Polarography differential curves

Fig. 3.4037 illustrates well the character of the curves and the gain in sensitivity on going from conventional DC via sampled DC and normal pulse to differential pulse polarography. It should be realized that, although the DPP curve AijAE) might seem an approximation of a derivative curve, we cannot speak of derivative polarography. 

Figure 3.16. Waveforms for pulse and differential-pulse polarography. Curves A and D Excitation signal applied to the working electrode. Curves B and E Instantaneous current observed at a single drop as a function of time. Curves C and F The resulting current-versus-voltage curves. In pulse polarography, square-wave voltage pulses of 40-msec duration are applied to the mercury drop, of drop-life mechanically controlled at 2.5 sec (A) ta,ti,ti,... represent successive drops. The overall rate of increase of the amplitude of the voltage pulses is about...

The detection limit for pulse polarography is typically about lO" Jl/, and about 10" Mfor the differential-pulse method—although, of course, detection limits do depend on the electrochemical properties of the substance analyzed, interferences, and other experimental variables. The detection limit for arsenic(III) by differential-pulse polarography, for example, has been reported to be 4 x 10" M (0.3 ppb), with a linear calibration curve up to 8 x 10" M. 

Each electroanalytical technique has certain characteristic potentials, which can be derived from the measured curves. These are the half-wave potential in direct current polarography (DCP), the peak potentials in cyclic voltammetry (CV), the mid-peak potential in cyclic voltammetry, and the peak potential in differential pulse voltammetry (DPV) and square-wave voltammetry. In the case of electrochemical reversibility (see Chap. 1.3) all these characteristic potentials are interrelated and it is important to know their relationship to the standard and formal potential of the redox system. Here follows a brief summary of the most important characteristic potentials. 

The intersects of the curves of the nearest oxidation states correspond to the macroscopic formal potentials and these values are in good agreement with the values determined by differential pulse polarography at the mercury electrode [126]. 

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