maximized the analytical response. NPP requires the application of a series of potential pulses at the working electrode for each drop. The pulse increases in potential with every drop. The current is sampled just before the end of the drops lifetime, minimizing the effect of the capacitance current on the faradaic current. As the potential pulse for each drop starts at a potential lower than the redox potential, no depletion of the analyte would have occurred. 

By using the mercury dropping electrode a single potential pulse is applied on each mercury drop, late in the drop-life. Three polarographic methods, namely normal pulse polarography (NPP), reverse pulse polar-ography (RPP) and dijferential pulse polarography (DPP), respectively, were developed for the use with DME. 

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